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Copyright © TWI Ltd 2015

Additive Manufacturing Seminar

24th September 2015

NDT Activities in Additive Manufacturing

Dr Dimos Liaptsis Principal Project Leader

TWI Technology Centre (Wales)


1. Pulsed laser generates surface waves.

2. Detection beam reflects off surface to record surface displacement information.

3. Interferometer compares the reflected beam with a reference beam.

4. AC signal output gives surface displacement as a function of time (A-scan).

Generation

Beam

Detection

Beam Surface waves

LUT introduction

Laser Ultrasonic Testing:


MN Ahsan, AJ Pinkerton & R Bradley (2011)

Background

• Known defects are very difficult to produce in LMD structures

• Was not possible to produce such defects for in-process

Intra-layer porosity:


Test Samples

Inconel 718 block with 500μm drilled hole Geometry of samples and FE models

• Machined Inconel 718 samples manufactured. • Hole diameters of 77, 100, 150, 250, and 500 μm. • Hole depths of 100 and 200 μm below top surface, centre of wall

section.


Testing Technique

Detection Generation

A ‘full-scan’ method was employed in order to detect sub-surface voids using Rayleigh waves:

• The generation laser is focused at x = 2 mm. • Surface displacement is measured every 1 mm, with the detection

laser focused at points from x = 3 mm to x = 18 mm. • The generation laser is moved along 1 mm and the detection process

repeated. • This continues until the generation laser is at x = 17 mm. • 136 A-scans acquired per sample.

Changes in surface displacement and Rayleigh wave frequency were investigated to look for indications of: • Void presence • Void size • Void depth

x = 0 mm x = 20 mm


Equipment & Set-Up

Generation laser: 1064 nm Nd:YAG 8 ns pulse 6.25 MW power Detection laser: 532nm Nd:YAG Constant wave 500 mW power

Focal lengths from 50 to 150 mm

Spot focus (700) μm or line focus (200 μm)

IOS wave-mixing interferometer with surface displacement sensitivity 4x10-7 nm (W/Hz)1/2

Oscilloscope measures intensity or ‘quality’ of reflected beam.

PC with LaserScan software records A-scans.

(a) generation, (b) detection, (c) samples


No flaw present. Surface wave is unaffected.

Sub-surface hole. 500 µm diameter 100 µm depth

Offline LUT data

Flaw Detection:


In-situ laser UT


Introduction

In-Process Inspection of LMD-p Manufacturing.


In-situ field trials

Proposed Set-up for In-Process NDT:

Deposition Laser UT


Configured for operation in the LMD system: • Both laser beams exit

perpendicular to the test surface.

• Distance between laser beams is adjustable.

• Includes eye-safe laser

beams for alignment.

• Remote controlled laser beam focusing.

Optical Design of LUT Head

Detection laser

Generation laser


Detail of LUT head


Laser Beam Focusing

Remote Controlled Laser Beam Focusing:

Inte

llig

ent

Optical Syste

ms

• Detection laser beam must be focused correctly to receive surface displacement signal.

• Intensity of reflected light is

monitored via DC voltage.

• Focus of generation laser beam maximises amplitude of ultrasonic waves.


Laser Safety Requirements


Preparing the LMD Platform


Mounting the NDT Optical Head

:


Placement of NDT Equipment

1. Laser for generation of ultrasound

2. Water-cooling and control

3. Laser for detection of ultrasound

4. Interferometer

5. Motors and controllers




4. Interferometer





4. Interferometer





4. Interferometer





4. Interferometer



In-situ NDT on Circular Wall

17.5 mm/s

140 mm


Results & Analysis

Limited Dynamic Performance:

4 mm/s

2 mm/s


Results & Analysis

8 mm/s

6 mm/s



Results & Analysis

10 mm/s

Stepped



Results & Analysis

Wave-Mixing Interferometry:

Dynamic performance limited by: • Type of crystal (depends on wavelength of laser beam)

• Power available for reference laser beam


Results & Analysis

Improving Dynamic Performance:

• AC/DC is measure of signal stability

• Increased power gives better stability at higher speeds

• Different crystal type also contributes


Results & Analysis

Environmental Noise:


Summary

• LUT head designed and installed in LMD system

• Safety requirements taken into account

• In-process inspection tested on a single geometry

• Capabilities and limitations of detection equipment verified

• Knowledge gained for future research


CT of AM parts


CT of an SLM Acoustic Suppressor

• Development of a novel Selective Laser Melting (SLM) noise suppressor

• Designed using analysis computation fluid dynamics

• CT used to verify build quality


CT images of an Acoustic Suppressor

Very good construction


CT of TiAl Alloy LMD Samples

• Development of a strengthened materials for the aerospace applications using Laser Metal Deposition

• Part of European Commission FP7 funded project, ‘Oxigen’

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