Investigation of Surface Integrity in Dry m

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ORIGINAL ARTICLE

Investigation of surface integrity in dry machining

of Inconel 718

Domenico Umbrello

Received: 20 April 2013 /Accepted: 12 July 2013# Springer-Verlag London 2013

Abstract Machining of advanced aerospace materials havegrown in the recent years although the diffucult-to-machinecharacteristics of alloys like titanium or nickel-based alloys

cause higher cutting forces, rapid tool wear, and more heat generation. Therefore, machining with the use of coolinglubricants is usually carried out. To reduce the productioncosts and to make the processes environmentally safe, thegoal is to move toward dry cutting by eliminating cuttingfluids. This objective can be achieved by using coated tool,

by increasing cutting speed, and by improving the product performance in term of surface integrity and product quality.The paper addresses the effects of cutting speed and feed onthe surface integrity during dry machining of Inconel 718alloy using coated tools. In particular, the influence of thecutting conditions on surface roughness, affected layer,

microhardness, grain size, and microstructural alterationwas investigated. Results show that cutting conditions havea significant effect on the parameters related to the surfaceintegrity of the product affecting its overall performance.

Keywords Machining . Inconel 718 . Surface integrity

1 Introduction

Nickel-based superalloys were created in the 1940s primarilyfor gas turbine application due to their long-time strength and

toughness at high temperature and more creep resistance property than available stainless austenitic steels. Nickel base superalloys are also used for other applications suchas marine equipment, nuclear reactors, etc. They are used in

these aggressive environments because of their ability tomaintain high resistance to corrosion, mechanical and ther-mal fatigue, mechanical and thermal shock, creep, and ero-

sion at elevated temperatures [1]. Within the commerciallyavailable nickel-based superalloys, Inconel 718 is the most frequently used for many applications: aircraft gas turbines,reciprocating engines, metal processing (e.g., hot work toolsand dies), space vehicles (e.g., aerodynamically heated skins,rocket engine parts), heat treating equipments, nuclear power

plants, chemical and petrochemical industries, and heat exchangers.

Although it is the most common nickel superalloy used inthe aerospace industry, some drawbacks are noticed andwithin these, the poor machinability is probably the worst.In fact, machining of nickel-based alloys generate high tem-

peratures at the cutting tool edge, impairing their perfor-mance as they are subjected to high compressive stressesacting on the tool tip. This leads to the plastic deformation of the tool edge, severe notching and flank wear [1, 2].

Severe wear are also due to the high hot hardness andstrength causes deformation of the cutting tool during ma-chining and the austenitic matrix of nickel alloys whichcauses rapid work hardening during machining. The poor thermal conductivity of nickel-based alloys, raises tempera-ture at the tool – workpiece interface during machining, thus,it accelerates the undesired tool wear and results in theshortening of cutting tool life [2, 3].

Therefore, all the cutting parameters, such as tool andcoating materials choice, tool geometry, machining strategy,cutting speed, feed rate, depth of cut, lubrication, etc., must

be controlled and selected in order to achieve an acceptabletool life and a correct surface integrity for the machined parts[4, 5]. Itakura et al. [6] conducted dry turning experiments toclearly identify the tool wear mechanisms when a commonlyused coated cemented carbide tool cuts Inconel 718. Jindalet al. [7] studied the relative merits of physical vapor depo-sition (PVD) TiN, TiCN, and TiAlN coatings on cemented

D. Umbrello (*)Department of Mechanical, Energy and Management Engineering,University of Calabria, Via P. Bucci, Cubo 45/C,87036 Rende, CS, Italye-mail: [email protected] URL: www.unical.it

Int J Adv Manuf Technol

DOI 10.1007/s00170-013-5198-0



carbide substrate (WC-6 wt% Co alloy) in the turning of Inconel 718. Prengel et al. [8] performed Inconel 718 turningtests with a coolant and different PVD-coated carbide cuttingtools.

Furthermore, in order to keep increasing the machining performance, different assistance methods have been recent-ly developed to replace the “conventional process” [9]. One

of them presents high-pressure jet assistance (HPJA), whichaims at upgrading conventional machining, using the ther-mal and mechanical properties of a high-pressure jet of water or emulsion directed into the cutting zone [10 – 12]. By ap-

plying a high-pressure fluid jet to the cutting zone, it is possible to achieve advantages such as significantly de-creased temperature in the cutting zone, prolonged tool life,and lower forces. These results have also shown improvedsurface integrity and better dimensional accuracy of the

produced aeroengine components [13].As recently described in a literature review proposed by

