- Email: [email protected]
Acoustic characterization of metals with columnar grains
Acoustic characterization columnar grains C.K. Jen”,
H. Soda?, Y.S.
of metals with
Liul, C. Neron”,
A. Ohnotgand
A. McLean+
* IMI, National Research Council, Boucherville, Quebec, Canada J4B 6Y4 ‘Department of Metallurgical & Materials Science, University of Toronto, Toronto, Ontario, Canada M5S lA4 : Department of Electrical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7 BDepartment of Metallurgical Engineering, Chiba institute of Technology, Narashino, Chiba-ken, Japan Received
7 July
1994; revised
18 January
1995
Acoustic characterization measurements of Sn, Pb, Al and Cu metallic alloys with highly oriented columnargrainsare presented. Using a 225 MHz line-focus-beam scanning acoustic microscope we have observed the ultrasonic anisotropy of a transversely isotropic symmetry for small grain width (-acoustic wavelength) samples. The long needle grain structures induce unique acoustic anisotropic characteristics, such as focussing and diverging, which have been illustrated in the guided longitudinal and shear acoustic wave pulse+cho measurements performed for rod geometry. The understanding of elastic anisotropy is of importance for non-destructive testing of such materials. Keywords: scopy
acoustic
characterization;
Polycrystalline metals are composed of numerous grains and each grain has its regular and single crystalline structure. These grains are, in general, randomly oriented and elastically anisotropic. However, in certain structures---such as nuclear power plants-highly oriented long needle-like grains appear in centrifugally cast stainless steels and austenitic stainless steel weldslm4. Such columnar grains introduce a different elastic anisotropy from those randomly oriented grains. The reported theoretical and experimental investigations1-4 have emphasized the effects of anisotropy on the ultrasonic testing of the austenitic stainless steel welds. The lack of information on anisotropy could, for instance, lead to false indications of defect locations’. In order to improve the ultrasonic non-destructive evaluation of metals having such elongated grain structures a fundamental understanding of the ultrasonic velocity anisotropy, which is the objective of this paper, is essential. Recently, Ahmed and Thompson’ have also theoretically studied the ultrasonic scattering loss mechanisms for the columnar grain configurations. In this work we use a newly invented Ohno continuous Casting (OCC) process6-’ to produce metallic tin (Sn), lead (Pb), aluminium (Al) and copper (Cu) alloy samples of columnar grain structures. Many copper and aluminium alloys with the columnar grains produced by this technology have been used commercially by the audio and video, and the automobile industries respectively’.
0041-624X/95/$09.50 c,: 1995 - Elsevier Science B.V. All rights reserved SSDI 0041-624X(95)00029-1
metallic
alloys;
grains;
acoustic
micro-
Other OCC-process produced metallic alloys are also utilized as heat exchanger tubes and pipings etc”. Thus, knowledge of the elastic anisotropy of metals with columnar grain structures is also of importance in order to perform properly the ultrasonic non-destructive evaluation on the products manufactured by the OCC process. A line-focus-beam scanning acoustic microscope’ ’ is used here to measure the ultrasonic anisotropy. Guided longitudinal and shear-type acoustic waves are applied to study the ultrasonic focussing or diverging effect of the waves propagating in these alloys, which have columnar grains.
Sample preparation and characterization using optical micrographs The columnar grain metal samples used for our experimental measurements were fabricated in the cylindrical rod shape by the above mentioned OCC process. In addition to the unique long needle grain structures, this process is capable of producing other novel cast features such as plate, net and near net-shape7, clad rod geometries’, extremely clean cast surfaces free from witness marks and good internal structures free from segregation and shrinkage cavities. The key characteristic that differentiates the OCC process from conventional continuous casting techniques is the delivery of molten
Ultrasonics
1995 Vol 33 No 3
181
Acoustic
characterization
of metals
with
columnar
grains:
C.K. Jen et al.
