Microstructural Evolution and Tensile Properties of Direct Metal...

i

Microstructural Evolution and Tensile Properties of Direct

Metal Laser Sintered (DMLS) CoCrMo and Direct Metal

Laser Deposited (DMLD) FSX-414 Cobalt base superalloys

A Dissertation

Submitted in Partial Fulfillment of the

Requirements for the degree of

Master of Engineering

In

Materials Engineering

By

Kaustubh Krishna Bawane

In collaboration with

GE India Technology Centre Pvt. Ltd., Bangalore, India

Under the guidance of

Prof. Dipankar Banerjee (IISc, Bangalore)

Dr. Dheepa Srinivasan (GE Power)

Department of Materials Engineering

Indian Institute of Science

Bangalore – 560012, India

June 2016

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ABSTRACT

Direct metal laser sintering (DMLS) and Direct Metal Laser Deposition (DMLD) are emerging

additive manufacturing or 3D printing technologies based on slicing a solid model into multiple

layers and building part layer by layer. Therefore parts with intricate shapes and cavities can be

built without need of dedicated tools and machining unlike conventional methods. This study

comprises microstructural characterization and tensile properties of DMLS CoCrMo and DMLD

FSX-414 cobalt based superalloys.

DMLS CoCrMo was investigated for microstructures and tensile properties in the as

printed and after heat treatments. As printed DMLS CoCrMo showed columnar dendritic

microstructure with the column width of 0.6 to 1 µm. STEM-EDS analysis showed Mo and Si

enrichment in the interdendritic region. Solution treatment at 1150oC showed fully equiaxed

grain structure due to breakdown elongated grains from as printed samples following Rayleigh

like instability. Solution treated samples also showed some remnants of the previous

interdendritic region. Extensive precipitation was observed along the grain boundaries as well as

inside grains after ageing treatment at 980oC. SEM-EDS mapping showed Mo and Si enrichment

in the precipitates with composition very similar to those observed in the interdendritic region.

Solution treatment resulted in decrease in room temperature tensile strength from 1378 MPa to

1114 MPa and increase in ductility from 5.7 to 15%, which was attributed to increase in grain

size from 0.6-1 µm (column width) in as printed to ~40 µm (grain size) in solution treated

samples. Room temperature tensile strength had dropped marginally to 982 MPa after ageing

treatment, implying grain size as major factor in determining strength over precipitation.

Considerable drop in ductility to 5.3% was reported after ageing treatment due to extensive

precipitation along grain boundaries. High temperature tensile properties were studied for

solution treated and aged specimens. Both of them showed considerable drop in tensile strength

and increase in ductility due to thermally activated mechanisms.

As deposited DMLD FSX-414 showed columnar dendritic structure with (Cr21W2)C6

precipitates in the interdendritic region (column width: 9-12 µm). DMLD FSX-414 was

subjected to three different solution treatment temperatures, viz. 1150oC, 1200

oC, 1250

oC, etc. in

order to evaluate the thermal stability of the alloy. Equiaxed microstructure with remnants of

interdendritic precipitates was observed after 1250oC treatment due to breakdown of as deposited

elongated grains following Rayleigh like instability. Both solution treatment at 1150oC and

ageing treatment at 980oC showed same columnar dendritic microstructure. Room temperature

tensile properties showed only marginal drop in tensile strength after solution (1150oC) and

ageing (980oC) heat treatment, which was attributed to negligible change in respective

microstructures. Solution (1150oC) and aged (980

oC) DMLD FSX-414 showed higher tensile

strength than Solution (1150oC) and aged (980

oC) Cast FSX-414 which was attributed to their

respective secondary dendrite arm spacing (4-6 µm for DMLD FSX-414 and 70 µm for Cast

FSX-414). All samples showed fully ductile fractures.

The results suggest possible applications of these techniques in the field of gas turbine

repair technology.

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ACKNOWLEDGEMENT

First of all, I would like to express my sincere gratitude to my supervisors Prof. Dipankar

Banerjee, IISc Bangalore and Dr. Dheepa Srinivasan for their patient, support and guidance

throughout this project. It is because of their valuable insights, suggestions that I could

successfully complete my dissertation. I express my deep sense of indebtedness to both my

supervisors for giving me this wonderful opportunity to work in GE. I feel privileged to be a part

of this GE-IISc collaboration. I also thank Prof. Abhik Chaudhury for his valuable inputs on my

project work.

I am thankful to Dr. Krishnamurthy Anand and Dr. Sundar Amancherla for allowing me

to use facilities at GE to carry out experiments. I also extend my gratitude to Prof. T.

Abinandanan, Chairman, Department of Materials Engineering and all other faculty members for

allowing me to use experimental facilities at IISc.

I would like to thank Mr. N Raghunandan and Mr. CA Jagadish from Intech DMLS,

Bangalore for providing DMLS CoCrMo parts and Dr. Bhaskar Dutta, DM3D Technologies,

Auburn, USA for providing DMLD FSX-414 parts.

I would like to express my sincere gratitude to Dr. Joydeep Pal and Mr. Dayananda

Narayana for their help in various aspects on my project. I extend my unlimited thanks to Mr.

Vinay Kunnathully for his help in TEM characterization. I am thankful to Mr. Hariharan S. for

his help in carrying out vacuum heat treatment. I would like to thank Mr. Shivanandappa Meti

and Mr. Lakshmikanth S. for assisting me in my work. My special thanks to Dr Amuthan Ramar

and Mr Prasanna for help with twin-jet polishing.

I wish to acknowledge the cooperation and assistance of technical and non-technical staff

of the Department of Materials Engineering, IISc for my project. My special thanks to M. S.

Sasidhara for his help in tensile testing.

I am grateful to Mr. Hariharan S, Dr. Prasad Raghupathruni, Ms Shalaka Shinde, Mr.

Subramahnyam Adabala, Mr. Aravind Prasanth, Mr. Joel Bhagyanath and all other GE

colleagues for constant support and encouragement and making my working experience very

comfortable.

My sincere thanks to all my labmates and classmates for their support and time to time

help during my stay at IISc.

I would like to extend my sincere thanks to my parents for their patience, love and

constant support.

iv

TABLE OF CONTENTS

ABSTRACT .................................................................................................................................... ii

ACKNOWLEDGEMENT ............................................................................................................. iii

LIST OF FIGURES ...................................................................................................................... vii

LIST OF TABLES ........................................................................................................................ xii

LIST OF ACRONYMS ............................................................................................................... xiii

1. INTRODUCTION ............................................................................................................... 14

1.1 Additive Manufacturing ............................................................................................. 14

1.2 The Land based turbine and Co alloys ....................................................................... 16

1.3 Metallurgy of Cobalt base alloys ................................................................................ 18

1.4 Direct Metal Laser Sintering (DMLS) ........................................................................ 20

1.4.1 Process Parameters of DMLS ....................................................................................... 21

1.4.2 Microstructural defects in DMLS processing ............................................................... 21

1.4.3 Microstructural evolution during DMLS processing .................................................... 22

1.4.4 Tensile properties of DMLS processed parts ............................................................... 23

1.5 Direct Metal Laser Deposition (DMLD) .................................................................... 24

1.6 Motivation and Objectives .......................................................................................... 25

2. MATERIALS AND EXPERIMENTAL PROCEDURE..................................................... 26

2.1 Materials and Heat treatment ......................................................................................... 26

2.2 Powder characterization ................................................................................................. 27

2.3 Processing conditions..................................................................................................... 27

2.4 Chemical Analysis – Inductively Coupled Plasma (ICP) .............................................. 28

2.5 Surface roughness .......................................................................................................... 28

2.6 X-ray Tomography........................................................................................................ 29

2.7 Metallographic procedure .............................................................................................. 29

2.8 Porosity area fraction ..................................................................................................... 30

2.9 Optical and Scanning Electron Microscopy .................................................................. 31

v

2.10 Transmission Electron Microscopy ............................................................................. 32

2.11 Electron Backscattered Diffraction (EBSD) ................................................................ 32

2.12 X-ray Diffraction (XRD) ............................................................................................. 32

2.13 X-ray Diffraction Sin2 ψ technique for residual stress measurement .......................... 33

2.14 Microhardness .............................................................................................................. 34

2.15 Tensile testing .............................................................................................................. 35

3. RESULTS - PART A: DIRECT METAL LASER SINTERING OF CoCrMo .................. 37


3.2 Chemical analysis .......................................................................................................... 38

3.3 Surface Roughness ......................................................................................................... 38

3.4 X-ray Microtomography of as printed DMLS CoCrMo ................................................ 39

3.5 Porosity .......................................................................................................................... 40

3.6 Microstructural Characterization of as printed DMLS CoCrMo ................................... 41

3.6.1 Optical microscopy ....................................................................................................... 41

3.6.2 Scanning Electron Microscopy ..................................................................................... 42

3.6.3 TEM Micrographs of As printed DMLS CoCrMo ....................................................... 44

3.7 Microstructural characterization of Solution heat treated DMLS CoCrMo .................. 47

3.8 Microstructural characterization of Sol HT + Aged DMLS CoCrMo ........................... 49

3.9 X-ray Diffraction ........................................................................................................... 51

3.10 Electron Backscattered Diffraction (EBSD) – Inverse Pole Figure maps ................... 53

3.11 X-ray Diffraction Sin2 ψ technique for residual stress measurements ......................... 54

3.12 Hardness ....................................................................................................................... 54

3.13 Tensile properties ......................................................................................................... 55

4. RESULTS - PART B: DIRECT METAL LASER DEPOSITION OF FSX-414 ................ 60


4.2 Chemical analysis .......................................................................................................... 61

4.3 X-ray Microtomography of as deposited DMLD FSX-414 ........................................... 61

4.4 Porosity .......................................................................................................................... 62

4.5 Microstructural Characterization of as deposited DMLD FSX-414 .............................. 62


vi


4.6 Microstructural characterization of Solution heat treated DMLD FSX-414. ................ 67



4.7 Microstructural characterization of Sol HT 1150oC + Aged DMLD FSX-414. ............ 70

4.8 Microstructural characterization of Sol HT+Aged Cast FSX-414. ............................... 72

4.9 X-ray Diffraction ........................................................................................................... 74

4.10 Hardness ....................................................................................................................... 75

4.11 Tensile properties ......................................................................................................... 77

5. DISCUSSION ...................................................................................................................... 81

5.1 Microstructural evolution in Direct Metal Laser Sintered CoCrMo .............................. 81

5.1.1 Porosity and microcracks .............................................................................................. 81

5.1.2 Macrostructure in as printed DMLS CoCrMo .............................................................. 81

5.1.3 Microstructure in DMLS CoCrMo ............................................................................... 82

5.2 Tensile properties of Direct Metal Laser Sintered CoCrMo .......................................... 86

5.3 Microstructural evolution in Direct Metal Laser Deposited FSX-414 .......................... 88

5.4 Tensile properties of Direct Metal Laser Deposited FSX-414 ...................................... 89

5.5 Comparison between DMLS CoCrMo and DMLD FSX-414 ....................................... 90

6. CONCLUSIONS AND FUTURE WORK .......................................................................... 92

6.1 Conclusions .................................................................................................................... 92

6.2 Future work .................................................................................................................... 93

BIBLIOGRAPHY ......................................................................................................................... 94

vii

LIST OF FIGURES

Figure 1.1 (a) Polymeric AM part showing kind of intricate designs that can be built; b) AM Fuel

Nozzle used in General Electric’s LEAP Jet Aircraft engine.

Figure 1.2 AM applications timeline of past, present and potential future applications [2]

Figure 1.3 Classification of Additive manufacturing (AM) techniques [3]

Figure 1.4 Land based gas turbine showing its three different sections namely, compressor,

combustor and hot gas path (Image source: gesol.com).

Figure 1.5 Equilibrium phase diagrams of (a) Co-Cr, (b) Co-Mo, (c) Co-Si systems [8]

Figure 1.6 Schematic showing Direct Metal Laser Sintering (DMLS) technology (Image source:

Custompart.net)

Figure 1.7 Relationship between DMLS process parameters and resulting properties [11].

Figure 1.8 Microsections of the Ti-6Al-4V specimens parallel to the building direction for

different beam powers: (a) 90 W, (b) 120 W, (c) 180 W [15].

Figure 1.9 Schematic diagram of generation of melt pools [21].

Figure 1.10 Schematic drawing showing Direct Metal Laser Deposition technology (Courtesy:

DM3D Technology) [3].

Figure 2.1 (a) Flat Direct Metal Laser Sintered (DMLS) CoCrMo coupon with cooling holes, (b)

Cylindrical DMLS CoCrMo for making tensile specimens, (c) Direct Metal Laser Deposited

(DMLD) FSX-414 on a cast nozzle

Figure 2.2 Procedure for determining hole roughness using optical microscopy

Figure 2.3 Nomenclature of various sections of DMLS CoCrMo and DMD FSX-414 component.

Figure 2.4 Method of % porosity evaluation on transverse section of DMLS component.

Figure 2.5 Method of % porosity evaluation on transverse section of DMLD FSX-414

component.

Figure 2.6 Method used for measuring residual stress on as printed DMLS CoCrMo part.

Figure 2.7 (a) DMLS CoCrMo and (b) DMLD FSX-414 components showing different locations

for taking hardness readings.

Figure 3.1 SE images showing size and morphology of as received CoCrMo powder, at (a) low

magnification (b) high magnification.

Figure 3.2 Particle size distribution of CoCrMo powder

Figure 3.3 Representative micrographs showing topography of (a) surface and (b) hole of as

printed DMLS CoCrMo.

Figure 3.4 2D X-ray Microtomography images of as printed DMLS CoCrMo with 0ᵒ tilt

(Voltage: 200 kV, Current: 500 µA).


(Voltage-200 kV, Current-500 µA).

viii

Figure 3.6 Porosity distribution on transverse section of as printed DMLS CoCrMo along the

build direction and corresponding unetched microstructure showing porosity at locations A, B, C

respectively.

Figure 3.7 Optical micrographs of transverse, front planar and base sections of as printed DMLS

CoCrMo, etched with 5% HCl - electrolytic – 6V.

Figure 3.8 Optical micrographs of transverse section of as printed DMLS CoCrMo at (a) low

magnification showing irregular pores at the interlayer boundaries and (b) high magnification

showing microcracks (etched with 5% HCl, electrolytic-6V)

Figure 3.9 SE image of as printed DMLS CoCrMo at (a) low magnification showing columnar

microstructure and domains (grains) and (b) high magnification one to one matching along melt

pool boundary (Etched with 5% HCl, electrolytic-6V).

Figure 3.10 BSE images of as printed DMLS CoCrMo at (a) low magnification, and (b) high

magnification showing bright contrast in the interdendritic region, etched with 5% HCl

(electrolytic-6V)

Figure 3.11 (a,b) High magnification BSE images of unetched as printed DMLS CoCrMo

showing interdendritic precipitates and ε-HCP (arrow) cutting across the columns.

Figure 3.12 TEM bright field image of as printed DMLS CoCrMo sample showing ε-HCP

phases in γ-FCC CoCrMo matrix.

Figure 3.13 High resolution-(HR) TEM image of as printed DMLS CoCrMo and its

corresponding FFT pattern showing existence of HCP phase

Figure 3.14 FFT pattern of entire HRTEM image from Figure 13 showing spots for both FCC

and HCP and their orientation relationship

Figure 3.15 High Angle Annular Dark Field (HAADF) STEM images of as printed DMLS

CoCrMo specimen showing elongated bright precipitates and globular black precipitates in the

interdendritic region.

Figure 3.16 Optical micrographs of Transverse, front planar and base sections of Sol HT DMLS

CoCrMo showing fully equiaxed grains on all sides, etched with 5% HCl, electrolytic – 6V

Figure 3.17 (a) Optical micrograph of Sol HT DMLS CoCrMo showing equiaxed microstructure

with average grain size of 44 µm and (b) corresponding grain size distribution.

Figure 3.18 (a) BSE image of unetched Sol HT DMLS CoCrMo showing twins and equiaxed

microstructure, (b) corresponding EDS spectrum.

Figure 3.19(a,b) High magnification BSE images of unetched Sol HT DMLS CoCrMo showing

remnants of previous interdendritic precipitates inside grains as well as precipitates along grain

boundaries.

Figure 3.20 Optical micrographs of Sol HT+Aged DMLS CoCrMo at (a)200x and (b)1000x

showing equiaxed grain structure and some precipitation (arrow), etched with 5% HCl –

electrolytic, 6V.

Figure 3.21(a,b) BSE images of Sol HT+Aged DMLS CoCrMo samples showing bright and dark

precipitates inside the grains as well as along the grain boundaries.

ix

Figure 3.22 (a) High magnification BSE image of Sol HT+Aged DMLS CoCrMo sample

showing bright and dark precipitate, and (b) corresponding EDS elemental mapping.

Figure 3.23 X-ray Diffractions patterns of (a) CoCrMo Powder, (b) as printed (c) Sol HT , (d)

Sol HT+aged, DMLS CoCrMo, all showing peaks for both γ-FCC and ε-HCP Cobalt phases

(Target: Cr-Kα -2.29 Aº)

Figure 3.24 EBSD IPF Maps of (a) As printed, (b) Solution heat treated DMLS CoCrMo

Figure 3.25 (a) Residual stress distribution on the surface of as printed CoCrMo, (b) Variation of

residual stress along the build direction

Figure 3.26 Hardness profile along build direction (a) as printed, (b) Sol HT, (c) Sol HT+aged,

DMLS CoCrMo.

