Ultrasonic Measurements of

Titanium Elasticity and

Attenuation in Welds and Plate

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Manufacturing Engineering

Ultrasonic Standards Group

Mechanical Production Metrology Division

Washington, DC 20234

May 1982

Final Report

Prepared for: Office of Naval Research • QC ials Division: Code 431

20Q ngton, DC 22217

, U56

62-2500

1902

NBSIR 82-2500

ULTRASONIC MEASUREMENTS OF

TITANIUM ELASTICITY AND

ATTENUATION IN WELDS AND PLATE

G. V. Blessing

NATIONAL BUMAD

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LIBSABT

JUL 7 1982

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Manufacturing Engineering

Ultrasonic Standards Group

Mechanical Production Metrology Division

Washington, DC 20234

May 1982

Final Report

Prepared for: Office of Naval Research

Materials Division: code 431

Washington, DC 22217

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director
/
TABLE OF CONTENTS

Abstract v

1. Introduction . . 1

2 . Experimental . . . . 1

2 .1 Samples . . 1

2.2 Ultrasonic Technique 3

3. Results 4

3.1 Uncontaminated Weld 4

3.2 Reference Materials 5 3.3 Contaminated Welds 5 3.4 Amplitude Scans 7

4. Conclusions 9

5. Recommendations 9

6 . Acknowledgments 9

7. References 10

iii
LIST OF FIGURES

Figure 1. Two magnifications of a cross-sectional view of a representative

Ti 6211 weld-plate sample, etched to reveal both the epitaxial

grain growth across the weld bead boundary and the plate texture.

Figure 2. Ultrasonic velocity measurement system utilizing the pulse-echo

overlap technique by means of an oscilloscope delta time feature.

Figure 3. (a) A double echo train of shear waves caused by texturing in a Ti 6211 plate sample. (b) An expanded view of the first echo

pair (inverted)

Figure 4. A series of C-scans at three discrete gain levels for the weldplate specimen of Figure 1, with an outline of the weld region

superimposed on the third trace.

LIST OF TABLES

Table 1. Oxygen Content in Ti 6211 Alloy for Raw Material and Four Weldment

Series. Numerical Values are ppm by Weight. [3].

Table 2. Ultrasonic Velocity Data (in m/s) as a Function of Propagation

Direction k and Polarization £ for the Uncontaminated Weld.

Table 3. Ultrasonic Velocity Data (in m/s) for Wave Propagation Perpendicular

to the Plate.

Table 4. Ultrasonic Longitudinal Velocities (in m/s) in Contaminated Welds

for k Parallel to 3.

Table 5. Ultrasonic Shear Velocities (in m/s) in Contaminated Welds for k

Parallel to 3 and for Polarization Directions 1 and 2.

IV
Abstract

Ultrasonic shear and longitudinal waves are used to evaluate the elasticity and attenuation of titanium weld and plate alloy. Wave speeds are used

to measure the materials’ elasticity and anisotropy, and the wave amplitude is

used to measure relative levels of scattering in the weld and plate regions.

Results obtained on a representative weld are compared with results obtained

on oxygen contaminated specimens

Key words: nondestructive evaluation; titanium plate; titanium welds; ultrasonic

C-scan; ultrasonic velocity; weld porosity.

v

1. INTRODUCTION

Titanium (Ti) alloy weldments are subject, during fabrication, to interstitial gas contamination which adversely affects their mechanical properties

and in turn degrades structural performance. While the contamination levels

which cause a problem are not well established, a need definitely exists to

detect small quantities of interstitial gas nondestructively . This report

presents the initial results obtained in an attempt to relate the elasticity

of Ti 621

1 alloy weldments measured ultrasonically to varying degrees of sample contamination. Baseline elasticity data were obtained on "good" or re- presentative weld and plate material, as were the relative scattering amplitudes of the ultrasonic waves. Feasibility for this approach has been demonstrated by other researchers [1,2]

2. EXPERIMENTAL 1 ' "

2 . 1 Samples

Ultrasonic shear and longitudinal velocity measurements were made on seven oxygen (0) contaminated weld specimens, on an uncontaminated weld, and on plate material. The seven double-Vee butt welds were fatigue crack test

specimens, fabricated with varying contamination levels by intermittent

atmospheric exposure of the weldments. By this process, a concentration of

interstitial 0 is expected at the surface of the approximately dozen weld

beads in a given weldment. Relative contamination levels were judged in-situ

by the bead coloration upon exposure. Subsequent metallurgical examination by

both bulk chemical analysis and surface microprobe techniques revealed a limited correlation of 0 levels with sample coloration [3].