Ulutan and Ozel [14], a large number of researches have

been carried out in order to investigate and to optimize themachining process of Inconel 718 alloy in terms of improv-ing quality and surface integrity of the components, increas-ing productivity and lowering cost. However, the mainworks concerning machining Inconel 718 were performedusing cutting fluids even when advanced cooling techniqueslike HPJA [10 – 12] or hybrid techniques [15] were involved.Thus, the effect of dry conditions, especially on the surfaceintegrity of machined products, was little investigated. Re-cently, Devillez et al. [16] analyzed the role played by thesuppression of cutting fluid on surface finish and residualstresses during turning of Inconel 718. They also executed

microhardness measurements at various locations on thecross section of the machined samples to determine themachining affected layer by plastic deformation. Their re-sults highlight that the workpiece machined in dry conditionwas hardened more than the one machined under wet condi-tions and the hardened layers were about 250μ m beneath themachined surface. Comparable microhardness values andgradients were measured by Pawade et al. [17] and byEzugwu and Tang [18].

Within the perspective to move toward dry cutting byeliminating cutting, this paper aims to investigate the effectsof cutting speed and feed on the surface integrity during dry

machining of Inconel 718 alloy using coated tools. Theeffects of the cutting conditions on surface roughness, affect-ed layer, grain size variations, and phase changes/modifica-tion were investigated.

2 Experimental procedure

Dry orthogonal cutting tests were conducted on Inconel 718(429±9 HV0.05) using a high-speed Mazak computer numerical

control (CNC) turning center and setting a configuration asschematically indicated in Fig. 1a. In particular, a bar of 347 mm as initial diameter was gently machined in order tocreate several disks characterized by thin wall geometry(10 mm depth and 2 mm thick) spaced 4 mm from each other.Coated DNMG Sandvik tool (ISO S-DNMG150616) was se-lected and mounted on a Sandvik DDJNR/L tool holder (pro-

viding rake and clearance angles of −6° and 4°, respectively) asshown in Fig. 1b.

In order to avoid effects on the machined surface relatedto transient condition (in either feed or speed) due to theorthogonal configuration, the experiments were executed inthe following sequence:

1. The tool was aligned and brought close to the rotatingworkpiece by single-stepping through the CNC program(i.e., only one block/line of program code is executedwith each press of the button by the operator).

2. All the instruments are set to “record” mode and the

CNC program is taken out of single-step mode.3. With the next press of the button the rest of the programis executed uninterrupted, i.e., the tool enters the work-

piece and continues cutting at the prescribed feed rate uptill the prescribed end-of-cut diameter, and then instan-taneously retracts at maximum feed. This insures that therubbing of the tool against the final machined workpiecesurface is minimal (though it may not be zero) and,equally importantly, invariant/constant for all the exper-imental conditions. Further, since the feed rate employedis in the low range (0.050 – 0.100 mm/rev) according tothe tool makers, and due to the large workpiece diameter

(347 mm at start and 338 at the end) the rpm correspond-ing to the cutting speeds employed are also low (47 – 66 rpm), it is possible for the CNC machine's hardwareto change from radially inward feed to outward extrac-tion almost instantaneously for all practical purposes.Hence, the transient effects were minimal, and this wasconfirmed from the force signals recorded during thecutting.

Disks were machined at varying of three cutting speedsand three feed rates as illustrated in Table 1; three replica-tions were performed for each test. It is important to under-line that the use of these ranges for the cutting parameters

were determined taking into account the tool maker recom-mendations for typical and industrial machining operationson nickel-based alloys. The cutting time of each test was 80 – 90 s in order to reach the mechanical and thermal steady-state conditions.

After machining, samples of 5×5 mm were sectioned bywire-EDM, then polished and etched for 35 s using Kalling'sreagent (number 2) to observe microstructural changes (affectedlayer and grain size) using a light optical microscope (×1,000).The surface roughness, Ra, of the machined workpiece was




measured using a Zygo® optical white light interferometry- based surface profilometer. The surface and subsurface hardnessvalues were also measured on a micro hardness indenter FutureTech F-7. Additionally, the X-ray diffraction (XRD) Bruker AXS D8 Discover with a quarter Ellurian cradle sample holder was used for investigating the microstructural phase composi-

tion of the machined surface. The X-ray diffractometer used Cu-K α radiation (λ=1.54184 Å, K α1/K α2=0.5) from a sourceoperated at 40 kV and 40 mA. Samples were accordingly

positioned at the center of plate into the X-ray goniometric inorder to ensure a correct beam irradiation. The 2θ scans werecarried out between 30° and 100° 2θ. The scan increment was0.05°; the corresponding acquisition time was accordinglyvaried.