Figure 3.27 Room temperature and high temperature tensile properties of as printed, Sol HT, Sol

HT+aged DMLS CoCrMo, (a) % Y.S., (b) UTS, (c) Ductility.

Figure 3.28 Engineering stress-Engineering strain curve of as printed DMLS CoCrMo tested at

room temperature

Figure 3.29 Engineering stress-Engineering strain curves of Sol HT DMLS CoCrMo tested at (a)

room temperature, (b) 925 oC

Figure 3.30 Engineering stress-Engineering strain curves of Sol HT+Aged DMLS CoCrMo

tested at (a) room temperature, (b) 925 oC

Figure 3.31 Fractographs of as printed DMLS CoCrMo tensile sample showing cracks.

Figure 3.32 Fractographs of Sol HT DMLS CoCrMo tensile samples showing mixed brittle and

ductile type failures.

Figure 3.33 Fractographs of Sol HT+Aged DMLS CoCrMo tensile samples showing

intergranular fracture in room both room temperature and high temperature tests.

Figure 4.1 SE images showing morphology of FSX-414 powder in the as received condition at

(a) low magnification and, (b) high magnification .

Figure 4.2 Particle size distribution of FSX-414 powder.

Figure 4.3 2D X-ray Microtomography images of as deposited DMLD FSX-414 samples with (a)

0o tilt and (b) 15

o tilt (Voltage: 200 kV, Current: 500 µA)

Figure 4.4 Porosity distribution along transverse section of as deposited DMLD FSX-414 and

Cast FSX-414, and corresponding unetched microstructure showing porosity at locations A, B, C

respectively.

Figure 4.5 Optical micrographs of the unetched as deposited DMLD FSX-414 showing

solidification cracks in (a) DMLD Part and (b) DMLD and Cast FSX-414 joint.

Figure 4.6 (a) Optical micrograph of as deposited DMLD FSX-414 showing dendritic

microstructure. (b) as deposited DMLD FSX-414 and Cast FSX-414 joint showing dendritic

growth direction relative to cast FSX-414 substrate, etched with 5% HCl, electrolytic-6V.

Figure 4.7 Stitched optical micrograph of as deposited DMLD FSX-414 showing dendritic

microstructure and domains/bundles of dendrites with same orientation.

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Figure 4.8 SEM micrographs of etched as deposited DMLD FSX-414 showing (a) primary

dendrites growing on the substrate cast FSX-414 and (b) domain boundary; etched with 5% HCl,

electrolytic – 6V

Figure 4.9 (a,b) BSE images of unetched as deposited DMLD FSX-414 showing columnar

structure with elongated bright and globular dark phases in the interdendritic region.

Figure 4.10(a) High magnification BSE image of as deposited DMLD FSX-414 showing bright

and dark precipitate, and (b) corresponding EDS elemental mapping.

Figure 4.11(a,b) Optical micrographs of Sol HT-1150oC DMLD FSX-414 showing fully

dendritic structure, etched with 5% HCl, electrolytic-6V.

Figure 4.12(a,b) Optical micrographs of Sol HT-1200oC DMLD FSX-414 showing dendritic

structure with the indication of the grain boundary, etched with 5% HCl, electrolytic-6V.

Figure 4.13(a,b) Optical micrographs of Sol HT-1250oC DMLD FSX-414 showing complete

breakdown of dendritic structure.

Figure 4.14(a,b) BSE images of Sol HT-1150oC DMLD FSX-414 showing interdendritic

precipitates.


precipitates.

Figure 4.16(a,b) BSE images of Sol HT-1250oC DMLD FSX-414 showing remnants

interdendritic precipitates.

Figure 4.17 (a) High magnification BSE image of Sol HT-1150oC DMLD FSX-414 showing

bright and dark precipitate, and (b) corresponding EDS elemental mapping.

Figure 4.18(a,b) Optical micrographs of Sol HT-1150oC+aged DMLD FSX-414 showing

dendritic microstructure, etched with 5% HCl, electrolytic – 6V.

Figure 4.19(a,b) BSE images of Sol HT-1150oC+aged DMLD FSX-414 samples showing bright

and dark precipitates in the interdendritic regions and ε-HCP bands crossing across the column.

Figure 4.20 (a) High magnification BSE image of Sol HT-1150oC+aged DMLD FSX-414

sample showing bright and dark precipitate, and (b) corresponding EDS elemental mapping.

Figure 4.21 Optical micrographs of Sol HT-1150oC+aged Cast FSX-414 at (a) low magnification

showing coarse dendritic structure, (b) high magnification showing interdendritic precipitates,

etched with 5% HCl – electrolytic, 6V.

Figure 4.22(a,b) BSE images of Sol HT-1150oC+aged Cast FSX-414 samples showing eutectic

phases in the interdendritic region.

Figure 4.23 (a,b) High magnification BSE image of Sol HT-1150oC+aged Cast FSX-414 sample

showing interdendritic precipitates and (b) corresponding EDS elemental mapping.

Figure 4.24 X-ray Diffractions patterns of (a) as deposited, (b) Sol HT-1150oC (c) Sol HT-

1150oC+aged, DMLD FSX-414, showing peaks for both γ-FCC and ε-HCP Cobalt phases


Figure 4.25 Hardness profile along build direction as deposited, Sol HT-1150oC, Sol HT-

1150oC+aged, DMLD FSX-414 and Cast FSX-414.

Figure 4.26 Hardness comparison between DMLD and Cast FSX-414.

xi

Figure 4.27 Variation in hardness with different solution heat treatment temperatures.

Figure 4.28 Room temperature tensile properties of As deposited, Sol HT+Aged DMLD and

Cast FSX-414, (a) 0.2% Y.S. and UTS, (b) % Elongation (ductility).

Figure 4.29 Fractographs of as deposited DMLD FSX-414 tensile sample showing (a) cracks at

low magnification and (b) dimples at high magnification.

Figure 4.30 Fractographs of Sol HT-1150oC+aged DMLD FSX-414 tensile samples showing (a)

curved facets at low magnification and (b) dimples at high magnification

Figure 4.31(a,b) Fractographs of Sol HT-1150oC+aged Cast FSX-414 tensile samples showing

dimples and big voids.

Figure 4.32 Engineering stress - Engineering strain curve for as deposited DMLD FSX-414.

Figure 4.33 Engineering stress - Engineering strain curve for Sol HT-1150oC+aged DMLD FSX-

414.

Figure 4.34 Engineering stress - Engineering strain curve for Sol HT-1150oC+aged Cast FSX-

414

Figure 5.1 Schematic representation of microstructural evolution in DMLS CoCrMo.

Figure 5.2 Schematic representation of athermal ε-HCP growing on γ-FCC Cobalt

Figure 5.3 Isothermal section of CoCrMo ternary diagram at 1200 0C.

Figure 5.4 Isothermal section of CoCrMo ternary diagram at 924oC.

Figure 5.5 Schematic representation of microstructural changes during solution and ageing heat

treatments.

Figure 5.6 Schematic representation of engineering stress vs. engineering strain curve for as

printed and solution treated DMLS CoCrMo (room temperature)

xii

LIST OF TABLES

Table 1.1 Comparison Between Various Additive Manufacturing Technologies [3]

Table 1.2 General Properties of Elemental Cobalt [5]

Table 1.3 Effect of notable alloying elements in Cobalt base alloys.

Table 1.4 Tensile properties of As printed DMLS CoCrMo-EOS Materials Data Sheet [30]

Table 2.1 Nominal chemical compositions of CoCrMo and FSX-414 alloys

Table 2.2 Heat treatment conditions for both DMLS CoCrMo and DMLD FSX-414

Table 2.3 Process parameters for Direct Metal Laser Sintering (DMLS) of CoCrMo and Direct

Metal Laser Deposited (DMLD) FSX-414

Table 2.4 Polishing steps followed for DMLS CoCrMo and DMLD FSX-414

Table 2.5 X-ray diffraction parameters

Table 2.6 Parameters for X-ray residual stress measurements

Table 2.7 Specimen geometries and testing conditions used for tensile testing

Table 3.1 Composition of CoCrMo powder analyzed using EDS

Table 3.2 Chemical composition of Direct Metal Laser Sintered (DMLS) CoCrMo and

corresponding nominal composition

Table 3.3 Surface roughness of as printed DMLS CoCrMo coupon

Table 3.4 Chemical analysis of various phases in as printed DMLS CoCrMo specimen using

TEM-EDS.

Table 3.5 EDS composition of solution heat treated specimen

Table 3.6 Composition of various phases in Sol HT+Aged DMLS CoCrMo

Table 3.7 Lattice parameters of FCC and HCP phases in various CoCrMo samples

Table 3.8 %Phase fraction of HCP phase in various CoCrMo samples

Table 4.1 Composition of FSX-414 powder analyzed using EDS

Table 4.2 Chemical composition of Direct Metal Laser Deposited (DMLD) FSX-414 and


Table 4.3 Chemical composition of various phases in As deposited DMLD FSX-414

Table 4.4 Chemical Composition of bright precipitates in DMLD FSX-414 samples solution

treated at various temperatures (all in weight %)

Table 4.5 Composition of various phases in Sol HT 1150oC + Aged DMLD FSX-414

Table 4.6 Composition of various phases in Sol HT 1150oC + Aged Cast FSX-414

Table 5.1 Size and morphology of grains in As printed, Sol HT, Sol HT+Aged DMLS CoCrMo

Table 5.2 Size and morphology of precipitates observed in As printed, Sol HT and Sol HT+Aged

DMLS CoCrMo

Table 5.3 Comparison between DMLS CoCrMo and DMLD FSX-414

xiii

LIST OF ACRONYMS

AM Additive Manufacturing

LENS Laser Engineered Net Shaping

DMLS Direct Metal Laser Sintering

DMLD Direct Metal Laser Deposition

SLM Selective Laser Melting

PBF Powder Bed Fusion

DED Directed Energy Deposition

SEM Scanning Electron Microscopy

TEM Transmission Electron Microscopy

HAADF High Angle Annular Dark Field

FFT Fast Fourier Transform

EDS Energy Dispersive X-ray Spectroscopy

Sol HT Solution heat treated at 1150oC

Sol HT+Aged Solution heat treated at 1150oC and aged at 980

oC

14

1. INTRODUCTION

1.1 Additive Manufacturing

Additive manufacturing (AM) or 3D printing technology is gaining lot of popularity in

various fields, right from electronics, biomedical to structural engineering components and

construction industry. American Society for Testing and Materials (ASTM – F2792) defines

additive manufacturing as ‘the process of joining materials to make objects from 3D model

data, usually layer upon layer, as opposed to subtractive manufacturing methodologies.’

Almost all AM techniques involve design of finished product using Computer Aided Design

(CAD), slicing this solid model into 2-dimensional layers and building the part layer by layer.

The materials used can be polymer, metal, ceramic, concrete or even biological tissues.

Figure 1.1 (a) Polymeric AM part showing kind of intricate designs that can be built; b) AM

Fuel Nozzle used in General Electric’s LEAP Jet Aircraft engine.

Additive Manufacturing is relatively recent manufacturing technology and has its roots in the

development of stereolithography technique used for polymer based materials in the 1980’s.

3D printing of metallic materials started around early 2000’s, when Optomec first

commercialized its Laser Engineering Net shaping (LENS) metal powder system based on

technology developed by Sandia National Labs.[1] Extensive research and numerous

processes have since been introduced to improve quality of products and efficiency of

process. Figure 1.2 gives timeline of past, present and potential future AM development and

applications.

15

Figure 1.2 AM applications timeline of past, present and potential future applications [2]

For years, constraints in fabrication methods have been primary obstacle for designers.

Advent of additive manufacturing has provided more flexibility in designs, and adding

complex features in product has now been possible without adversely affecting cost,

production rate or quality. Moreover, designers have the key to success of AM, as they can

come up with more and more sophisticated designs which were earlier limited by

conventional manufacturing.

AM has proven highly profitable in cases of low volume production, parts with highly

intricate features and where changes in designs are frequent. Excellent efficiency, low cost,

energy savings, low wastage and customizability are main advantages of AM. However, for

higher volume of production AM is considerably slower and also high initial investments,

discontinuous process cycle, and limited build size are the major problem in getting

companies to use AM.[2]

According to ASTM, Additive Manufacturing (AM) techniques for metallic materials can be

classified into two categories: Directed Energy Deposition (DED) and Powder based fusion

16

(PBF) as shown in Figure 1.3. Directed energy deposition involves injecting material into the

weld pool while Powder based fusion technology involves scanning layer of powder on the

build platform with a heat source.[3] Table 1.1 shows comparison between various aspects of

three different AM processes.

This work deals with additive manufacturing of CoCrMo and FSX-414 Cobalt base

superalloys for gas turbine applications.

Figure 1.3 Classification of Additive manufacturing (AM) techniques [3]

1.2 The Land based turbine and Co alloys

Typical land based gas turbine can be divided into three sections, viz. compressor, combustor

and turbine as shown in Figure 1.4. In combustor fuel is burnt with the help of compressed air

from compressor and thus it is the hottest section. In turbine or hot gas path, there is an

assembly of nozzles and rotors. Due to specific aerodynamic shape of nozzles, hot gas from

combustor flows in a particular way that drives the rotor next to it efficiently. Due to

proximity to combustor and the hot gases coming from it, the hot gas path component

experiences high temperatures. Hot gas path components were made of high temperature

resistant cobalt base superalloy such as FSX-414.

Additive Manufacturing

Directed Energy Deposition (DED)

Direct Metal Deposition (DMD)

Laser Engineered Net Shaping (LENS)

Direct Manufacturing (Electron beam and metal wire)

Powder Bed Fusion (PBF)

Direct Metal Laser Sintering (DMLS)

Selective Laser Melting (SLM)

Electron Beam Melting (EBM)

17

Table 1.1 Comparison Between Various Additive Manufacturing Technologies [3]

Figure 1.4 Land based gas turbine showing its three different sections namely, compressor,

combustor and hot gas path (Image source: gesol.com).

Compressor

Combustor

Hot gas path

18

1.3 Metallurgy of Cobalt base alloys

Cobalt has been in service of mankind for last 5000 years, first used by early Egyptians as

blue pigment for glaze. Cobalt ranks 33rd

in abundance.[4] Cobalt has both metallurgical and

non-metallurgical uses. Non-metallurgical uses involves paint pigments, radioactive source,

batteries, varnishes, inks, magnetic recording media, ground coats for porcelain enamels, and

catalysts for chemical and petroleum industries.[5] Metallurgical uses of cobalt exploit its

properties such as high temperature strength, biocompatibility, high wear and corrosion

resistance, magnetic properties, low expansion coefficient etc. It is widely used in gas turbine

nozzles, jet engine blades and vanes and hardfacing wear resistant applications.[6] General

properties of elemental cobalt are listed in Table 1.2.

Table 1.2 General Properties of Elemental Cobalt [5]

Density 8.85 g/cc

Melting Point 1493 °C

Curie Temperature 1127 °C

Coefficient of Thermal Expansion 13.8 μm/m.K

Thermal Conductivity 69 W/m.K

Electrical resistivity 7.8 μΩ.cm

Elastic modulus 211 GPa

0.2% Yield strength 305 to 345 MPa

Tensile strength 800 to 875 MPa

Elongation 15 to 30%

Cobalt has HCP crystal structure at room temperature (ϵ-Cobalt) and shows allotropic

transformation to FCC structure (α-Cobalt) at 417 °C temperature. Alloying elements are

categorized into two types, those that stabilize HCP structure such as molybdenum, tungsten,

chromium and silicon etc. These elements increase the transformation temperature and

decrease the stacking fault energy in FCC Co. Thus for CoCrMo alloy, allotropic

transformation temperature increases to near 970 °C owing to the high percentage of HCP

stabilizing elements.[7] Other type of elements such as carbon, iron and nickel stabilize FCC

structure and has opposite effects on transformation temperature and stacking fault energy in

FCC Co. Even though equilibrium phase diagrams (Figure 1.5) shows stable HCP phase at

room temperature, the transformation from FCC to HCP is extremely sluggish for pure

cobalt. Thus ϵ or α+ϵ phases are rarely observed in pure cobalt under normal cooling

conditions and metastable α (FCC) is more common.[5] Table 1.3 shows effect of various

alloying elements on cobalt base alloys.

19

Figure 1.5 Equilibrium phase diagrams of (a) Co-Cr, (b) Co-Mo, (c) Co-Si systems [8]

Table 1.3 Effect of notable alloying elements in Cobalt base alloys.