Table 1 compares 0 content data for the raw material and for the series

of four attempted levels of contamination. The bulk chemical analyses indicate a (expected) consistently larger 0 content in the welds as compared to

the raw material, with a contamination range approaching 500 ppm. The surface

microprobe measurements, on the other hand, indicate a very limited increase

in 0, and for only two of the weld series.

Table 1. Oxygen Content in Ti 6211 Alloy for Raw Material and Four Weldment

Series. Numerical Values are ppm by Weight [3].

Material

Bulk Chemical

Analysis

Surface Nuclear

Microprobe

Coloration

Judgment

Base Plate 700 803 ______ Weld Wire 730 — —

Weld Series 1 860 783 Straw

2 780 770 Yellow-Blue

3 980 860 Blue

4 1190 915 Gray

Ti 6211 is an alpha stabilized alloy with 6Al-2Nb-lTa-0. 8Mo percent atomic

weights.

1
Fig. 1. Two magnifications of a cross sectional view of a representative

Ti 6211 weld-plate sample, etched to reveal grain growth across

the weld bead boundary and plate texture.

2
The seven specimens for ultrasonic measurements, taken from the four weld

series, were machined flat in the weld region to a thickness of 20 mm for fatigue tests. The specimens were fatigued prior to their ultrasonic evaluation. However, in most cases the cracks were located at the plate-weld boundary, and in no case were they judged to interfere with the velocity measurements. A cross section of the uncontaminated weld, of the same double-Vee

fabrication as the contaminated specimens, is shown in Figure 1. The 2^, and 3^ directions respectively identify the bead, the in-plane normal to the

bead, and the plate normal directions. Extensive grain growth in the 3

* direction may be observed across the bead boundaries. After etching, the weld bead

heat affected zones, caused by partial melting of the previous weld line during successive passes, appear as white cusps due to their contrasting phase

structure. This weld region was subsequently cut from the specimen, and ma- chined parallel and flat for the ultrasonic velocity measurements. A dry density measurement yielded a value of 4.46 gm/cm^, in reasonable agreement with

4.44 gm/ cm^ calculated from the atomic weight percentages of alloy constituents.

2.2 Ultrasonic Technique

Longitudinal and shear wave velocities were obtained from time-of-f light

measurements using the pulse echo overlap technique illustrated in Figure 2.

A resolution of one part in 10^ for delta time exceeded the sample dimensional

uniformity, and allowed for velocity measurements with a nominal precision of + 0.1% for longitudinal waves, and + 0.2% for shear waves. In general,

several measurements were made across a given specimen geometry to evaluate

material homogeneity. The longitudinal measurements were made by immersion

using a 25 mm water path length, and the shear by direct contact using a high

viscosity couplant. The longitudinal waveform was a 3 to 4 cycle tuned pulse

at 10 MHz, and the shear a 1.5 cycle broadband pulse at 5 MHz. Both transducer elements were ceramic, the longitudinal of diameter 6 mm and the shear

13 mm.

Figure 2. Ultrasonic velocity measurement system utilizing the pulse echo

overlap technique by means of an oscilloscope delta time feature.

3
3. RESULTS

A detailed evaluation of longitudinal and shear wave velocities in the

uncontaminated weld will first be presented. This will be followed by the ve- locities obtained on the plate and two reference materials for alloy comparison and system calibration, and then on the series of contaminated welds.

Finally, signal amplitude scans (C-scans) of the uncontaminated weld and plate

sample will be presented.

3.1 Uncontaminated Weld

Table 2 summarizes the velocity results for the uncontaminated or representative weld with wave propagation k in the three principal directions defined in Figure 1. The first data column presents the longitudinal velocity

<V]_> n averaged across the sample, where n is the number of measurements made.

The second column is the percent variation observed for each propagation direction as the 6 mm diameter transducer was discretely scanned across the sam- ple surface. Two values of (extremum) shear velocities <Vg> are given as a function of k in the next column, corresponding to a particular polarization,

e, or particle displacement. The extremums were observed to occur for e parallel to the respective principal directions, indicating a (not unexpected)

material symmetry in those directions. The last column gives the percent

elastic anisotropy determined from the ratio of the shear moduli^ for the two

propagation directions. While Vg was not measured at different positions

across the sample surface, one would expect the percent variation to be com- parable or greater than that observed for V

L

. In fact, the repeatability of

the V

g measurement was itself limited to + 0.5% in some instances, showing a sensitivity to both repeat positioning and polarization orientation.

Table 2. Ultrasonic Velocity Data (in m/s) as a Function of Propagation

Direction k and Polarization e for the Uncontaminated Weld.

k VL

% Variation V s ( e ) % Anisotropy

1 <6150> 6 0.08 <3168>9 (2)

<3200>9 (?)