3 Experimental results and discussion

3.1 Surface roughness

The surface roughness values, Ra, of the machined samplewere measured five times for each test to evaluate the char-acteristic of the machined surface. As shown in Fig. 2, thesurface roughness measurements for all the test conditionsare always below 0.3 μ m, which is a very good finish surfacequality. It has been shown that, a smooth surface with better surface roughness would prevent the initiation of cracksunder cyclic loads [19].

Figure 2 also shows a mapped region called “turningreplaces grinding” where only cutting speed of 70 m/min

produces comparable Ra values with grinding.

3.2 Microhardness

Figure 3 shows the variation in microhardness values for thedifferent experimental conditions employed. In particular,the results highlight that, in all of the investigated cases,the surface hardness is higher than that of the bulk material.Also, the value of the ratio HV0.05max/HV0.05initial increaseswith the increasing of both cutting speed and feed rate.Furthermore, higher cutting speed and feed rate allow thematerial to reach a deeper hardness variation. The depthcontaining hardness values greater than the one in the bulk material ranges from 60 – 65 μ m for test ID 1 up to 120 – 130 μ m for test ID 9.

3.3 Grain size and affected layer

The structure of each machined surface and subsurface has been measured by optical microscope (×1,000). It was foundthat all the examined samples presented a refinement of sizenear to the machined surface and beneath it (the initial grainsize in the bulk averages at 18 μ m). Figure 4 shows theoptical images for tests ID 1 and ID 9, while Fig. 5 shows

Fig. 1 a Scheme of theorthogonal machining; b

obtained disks from Inconel718 bar and coated DNMGSandvik tool positioned for theorthogonal machining operation

Table 1 Experimental machining test conditions

Cutting speed

50 m/min 60 m/min 70 m/min

Feed rate [mm/rev] 0.050 ID 1 ID 4 ID 7

0.075 ID 2 ID 5 ID 8

0.100 ID 3 ID 6 ID 9

Turning replaces grinding

Fig. 2 Surface roughness, Ra, on machined samples at varying of cutting speeds and feed rates




Fig. 3 Surface and subsurface hardness profiles at varying of cuttingspeed: a 50 m/min, b 60 m/min, and c 70 m/min

affected “featureless” layer

affected “featureless”

layer

a bFig. 4 Optical images of themachined surface and subsurface:a test ID 1 and b test ID 9

Grain size on the bulk material



a

b

c

Fig. 5 Grain size evolution near the machined surface and below it: a50 m/min, b 60 m/min, and c 70 m/min




grain size variation from machined surface to the bulk ma-terial for all the investigated tests. Results clearly show that the cutting conditions influence the final microstructure of the machined product. In fact, the grain size becomes smaller

when higher cutting speeds and feed rates are utilized.Moreover, in several tests, the grain size on the ma-

chined surface cannot be revealed by optical microscopeeven when the largest magnification was used (×1,000)since affected featureless structures appear. The appear-ance of the featureless layers formed under machining(Fig. 4b) were similar to the white layers in the machinedsurfaces of nickel-based superalloy IN -100 [20], wheresignificant grain refinement to nanocrystalline level wasfound due to dynamic recrystallization.

Estimation of this layer for all the samples was executed by Image Pro Plus software; results are reported in Fig. 6. Asobserved, the affected featureless layer ranges between 3.5and 6 μ m and it increases with both the cutting speed andfeed rate. However, the influence of cutting speed is moresignificant when feed higher than 0.075 mm/rev is utilized.

Finally, it is important to underline that depth containing

grains slightly smaller than the one in the bulk materialranges from 75 μ m for test ID 1 up to 120 – 130 μ m for testscharacterized by the higher feed rate (IDs 3, 6, and 9). Thesedepths are in accordance with those found by analyzing themicrohardness (Fig. 3).

3.4 XRD phase analysis

Figure 7 shows diffraction patterns of Inconel 718 for severalmachined samples and for the as received material. In partic-ular, Fig. 7a reports the influence of the feed rate for the cutting

speed of 50 m/min (tests IDs 1, 2 and 3), while Fig. 7b showsthe effect of the feed rate at the higher cutting speed (tests IDs7, 8, and 9). The X-ray phase analysis on the as receivedmaterial shows five peaks at 43.7°, 50.9°,75.3°, 91.3°, and96.4° which, according to Bragg's law, correspond to Ni alloyin a face-centered cubic (FCC) structure at (111), (200), (220),(311), and (222) Miller's indices, respectively [21].