Element Property

C It produces strengthening by formation of various carbides of type M7C3, M23C6, M6C,

MC etc.[9] Carbide distribution, morphology, type, amount depends on composition,

heat treatment and cooling rates. Use of high carbon alloys usually limits

manufacturing process to hot working only, for cold working carbon content must be

less than 0.15%. Excess carbon in cobalt tends to decrease ductility.[6]

Cr For resistance to oxidizing and sulfidizing environments chromium is preferred

alloying element. It improves hot corrosion resistance and also acts as solid solution

strengthener for cobalt. Chromium also forms carbides of type M7C3 and M23C6. These

carbides are effective in pinning dislocations and thereby improving strength. Being a

HCP stabilizer chromium decreases stacking fault energy thereby making cross-slip

and glide of dislocations even more difficult.[6] But when chromium is added in excess

(>58%), undesirable TCP (Topologically Closed Packed) phases can form and

degrades properties.[9]

(a)

(c)

(b)

20

Mo, W Both are excellent solid solution strengtheners by virtue of their large atomic size and

are generally added in Co-Cr alloys.[5] They also give strengthening effect by

producing intermetallic like Co3M and carbides like M6C. When present in small

amount they substitute for chromium in M23C6 carbides.[10] Molybdenum has been

found to enhance wear resistance and corrosion resistance of cobalt-base alloys

Ni It stabilizes FCC phase at room temperature and also inhibits stacking fault formation

in FCC cobalt. It produces strengthening by formation of intermetallic compound

Ni3Ti. It improves forgeability of cobalt-base alloys. But when added in excess it

lowers the corrosion resistance.[9]

Ta, Nb,

Ti

It produces strengthening due to formation of intermetallic compound Co3M and MC

type carbides. It also produces solid solution strengthening effect.[6]

B, Zr They produce strengthening by effect on grain boundaries and precipitate formation.

They also increases stress rupture strength of alloy. Zirconium forms MC type carbides

and boron promotes formation of borides with other alloying elements.[9]

1.4 Direct Metal Laser Sintering (DMLS)

Figure 1.6 Schematic showing Direct Metal Laser Sintering (DMLS) technology (Image

source: Custompart.net)

DMLS is an AM technique that uses Yb (Ytterbium) Fiber laser (200 or 400 W) to sinter

metal powder particles (Figure 1.6). It was developed by EOS, Munich, Germany. DMLS has

capabilities to produce small batches of dimensionally accurate and structurally sound

metallic parts. Steps involved in this process are mentioned below [3]:

3D CAD model of the component is prepared and digitally sliced into 2D layer model.

Substrate is fixed on a build platform.

21

Build chamber is filled with inert gas and oxygen content is reduced to the desired level

to avoid oxidation.

Scraper is used to transfer thin layer of powder from supply cylinder to the substrate.

Laser beam scans powder bed following the CAD data of the component.

This process is repeated for the next layers until component is completely built.

1.4.1 Process Parameters of DMLS

Process parameters of DMLS play an important role in determining quality of the final

component. Figure 1.7 shows relationship between process parameters and their influence on

the resulting properties.

Figure 1.7 Relationship between DMLS process parameters and resulting properties [11].

1.4.2 Microstructural defects in DMLS processing

Major microstructural defects observed in additive manufactured parts are lack of fusion,

porosity, part distortion, microcracks and delamination [12]. Among these, the most common

defects in DMLS processed parts are mainly porosity and microcracks [13]–[17]. Zhou et al

[17] studied the 3D morphology of defects in selective laser melted CoCrMo alloy using

synchrotron based micro-CT. Two types of defects were observed viz., (i) defects in single

powder layer and (ii) defects in multiple powder layers. Defects were observed to have

complex 3D shape anisotropy. These defects were attributed to melt pool

dynamics/oscillations and melt track instabilities. Baurei et al [15] systematically studied

22

defect generation mechanism using the numerical modelling in EBM (Electron Beam

Melting) processed Ti-6Al-4V parts. It has been reported that small faults in the molten layer

can expand into large channel like defects over multiple layers as shown in Figure 1.8a.

These types of pores were eliminated by process control methods such as increasing laser

power (Figure 1.8). Small near spherical pores were also observed as shown in Figure 1.8c.

Formation of these small pores was attributed to entrapped gas within gas atomized

powders[14], [15] and bubbles from metallic evaporation due to the high power laser beam

[17].

Microcracks were observed in various DMLS processed alloys [13], [16]. Cracks

formation is attributed mainly to the solidification shrinkage in upper molten layer which is

restricted by cooler substrate or earlier layers [13]. Quian et al’s [16] work on selective laser

melted CoCrMo showed that in spite of the microcracks and other defects, mechanical

properties are still better than its cast counterpart.

Figure 1.8 Microsections of the Ti-6Al-4V specimens parallel to the building direction for

different beam powers: (a) 90 W, (b) 120 W, (c) 180 W [15].

1.4.3 Microstructural evolution during DMLS processing

Considerable work has been carried out to understand microstructural evolution during

DMLS processing of various alloys. It was observed that many DMLS / SLM processed

materials such as titanium, cobalt and nickel based superalloys shows similar macrostructure

consisting of a series of melt pools stacked over each [13], [16], [18]–[21]. Yan et al [21]

described the macrostructural features using the schematic as shown in Figure 1.9. The

Gaussian energy distribution of the laser beam is the main cause of the arc shaped melt pool

in the structure. The greatest intensity at the center of the beam produces deep melt pools. In

order to accomplish good bonding between layers and high densification, the generated melt

pool overlaps with previous layer as well as neighboring scan tracks.

23

Figure 1.9 Schematic diagram of generation of melt pools [21].

Many researchers working on DMLS/SLM of various nickel based and cobalt based

alloys observed cellular / columnar microstructure with the segregation of few alloying

elements towards cell / column boundaries[18], [20], [22]–[27]. Few researchers have carried

out in depth analysis of the microstructural features of as printed DMLS/SLM CoCrMo.

Quian et al [16] studied the microstructures and mechanical properties of the SLM processed

biomedical CoCrMo alloy. The presence of fine (~1 µm) cellular subgrains was observed

inside much larger single crystal grains. These single crystal grains were basically clusters of

these fine cellular grains which grew coherently along one crystallographic orientation.

Molybdenum enrichment was observed at the intercellular boundaries. Takaichi et al [27]

also observed columnar structure with ~2.7 µm diameter for SLM processed CoCrMo alloy.

Needle like precipitates enriched with Chromium and Molybdenum were observed at the

interdendritic boundaries. Takaichi thought needle like precipitate could be the σ phase based

on the ternary phase diagram of Co-Cr-Mo system. Both the researchers also observed the

presence ε (HCP) martensite phase in their XRD patterns.

Barucca et al [18] studied microstructural evolution in as printed DMLS CoCrMoW

biomedical alloy. Columnar structure was observed with the diameter ranging from 300-400

nm. TEM analysis revealed that the columnar structure is mainly due to aggregation of

athermal ε-HCP martensite phase. Small quantity of metal carbide of type M23C6 was also

observed. Mengucci et al [23] carried out detailed TEM analysis of columnar structure in

DMLS CoCrMoW alloy. Elongated precipitates with HCP structure and composition

resembling Co3(Mo,W)2Si were observed at the column boundaries. STEM-EDS analysis

also confirmed the presence of small dark spherical Si-rich inclusions close to these

precipitates. Such Si-rich inclusions were also observed in the microstructures of as cast

biocompatible CoCrMo alloy in the study by Giacchi et al. [28].

1.4.4 Tensile properties of DMLS processed parts

Room temperature tensile properties of as printed DMLS CoCrMo are reported in the

literature. Table 1.4 shows the average tensile properties of the as printed DMLS CoCrMo

specimens tested along build direction (vertical, Z-axis) and perpendicular to the build

direction (horizontal, XY plane). Considerable mechanical anisotropy as shown in Table 1.4

24

was also observed by Takaichi et al [27] for SLM CoCrMo and Vilaro et al [29] for Nimonic

263 nickel based superalloy. Tensile properties were also observed to vary with the

processing parameters [27].

Mengucci et al [23] observed mixed areas of ductile failure as well as quasi cleavage

facets for fracture surfaces of as printed DMLS CoCrMoW tensile specimens (room

temperature). High UTS and hardness was attributed to intricate network of ε-HCP martensite

phase in the γ-FCC matrix. Quian et al [16] also observed similar cleavage facets (brittle) at

low magnification and some dimples (ductile) at high magnification on as printed SLM

CoCrMo fracture surfaces.

Table 1.4 Tensile properties of As printed DMLS CoCrMo-EOS Materials Data Sheet

[30]

Property Vertical direction (Z) Horizontal direction (XY)

0.2% Yield strength 800±100 MPa 1060±100 MPa

UTS 1200±150 MPa 1350±100 MPa

% Elongation at break 24±4 11±3

Modulus of elasticity 190±20 GPa 200±20 GPa

Most of the existing literature for DMLS processed CoCrMo alloy deals with its

biomedical applications. The suitability of DMLS processed CoCrMo alloy for high

temperature structural applications has not been tested so far. Also the effect of solution and

ageing heat treatment on microstructure and mechanical properties at RT and high

temperature is not reported in literature.

1.5 Direct Metal Laser Deposition (DMLD)

DMLD is an AM technique which works by injecting powder into the melt pool created with

laser rather than sintering a powder bed [3]. DMLD process can operate with local shielding

and doesn’t require inert gas chamber for less reactive metals such as Nickel and Cobalt

alloys. Figure 1.10 shows schematic representation of DMLD process. Steps involved in

DMLD process are given below [3]:

1. Substrate or existing block is placed on the work table.

2. The process nozzle with concentric laser beam is focused on the surface to create melt

pool.

3. Coaxial nozzle is used to feed powder into the melt pool

4. Process nozzle moves at a constant speed and follows a predetermined tool path created

using CAD data.

5. Melt pool solidifies when nozzle moves away forming a layer of solidified metal.

25

6. The process is repeated and part is built layer by layer.

Figure 1.10 Schematic drawing showing Direct Metal Laser Deposition technology

(Courtesy: DM3D Technology) [3].

1.6 Motivation and Objectives

This thesis presents the use of two popular additive manufacturing techniques, i.e., Direct

Metal Laser Sintering (DMLS) on CoCrMo and Direct Metal Laser Deposition (DMLD) on

FSX-414 for the possible use in repair of hot gas path components in industrial turbines. The

focus of this work is on microstructural characterization, room temperature and high

temperature tensile properties of AM parts with the view to evaluate suitability of this

technique for the current high temperature application.

Objectives of the thesis are as follows:

Study the microstructural evolution and tensile properties of DMLS processed CoCrMo

Study the effect of solution and ageing heat treatments on microstructures and tensile

properties of the DMLS CoCrMo alloys.

Study of microstructural evolution and tensile properties of DMLD processed FSX-414

Study of solution and ageing heat treatment response on microstructure and room

temperature tensile properties of DMLD processed FSX-414.

26

2. MATERIALS AND EXPERIMENTAL

PROCEDURE

2.1 Materials and Heat treatment

The material used for this study comprised two Cobalt based alloys viz., CoCrMo and

FSX-414, whose nominal composition is listed in Table 2.1. The CoCrMo alloy was

processed with direct metal laser sintering (DMLS), a powder bed fusion technique, at Intech

DMLS, Bangalore. This part will henceforth be referred to as DMLS CoCrMo through the

rest of this report. The FSX-414 alloy was processed with direct metal laser deposition

(DMLD), a directed energy deposition technique, at DM3D Technologies, USA and thus will

be referred to as DMLD FSX-414 in this thesis.

Both DMLS CoCrMo and DMLD FSX-414 parts were subjected to solution heat

treatment in vacuum furnace at 1050 ᵒC for 4 hours followed by aging heat treatment at 980

ᵒC for 4 hours. Heat treatment conditions (Table 2.2) were chosen in order to mimic the heat

treatment of the actual nozzle. As-printed, solution heat treated and aged specimens from

both DMLS CoCrMo and DMLD FSX-414 were cut using abrasive wheel cutter and wire

Electrical Discharge Machining (EDM) for detailed microstructural and mechanical

characterization.

Table 2.1 Nominal chemical compositions of CoCrMo and FSX-414 alloys

Elements Co Cr Mo Ni W Mn Si C Fe

CoCrMo Bal. 28.7 7 - - 0.9 0.9 0.1 -

FSX-414 Bal. 29.8 - 10.6 7 0.9 06 0.2 1

Table 2.2 Heat treatment conditions for both DMLS CoCrMo and DMLD FSX-414

Heat treatment Conditions

Solution heat treatment 1050ᵒC in vacuum furnace for 4 hours followed by

argon fan quench

Solution heat treatment + Ageing 980ᵒC in vacuum furnace for 4 hours followed by

furnace cool

27

2.2 Powder characterization

Particle size distribution (PSD) analysis was performed on both CoCrMo and FSX-414

powders using ‘Mastersizer 2000E’ laser diffraction based powder size analyzer. SEM-EDS

analysis was performed on Zeiss EVO18 Scanning Electron Microscope along with Oxford

link energy dispersive spectroscopy (EDS) to characterize morphology and composition of

the powders. Preliminary phase identification was carried out using X-ray diffraction on the

powders using a Rigaku Miniflex600 (Cr Kα – 2.29 Aᵒ wavelength).

2.3 Processing conditions

Table 2.3 enlists process parameters used for both Direct Metal Laser Sintering (DMLS) of

CoCrMo and Direct Metal Laser Deposition (DMLD) of FSX-414 alloy. Flat DMLS

CoCrMo coupons (125 mm×43 mm×10 mm) were printed with holes in order to monitor

roughness of both flat as well as curved surface (within holes). Cylindrical DMLS CoCrMo

coupons (length-95 mm, dia-15 mm) were also made to prepare tensile specimens. In case of

DMLD, FSX-414 powder was deposited directly on the investment cast FSX-414 nozzle.

Figure 2.1 shows representative macro photographs of as printed DMLS CoCrMo and

DMLD FSX-414 parts.

Table 2.3 Process parameters for Direct Metal Laser Sintering (DMLS) of CoCrMo and

Direct Metal Laser Deposited (DMLD) FSX-414

Source INTECH-DMLS DM3D Technologies

Material CoCrMo FSX-414

Laser power 290 W 1000 W

Laser beam diameter 80 µm 2 mm

Powder feeding rate - 15 g/min

Layer thickness 40 µm ~ 400 µm

Hatch distance 110 µm -

Scanning speed 950 mm/s 8.3 mm/s

Powder size 10 to 50 µm 35-95 µm

Powder source EOS Praxair

Time for printing

Mini Coupons (24 in one run)

48-50 hours 80-90 mins

28

Figure 2.1 (a) Flat Direct Metal Laser Sintered (DMLS) CoCrMo coupon with cooling holes,

(b) Cylindrical DMLS CoCrMo for making tensile specimens, (c) Direct Metal Laser

Deposited (DMLD) FSX-414 on a cast nozzle

2.4 Chemical Analysis – Inductively Coupled Plasma (ICP)

The chemical composition of the DMLS CoCrMo, DMLD FSX-414 and cast FSX-414

nozzle was measured using an Inductively Coupled Plasma (ICP) technique.

2.5 Surface roughness

The first step in the characterization of any additively manufactured coupon is the part

surface roughness which was measured using a Zeiss Surfcom 1800D surface profilometer.

The Ra roughness parameter which is arithmetic average of heights of each point on the

surface was chosen for the measurements.

𝑅𝑎 = 1

𝑛 ∑|𝑦𝑖|

𝑛

𝑖=1

n = No. of points, y = height at particular point

(a)

(c)

(b)

Buil

d d

irec

tion

Buil

d d

irec

tion

DMLD FSX-414

Buil

d d

irec

tion

125 mm×43 mm×10 mm

Length-95 mm, Dia-15 mm

130 mm×25 mm×10 mm

Cast FSX-414 nozzle

29

Roughness of cooling holes:

Measurement of roughness of cooling holes was not possible using profilometer. Thus

section perpendicular to cooling holes was cut, mounted and subjected to metallographic

preparation for investigation using optical microscope as shown in Figure 2.2.

Figure 2.2 Procedure for determining hole roughness using optical microscopy

2.6 X-ray Tomography

X-ray Tomography was performed using GE Phoenix v-tome-xs machine with a 240 kV/320

W microfocus tube (resolution – 7 to 10µm). 3D Computed Tomography was not possible

because of dimensional constraints of the part. Hence, 2D X-ray images were taken at

different angles to determine the presence of voids and cracks in DMLS CoCrMo and DMLD

FSX-414 samples which could seriously affect mechanical behavior of the printed

component.

2.7 Metallographic procedure

Various cross sections of the part shown in Figure 2.3 were cut and hot mounted using

phenolic resin in Buehler SimpliMet 3000 machine. Samples were polished using Struers

automatic polishing machine (Tegramin 25). Table 2.4 enlists sequence of polishing steps

followed. All the samples were subjected to ultrasonic cleaning in a water-detergent solution

to remove colloidal silica particles entrapped in voids.

Cooling holes

Cut section

30

Figure 2.3 Nomenclature of various sections of DMLS CoCrMo and DMD FSX-414

component.