2.

2 <6158>4 0.15 <3168> 4 (T)

< 317 3> 4 (?)

0.3

3 <6153> 6 0.05 <3183> 3

(1.)

<317 4>g (?)

0. 5

The large velocity variations for propagation in a given direction, re- flecting material inhomogeneity, prevent us from making a detailed comparison

of the elasticity as a function of direction. Qualitatively we can say that

the longitudinal velocity, which is dominated by the compressional elasticity,

varies little as a function of direction. The shear velocity (and therefore

modulus) varies much more, as a function both of propagation and polarization

direction, with a maximum anisotropy occurring for k parallel to the 1_ direction. Much of the shear data can be explained by assuming a unidirectional

symmetry model with the direction of maximum stiffness lying in the (,1,3,)

plane, near the ^3 direction. This assumption for the weld texture is consis4
tent with metallurgical examinations which reveal grain growth in the 3 direction, and with an expected component of dendritic growth along the bead (1)

direction.

3.2 Reference Materials

It is of interest to compare the weld elasticity with the raw plate ma- terial and with another titanium alloy. The data for Ti 6211 and a more com- mon alloy Ti 6A1 4V plate are given in Table 3 for wave propagation perpendicular to the plane of the plate. In addition, velocity values obtained in a single crystal of X-cut quartz are presented as reference points for system

accuracy and subsequent measurement precision. The longitudinal data show an approximate one percent increase in compressional modulus for the plates com- pared to the weld. In Ti 6211, both the stiffer shear component and the anisotropy are greater in the plate than in the weld. One plate specimen

(attached to weld 16) revealed an anisotropy of 6.5% which illustrates itself

by the double-echo pattern of Figure 3. The separation of the incident shear

energy ( k perpendicular to the rolled plate) into two distinct modes of wave

propagation is easily observed, revealing substantial plate texturing.

Finally, wave velocities measured parallel to the plane of Ti 6211 plate

yielded values comparable to those in Table 3.

Table 3. Ultrasonic Velocity Data (in m/s) for Wave Propagation

Perpendicular to the Plate.

Material VL

% Variation V S % Anisotropy

Ti 6211 Plate <6196>6 0.13 <3179>2 (1)

<3220> 2 (2)

2.6

Ti 6A14V Plate <6191>2 — 3190 (1)

3194 (?)

0.2

Quartz X-cut <5752>4 — 3299 (slow mode)

3.3 Contaminated Welds

Table 4 lists the longitudinal velocities obtained on the seven contaminated welds for wave propagation in the 3_ direction. The specimens have two

digit descriptors, the first corresponding to the weld series and the second

to the particular sample cut from the parent series. While a measurement precision of 0.06% is able to differentiate a relatively low velocity value for

specimen 22, the large variations observed within each sample preclude ascribing much significance to that difference. The results were similar for the

shear velocities (0.1% measurement precision) given in Table 5: no correlation of wave speed with weld series could be observed.

^Shear Modulus = Density x Vg2

5
Amplitude

—

Amplitude

( a, Time — (5 ^s/DIV)

<b, Time —(0.2 ^s/DIV)

Fig. 3. (a) A double echo train of shear waves caused by texturing in a Ti 6211 plate sample. (b) An expanded view of the first echo

pair (inverted).

6
Table 4 Ultrasonic Longitudinal Velocities (in m/s) in Contaminated Welds

for k Parallel to 3.

Specimen <VL>6 % Variation

16 6180 0.5

22 6167 0.3

24 6178 0.1

35 6181 0.2

36 6180 0.2

45 6180 0.1

46 6180 0.2

Table 5 Ultrasonic Shear Velocities (in m/s) in Contaminated Welds for

k Parallel to 3 and for Polarization Directions 1 and 2.

Specimen# VA> VA> % Anisotropy

16 3211 3193 1.5

22 3216 3188 1.0

24 3214 3187 1.1

35 3216 3192 1.5

36 3216 3197 0.9

45 3215 3198 0.9

46 3215 3190 1.1

3 . 4 Amplitude Scans

In addition to the velocity measurements, a series of pulse-echo backwall

reflection scans (C-scans) were made on the reference weld-plate specimen,

with wave propagation in the ^ direction. A 10 MHz transducer focused at 300

mm in water was used for enhanced lateral resolution. Two significant features in the three representative C-scans of Figure 4 deserve attention. The

first is a greater attenuation of ultrasound in the plate than in the weld,

which contrasts sharply with the situation commonly observed in austenitic

steels. For example, while the plate region has become nearly opaque to

ultrasound at the -34 dB gain setting, the weld region remains identifiable.

