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Process Control for Additive Manufacturing
Lesson 4Laser Ultrasonics
Objective
Explore real time, in-situ, non-contact porosity monitoring during additive
manufacturing
2Laser Ultrasonic Quality Control
Outline
• Laser ultrasonic testing (LUT)• Porosity and Rayleigh wave velocity• Texture and wave velocity• Monitoring system design
3Laser Ultrasonic Quality Control
Advantages Laser Ultrasonic Testing
• Non-contact, local information, real time, in-situ measurement
• Interrogates individual build layers based on frequency (or wavelength)
• Surface acoustic wave velocity, attenuation, and resonance is sensitive to,– Delamination– Porosity– Inclusions– Cracks
4Laser Ultrasonic Quality Control
• Generation laser heats sample locally and leading to thermal expansion
• Local thermal expansion relaxes by creating bulk and surface acoustic (Rayleigh) waves
• Detection laser monitors the surface displacement (or velocity) at remote distance from generation laser point
Laser Ultrasonic Quality Control 5
Laser Ultrasonic Testing (LUT) Approach
Laser Ultrasonic Quality Control 6
Surface Acoustic Wave (SAW) Metrology
Substrate
AM Coating
• Confined to propagate in the near surface region of material• Penetration depth (60µm at 50MHz in 316L) increases with
wavelength• If elastic properties or density varies with depth, SAW velocity
varies with wavelength (or frequency)
Porosity Measurement
• Measure Rayleigh wave (SAW) velocity behind the build head • Correlate velocity to change in material density and elastic
modulus
7
Bulk Stainless Steel 316L
Laser Ultrasonic Quality Control
Overhead view of build plane
Porosity Measurement
8
0.0 0.1 0.2 0.3 0.4 0.5-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
Rel
ativ
e A
mpl
itude
Time (µs)
A B
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
0.0
0.2
0.4
0.6
0.8
1.0
Cor
rela
tion
Coe
ffici
ent
Time (µs)
From 10 measurements:Mean Rayleigh velocity = 2903 m/s Standard deviation = 8.5 m/s
Laser Ultrasonic Quality Control
Representative waveforms obtained on bulk sample:Cross-correlation of acoustic signals:
Modeling
9
Factors that affect the material properties and thus surface wave velocity:
• Porosity (volume fraction and size)• Material texture
Laser Ultrasonic Quality Control
Self Consistent Modeling
10
Assuming the wave velocity only depends on the volume fraction of voids,
Laser Ultrasonic Quality Control
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
164
168
172
176
180
184
188
192
196
Porosity (%)
Elas
tic M
odul
us (G
Pa)
2600
2650
2700
2750
2800
2850
Surface Wave Velocity (m
/s)
Choren J.A., Heinrich S.M., Thorn-Silver M.B., “Young’s modulus and volume porosity relationships for additive manufacturing applications”, J. Mater. Sci. 48, pp. 5103-5112, (2013)
Ep = E0(1-P)2/(1+yP)y = 2-3ν0
Linear slope of velocity-porosity curve = -31 m/s per 1% porosity
Ep = Young modulusof porous sampleE0 = Young modulusof porous sampleP = porosityv0 = Bulk Poisson’s ratio
Archimedes Measurement - Void Volume
• Bulk sample density: 8027 kg/m3
• AM sample density: 7962 kg/m3
• Porosity: 0.81%
11
0.0 0.1 0.2 0.3 0.4 0.5-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
0.08
Rel
ativ
e A
mpl
itude
Time (µs)
A B
‧Predicted wave velocity: 2838 m/s (Based on theoretical model)‧Measured wave velocity: 2834 m/s (Based on measured waveforms)
Representative waveform obtained on AM sample:• Assume SAW measurement is affected by pores only
Laser Ultrasonic Quality Control
Texture & Wave Velocity
• Rayleigh wave velocity varies as wave propagates along different directions relative to the rolling axis of sample
12
Mean WaveVelocity(m/s)
Standard Deviation
(m/s)
Parallel 2903 8.5
Perpendicular 2818 19.3
2700