Table 2.4 Polishing steps followed for DMLS CoCrMo and DMLD FSX-414

Abrasive type Suspension Time (min) Force applied (N)

MD Piano 220 Water 3:00 50

Allegro Diamond – 9 µm 5:00 40

Dac Diamond – 6 µm 3:00 30

Mol Diamond – 3 µm 2:00 30

Nap Diamond – 1 µm 2:00 20

Chem Colloidal Silica 5:00 50

2.8 Porosity area fraction

Porosity was evaluated on transverse section of DMLS component using Nikon Optical

Microscope (Eclipse MA200) and Clemex Image analysis software. Stage pattern was created

on the image using Clemex software. Sample stage was made to navigate automatically on

each block of the pattern and calculate porosity area fraction by autofocus and auto-gray scale

methods using a macro code in Clemex software. Porosity distribution was plotted on

transverse section of coupon along the build direction. Figure 2.4 & 2.5 shows method of %

porosity evaluation for DMLS and DMLD components respectively.

Buil

d d

irec

tion

Transverse section

Base

Front planar

Longitudinal

direction

31

Figure 2.4 Method of % porosity evaluation on transverse section of DMLS component.

Figure 2.5 Method of % porosity evaluation on transverse section of DMLD FSX-414

component.

2.9 Optical and Scanning Electron Microscopy

Microstructural investigation was carried out on the Nikon Eclipse Optical microscope.

Samples were etched using 5% HCl (Electrolytic, 6V) for 10 seconds. Optical micrographs of

as-printed as well as heat treated DMLS CoCrMo and DMLD FSX-414 parts were taken at

various magnifications. Grain size measurement and distribution was carried out using

ImageJ software.

Detailed microstructural characterization was performed using scanning electron

microscopy (SEM). Zeiss SIGMA (Field Emission) microscope was used at an accelerating

voltage of 20 kV and working distance of 8.5 mm. BSE images were taken on the unetched

specimens to get qualitative information on extent of elemental segregation. An Oxford –

LINK system EDS (Energy dispersive spectroscopy) attached to the microscope was used for

getting compositions of samples and various phases within it. EDS mapping was also

performed to systematically identify various phases present in the sample.

0 40 mm

0 40 mm 60 mm

Stage Pattern

Build direction

DMLD FSX-414 Cast FSX-414

Build direction

32

2.10 Transmission Electron Microscopy

Transmission Electron Microscopy was carried out to identify composition and crystal

structure of fine precipitates and phases in the samples. TEM samples were prepared by first

cutting thin section (around 300 µm) using ‘Buehler Isomet Low speed saw’ and then

mechanically polishing on SiC paper ranging from 1200 grit to 4000 grit. Mechanical

polishing was done till sample reaches thickness of 80 µm. Streurs Twin jet electropolishing

machine was used to make electron transparent hole in the sample. FEI Technai F30 was used

for generating TEM bright field, HAADF and high resolution images. EDAX energy

dispersive spectroscopy attached to TEM was used to get compositions of phases present.

2.11 Electron Backscattered Diffraction (EBSD)

Electron Backscattered Diffraction (EBSD) analysis was performed to determine texture

evolution during 3D printing as well as texture changes after the heat treatment. EBSD was

carried out on both DMLS CoCrMo and DMLD FSX-414 samples using HKL EBSD link

system attached to Zeiss EVO18 Scanning Electron Microscope with step width of 1µm.

Samples for EBSD were prepared using normal metallographic procedure followed by

additional fine polishing in colloidal silica in Beuhler Vibromet polishing machine. EBSD

orientation maps were generated using TSL OIM analysis software.

2.12 X-ray Diffraction (XRD)

The Rigaku Miniflex 600 X-ray diffractometer with Cu Kα target (wavelength-1.54 Aᵒ) and

Ni-filter was used for identifying phases in samples. XRD analysis was done using PDXL

software using ICDD database. XRD parameters are listed in Table 2.5.

Table 2.5 X-ray diffraction parameters

Parameter Values

Starting angle (deg) 10

Finishing angle (deg) 120

Step size (deg) 0.005

Speed (deg/min) 0.5

Wavelength (Å) 1.54

Voltage (kV) 40

Current (mA) 15

Slit width (mm) 10

33

2.13 X-ray Diffraction Sin2 ψ technique for residual stress measurement

Residual stress along both the build and longitudinal direction was measured using a Rigaku

Automate II Micro-area X-ray residual stress. Surface was electropolished in order to reduce

roughness and to remove the impurities. In this study, residual stress was measured at various

locations on the electropolished surface of as printed DMLS CoCrMo as shown in the Figure

2.6. Sample stage was made to navigate automatically on selected grid to calculate residual

stress at every intersection. 2D residual stress distribution along the build direction was

plotted in Origin8.5 software.

Residual stress measurement was carried out using (220) diffraction peak of FCC

Cobalt with a Chromium Kα source (wavelength-2.29 Aᵒ). Table 2.6 enlists various

parameters used for measurement of residual stress.

Table 2.6 Parameters for X-ray residual stress measurements

Parameters Values

X-ray Source Cr - kα (λ = 2.29 Aᵒ)

Generator settings 40 kV, 40 mA

Diffraction peak (220) at 130ᵒ

Scanning range 127ᵒ to 133ᵒ

Step width 0.1ᵒ

Counting time 30 s

Collimator size 1 mm

ψ – angles 0, 10, 15, 20, 25, 30, 35, 40, 45, 50

Stress constant -720.28 MPa/deg

Young’s modulus 241 GPa

Poisson’s ratio 0.3

Absorption coefficient 2389 1/cm

Kα2 elimination ratio 0.5

34

Figure 2.6 Method used for measuring residual stress on as printed DMLS CoCrMo part.

2.14 Microhardness

The Vickers hardness was carried out using Shimadzu Micro Hardness tester with 300 gm

load and 10 sec dwell time. Standard formula for Vickers hardness is given below:

HV = 1.854

𝑑2 𝑃

Where, P is force applied in kgf and d is the mean diagonal of the indentation.

Hardness readings were taken on the transverse sections of DMLS CoCrMo coupon at the

three different locations viz. 10, 50 and 90 % spans (Figure 2.7a). Hardness profile was

plotted along build direction for all these three locations. While for DMLD FSX-414

hardness readings were taken on only 10% span section as shown in Figure 2.7b.

Electropolished surface

Buil

d d

irec

tion

30 m

m

10 mm

DMLS CoCrMo

Longitudinal direction

35

Figure 2.7 (a) DMLS CoCrMo and (b) DMLD FSX-414 components showing different

locations for taking hardness readings.

2.15 Tensile testing

Both room temperature and high temperature (925ᵒC) tensile tests were carried out on DMLS

CoCrMo, DMLD FSX-414 and Cast FSX-414 samples. Varied specimen geometries were

used owing to the material constraint as shown Table 2.7.

Table 2.7 Specimen geometries and testing conditions used for tensile testing

Room temperature High temperature

DMLS

CoCrMo

Flat tensile specimen (ASTM standard) Round tensile specimen (ASTM

standard)

Gauge length 1” Gauge length 1”

Strain rate upto 2% 0.005 in/in/min Strain rate upto

2%

0.005 in/in/min

Strain rate after 2% 0.05 in/min Strain rate after

2%

0.05 in/min

DMLD

FSX-414

Micro-tensile specimen

90% span

50% span

10% span

Buil

d d

irec

tion

10% span

Cast

FSX-414

DMLD

FSX-414

36

t=0.5mm

-

Gauge length 6 mm

Strain rate 0.006 mm/s

Cast

FSX-414

Micro-tensile specimen

(same as above)

- Gauge length 6 mm

Strain rate 0.006 mm/s

37

3. RESULTS - PART A: DIRECT METAL

LASER SINTERING OF CoCrMo

This chapter presents outcome of the experimental work carried out to characterize DMLS

processed CoCrMo and DMLD processed FSX-414 in order to evaluate the suitability of

these processes for possible gas turbine applications. The results are described in two parts:

DMLS of CoCrMo and DMLD of FSX-414. First part describes microstructural evolution in

as-printed DMLS CoCrMo, effect of solution treatment and ageing on microstructure and

both room temperature and high temperature tensile properties. Second part describes

microstructural evolution in as deposited DMLD FSX-414, effect of solution treatment and

ageing on microstructure and room temperature tensile properties.


CoCrMo powder was analyzed for the morphology, composition and particle size. SEM

micrograph of powder in Figure 3.1 reveals a spherical morphology with particle size in the

range of 5-45 µm. Table 3.1 shows the composition of CoCrMo powder analyzed using EDS.

Particle size was calculated using Mastersizer 2000E laser diffraction based powder size

analyzer. Particle size distribution in Figure 3.2 shows the powder size is normally distributed

with average size of 24 µm.

Figure 3.1 SE images showing size and morphology of as received CoCrMo powder, at (a)

low magnification (b) high magnification.

Table 3.1 Composition of CoCrMo powder analyzed using EDS

Elements Co Cr Mo Mn Si

Wt % 62.81 28.34 6.97 0.75 0.77

At% 60.7 30.9 4.12 1.66 0.84

(a) (b)

~ 24 µm

38

Figure 3.2 Particle size distribution of CoCrMo powder

3.2 Chemical analysis

Table 3.2 gives chemical composition of as printed DMLS CoCrMo analyzed using

Inductively Coupled Plasma (ICP). The measured composition is roughly similar to the

nominal CoCrMo composition except for iron and carbon which are little bit higher.

Table 3.2 Chemical composition of Direct Metal Laser Sintered (DMLS) CoCrMo and


Elements Co Cr Mo Ni Mn Si C Fe

CoCrMo (wt%) 62.5 27.66 6.42 0.1 0.75 0.56 0.18 1.12

Nominal

CoCrMo[30]

(wt%)

60-65 26-30 5-7 0.1 1.0 1.0 0.16 0.75

3.3 Surface Roughness

Table 3.3 shows surface roughness readings in Ra parameter measured along the build and

longitudinal direction. The surface roughness ranges between 3-5 µm, Ra along the vertical

direction and 1-5 µm along the horizontal direction. Figure 3.3 shows representative optical

micrographs of transverse sections of surface and hole. Micrograph of hole (Figure 3.3b)

shows very smooth surfaces even on the curved surface.

39

Table 3.3 Surface roughness of as printed DMLS CoCrMo coupon

Roughness (µm,

Ra)

Reading 1

(µm, Ra)

Reading 2

(µm, Ra)

Reading 3

(µm, Ra)

Reading 4

(µm, Ra)

Average

(µm, Ra)

Vertical

direction 4.34 4.08 3.97 3.81

3.59 ± 1.04

Horizontal

direction 1.00 4.54 3.43 3.53

Figure 3.3 Representative micrographs showing topography of (a) surface and (b) hole of as

printed DMLS CoCrMo.

3.4 X-ray Microtomography of as printed DMLS CoCrMo

2D X-ray microtomography images at 0ᵒ and 30ᵒ sample tilt are shown in Figure 3.4 and 3.5

respectively. X-ray images do not show any indication of significant voids or cracks with the

size big enough to get resolved with X-ray microtome (resolution: 10-15 µm in the as printed

samples. Black and white contrast in both 0ᵒ and 30ᵒ tilt image is due to the holes (Figure

3.4) present in the samples. Two different tilts were chosen because some features can be

invisible in one of the image if they are parallel to the X-ray beam.

(a) (b)

40


(Voltage: 200 kV, Current: 500 µA).


(Voltage-200 kV, Current-500 µA).

3.5 Porosity

Figure 3.6 shows porosity distribution along the transverse section of the as printed DMLS

CoCrMo coupon. The average porosity is around 0.043±0.04% and also porosity does not

vary much along the build direction. Unetched micrograph in different locations viz, A, B

and C show pores has fine, spherical morphology. Some pores with irregular morphology can

41

be seen in location C. Interlayer porosity which is commonly observed in the various DMLS

processed alloys is absent in the given sample. The processed parameters have been

optimized to enable a dense component.

Figure 3.6 Porosity distribution on transverse section of as printed DMLS CoCrMo along the

build direction and corresponding unetched microstructure showing porosity at locations A,

B, C respectively.

3.6 Microstructural Characterization of as printed DMLS CoCrMo

3.6.1 Optical microscopy

Figure 3.7 shows optical micrographs of transverse, front planar and base sections of the as

printed DMLS CoCrMo. Size of this pool is very small around ~120 µm wide and ~60 µm

measured using ImageJ software. Transverse section macrostructure in Figure 3.7 clearly

shows solidified melt pools stacked layer by layer. Solidified scan paths can be seen in the

base macrostructure. It can be observed that the angle between two scan paths is 67ᵒ as

shown in Figure 3.7. The optical micrographs of transverse section in Figure 3.8a show

location of irregular pores as close to the melt pool boundary. High magnification optical

micrograph in Figure 3.8b clearly shows microcracks. It can be seen that microcracks are

almost perpendicular to the melt pool boundary.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 5 10 15 20 25 30 35 40

% P

oro

sity

are

a fr

acti

on

Distance from the base

A B

C

A B C

42

Figure 3.7 Optical micrographs of transverse, front planar and base sections of as printed

DMLS CoCrMo, etched with 5% HCl - electrolytic – 6V.

3.6.2 Scanning Electron Microscopy

Figure 3.9 shows SEM micrographs of as printed DMLS CoCrMo samples. Columnar

structure is evident inside the solidified melt pool. Melt pool boundaries can be seen clearly.

Figure 3.9b shows a one to one matching of columns across the melt pool grain boundaries.

Thus columns growing inside melt pool during solidification seem to be adopting orientation

of columns in earlier layers. Similarly oriented columns forms domains inside the melt pool

as can be seen in Figure 3.9a.

~60

µm

43

Figure 3.8 Optical micrographs of transverse section of as printed DMLS CoCrMo at (a) low

magnification showing irregular pores at the interlayer boundaries and (b) high magnification

showing microcracks (etched with 5% HCl, electrolytic-6V)

Figure 3.10 shows BSE images of etched as printed DMLS CoCrMo sample. The

primary dendrite arm spacing (PDAS) is around 600 nm to 1000 nm and shows no branching

into secondary dendritic arms throughout the sample. Unetched BSE images in the Figure

3.11 show the presence of elongated bright precipitates in the interdendritic regions. This

bright contrast in the interdendritic region indicates possible elemental segregation along the

column width. Dark globular phases can be observed sometimes in conjunction with bright

precipitates and sometimes isolated in the matrix. Elongated bright precipitates have width of

around 30-70 nm and globular dark phases have diameter of around 20-70 nm measured

using ImageJ software. In order to further discern location, size, morphology and

composition of various phases, transmission electron microscopy was carried out.

Figure 3.9 SE image of as printed DMLS CoCrMo at (a) low magnification showing

columnar microstructure and domains (grains) and (b) high magnification one to one

matching along melt pool boundary (Etched with 5% HCl, electrolytic-6V).

(b) (a)

Melt pool boundary

(b) (a)

44

Figure 3.10 BSE images of as printed DMLS CoCrMo at (a) low magnification, and (b) high

magnification showing bright contrast in the interdendritic region, etched with 5% HCl

(electrolytic-6V)

Figure 3.11 (a,b) High magnification BSE images of unetched as printed DMLS CoCrMo

showing interdendritic precipitates and ε-HCP (arrow) cutting across the columns.

3.6.3 TEM Micrographs of As printed DMLS CoCrMo

Transmission electron microscopy was carried out on the as printed DMLS CoCrMo sample

to find out location, size and morphology of various inter-dendritic and intra-dendritic phases.

Figure 3.12 shows TEM bright field image of intra-dendritic region of the sample at [011]

zone axis. Dark plates like phases are in the 70.52o angle. Fringe contrast can also be

observed. High resolution (HR) TEM image and corresponding Fast Fourier Transform (FFT)

pattern in Figure 3.13 clearly shows the presence of fully coherent alternate bands of ε-HCP

and γ-FCC phase in the region of dark plate phase. The width of the ε-HCP band is around

9.2 nm. Figure 3.14 shows FFT pattern of entire HRTEM image (Figure 3.13). This pattern

confirms (111) γ || (0001)ε, [011]γ || [1210]ε , i.e., Shoji-Nishiyama orientation relationship

between γ-FCC and ε-HCP phases.

(a) (b)

(a) (b)

(111)

45

High Angle-Annular Dark Field (HAADF) STEM images in Figure 3.15(a,b) shows

precipitates with elongated and globular morphology in the interdendritic region similar to

the BSE image. Bright and dark contrast of elongated and globular precipitates respectively

clearly indicates possible elemental segregation in the interdendritic region. TEM-EDS

analysis was carried out in order to get chemical compositions of these precipitates. Table 3.4

enlists chemical composition of bright, dark precipitates and matrix in both weight% and

atomic% units. It can be observed that interdendritic bright precipitates are Mo and Si rich,

while globular dark precipitates and Si rich.

Figure 3.12 TEM bright field image of as printed DMLS CoCrMo sample showing ε-HCP

phases in γ-FCC CoCrMo matrix.

Figure 3.13 High resolution-(HR) TEM image of as printed DMLS CoCrMo and its

corresponding FFT pattern showing existence of HCP phase

70.52ᵒ

30.1ᵒ

46

Figure 3.14 FFT pattern of entire HRTEM image from Figure 13 showing spots for both FCC

and HCP and their orientation relationship

Figure 3.15 High Angle Annular Dark Field (HAADF) STEM images of as printed DMLS

CoCrMo specimen showing elongated bright precipitates and globular black precipitates in

the interdendritic region.