7
-34dB

Fig. 4. A series of C-scans at three discrete gain levels for the weld-plate

specimen of Fig. 1, with an outline of the weld region superimposed

on the third trace.
The second feature is the large range in signal amplitude for both the plate

and weld regions. A total range of about 30 dB from initial ultrasonic penetration to complete transmission was observed at this frequency and sample

thickness.

Relative amplitude measurements were also made for wave propagation at

discrete positions in the 3^ direction. Again the plate was observed to have a much larger attenuation than the weld, by about a factor of two.

4. CONCLUSIONS

Ultrasonic shear and longitudinal wave velocity measurements on a representative Ti 6211 weldment revealed an anisotropy due to preferential grain

growth, implying an elastic axis of symmetry to lie between the bead direction

and a perpendicular to the plane of the welded plates. A comparison with the

plate elasticity showed slightly less stiffness (approximately one percent)

and a smaller anisotropy. The elastic anisotropy due to texturing in the

rolled plate was very noticeable, in one case exceeding six percent. Amplitude

measurements by both direct contact and immersion C-scan revealed an appreciably greater attenuation in the plate than in the weld for the 0.5mm ultrasonic wavelengths employed.

Perhaps the greatest challenge to the ultrasonic inspection of Ti 6211

alloy is the material variability or inhomogeneity that exists on the scale of

ultrasonic wavelengths normally employed. For example, sound speed differences that might be readily detected with available measurement precision, and

related to sample contamination levels, were in fact overwhelmed by variations

within the samples themselves at 5 and 10 MHz. No correlation of weld sound

speed with the limited 0 contamination levels could be observed for the sample

set provided. However, inconsistent contamination level analyses raises a question for sample veracity. It is encouraging to note that the contaminated

weld sound speeds were consistently larger than that for the uncontaminated

weld. This is expected for titanium additionally hardened by an increased

alpha phase level via interstitial 0 stabilization.

5. RECOMMENDATIONS

A primary recommendation for future work is to generate a well characterized sample set possessing a large range of contamination levels, especially

to include the worst case level. In addition to the approach taken here, the

temperature and frequency dependence of ultrasonic wave propagation may be

worth pursuing. For example, elevated temperature studies may reveal a greater sensitivity of the sound speed to gaseous contamination as the matrix elasticity is reduced. Secondly, using a lower frequency (larger wavelength)

would serve to desensitize the elasticity measurements to the effects of large

grains and texturing while remaining sensitive to interstitial gas content.

6.

ACKNOWLEDGEMENTS

The author wishes to acknowledge fruitful discussions with Prof. R. E.

Green, Jr. of Johns Hopkins University, and Dr. N. N. Hsu of the National

Bureau of Standards. This work was supported by the Naval Research

Laboratory, Code 8531, Washington, D. C.

9
7. REFERENCES

1 . Hsu N. ; Conrad, H. Ultrasonic wave velocity measurements on titanium-oxygen alloys, Scripta Metallurgica _5: 905-908; 1971.

2. Buck, 0.; Thompson, D. 0.; Paton, N. E. ; Williams, J. C. The relation

between internal friction spectra and the athermal 0 w transformation in

Ti-V, Ti-V-0, and Ti-V-H alloys. Proc. of the fifth intern, conf. on internal

friction and ultrasonic attenuation in crystalline solids; 1973 August 27-30;

Aachen, Germany. 451-458.

3. Hardy, R. Jr., Toughness of air contaminated Ti 6Al-2Nb-lTa-0. 8Mo

,

David Taylor Naval Ship Res. and Develop. Center (U.S.)

Internal Report SME-79/34; 1979. 16p.

10
N BS«1 14A (rev. 2 -sc) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 82-2500

2. Performing Organ. Report No. 3. Publication Date

May 1982

4. TITLE AND SUBTITLE

Ultrasonic Measurements of Titanium 6211 Weld and Plate

5. AUTHOR(S)

G.V. Blessing

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Office of Naval Research

Materials Division: Code 431

Washington, DC 22217

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant in formation. If document includes a significant

bi bl iography or literature survey, mention it here)

Ultrasonic shear and longitudinal waves are used to evaluate the elasticity and

attenuation of titanium weld and plate alloy. Wave speeds are used to measure

the materials’ elasticity and anisotropy, and the wave amplitude is used to

measure relative levels of scattering in the weld and plate regions. Results

obtained on a representative weld are compared with results obtained on oxygen

contaminated specimens.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s, nondestructive evaluation; titanium plate; titanium welds; ultrasonic C-scan;

ultrasonic velocity; weld porosity.

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