Lesson 4 Laser Ultrasonics · Advantages Laser Ultrasonic Testing • Non-contact, local information, real time, in-situ measurement • Interrogates individual build layers based

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


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


• 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


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


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


Self Consistent Modeling

10

Assuming the wave velocity only depends on the volume fraction of voids,


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%

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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


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


316L AM & Wave Velocity Accuracy


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)


• 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


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


Attenuation of high frequency surface acoustic waves leads to wave distortion observed in LENS samples

Frequency Effect – 316L Control With 3% Porosity


Lower frequencies more sensitive to deeper structural discontinuities like pores and delamination.

SS316 Bulk & 3% Porosity Control


10 MHzOver holes

10 MHzBulk

Mean SAW velocity (m/s) 2754 2890

Standard deviation (m/s) 46 42

Baseline Monitoring Interferometer Design


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

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Adaptive Photorefractive Holographic Interferometry:

• Suitable for LUT on rough surfaces • Self vibration compensation• Electronic stabilization not needed


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.)


Resources

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Northern Illinois University (NIU)Additive Manufacturing LabFederico Sciammarella, Director

[email protected]

National Institute of Standards and Technology (NIST)Engineering Laboratory

Kevin Jurrens, Deputy Division [email protected]

Northwestern UniversityMechanical Engineering Department

McCormick School of EngineeringSeyi Balogun

[email protected]


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.

mailto:[email protected]



Documents

Lesson 4 Laser Ultrasonics · Advantages Laser Ultrasonic Testing • Non-contact, local information, real time, in-situ measurement • Interrogates individual build layers based