(a) (a)

47

Table 3.4 Chemical analysis of various phases in as printed DMLS CoCrMo specimen

using TEM-EDS.

Elements Co Cr Mo Mn Si

Bright

precipitate

Wt% 44.2 27.4 22.3 2.7 3.3

At% 44.7 31.4 13.0 3.0 7.0

Dark

precipitate

Wt% 55.1 28.4 7.6 3.0 5.8

At% 51.2 30.0 4.4 3.1 11.3

Matrix Wt% 60.4 26.4 7.4 3.0 1.8

At% 58.4 29.1 4.5 3.9 3.7

3.7 Microstructural characterization of Solution heat treated DMLS

CoCrMo

Figure 3.16 shows optical micrographs of transverse, front planar and base section of solution

heat treated DMLS CoCrMo. The microstructure shows fully equiaxed grains on all sections

in contrast to the as printed DMLS CoCrMo. The average grain size is around 44 µm. Figure

3.17 shows the optical micrograph of solution treated DMLS CoCrMo and its corresponding

grain size distribution. Grain size ranges between 5 to 100 µm. The dendritic structure and

melt pool boundaries in as printed sample has homogenized completely after heat treatment

and grain size shows no correlation with either powder size or melt pool size.

Figure 3.16 Optical micrographs of Transverse, front planar and base sections of Sol HT

DMLS CoCrMo showing equiaxed grains on all sides, etched with 5% HCl, electrolytic – 6V

48

Figure 3.17 (a) Optical micrograph of Sol HT DMLS CoCrMo showing equiaxed

microstructure with average grain size of 44 µm and (b) corresponding grain size distribution.

BSE images of the solution treated samples show in Figure 3.18a shows twinning in the

equiaxed microstructure. Dark and bright contrast is due to orientation difference between the

grains. A representative EDS spectrum in Figure 3.18b and corresponding chemical

composition in Table 3.5 shows similar chemistry as that of nominal CoCrMo alloy. High

magnification BSE images of unetched solution treated samples in Figure 3.19 shows the

elliptical shaped bright precipitates on the grain boundaries having contrast very similar to

what observed for elongated bright precipitates in as printed DMLS CoCrMo (Figure 3.11).

Also some bright contrast can be seen inside the grains as well (Figure 3.19b). Dark globular

phases can also be observed and are very similar to that observed in as printed DMLS

CoCrMo in Figure 3.11.

Figure 3.18 (a) BSE image of unetched Sol HT DMLS CoCrMo showing twins and equiaxed

microstructure, (b) corresponding EDS spectrum.

0

5

10

15

20

25

30

5 20 35 50 65 80 95

No. of

gra

ins

Grain size (µm)

Grain size distribution (a) (b)

(a) (b)

49

Figure 3.19(a,b) High magnification BSE images of unetched Sol HT DMLS CoCrMo

showing remnants of previous interdendritic precipitates inside grains as well as precipitates

along grain boundaries.

Table 3.5 EDS composition of solution heat treated specimen

Elements Co Cr Mo Si Mn C

Wt% 62.83 28.23 6.95 0.82 0.81 0.37

At% 60.70 30.91 4.12 1.67 0.84 1.75

3.8 Microstructural characterization of Sol HT + Aged DMLS CoCrMo

In order to evaluate the high temperature stability, the solution treated CoCrMo was given an

ageing heat treatment. Figure 3.20(a,b) shows optical micrographs of Sol HT+aged DMLS

CoCrMo sample. Microstructure shows equiaxed grains with similar grain size as that of

solution heat treated samples. Precipitation of various phases inside grains and along the

grain boundaries can be seen after aging (Figure 3.20b). SEM-EDS analysis was carried out

to discern size, morphology and composition of the precipitates. Figure 3.21 shows BSE

images of the aged samples clearly showing various bright and dark precipitates. Dark

precipitates have globular morphology everywhere similar to what observed in as printed and

solution heat treated specimens. Most of the intragranular bright precipitates have plate

morphology while some are globular as well. Almost all the intergranular bright precipitates

have either globular or elliptical morphology similar to the intergranular precipitates in

solution treated samples (Figure 3.19). It can be seen that precipitates cover almost the entire

grain boundary. Table 3.6 gives chemical composition of the matrix and both bright and dark

precipitates. Composition wise there is no difference between plate shaped and globular

bright precipitates. Chemical composition of precipitates in Table 3.6 shows that though both

(a) (b)

50

the bright precipitates are rich in Molybdenum and Silicon. Dark phase is rich in Silicon and

Manganese. EDS mapping of high magnification BSE image in Figure 3.22 confirms the

Molybdenum and Silicon enrichment in the bright precipitates.

Figure 3.20 Optical micrographs of Sol HT+Aged DMLS CoCrMo at (a)200x and (b)1000x

showing equiaxed grain structure and some precipitation (arrow), etched with 5% HCl –

electrolytic, 6V.

Figure 3.21(a,b) BSE images of Sol HT+Aged DMLS CoCrMo samples showing bright and

dark precipitates inside the grains as well as along the grain boundaries.

Table 3.6 Composition of various phases in Sol HT+Aged DMLS CoCrMo

Elements Co Cr Mo Mn Si C

Bright

precipitate

Wt% 51.2 27.5 17.3 1.5 2.6 -

At% 51.1 31.2 10.7 1.6 5.5 -

(a) (b)

(a) (b)

51

Figure 3.22 (a) High magnification BSE image of Sol HT+Aged DMLS CoCrMo sample

showing bright and dark precipitate, and (b) corresponding EDS elemental mapping.

3.9 X-ray Diffraction

Phase identification was carried out using XRD, as shown in Figure 3.23. X-ray diffraction

peak identification was done using two ICDD cards, one for FCC-Co and other for HCP Co.

XRD patterns of all four samples viz, powder, as printed and solution heat treated and Sol

HT+aged DMLS CoCrMo, etc. shows peaks for both the γ-FCC and ε-HCP Cobalt phases.

Dark

precipitate

Wt% 47.6 38.4 10.9 2.0 1.2 -

At% 46.5 42.5 6.5 2.1 2.4 -

Matrix Wt% 63.0 28.4 6.8 0.6 0.8 0.3

At% 60.9 31.1 4.07 0.6 1.6 1.7

(a)

(b)

52

Table 3.7 shows lattice parameters of both γ-FCC and ε-HCP phases in all samples. %Phase

fraction of ε-HCP phase in all the samples is shown in the Table 3.8. % HCP was calculated

using Sage and Guillad [31] equation:

Figure 3.23 X-ray Diffractions patterns of (a) CoCrMo Powder, (b) as printed (c) Sol HT , (d)

Sol HT+aged, DMLS CoCrMo, all showing peaks for both γ-FCC and ε-HCP Cobalt phases


Table 3.7 Lattice parameters of FCC and HCP phases in various CoCrMo samples

Specimen Lattice parameter (Ao)

γ-FCC ε-HCP

CoCrMo powder a= 3.5794 a= 2.5310, c= 4.7773

As printed DMLS CoCrMo a= 3.5787 a= 2.5305, c= 4.7742

Sol HT DMLS CoCrMo a= 3.5846 a= 2.5347, c= 4.7867

0

50

100

150

200

250

300

350

400

10 30 50 70 90 110 130 150

Inte

nsi

ty (

arbit

rary

unit

s)

2Ɵ

As printed

Sol HT

Sol HT+Aged

CoCrMo Powder

(a)

(b)

(c)

(d)

* *

* * *

*

*

2

2

2

2

2

2

2

2

1

1

1

1

1, 2

1, 2

1, 2

1, 2

1, 2

1, 2

1, 2

1, 2

*

1 – γ(FCC) Co

ICDD: 00-015-806

2 – ε(HCP) Co

ICDD: 00-005-726

*Unidentified

53

Sol HT + Aged DMLS CoCrMo a= 3.5743 a= 2.5274, c= 4.7493

Pure Cobalt [Source:

periodictable.com]

a= 3.5447 a= 2.5071, c= 4.0695

Table 3.8 %Phase fraction of HCP phase in various CoCrMo samples

Specimen % Phase fraction of ε-HCP phase

CoCrMo powder 45.1

As printed DMLS CoCrMo 19.7

Sol HT DMLS CoCrMo 13.6

Sol HT+Aged DMLS CoCrMo 28.6

3.10 Electron Backscattered Diffraction (EBSD) – Inverse Pole Figure

maps

Figure 3.24 shows EBSD IPF maps of as printed and solution treated DMLS CoCrMo. Large

elongated grains can be observed in the as printed EBSD pattern. Heat treated pattern shows

equiaxed grains with twins. No preferred orientation can be observed in both as printed and

heat treated microstructures.

Figure 3.24 EBSD IPF Maps of (a) As printed, (b) Solution heat treated DMLS CoCrMo

(a) As printed DMLS CoCrMo (b) Solution heat treated DMLS CoCrMo

54

3.11 X-ray Diffraction Sin2 ψ technique for residual stress measurements

Residual stress analysis was carried out on the electropolished surface of as printed DMLS

CoCrMo. Figure 3.25a shows the distribution of residual stress on the surface of the sample.

Residual stress ranged between 550 MPa to 950 MPa. It can be seen that residual stress

decrease gradually along the build direction (Figure 3.25b).

Figure 3.25 (a) Residual stress distribution on the surface of as printed CoCrMo, (b)

Variation of residual stress along the build direction

3.12 Hardness

Hardness profiles were plotted against build direction for transverse sections from different

spans as can be seen in Figure 3.26. Hardness value of as printed sample are between 420-

460 HV and after solution heat treatment it decreases to 340-380 HV. Hardness almost

remains constant after ageing treatment.

450

600

750

900

1050

2 4 6 8 10 12 14 16 18 20

Res

idual

str

ess

(MP

a)

Distance from base (mm)

Build direction

(a)

(b)

55

Figure 3.26 Hardness profile along build direction (a) as printed, (b) Sol HT, (c) Sol

HT+aged, DMLS CoCrMo.

3.13 Tensile properties

Figure 3.27 shows 0.2% yield strength, UTS and ductility data for room temperature and high

temperature (925ºC) tensile tests. As printed samples were tested only at room temperature

and shows highest yield strength and UTS as compared to Sol HT and Sol HT+Aged

samples. Sol HT and Sol HT+Aged samples were tested at both room temperature and at

925oC. For room temperature tensile tests, Sol HT samples shows very high ductility as

compared to as printed or Sol HT+Aged specimens. For high temperature tests, ductility of

Sol HT samples is marginally higher than Sol HT+aged samples. 0.2% Y.S. and UTS values

of the Sol HT and Sol HT +Aged samples are almost similar for both room temperature and

high temperature testing. Sol HT samples shows Sol HT+aged sample shows around 70-80%

decrease in both 0.2% yield strength and UTS, and 300% increase in ductility when tested at

925 ºC as compared to room temperature tests. Figures 3.28, 3.29, 3.30 show engineering

300

340

380

420

460

0 10 20 30 40

Har

dnes

s (H

V)


10% span

50% span

90% span

300

340

380

420

460

0 10 20 30 40

Har

dnes

s (H

V)


10% span

50 % span

90 % span

300

340

380

420

460

0 10 20 30 40

Har

dn

ess

(HV

)


Build direction

(b) Solution heat treated

Build direction Build direction

(c) Sol heat treated

+ aged DMLS

10 % span

(a) As printed

56

stress vs. engineering strain curves for as printed, Sol HT and Sol HT+aged DMLS CoCrMo

respectively. Figure 3.31 shows fracture surface of as printed DMLS CoCrMo tested at room

temperature. Cracks and transgranular facets can be observed. Fractographs of room

temperature tested Sol HT DMLS CoCrMo is shown Figure 32(a,b). Cracks can be observed

in low magnification (Figure 32a). High magnification fractographs in Figure 32b shows fine

dimples. Figure 32c,d shows fractographs of Sol HT samples tested at high temperature. Here

also high magnifaction image shows fine dimples. Figure 33a,b shows fractographs of room

temperature tested Sol HT+Aged DMLS CoCrMo. Cracks and facets of grains can be

observed. Fractographs of high temperature tested Sol HT+aged samples shows intergranular

cracks at low magnification and dimples at high magnification (Figure 33c,d).

Figure 3.27 Room temperature and high temperature tensile properties of as printed, Sol HT,

Sol HT+aged DMLS CoCrMo, (a) % Y.S., (b) UTS, (c) Ductility.

0

300

600

900

1200

1500

As printed Sol HT Sol HT +

Aged

0.2

% Y

.S. (M

Pa)

0

300

600

900

1200

1500

As printed Sol HT Sol HT +

Aged

UT

S (

MP

a)

0

5

10

15

20

25

30

As printed Sol HTSol HT + Aged

% E

longat

ion

0.2% Yield strength

Ductility

Ultimate Tensile Strength

(a) (b)

(c)

57

Figure 3.28 Engineering stress-Engineering strain curve of as printed DMLS CoCrMo tested

at room temperature

Figure 3.29 Engineering stress-Engineering strain curves of Sol HT DMLS CoCrMo tested at

(a) room temperature, (b) 925oC

Figure 3.30 Engineering stress-Engineering strain curves of Sol HT+Aged DMLS CoCrMo

tested at (a) room temperature, (b) 925oC

0

250

500

750

1000

1250

1500

0 2 4 6 8

Str

ess

(MP

a)

Strain %

0

200

400

600

800

1000

1200

0 20 40

Str

ess

(MP

a)

Strain %

0

50

100

150

200

250

0 1 2

Str

ess

(MP

a)

Strain %

0

200

400

600

800

1000

0 0.5 1 1.5 2

Str

ess

(MP

a)

Strain %

0

30

60

90

120

150

180

0 0.5 1 1.5 2

Str

ess

(MP

a)

Strain %

(a)

(a)

(b)

(b)

58

Figure 3.31 Fractographs of as printed DMLS CoCrMo tensile sample showing cracks.

Figure 3.32 Fractographs of Sol HT DMLS CoCrMo tensile samples showing mixed brittle

and ductile type failures.

(a) (b)

(a)

(c) (d)

(b)

Room

tem

per

ature

925ºC

R

oom

tem

per

ature

59

Figure 3.33 Fractographs of Sol HT+Aged DMLS CoCrMo tensile samples showing

intergranular fracture in room both room temperature and high temperature tests.

(a) (b)

(c) (d)

Room

tem

per

ature

925ºC

60

4.RESULTS - PART B: DIRECT METAL

LASER DEPOSITION OF FSX-414


FSX-414 powder was analyzed for the morphology, composition and particle size. SEM

micrograph of powder in Figure 4.1 reveals spherical morphology with particle size in the

range of 35-95 µm. Table 4.1 shows composition of FSX-414 powder analyzed using EDS.

Particle size was calculated using laser diffraction based Mastersizer 2000E analyzer. Particle

size distribution in Figure 4.2 shows powder is normally distributed with average size of

around 60 µm.

Figure 4.1 SE images showing morphology of FSX-414 powder in the as received condition

at (a) low magnification and, (b) high magnification .

Table 4.1 Composition of FSX-414 powder analyzed using EDS

Elements Co Cr Ni W Mn Si

Wt % 50 29.3 10.7 7.1 1.7 1.2

Std dev. 0.3 0.2 0.2 0.3 0.1 0.1

(a) (b)

61

Figure 4.2 Particle size distribution of FSX-414 powder.

4.2 Chemical analysis

Table 4.2 gives chemical composition of as printed DMLD FSX-414 analyzed using

Inductively Coupled Plasma (ICP). Measured composition is roughly similar to the nominal

FSX-414 composition except for tungsten which is little bit higher.

Table 4.2 Chemical composition of Direct Metal Laser Deposited (DMLD) FSX-414 and


Elements Co Cr Ni W Mn Si C Fe

FSX-414 (wt%) 48.9 29.73 10.30 9.90 - - 0.18 0.8

Nominal FSX-

414[30] (wt%)

Bal. 28.5-

30.5

9.5-

11.5

6.5-7.5 0.4-1.0 0.5-1.0 0.2-0.3 1.0

4.3 X-ray Microtomography of as deposited DMLD FSX-414

2D X-ray microtomography images at 0ᵒ and 15ᵒ sample tilt are shown in Figure 4.3. X-ray

images do not show any indication of significant voids or cracks with size big enough to be

resolved with X-ray Microtome (Resolution ~ 10-15 µm) in the as printed samples. Two

different tilts were chosen because some features can be invisible in one of the image if they

are parallel to the X-ray beam.

62

Figure 4.3 2D X-ray Microtomography images of as deposited DMLD FSX-414 samples

with (a) 0o tilt and (b) 15

o tilt (Voltage: 200 kV, Current: 500 µA)

4.4 Porosity

Figure 4.4 shows porosity distribution along the transverse section of the as deposited DMLD

FSX-414. The average porosity in the DMLD part and Cast FSX-414 part is around 0.24 ±

0.06% and 0.4 ± 0.09% respectively. Porosity is not varying much along the build direction.

Unetched micrographs (all 100×) in different locations viz, A, B and C in Figure 4.4 shows

some irregular pores with entrapped unmelted particles as well as spherical pores in the as

deposited DMLD FSX-414 (locations A,B). Pores in cast substrate part (location C) are fine

and densely populated while pores in DMLD part are relatively coarse and sparsely

populated.