2750
2800
2850
2900
2950
3000
0 2 4 6 8 10 12
Wav
e Ve
loci
ty (m
/s)
Data Number
Rolled 316L Sheet
Perpendicular
Parallel
A B
0.50.5
A B
0.50.5
Perpendicular
Parallel
Generation Laser(Line wave source)
Detection Laser
Rolling Mark
Wave Propagation
(mm)
(mm)
Wave Propagation
Laser Ultrasonic Quality Control
316L AM & Wave Velocity Accuracy
Laser Ultrasonic Quality Control 13
0 1 2 3 4 52500
2600
2700
2800
2900
3000
3100
3200
Porosity (%)
Rayl
eigh
wav
e ve
loci
ty (m
/s)
Source: CW Laser
Bulk
C.s.A.1~3
#6
#5
#4
C.s.C.2C.s.C.1
#1
#2
Theory
Accuracy of velocity measurement - Reference sample (rolled 316L)
~ 10m/s (or 0.3%) - LENS samples: 100-200m/s (~ 7%)
Minimum detectable porosity = 0.3% (based on velocity-porositysensitivity curve and 10m/s velocityaccuracy
Bulk porosity estimate based on Archimedes method
Acoustic technique measures near surface porosity ( < 60µmpenetration depth at 50MHz)
Laser Ultrasonic Quality Control 14
• Velocity Accuracy Limited By:• Electronic noise in the optical detectors• Jigger in the electronic trigger between data
acquisitions
316L AM & Wave Velocity Accuracy
Narrowband LUT Measurements in 3% Control Sample vs. Frequency
Laser Ultrasonic Quality Control 15
Scanning laser source over the hole reduces amplitude of acoustic signal
Scanning laser source over the hole reduces amplitude of acoustic signalto noise floor
Attenuation of high frequency surface acoustic waves limits the wave interaction distance with voids
Compatible 3% Control & AM 2.8% Porosity
Laser Ultrasonic Quality Control 16
Attenuation of high frequency surface acoustic waves leads to wave distortion observed in LENS samples
Frequency Effect – 316L Control With 3% Porosity
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Lower frequencies more sensitive to deeper structural discontinuities like pores and delamination.
SS316 Bulk & 3% Porosity Control
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10 MHzOver holes
10 MHzBulk
Mean SAW velocity (m/s) 2754 2890
Standard deviation (m/s) 46 42
Baseline Monitoring Interferometer Design
Laser Ultrasonic Quality Control 19
Surface roughness and curvature degrades ultrasound detection sensitivity In the current experimental configuration
Adaptive Photorefractive Holographic Interferometry:
• Suitable for LUT on polished surfaces • Vibration compensation needed
20
Adaptive Photorefractive Holographic Interferometry:
• Suitable for LUT on rough surfaces • Self vibration compensation• Electronic stabilization not needed
Laser Ultrasonic Quality Control
Photorefractive Interferometer
Challenges
• Quantify feature sensing capability on well characterized static control samples and AM samples in lab
• Integrate LUT apparatus into a LENS system to validate lab capabilities for QC
• Develop comprehensive stainless steel technical data package for standard development (or company specification) by round robin testing
• Extend standard to other engineering alloys (titanium, nickel, aluminum, tungsten, etc.)
21Laser Ultrasonic Quality Control
Resources
22
Northern Illinois University (NIU)Additive Manufacturing LabFederico Sciammarella, Director
National Institute of Standards and Technology (NIST)Engineering Laboratory
Kevin Jurrens, Deputy Division [email protected]
Northwestern UniversityMechanical Engineering Department
McCormick School of EngineeringSeyi Balogun
Laser Ultrasonic Quality Control
This work was performed under the following financial assistance award 70NANB13H194 from the U.S. Department of Commerce, National Institute of Standards and Technology. The views expressed do not necessarily reflect the official policies of NIST; nor does mention by
trade names, commercial practices, or organizations imply endorsement of the U.S. Government.