According to the Hanalwalt method [22] phase and crystalstructure are defined by the peaks intensity of powder pattern.In particular, the sequence of the three strongest lines is

Fig. 6 Thickness of the affected layer measured on machined samplesat varying of cutting speeds and feed rates

a b(111)

(200)

(220) (311) (222)

(111)

(200)

(220) (311) (222)

Fig. 7 X-ray phase analysis of machined Inconel 718 samples at varying




responsible of the different phase and crystal structure. Testsfrom ID 1 to ID 6 do not highlight any phase modification,since their XRD patterns show that the three strongest inten-sities are ordered as the virgin sample [(111), (200), (311) ].Then, by applying the Hanalwalt search technique the virginmaterial and the machined samples from ID 1 to ID 6 arecharacterized by a gamma prime, γ ′, structure (PDF 18 – 872).Gamma prime, the first of the two phases to precipitate duringheat treatment, is a coherent, ordered Ni3( A1,Ti) face-centeredcubic structure with nickel present on the cube faces andniobium on the corners of the unit cell.

In contrast, tests carried out at the higher cutting speed (ID7, 8, and 9) show a different sequence of the three strongest intensities [(111), (311), (200)], indicating that the structureon the machined samples is affected by the cutting process.This evidence is clearer when the highest cutting speed and

feed rate are considered (test ID 9): the presence of precip-itating phase (004) γ ″ at 48.5° 2θ can be observed (Fig. 8).

Precipitating phase γ ″, together with γ ′, are responsiblefor the heat resistance properties of the matrix gamma (γ )

phase [23]. Gamma double prime, which nucleates andcoarsens on the γ ′ particles, is a coherent but misfitting andordered metastable body-centered tetragonal Ni3 Nb structurewith the nickel atoms sitting on the faces and niobium,titanium, and aluminum on the corners in the body center sites. Both γ ′ and γ ″ enhance the mechanical properties of the Inconel 718 alloy by anti-phase boundary strengtheningand coherency strains [24 – 26] although the metastability of the primary strengthening (γ ″, gamma double prime) phaseis typically unacceptable for applications above about 650 °C. As a result, other more costly and difficult to processalloys, like Waspalloy, are used in such applications [27].

(200)

(220)(311)

(222)

(004) ‘‘

(004) ‘‘

(111) (200)

γ

γ

Fig. 8 Comparison of XRD patterns between the as receivedsample and the machined sampleat 70 m/min and 0.1 mm/rev(ID 9)

Fig. 9 XRD peak and width of Ni alloy in a FCC structure at (111) of both machined and as received samples under a different cutting speeds for thefeed rate of 0.1 mm/rev and for b different feed rates for the cutting speed of 70 m/min




Finally, another cardinal aspect to be highlighted from theXRD analysis is related to the influence of the cutting pa-rameters on the peak relative intensity and its width.According to Herbert et al. [28], different intensity and widthrepresent different grain size. As it is clearly seen in Fig. 9,grain refinements are observed when higher cutting speedsand feed rates are utilized. Moreover, for the cutting param-

eters employed in this research, the influence of the feed rateseems to be more predominant on grain refinements than thecutting speed. In fact, observing Fig. 9b, the peaks fitting of the machined samples are almost similar while some differ-ences can be seen when the cutting speed is varied (Fig. 9a),highlighting that moderate cutting speed does not allow toreach a high grain refinement.

4 Conclusions

In this paper, an experimental study is proposed for investigat-ing the dry machining of Inconel 718 alloy in terms of surfaceintegrity indicators (surface roughness, microhardness, affect-ed layer, grain size and phase changes); consequently, thefollowing conclusion can be drawn:

& Surface roughness in machining Inconel 718 alloy arecomparable with those obtained by finishing processes(e.g., grinding process), when coated tool are used, andcutting speeds higher than 70 m/min and low feed ratesare chosen.

& Higher cutting speed and feed rate allow the material toreach a higher surface hardness and a deeper hardness

variation.& All the examined samples presented a refinement of

size and, for certain conditions, the grain size on themachines surface cannot be revealed by optical micro-scope since affected featureless structures appear. Theappearance of this layer formed under machining un-derline that significant grain refinement occurred dueto dynamic recrystallization.

& XRD observations highlight that there is a phase changeon the machined surface for tests carried out at 70 m/min.Also, XRD results show that the peak relative intensityand its width are influenced by the cutting process pa-

rameters. In particular, high grain refinements are ob-served when higher cutting speeds and feed rates areutilized. Moreover, the influence of the feed rate seemsto be more predominant on grain refinements than thecutting speed.

& XRD pattern for test ID 9 (70 m/min and 0.1 mm/rev)shows the presence of precipitating phase γ ″ which,together with γ ′, are responsible for the heat resistance

properties of the matrix gamma (γ ) enhancing the me-chanical properties of the Inconel 718 alloy.

Acknowledgments The author gratefully thanks Mr. J. Backus fromKentucky Geological Survey for his help with the XRD measurements.The author also acknowledges the undergraduate student Diego Maidafor his contribution with the analysis of the microscope images.

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