4.5 Microstructural Characterization of as deposited DMLD FSX-414


The optical micrograph of unetched DMLD FSX-414 in Figure 4.5 clearly shows cracks in

the DMLD part as well as in the DMLD-Cast FSX-414 joint. Optical micrographs of the

etched section of DMLD and DMLD-Cast FSX-414 joint in Figure 4.6a & b show fine

dendritic microstructure. Primary dendrite arm spacing (PDAS) is around 8-12 µm while

secondary dendrite arm spacing (SDAS) is around 5-6 µm. Dendritic columns in Figure 4.6b

are almost perpendicular to the substrate. Layer thickness is around 400 µm as shown in the

stitched optical micrograph in Figure 4.7. Domains or bundles of dendrites with same

orientation can be observed in Figure 4.7.

(a) (b)

63

Figure 4.4 Porosity distribution along transverse section of as deposited DMLD FSX-414 and

Cast FSX-414, and corresponding unetched microstructure showing porosity at locations A,

B, C respectively.

Figure 4.5 Optical micrographs of the unetched as deposited DMLD FSX-414 showing

solidification cracks in (a) DMLD Part and (b) DMLD and Cast FSX-414 joint.

0

0.2

0.4

0.6

0 10 20 30 40 50 60

% P

oro

sity

Distance from Trailing Edge (mm)

Cast FSX-414

A

B

C

A B C

Build direction

DMLD FSX-414

64

Figure 4.6 (a) Optical micrograph of as deposited DMLD FSX-414 showing dendritic

microstructure. (b) as deposited DMLD FSX-414 and Cast FSX-414 joint showing dendritic

growth direction relative to cast FSX-414 substrate, etched with 5% HCl, electrolytic-6V.

Figure 4.7 Stitched optical micrograph of as deposited DMLD FSX-414 showing dendritic

microstructure and domains/bundles of dendrites with same orientation.


SEM micrographs in Figure 4.8a show primary dendrites growing on the cast FSX-414

substrates. Boundary between domains i.e. bundles of dendrites with same orientation is

evident in the Figure 4.8b.

(a) (b)

DMLD FSX-414

Cast FSX-414

Layer

thic

kn

ess

~ 4

00 µ

m

Domain 1

Domain 2 Domain 3

65

Figure 4.9a,b shows BSE images of the unetched DMLD FSX-414. The interdendritic

region is in light contrast indicating segregation of high atomic number elements. Fine

globular dark phases can be observed in the interdendritic region often surrounded by a

region of light contrast, as well as well as within the dendrites. A fine network of plate like

precipitates is present within the dendrites. The high magnification micrograph of the

interdendritic region in Figure 4.10a shows the interdendritic region consists of a 2-phase

mixture with phases in light contrast in a matrix of darker contrast. EDS mapping in Figure

4.10b shows the segregation of chromium, tungsten and carbon to the interdentritic region.

The dark globular precipitate is rich in Silicon, Manganese and Oxygen. Table 4.3 enlists

chemical composition of interdendritic region and dark phase as well as matrix measured

using EDS.

Figure 4.8 SEM micrographs of etched as deposited DMLD FSX-414 showing (a) primary

dendrites growing on the substrate cast FSX-414 and (b) domain boundary; etched with 5%

HCl, electrolytic – 6V

Figure 4.9 (a,b) BSE images of unetched as deposited DMLD FSX-414 showing columnar

structure with elongated bright and globular dark phases in the interdendritic region.

(a) (b)

(a) (b)

66

Figure 4.10(a) High magnification BSE image of as deposited DMLD FSX-414 showing


Table 4.3 Chemical composition of various phases in As deposited DMLD FSX-414

Elements Co Cr Ni W Mn Si O

Bright Wt% 17.0 59.6 3.0 18.3 1.9 0.2

Dark Wt% 36.5 34.2 7.2 9.1 3.6 3.5 5.8

Matrix Wt% 51.4 28.5 10.9 7.1 1.3 0.8 -

(a)

(b)

67

4.6 Microstructural characterization of Solution heat treated DMLD FSX-

414.


Solution heat treatment was done at three different temperature namely, 1150oC, 1200

oC, and

1250oC. Figure 4.11 shows optical micrographs of solution heat treated DMLD FSX-414. A

fully dendritic structure is retained after heat treatment at 1150oC (Figure 4.11a). The

microstructure of the 1200oC treated sample in Figure 4.12 shows dendritic structure with the

breakdown of the interdendritic regions. Some indication of the grain boundaries can be seen

as well. Microstructure of samples solution treated at 1250oC in Figure 4.13 shows complete

breakdown of the dendritic structure. There are some small equiaxed grains in combination

with large elongated grains. Figure 4.13b highlights the curvature in the grain boundaries of

the two elongated grains. Figure 4.13a shows the remnants of the interdendritic regions

(arrow).


Figure 4.14 shows BSE images of the unetched DMLD FSX-414 sample solution treated at

1150oC. The continuity of the interdendritic region has broken down. The dark contrast

within the grains points to a remnant of microsegeration. BSE image of 1200oC treated

sample in Figure 4.15 indicates that the former interdendritic regions have broken down into

secondnd phases in light contrast. BSE image of 1250oC treated samples in Figure 4.16

shows only globular bright precipitates. A fine equiaxed network of bright and dark regions

bound by veins of light contrast appears within the grains in both the 1200oC and 1250

oC heat

treated samples. Globular dark precipitates can be observed in all 1150oC, 1200

oC and

1250oC treated samples and does not show any perceivable change in their volume fraction

and size.

Table 4.4 enlists composition of bright precipitates for all solution heat treatment

temperatures. The composition is roughly similar for all treatments. It can be observed that

the precipitates are changing their morphology from elongated to globular following a

systematic breakdown as a response to increase in the solution treatment temperature. These

precipitates are mainly rich in chromium, tungsten and carbon and the composition is roughly

similar for all treatments. EDS mapping of 1150oC treated specimen in Figure 4.17b confirms

the enrichment of Cr, W and C in the bright precipitates. Figure 4.17b also shows that

globular dark precipitates are rich in Mn, Si and O similar to as deposited samples. The high

magnification BSD image in Figure 4.17a also shows the presence of a network of plate

shaped precipitates.

68

Figure 4.11(a,b) Optical micrographs of Sol HT-1150

oC DMLD FSX-414 showing fully

dendritic structure, etched with 5% HCl, electrolytic-6V.


oC DMLD FSX-414 showing dendritic

structure with the indication of the grain boundary, etched with 5% HCl, electrolytic-6V.


oC DMLD FSX-414 showing complete

breakdown of dendritic structure.

(a) (b)

(a) (b)

(a) (b)

69

Figure 4.14(a,b) BSE images of Sol HT-1150

oC DMLD FSX-414 showing interdendritic

precipitates.


precipitates.

Figure 4.16(a,b) BSE images of Sol HT-1250oC DMLD FSX-414 showing remnants

interdendritic precipitates.

(a) (b)

(a) (b)

(a) (b)

70

Figure 4.17 (a) High magnification BSE image of Sol HT-1150

oC DMLD FSX-414 showing


Table 4.4 Chemical Composition of bright precipitates in DMLD FSX-414 samples

solution treated at various temperatures (all in weight %)

Sol HT Co Cr Ni W Mn Si

1150oC 15.0 61.2 2.7 18.5 2.2 0.4

1200oC 14.5 64 2.2 17 2.1 0.1

1250oC 15.3 63.8 2.0 16.7 2.0 0.2

4.7 Microstructural characterization of Sol HT 1150oC + Aged DMLD

FSX-414.

Ageing treatment (980oC) was given to solution treated (1150

oC) DMLD FSX-414 samples

to evaluate its high temperature stability. Optical micrographs in Figure 4.18 shows dendritic

microstructure similar to solution heat treated samples. Figure 4.19 shows BSE images of

(a)

(b)

71

unetched aged DMLD specimens. The inderdendritic regions are in bright contrast and

globular dark precipitates can be seen. High magnification BSE image in Figure 4.19b also

shows intersecting bands of ε-HCP martensite (as confirmed later by XRD). EDS mapping in

Figure 4.20b shows Cr, W and C enrichment in the elongated bright precipitates while Mn, Si

and O enrichment in dark globular precipitates. Precipitates were similar in composition and

morphology to that of as deposited and solution treated ones and ageing treatment has not

resulted in any new phase. Table 4.5 shows composition of precipitates measured using EDS.


oC+aged DMLD FSX-414 showing

dendritic microstructure, etched with 5% HCl, electrolytic – 6V.


oC+aged DMLD FSX-414 samples showing

bright and dark precipitates in the interdendritic regions and ε-HCP bands crossing across the

column.

(a) (b)

(a) (b)

72

Figure 4.20 (a) High magnification BSE image of Sol HT-1150

oC+aged DMLD FSX-414

sample showing bright and dark precipitate, and (b) corresponding EDS elemental mapping.

Table 4.5 Composition of various phases in Sol HT 1150oC + Aged DMLD FSX-414

4.8 Microstructural characterization of Sol HT+Aged Cast FSX-414.

Optical micrographs of Sol HT 1150oC + Aged Cast FSX-414 samples Figure 4.21 shows

very coarse dendritic structure with secondary dendrite arm spacing (SDAS) of around

around 70 µm. BSE images were taken in order to identify interdendritic phases as shown in

Figure 4.22. Figure 4.22 shows that the interdendritic region has typical irregular eutectic

type morphology and globular dark phases distributed all over the sample. EDS mapping in


Bright Wt% 12.5 64.2 1.9 19.4 2.0 0.1 -

Dark Wt% 12.4 41.8 2.7 3.2 19.4 1.1 19.2

Matrix Wt% 48.5 27.7 10.2 6.3 1.4 0.9 -

(a)

(b)

73

Figure 4.23b confirms segregation of chromium, tungsten and carbon in the interdendritc

bright precipitates, while globular dark phases are enriched in silicon, manganese and

oxygen. Table 4.6 enlists the composition of the phases present. Except the size, there is no

difference between the precipitates observed in DMLD FSX-414 and Cast FSX-414, both of

which have roughly similar composition.

Figure 4.21 Optical micrographs of Sol HT-1150

oC+aged Cast FSX-414 at (a) low

magnification showing coarse dendritic structure, (b) high magnification showing

interdendritic precipitates, etched with 5% HCl – electrolytic, 6V.


oC+aged Cast FSX-414 samples showing

eutectic phases in the interdendritic region.

(a) (a) (b)

(a) (b)

74

Figure 4.23 (a,b) High magnification BSE image of Sol HT-1150oC+aged Cast FSX-414

sample showing interdendritic precipitates and (b) corresponding EDS elemental mapping

.

Table 4.6 Composition of various phases in Sol HT 1150oC + Aged Cast FSX-414

4.9 X-ray Diffraction

Phase identification was carried out using XRD. X-ray diffraction peak identification was

done using two ICDD cards, one for FCC-Co and other for HCP Co. XRD patterns of As

deposited, Sol HT-1150oC and Sol HT-1150

oC+aged DMLD FSX-414, etc. shows peaks for

both the γ-FCC and ε-HCP Cobalt phases as shown in Figure 4.24.


Bright Wt% 12.8 64.8 2.2 18 2.2 - -

Dark Wt% 40 29.2 8.5 0.9 2.8 8.2 10.5

Matrix Wt% 50.6 29.1 10.7 7.1 1.4 0.8 -

(a)

(b)

75

Figure 4.24 X-ray Diffractions patterns of (a) as deposited, (b) Sol HT-1150

oC (c) Sol HT-

1150oC+aged, DMLD FSX-414, showing peaks for both γ-FCC and ε-HCP Cobalt phases


4.10 Hardness

Hardness profiles were plotted against build direction for transverse sections from 10% span

as can be seen in Figure 4.25. Hardness value of as deposited sample is between 310- 330 HV

and after solution heat treatment at 1150oC, it decreases to 340-380 HV. Hardness roughly

remains same after ageing treatment. Hardness values of DMLD part are 30-50 HV higher

than the cast FSX-414 for all the treatments as shown in Figure 4.26. Figure 4.27 shows

variation in hardness with increasing solution heat treatment temperature. The hardness

decreases with increasing solution treatment temperature. Hardness value of Solution HT-

1250oC is 100 HV lower than that of as deposited sample.

0

50

100

150

200

250

300

10 30 50 70 90 110 130 150

Inte

nsi

ty (

arb.

unit

s)

2θ

As deposited

Sol HT-1150 C

Sol HT+Aged

(a)

(b)

(c)

*

*

2

2

2

2

2

1

1

1, 2

1, 2

1, 2

1, 2

1, 2

1, 2

1 – γ(FCC) Co

ICDD: 00-015-806

2 – ε(HCP) Co

ICDD: 00-005-726

*Unidentified

76

Figure 4.25 Hardness profile along build direction as deposited, Sol HT-1150

oC, Sol HT-

1150oC+aged, DMLD FSX-414 and Cast FSX-414.

Figure 4.26 Hardness comparison between DMLD and Cast FSX-414.

250

270

290

310

330

350

370

390

0 10 20 30 40 50 60

Har

dnes

s (H

V)

Distance from Trailing Edge (mm)

As Deposited

Sol HT

Sol HT + Aged

0

50

100

150

200

250

300

350

400

450

As deposited Sol HT 1150 C Sol HT 1150 C +

Aged

Har

dnes

s (H

V)

DMLD Cast FSX-414

Build direction

DMLD FSX-414 Cast FSX-414

77

Figure 4.27 Variation in hardness with different solution heat treatment temperatures.

4.11 Tensile properties

Figure 4.28 shows 0.2% yield strength, UTS for room temperature tensile tests. As deposited

samples shows highest yield strength and UTS. Marginal drop in UTS and 0.2% Y.S. can be

seen after solution and ageing treatment on as deposited DMLD FSX-414. Figure 4.29 shows

fracture surfaces of the as deposited samples. Some curved facets and cracks can be seen at

lower magnification (Figure 4.29a). Fine dimples can be observed in the higher magnification

fractographs (Figure 4.29b). Similar curved facets at low magnification and dimples at high

magnification can be observed in fractographs of solution treated and aged samples (Figure

4.30). Both as deposited and Sol HT-1150oC+aged DMLD samples show higher 0.2% Y.S.

and UTS than that of cast FSX-414 sample as shown in Figure 4.28. Fractographs of Cast

FSX-414 in Figure 4.31 shows mainly dimples and some big voids. Figure 4.32, 4.33, 4.34

shows engineering stress-engineering strain curves for as deposited, Sol HT-1150oC+aged

DMLD FSX-414, Sol HT-1150oC+aged Cast FSX-414,

0

100

200

300

400

As Deposited HT - 1150 C HT - 1200 C HT - 1250 C

Har

dn

ess

(HV

)

0

200

400

600

800

As deposited Sol HT 1150 C+Aged

DMLD FSX-414

Sol HT 1150 C+Aged

Cast FSX-414

Str

enght

(MP

a)

0.2% Y.S. UTS(a)

78

Figure 4.28 Room temperature tensile properties of As deposited, Sol HT+Aged DMLD and

Cast FSX-414, (a) 0.2% Y.S. and UTS, (b) % Elongation (ductility).

Figure 4.29 Fractographs of as deposited DMLD FSX-414 tensile sample showing (a) cracks

at low magnification and (b) dimples at high magnification.

Figure 4.30 Fractographs of Sol HT-1150

oC+aged DMLD FSX-414 tensile samples showing

(a) curved facets at low magnification and (b) dimples at high magnification

0

10

20

30

40

As Deposited Sol HT + Aged DMLD

FSX-414

Sol HT+Aged Cast

FSX-414

Du

ctil

ity

(a) (b)

(a) (a) (b)

(b)

79

Figure 4.31(a,b) Fractographs of Sol HT-1150oC+aged Cast FSX-414 tensile samples

showing dimples and big voids.

Figure 4.32 Engineering stress - Engineering strain curve for as deposited DMLD FSX-414.

Figure 4.33 Engineering stress - Engineering strain curve for Sol HT-1150

oC+aged DMLD

FSX-414.

0

200

400

600

800

1000

0 10 20 30 40

Str

ess

(MP

a)

Strain %

0

200

400

600

800

1000

0 10 20 30 40

Str

ess

(MP

a)

Strain %

(a) (a) (b)

80

Figure 4.34 Engineering stress - Engineering strain curve for Sol HT-1150

oC+aged Cast

FSX-414

0

200

400

600

800

0 5 10 15 20 25 30

Str

ess

(MP

a)

Strain %

81

5. DISCUSSION

This chapter presents an analysis and discussion of the results of DMLS CoCrMo and DMLD

FSX-414. The results are discussed in five sections: first two sections describe

microstructural evolution and tensile properties of DMLS CoCrMo, later two sections include

microstructural evolution and tensile properties of DMLD FSX-414. The final section

presents the comparison between solidification behavior of DMLS and DMLD processes.

5.1 Microstructural evolution in Direct Metal Laser Sintered CoCrMo

5.1.1 Porosity and microcracks

Porosity and microcracks can be observed in as printed DMLS CoCrMo samples as examined

in this thesis (Figure 3.6 & 3.8). Most of pores are fine and spherical, while some have

irregular morphology. Fine and spherical pores can form due to entrapped gas within gas

atomized powders and bubbles[14], [15] from metallic evaporation due to high power laser

beam [17]. Location of irregular pores is mainly at the interlayer boundaries as shown in

Figure 3.8. Thus irregular can be due to incomplete remelting of the previous layer. However

the extent of porosity in sample is not very high and processing parameters have been

optimized properly to enable a dense component. Microcracks are almost perpendicular to the

melt pool boundary. Solidification shrinkage in upper molten layer which is restricted by

cooler substrate or previous layers is the reason behind formation of microcracks [13]

5.1.2 Macrostructure in as printed DMLS CoCrMo

The DMLS process is very similar to the laser welding process. The molten metal pool is

created when the laser beam with around 80 µm diameter hits the thin CoCrMo powder layer

with thickness of 40 µm. Since size of this pool is very small which is around 120 µm wide

and 60 µm deep as shown in Figure 3.7, it solidifies very rapidly owing to the high rate of

heat extraction from the cooler substrate / previous layers. The laser beam has maximum

intensity in the center which gradually decreases towards the edge of beam due to its

Gaussian energy distribution. This is the main reason for the formation of arc shaped melt

pools [21]. The laser beam creates a heat distribution profile with maximum temperature at

the center, which can melt the powder in area more than its size. Thus width of the melt pool

is higher than the laser beam diameter. Subsequently, laser beam scans the powder layer in a

predefined path and process is repeated for next layers. Melt pool boundaries as well as scan

paths can be seen transverse and base microstructures respectively in Figure 3.7. Schematic

representation of microstructural evolution during DMLS processing is shown in Figure 5.1.

Remelting of previous layers as well as adjacent melt tracks is important in order to achieve

good bonding between them. Depth of melt pool (~60 µm) is higher than the powder layer

thickness (40 µm). Microstructure of front planar section in Figure 3.7 shows more width.

82

Since the section taken for microstructure may not be perpendicular to the melt tracks, melt

apparent pool widths in the metallographic sections can be larger than actual widths as seen

in Figure 5.1.

The laser scanning direction is rotated by 67o

for every new powder layer. This can be

observed in the base microstructure in Figure 3.7. Changing scan direction by 67o decreases

the residual stress and porosity in the sample [17].

Figure 5.1 Schematic representation of microstructural evolution in DMLS CoCrMo.

5.1.3 Microstructure in DMLS CoCrMo

As printed DMLS CoCrMo show columnar microstructure with molybdenum and silicon

enrichment in the interdendritic regions. Meacock et al [32], Quian et al [16], Takaichi et al

[27] observed similar molybdenum and silicon rich phases in the interdendritic regions in

their studies on DMLS / SLM CoCrMo. Silicon rich dark globular phase in DMLS CoCrMo

(Figure 3.11) were also observed by Mengucci et al [23] for DMLS CoCrMoW and Giacchi

et al [33] for Cast CoCrMo. Identification of elongated bright precipitate in the interdendritic

regions requires extensive TEM characterization and will be studied in greater detail in

future. Mengucci et al[23] reported similar microstructural features with molybdenum and

silicon enriched interdendritic precipitate for DMLS CoCrMoW alloy. The precipitate was

identified as Co3(Mo,W)2Si phase with HCP crystal structure. Owing to the similar

composition used in this study except the tungsten content, the most probable phase in the

interdendritic region can be Co3Mo2Si.

Remelting of

previous layer and

adjacent melt tracks

CoCrMo

powder layer Large width of

melt pool due

to inclined melt

track.

83

TEM analysis of as printed DMLS CoCrMo reveals presence of ε-HCP martensite in

the form of plates as shown in Figure 3.12 & 3.13. XRD analysis also shows 19.7% HCP in

as printed sample (Table 3.8). Presence of athermal ε-HCP martensite phase and its formation

mechanism in CoCrMo alloy is widely reported in literature [7], [19], [23], [27], [31], [32],

[34]–[37]. ε-HCP phases grow on 111 planes of γ-FCC matrix. Since 111 planes have

70.71o

angle for [011] zone axis, ε-HCP phases can be observed with same angle as shown in

Figure 3.12. Fringe contrast in Figure 3.12 should be due the ε-HCP phases inclined to the

surface. Schematic representation of HCP phase growing on the 111 plane of the FCC is

shown in Figure 5.2. In pure cobalt FCC to HCP transformation is around 417oC. But due to

HCP stabilizer elements like Cr, Mo, Si, etc, transformation temperature can increase.

Because of sluggish kinetic of FCC-HCP transformation, samples contains majority of

metastable γ-FCC phase.

Figure 5.2 Schematic representation of athermal ε-HCP growing on γ-FCC Cobalt

The microstructure of the solution heat treated sample is fully equiaxed as shown in Figure

3.16. Table 5.1 shows size and morphology of grains in as printed, solution treated and aged

DMLS CoCrMo. Grains (or domains) in as printed sample are elongated. Solution heat

treatment can cause perturbations in the grain boundaries following Rayleigh instability

criteria, which finally results in breakdown of grain into several equiaxed grains. Rayleigh

instability is based on the principle in which elongated objects breaks into spheres in order to

decrease the overall surface energy. This process is aided by diffusion at high temperature

treatment. Grain sizes in Table 5.1 support this mechanism, where average width in as printed

grains is approximately similar to the grain size of solution treated condition. Figure 5.5

shows a schematic representation of equiaxed grain formation.

Table 5.2 shows size, location and morphology of various precipitates in the as

printed, Sol HT and Sol HT+Aged DMLS CoCrMo. The bright precipitate in solution treated

sample (Figure 3.19) is due to the remnants of the previous interdendritic region (Figure 3.11)

Zone Axis

= [011]

(11

1)

Pla

ne

x

y

z

(0001) Basal plane

84

indicating incomplete homogenization. Few globular bright precipitates can also be observed

along the grain boundaries. Size of these precipitates is around 235 nm, which is higher than

the width of interdendrtic precipitates from as printed structure (~80 nm) as shown in Table

5.2. Globular dark precipitates in solution treated samples must be the silicon rich inclusions

from the as printed samples. Microstructure of solution treated and aged samples shows

bright and dark precipitates covering almost entire grain boundary. BSE images do not show

any evidence of the interdendritic segregation (Figure 3.21). Thus ageing seems to have

dissolved these regions completely. The precipitates along the grain boundary are very

similar to those observed in solution heat treated sample except they are little coarser (Table

5.2). Ageing treatment has nucleated more bright precipitates along the grain boundary

(globular) as well as inside grains (plate-like). Plate like bright precipitates can be observed

predominantly along the twin boundaries and ε-HCP plates as shown in Figure 3.21. Both

grain boundary bright precipitate and plate like precipitate inside grains are molybdenum and

silicon rich as shown in EDS mapping in Figure 3.23. Figure 5.3 shows isothermal section of

the CoCrMo ternary phase diagram at 1200oC. Current alloy composition is exactly at the

boundary between γ and σ (Co9Mo15) phase. Thus it is expected that samples will have

certain amount of σ phase solution treatment at 1150 oC. Isothermal section of CoCrMo

ternary phase diagram at 924oC also shows presence of ε and σ phase. XRD analysis shows

increase in volume fraction of ε-HCP percentage as after ageing at 980oC as compared to

solution treated DMLS CoCrMo as shown in Table 3.8. Molybdenum and silicon rich

precipitates in Sol HT+Aged DMLS CoCrMo samples show very similar composition as that

of interdendritic precipitates. Thus, though ternary diagram predict the presence of σ-phase in

Sol HT and Sol HT+Aged samples, silicon might have shifted the equilibria towards the same

precipitate as that of interdendritic precipitate (probably Co3Mo2Si). The proper identification

of precipitates in as printed and heat treated DMLS CoCrMo requires extensive TEM

characterization which will be studied in greater detail in future. Figure 5.5 shows schematic

representation of microstructural evolution after solution and ageing heat treatments.

Table 5.1 Size and morphology of grains in As printed, Sol HT, Sol HT+Aged DMLS

CoCrMo

Treatment Grain morphology Grain / Domain size (µm)

Range Average value

As printed Elongated Width 10-70 µm ~39 µm

Length 40-290 µm ~104 µm

Sol HT Equiaxed 5-90 µm ~44 µm

Sol HT+Aged Equiaxed 5-90 µm ~43 µm

85

Table 5.2 Size and morphology of precipitates observed in As printed, Sol HT and Sol

HT+Aged DMLS CoCrMo

Treatment Morphology

and contrast

Location Precipitate size

Range Average

As printed Elongated

bright

Interdendritic

region

Width (nm) 50-105 nm ~ 80 nm

Globular dark Everywhere Diameter

(nm)

15-100 nm ~35 nm

Sol HT Globular /

Elliptical

bright

Grain boundary Diameter

(nm)

100-400

nm

~235

nm


(nm)

40-160 nm ~86 nm

Sol

HT+Aged

Globular /

Elliptical

bright

Predominantly

Along grain

boundary, Some

inside grains

Diameter

(nm)

175-725

nm

~390

nm

Plate bright Inside grains –

along twin

boundaries and ε-

plates

Width 70-120 nm ~95 nm

Length 1000-3000

nm

~1480

nm


(nm)

26-325 nm ~160

nm

Figure 5.3 Isothermal section of CoCrMo ternary diagram at 1200

0C.

86

Figure 5.4 Isothermal section of CoCrMo ternary diagram at 924

oC.

Figure 5.5 Schematic representation of microstructural changes during solution and ageing

heat treatments.

5.2 Tensile properties of Direct Metal Laser Sintered CoCrMo

Strengthening can take place due to work hardening, grain boundary strengthening (hall petch

effect), solid solution strengthening and precipitate strengthening. The equation 1 gives the

relation for the yield strength of the material.

As printed Sol HT Sol HT + Aged

87

σ = σo + σgs + σss + σpp

where, σo = Y.S. of a single crystal

σgs = increase in strength due to hall petch effect

σss -= increase in strength due to solid solution stregntheing

σpp = increase in strength due to precipitate strengtheining

As printed samples shows highest 0.2% Y.S. It can be observed that it decreases considerably

after solution heat treatment and remains unaffected after ageing. σss will be same for as

printed, solution and ageing treated samples due to same chemical composition. Due to the

difference in grain size, i.e. ~800 nm (column width) for as printed and ~40 µm (equiaxed

grain size) for solution heat treated, σgs is possibly the dominant factor for high strength in as

printed DMLS CoCrMo as compared to heat treated. After ageing there is almost no change

in the grain size. However extensive precipitation along the grain boundaries as well as inside

grains does not seem to play any role in increasing the strength of the alloy. Thus σpp has

negligible contribution towards overall yield strength.

Fractographs of as printed samples showing facets and cracks indicate cleavage type

fracture. Thus as printed samples have a brittle failure mode at room temperature. Low

ductility values of as printed samples confirm the same. Solution treated samples shows

relatively higher ductility for room temperature tensile tests. Figure 5.6 shows schematic

representation of engineering stress vs. engineering strain curve for room temperature tested

as printed and solution treated DMLS CoCrMo. Very high UTS of as printed sample as

compared to cleavage stress of CoCrMo must be the reason for its brittle failure mode.

Figure 5.6 Schematic representation of engineering stress vs. engineering strain curve for as

printed and solution treated DMLS CoCrMo (room temperature)

As printed

Sol HT

Cleavage stress

Very low ductility Relatively higher ductility

Strain %

Str

ess

(MP

a)

88

The presence of intergranular cracks and dimples in the fractographs of room

temperature tested samples reveal mixed ductile and brittle failure modes. For room

temperature tests, Sol HT+Aged samples shows considerably low ductility values than

solution heat treated samples. Fractographs shows cracks and reveals mainly intergranular

brittle fracture. This can be attributed to the intergranular precipitate phases, which might

have paved easy way for the propagation of intergranular cracks.

Low 0.2% Y.S. and UTS at high temperature tests is mainly because thermally

activated mechanisms assists deformation and decrease strength of the material [38]. In many

FCC materials UTS is more temperature dependent than 0.2% Y.S [38]. Figure 3.27 shows

UTS is dropping more rapidly than 0.2% when tested at high temperature for both Sol HT

and Sol HT+Aged samples. This phenomenon along with decrease in strain hardening

exponent with increasing temperature [38] causes flattening of Stress-strain curve when

tested at higher temperature (Figure 3.29 & 3.30). Both strength (0.2% Y.S., UTS) and

ductility is almost similar for high temperature tested Sol HT and Sol HT+Aged samples. The

mixed ductile and brittle mode can be observed in the fractographs of both high temperature

tested Sol HT and Sol HT+Aged samples (Figure 3.32 & 3.33). The grain boundary

precipitates in Sol HT+Aged samples does not harm the high temperature tensile properties

especially ductility unlike room temperature tests.

5.3 Microstructural evolution in Direct Metal Laser Deposited FSX-414

Microcracks and porosity can be observed in Figure 4.5. Microcracks are due to solidification

shrinkage which is restricted by the cooler substrate. Most of the pores are spherical, thus

might have originated from gas entrapped within gas atomized powders or due to metallic

vapors caused by high power laser beam same as what is observed for DMLS CoCrMo.

Microstructure of the as deposited samples shows very fine dendritic structure.

Dendritic columns in Figure 4.6b are almost perpendicular to the substrate. Melt pool cools

rapidly owing to its very small size relative to the substrate. The rapid heat extraction from

the substrate during solidification causes columnar dendrites to grow in a direction opposite

to that of heat flux. This phenomenon is also observed by Dinda et al [39], Bi e al [40] and

Hussein et al [41] for DMLD on Nickel based superalloys. Epitaxial growth can be observed

in Figure 4.7 (domain 3). Partially remelted grains from the previous layers acts as pre-nuclei

for the directional columns and thus leading to epitaxial growth [39].

Effect of varying solution heat treatment can be seen in Figures 4.11, 4.12 & 4.13.

Dendritic structure finally breaks down into equiaxed at 1250oC. Highlighted grains (no. 1

and 2) in Figure 4.13 shows curved boundaries for some elongated grains. Thus Rayleigh

instability criterion is acting on these boundaries to make them curved and eventually meet to

form equiaxed dendrites. Grain no. 1&2 in Figure 4.13 are actually in the process of

breakdown and just require extra thermodynamic driving force. Thus the mechanism of

breakdown is similar to what is observed for DMLS CoCrMo as shown in Figure 5.3. BSE

images for all three solution treatment temperature in Figures 4.14, 4.15 & 4.16 shows

89

systematic breakdown of the interdendritic precipitates following Rayleigh instability

criterion. Solution treatment at 1250oC does not dissolve the interdendritic precipitates fully,

and remnants can be seen in a globular form as shown in Figure 4.16. Hardness values

decreases considerably with increasing solution treatment. (Figure 4.27) This can be

attributed to lower volume fraction of precipitates due to partial dissolution and breakdown of

the dendritic structures.

BSE images and corresponding EDS mapping of as deposited samples in Figure 4.9 &

4.10 show Cr, W and C rich elongated bright precipitates in the interdendritic region. EDS

mapping of Sol HT+Aged Cast FSX-414 in Figure 4.23 also shows Cr, W and C rich

precipitates in the interdendritic region. Interdendritic phases in cast FSX-414 were identified

previously by Foster et al [42] and Mezzedimi et al [43] as Cr21W2C6 (M23C6 type).

Chemical composition (atomic %) of bright phase in both as deposited (Table 4.3) and Sol

HT+Aged cast FSX-414 (Table 4.6) samples corresponds to the presence of Cr21W2C6

precipitates.

Microstructure of Solution treated (1150oC) shows dendritic structure. EDS mapping

(Figure 4.17) and composition (Table 4.4) confirms the presence of Cr21W2C6 precipitates.

Solution treatment at higher temperatures (1200oC, 1250

oC) show similar composition of the

precipitates (Table 4.4) but with different morphology due to the breakdown. Microstructure

and corresponding EDS mapping of Sol HT-1150oC+Aged samples in Figure 4.20 also shows

same elongated Cr21W2C6 precipitates in the interdendritic region. No new precipitate can be

observed. Thus both solution treatment and ageing has almost no perceivable effect on the

microstructures. It has been reported that commercial heat treatment cycles for cast FSX-414

does not bring about complete carbide precipitation and more types of carbide precipitates

during service at high temperature [44].

Globular dark phases can be observed in all treatments of DMLD FSX-414 and Cast

FSX-414. EDS mapping and chemical composition suggests Si, Mn and O enrichment in

these phases (Figures 4.10, 4.17, 4.20, 4.23). These particles can be Si/Mn oxide inclusions

which form during solidification.

5.4 Tensile properties of Direct Metal Laser Deposited FSX-414

As deposited samples shows highest 0.2% Y.S. and UTS. Fractographs of as deposited in

Figure 4.29 samples show curved facets which seems to be following domain boundaries and

cracks at low magnification and dimples at high magnification reveal ductile type fracture.

Similar fractographs were observed for Sol HT+Aged samples (Figure 4.30) thus also show

fully ductile fracture. 0.2% Y.S and UTS of the Sol HT+Aged DMLD FSX-414 is only

marginally lower than that of as deposited owing to the similar microstructure as shown in

Figure 4.28a. Higher strength of the Sol HT-1150oC+aged DMLD FSX-414 as compared to

Sol HT 1150oC +Aged Cast FSX-414 is attributed to difference in grain size, i.e., 4-5 µm for

Sol HT-1150oC+aged DMLD FSX-414 and 70 µm for Sol HT 1150

oC +Aged Cast FSX-414.

Fractographs of the Sol HT 1150oC +Aged Cast FSX-414 samples in Figure 4.31 shows

dimpled rupture indicating fully ductile failure mode. All samples, as deposited and Sol HT

1150oC + Aged DMLD FSX-414 and Sol HT 1150

oC + Aged Cast FSX-414 shows

considerable ductility as shown in Figure 4.28b.

90

5.5 Comparison between DMLS CoCrMo and DMLD FSX-414

Table 5.3 shows comparison between various microstructural and mechanical properties

aspects of DMLS CoCrMo, DMLD FSX-414 and cast FSX-414. Though both CoCrMo and

FSX-414 are two cobalt based superalloys, very similar in composition except their Mo and

W contents respectively. It can be observed that ε-HCP phase is present in both DMLS

CoCrMo and DMLD FSX-414 in varying quantities in metastable γ-FCC matrix. It can be

seen that the difference of cooling rates between DMLS (106 oC/s) and DMLD (10

3 oC/s) is

reflected in the primary dendritic arm spacing of respective solidification microstructures

(DMLS: 0.6-1.0 µm, DMLD: 9-12 µm).

Table 5.3 Comparison between DMLS CoCrMo and DMLD FSX-414

DMLS CoCrMo DMLD FSX-414 Cast FSX-

414

Cooling rate 106 oC/s [45] 10

3 oC/s [26] -

As

printed/deposite

d microstructure

Columnar dendritic

PDAS = 0.6-1.0 µm

Columnar dendritic

PDAS = 9-12 µm

Interdendritic

region Rich in Mo and Si (Cr21W2)C6 (Cr21W2)C6

ε-HCP Present Present -

Solution treated

(1150oC)

microstructure

Equiaxed grains with size ~

44 µm, Few remnants of

interdendritic precipitates

1150oC

Columnar

dendritic

PDAS = 9-12

µm

-

1200oC

Partially

dissolved

interdendritic

region

-

1250oC

Equiaxed grains

with remnants

of interdendritic

precipitates

-

Solution treated

(1150oC) + Aged

(980oC)

Equiaxed grains with size ~

44 µm, Extensive

precipitation of Mo and Si

rich precipitates along

grains boundaries as well

as inside grains.

Columnar dendritic

PDAS = 9-12 µm

SDAS = 4-6 µm

Dendritic

SDAS = 70

µm

91

DMLS CoCrMo DMLD FSX-414 Cast FSX-

414

As

printe

d

Sol

HT

Sol

HT+Age

d

As

deposited

Sol

HT

1150 oC

Sol HT

1150 oC +

Aged

Sol HT

1150 oC +

Aged

Hardness (HV) 467 365 346 353.2 323.9 330 306.5

Tensile strength

(MPa) 1378.9 1114 982.5 887.7 - 805.7 729.9

Ductility (%

Elongation) 5.7% 15% 5.3%

92

6. CONCLUSIONS AND FUTURE WORK

This thesis presents detailed microstructural characterization and tensile properties of direct

metal laser sintered (DMLS) CoCrMo and direct metal laser deposited (DMLD) FSX-414.

The conclusions drawn from studies are as follows:

6.1 Conclusions

Part A: Direct Metal Laser Sintering (DMLS) of CoCrMo

Microstructure inside the melt pool shows very fine columnar structure with the Mo and

Si segregation in the interdendritic regions.

Columns with same orientation which can grow across the melt pool boundaries form

elongated domains / grains.

TEM studies reveals presence of athermal ε-HCP martensite which forms due to rapid

solidification.

Fully equiaxed grain structure after solution heat treatment is due to breakdown of the

elongated grains from the as printed samples following Rayleigh like instability.

Precipitates can be observed after ageing heat treatment both along the grain boundaries

as well as inside grains.

Hardness and tensile strength decreases after the solution heat treatment, which can be

attributed to increase in grain size (Hall-Petch effect).

Hardness and tensile strength remains same after ageing treatment, thus precipitates in the

ageing treatment seems to be not playing any role in increasing the strength and grain size

is dominant factor in determining strength.

Intergranular failure seen with very low ductility after ageing can be attributed to

extensive precipitates along the grain boundaries.

Part B: Direct Metal Laser Deposition (DMLD) of FSX-414

As deposited structure shows fine columnar dendritic structure with (Cr21W2)C6

segregation at the interdendritic regions.

Columns with same orientation which can grow across the layers form elongated

domains/grains.

Dendritic structure breaks down during solution treatment at 1250oC temperature

following Rayleigh like instability.

With increase in solution heat treatment, hardness decreases from 353.2 HV for as

deposited samples to 258 HV for 1250oC treated samples.

Precipitates also breakdown following Rayleigh like instability.

No change in microstructure was observed after solution and ageing treatments.

93

Hardness and tensile properties decreases only marginally due to Sol HT 1150oC +

Ageing treatment can be attributed to almost no change in microstructure.

Hardness and tensile strength of both as deposited and Sol HT 1150oC + Aged DMLD

FSX-414 is higher than that of Sol HT 1150oC + Aged Cast FSX-414 can be attributed to

the grain size (SDAS).

Both as deposited, Sol HT 1150oC + Aged DMLD FSX-414 samples shows significant

ductility.

6.2 Future work

TEM analysis in order to identify various precipitates in as printed, solution treated and

aged DMLS CoCrMo.

High temperature tensile testing of DMLD FSX-414 samples.

Creep and fatigue studies on both DMLS CoCrMo and DMLD FSX-414 in order to

evaluate its suitability in high temperature gas turbine applications.

94

BIBLIOGRAPHY

[1] T. Wohlers and T. Gornet, “History of additive manufacturing,” Wohlers Rep. 2014,

pp. 1–23, 2014.

[2] R. A. of Engineering, “Additive manufacturing : opportunities and constraints,” R.

Acad. Eng., no. May 2013, p. 21, 2013.

[3] F. H. Froes and B. Dutta, “Additive Manufacturing of Titanium Alloys,” Adv. Mater.

Res., vol. 1019, no. February, pp. 19–25, 2014.

[4] Cobalt Facts, “Cobalt in Metallurgical Uses,” Cobalt Facts, pp. 8–22, 2006.

[5] J. R. Davis and A. S. M. I. H. Committee, Nickel, Cobalt, and Their Alloys. 2000.

[6] W. Baldwin, “Metallography and Microstructures Handbook,” ASM Int., vol. 9, p.

2733, 2004.

[7] A. J. Saldivar-Garcia and H. F. Lopez, “Temperature Effects on the Lattice Constants

and Crystal Structure of a Co-27Cr-5Mo Low-Carbon Alloy,” Metall. Mater. Trans. A,

vol. 35, no. August, pp. 2517–2523, 2004.

[8] ASM International, ASM Handbook, Volume 3, Alloy Phase Diagrams. 2004.

[9] M. Agarwala, D. Bourell, J. Beaman, H. Marcus, and J. Barlow, “Direct selective laser

sintering of metals,” Rapid Prototyp. J., vol. 1, no. 1, pp. 26–36, 1995.

[10] D. Clark, M. R. Bache, and M. T. Whittaker, “Shaped metal deposition of a nickel

alloy for aero engine applications,” J. Mater. Process. Technol., vol. 203, no. 1–3, pp.

439–448, 2008.

[11] a D. Submitted, F. O. R. The, D. Of, and D. Of, “Process Parameter Optimization for

Direct Metal Laser Sintering ( DMLS ),” Optimization, 2005.

[12] K. Monroy, J. Delgado, and J. Ciurana, “Study of the pore formation on CoCrMo

alloys by selective laser melting manufacturing process,” Procedia Eng., vol. 63, pp.

361–369, 2013.

95

[13] N. J. Harrison, I. Todd, and K. Mumtaz, “Reduction of micro-cracking in nickel

superalloys processed by Selective Laser Melting: A fundamental alloy design

approach,” Acta Mater., vol. 94, pp. 59–68, 2015.

[14] H. Gong, K. Rafi, H. Gu, T. Starr, and B. Stucker, “Analysis of defect generation in

Ti-6Al-4V parts made using powder bed fusion additive manufacturing processes,”

Addit. Manuf., vol. 1, pp. 87–98, 2014.

[15] A. Bauereiß, T. Scharowsky, and C. Körner, “Defect generation and propagation

mechanism during additive manufacturing by selective beam melting,” J. Mater.

Process. Technol., vol. 214, no. 11, pp. 2497–2504, 2014.

[16] B. Qian, K. Saeidi, L. Kvetková, F. Lofaj, C. Xiao, and Z. Shen, “Defects-tolerant Co-

Cr-Mo dental alloys prepared by selective laser melting,” Dent. Mater., vol. 31, no. 12,

pp. 1435–1444, 2015.

[17] X. Zhou, D. Wang, X. Liu, D. Zhang, S. Qu, J. Ma, G. London, Z. Shen, and W. Liu,

“3D-imaging of selective laser melting defects in a Co-Cr-Mo alloy by synchrotron

radiation micro-CT,” Acta Mater., vol. 98, pp. 1–16, 2015.

[18] G. Barucca, E. Santecchia, G. Majni, E. Girardin, E. Bassoli, L. Denti, a. Gatto, L.

Iuliano, T. Moskalewicz, and P. Mengucci, “Structural characterization of biomedical

Co–Cr–Mo components produced by direct metal laser sintering,” Mater. Sci. Eng. C,

vol. 48, pp. 263–269, 2015.

[19] E. Girardin, G. Barucca, P. Mengucci, F. Fiori, E. Bassoli, A. Gatto, L. Iuliano, and B.

Rutkowski, “Biomedical Co-Cr-Mo Components Produced by Direct Metal Laser

Sintering,” Mater. Today Proc., vol. 3, no. 3, pp. 889–897, 2016.

[20] P. Kanagarajah, F. Brenne, T. Niendorf, and H. J. Maier, “Inconel 939 processed by

selective laser melting: Effect of microstructure and temperature on the mechanical

properties under static and cyclic loading,” Mater. Sci. Eng. A, vol. 588, pp. 188–195,

2013.

[21] C. Yan, L. Hao, A. Hussein, P. Young, J. Huang, and W. Zhu, “Microstructure and

mechanical properties of aluminium alloy cellular lattice structures manufactured by

direct metal laser sintering,” Mater. Sci. Eng. A, vol. 628, pp. 238–246, 2015.

[22] K. N. Amato, S. M. Gaytan, L. E. Murr, E. Martinez, P. W. Shindo, J. Hernandez, S.

Collins, and F. Medina, “Microstructures and mechanical behavior of Inconel 718

fabricated by selective laser melting,” Acta Mater., vol. 60, no. 5, pp. 2229–2239,

2012.

96

[23] P. Mengucci, G. Barucca, A. Gatto, E. Bassoli, L. Denti, F. Fiori, E. Girardin, P.

Bastianoni, B. Rutkowski, and A. Czyrska-Filemonowicz, “Effects of thermal

treatments on microstructure and mechanical properties of a Co-Cr-Mo-W biomedical

alloy produced by laser sintering,” J. Mech. Behav. Biomed. Mater., vol. 60, pp. 106–

117, 2016.

[24] L. Rickenbacher, T. Etter, S. Hövel, and K. Wegener, “High temperature material

properties of IN738LC processed by selective laser melting (SLM) technology,” Rapid

Prototyp. J., vol. 19, no. 4, pp. 282–290, 2013.

[25] Q. Jia and D. Gu, “Selective laser melting additive manufacturing of Inconel 718

superalloy parts: Densification, microstructure and properties,” J. Alloys Compd., vol.

585, pp. 713–721, 2014.

[26] P. Nie, O. A. Ojo, and Z. Li, “Numerical modeling of microstructure evolution during

laser additive manufacturing of a nickel-based superalloy,” Acta Mater., vol. 77, pp.

85–95, 2014.

[27] A. Takaichi, T. Nakamoto, N. Joko, N. Nomura, Y. Tsutsumi, S. Migita, H. Doi, S.

Kurosu, A. Chiba, and N. Wakabayashi, “Microstructures and mechanical properties

of Co–29Cr–6Mo alloy fabricated by selective laser melting process for dental

applications,” J. Mech. Behav. Biomed. Mater., vol. 21, pp. 67–76, 2013.

[28] J. V Giacchi, C. N. Morando, O. Fornaro, and H. A. Palacio, “Microstructural

characterization of as-cast biocompatible Co – Cr – Mo alloys,” vol. 2, 2010.

[29] T. Vilaro, C. Colin, J. D. Bartout, L. Nazé, and M. Sennour, “Microstructural and

mechanical approaches of the selective laser melting process applied to a nickel-base

superalloy,” Mater. Sci. Eng. A, vol. 534, pp. 446–451, 2012.

[30] EOS, “Material data sheet EOS CobaltChrome MP1,” vol. 49, no. 0, pp. 1–6, 2011.

[31] a. D. J. Saldívar García, a. M. Medrano, and a. S. Rodríguez, “Formation of hcp

martensite during the isothermal aging of an fcc Co-27Cr-5Mo-0.05C orthopedic

implant alloy,” Metall. Mater. Trans. A, vol. 30, no. 5, pp. 1177–1184, 1999.

[32] C. G. Meacock and R. Vilar, “Structure and properties of a biomedical Co-Cr-Mo

alloy produced by laser powder microdeposition,” vol. 21, no. 2, pp. 88–95, 2009.

[33] J. V Giacchi, O. Fornaro, and H. Palacio, “Microstructural evolution during solution

treatment of Co – Cr – Mo – C biocompatible alloys,” vol. 8, pp. 3–11, 2012.

97

[34] a D. J. Saldı, a S. Rodrı, A. de J. Saldıvar Garcıa, A. Manı Medrano, and A. Salinas

Rodrıguez, “EFFECT OF SOLUTION TREATMENTS ON THE FCC / HCP

ISOTHERMAL MARTENSITIC TRANSFORMATION IN Co-27Cr-5Mo-0 . 05C

AGED AT 800 ° C,” Scr. Mater., vol. 40, no. 6, pp. 717–722, 1999.

[35] K. Yamanaka, M. Mori, Y. Koizumi, and A. Chiba, “Local strain evolution due to

athermal ??????? martensitic transformation in biomedical CoCrMo alloys,” J. Mech.

Behav. Biomed. Mater., vol. 32, pp. 52–61, 2014.

[36] M. Mori, K. Yamanaka, S. Sato, and A. Chiba, “Texture evolution and mechanical

anisotropy of biomedical hot-rolled Co – Cr – Mo alloy,” vol. 51, pp. 205–214, 2015.

[37] C. Song, H. Park, H. Seong, and H. F. López, “Development of athermal and

isothermal ε-martensite in atomized Co-Cr-Mo-C implant alloy powders,” Metall.

Mater. Trans. A Phys. Metall. Mater. Sci., vol. 37, no. 11, pp. 3197–3204, 2006.

[38] H. Fujita and S. Ueda, “Stacking faults and fcc (γ)→ hcp (ϵ) transformation in 188-

type stainless steel,” Acta Metall., 1972.

[39] G. P. Dinda, a. K. Dasgupta, and J. Mazumder, “Laser aided direct metal deposition of

Inconel 625 superalloy: Microstructural evolution and thermal stability,” Mater. Sci.

Eng. A, vol. 509, no. 1–2, pp. 98–104, 2009.

[40] G. Bi, C. N. Sun, H. C. Chen, F. L. Ng, and C. C. K. Ma, “Microstructure and tensile

properties of superalloy IN100 fabricated by micro-laser aided additive

manufacturing,” Mater. Des., vol. 60, pp. 401–408, 2014.

[41] N. I. S. Hussein, J. Segal, D. G. McCartney, and I. R. Pashby, “Microstructure

formation in Waspaloy multilayer builds following direct metal deposition with laser

and wire,” Mater. Sci. Eng. A, vol. 497, no. 1–2, pp. 260–269, 2008.

[42] F. Alifui-Segbaya, J. Lewis, D. Eggbeer, and R. J. Williams, “In vitro corrosion

analyses of heat treated cobalt-chromium alloys manufactured by direct metal laser

sintering,” Rapid Prototyp. J., vol. 21, no. 1, pp. 111–116, 2015.

[43] K. Ando, T. Omori, J. Sato, Y. Sutou, K. Oikawa, R. Kainuma, and K. Ishida, “Effect

of Alloying Elements on fcc/hcp Martensitic Transformation and Shape Memory

Properties in Co-Al Alloys,” Mater. Trans., vol. 47, no. 9, pp. 2381–2386, 2006.

[44] D. L. Oates, “Microstructural Changes as a Time Temperature Indicator in Cobalt

Superalloys and a NiCoCrAITaY coating by,” no. July, 2007.

98

[45] Y. Li and D. Gu, “Parametric analysis of thermal behavior during selective laser

melting additive manufacturing of aluminum alloy powder,” Mater. Des., vol. 63, pp.

856–867, 2014.

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