metal into a heated mould, the temperature of which is held just above the solidification temperature of the metal to be casth.‘. In conventional continuous casting systems. the mould is water cooled; thus, crystals nucleate on the mould surface and grow towards the inner part of the casting and the direction of crystal growth is perpendicular to the casting direction. In the OCC process, there is no nucleation on the mould surface due to the external heat applied to the mould, and the heat is only extracted through the strand being cast: as a result, the crystal growth is parallel to the casting direction. This gives rise to the change in solidification morphology, which significantly influences metallurgical and mechanical properties of the solidified materials. Due to unidirectional solidified grain structures, such metals have good workability and resistivity to corrosion’“. Fi(prtxs IN, lb. I’tr, 30 and 3h show the optical micrographs of Al-45%Cu, Sn-7.5%Pb and 99.99%Cu samples cast by the OCC technique in the Z-X and X’- Y’ planes, where Z is the casting direction. The X, Y and X’, Y’ axes are orthogonal and in the same plane, which is perpendicular to the Z axis. F~JUWS4 and 5 show the optical images of a 99.9%Sn and 99.9%Cu produced by the conventional fabrication technique respectively in the X’-Y’ plane. Note that it is very difficult to produce small Al-45%Cu alloy rods by the conventional fabrication
Figure 2 Optical micrographs of an OCC S-7.5% Pb sample: (a) Z-X plane; (b) X’-Y’ plane. Z is the casting direction
technique”. As expected, the random oriented grains can be seen from Figurrs 4 and 5 and no additional micrograph is necessary to illustrate such a structure. As for OCC samples with columnar grains we use two planes (X-Z and X’-Y’) to show the grain features. In the Z-X plane, as shown in Figures lm-34 the preferred and highly oriented grain structure can be observed. Cross-sections of the cast metal rods in the X’-Y’ plane are given in FigJ1iw.s lb-3h. For the Al-45%Cu and Sn-7S%Pb the dendrite microstructures are also visible. Acoustic acoustic
Figure 1 Optical micrographs of an OCC AI-45% Cu sample: (a) Z-X plane; (b) X-Y plane. Z is the casting direction
182
Ultrasonics
1995
Vol 33 No 3
characterization waves
using
leaky
In order to investigate the acoustic anisotropy, a 225 MHz line-focus-beam scanning acoustic microscope (LFBSAM) is used to evaluate the leaky surface acoustic waves (LSAW), I&w, and/or leaky surface skimming compressional wave (LSSCW) velocity, I/Lsscw, variations’ ‘.i2. At this frequency, the compressional wave wavelength in water is 6.6pm. The spatial resolution of the LFBSAM depends on the sample, however and is around lOOOpm, by a few hundred micrometres, due to the defocussing mechanism of the qz) measurement’i. Since LSAW and LSSCW have predominantly shear and longitudinal wave components respectively, I&,, and V,,,,, profiles can be regarded as those of shear and
Acoustic
characterization
of metals
with
columnar
grains:
C.K. Jen
et al.
longitudinal wave velocity l3 . Because the microscope lens has been designed in such a way that it is not able to measure a leaky wave velocity of less than 2100m s-r, to slow velocity materials, only V,,,,, measurements are shown. Figures 6 and 7 show the VtSScWvariations in the X’-Y’ plane for 99.9% tin with an average grain size of around 130 pm, and 99.9% copper with an average grain
Figure 5 Optical micrographs of a 99.9% conventional fabrication method
Cu sample
made
by a
3.5
3.1 -100
Figure 3 Optical micrographs of an OCC Cu sample: plane; (b) X-Y plane. Z is the casting direction
Figure 4 Optrcal micrographs of a 99.9% conventional fabrication method
Sn sample
(a)
made
100
-50
Figure 6 Sn sample
Measured V,,,,, shown in Figure
variations
in the x’-Y’
plane for the
Figure 7 Cu sample
Measured VLsscw variations shown in Figure 5
in the X’-Y’
plane for the
4
Z-X
by a
size of around 10 pm, made by the conventional fabrication technique. We can see the near isotropic velocity profiles that have also been observed for the Z-X plane. For the OCC samples Figures 8a, 8b and 9 show the VLSAW and variations of the Al-45%Cu and the vLSScW VLSSCW of the Sn-7S%Pb respectively. The upper curve is for the X’-Y’ and the lower is for the Z-X plane. The casting direction is along the Z-axis. We can see that, in
Ultrasonics
1995
Vol 33 No 3
183
Acoustic
characterization
of metals with columnar
grains: C.K. Jen et
the with a grain of around 15 pm behave like a transversely material and show the anisotropic features in the Z-X plane for both V,,,, and I&,,. For the Sn-7.5%Pb sample, width
-100
2.8 r-1
0’
1:: LL-__.d -100
-50
(dDe, W,,;
@’ 3.0,,
2.9
0
-50
, ,I,
3.sc 100
50
100
.‘..,,
4
50
100
(degrees)
.,,,,,,,.,,*,
3.4
O
,,I,,
, ,,,,
I,
1
3.3 I ‘,
il
3.2 3.1
t* -100
-50 @
(digrees)
Figure 9 Measured VLsscw variations for the OCC S-7.5% Pb sample shown in Figure 2; upper curve: in the X’-Y’ plane; lower curve in the Z-X plane. Z refers to the casting direction
-100 a
-100
100
-50 @
(dOegrees;O
0’
(ckgrees;
100
-50 O
6m2m 6.0 I,
w
>
!f
5.8 5.6
b
-100
-50 @
(dkpes)
50
100
Figure 8 Measured (a) VLsnw and (b) VLsscw variations for the OCC Al-4596 Cu sample shown in Figure I; upper curve: in theX’-Y’ plane; lower curve: in theZ-Xplane. Z refers to the casting direction
184
Ultrasonics 1995 Vol 33 No 3
which is still small compared with the spatial resolution of the LFBSAM, the V,,,, profile is somewhat similar to a transversely isotropic material but with a minor texture. For the OCC copper sample, since the grain width (-hundreds of pm) is comparable with the spatial resolution of the LFBSAM, we start to see clearly the both in the X’-Y’ and anisotropic V,,,,. characteristics Z-X planes shown respectively in the upper and lower curves of Figure IO. Thus, the grain width is an important parameter for the consideration of ultrasonic nondestructive evaluation of metals with columnar grains. Note that if the grain width is much larger than the spatial resolution of the LFBSAM, then the elastic anisotropy of each single grain can be probed. In addition, Al and Cu are cubic, and Sn is tetragonal, and there is a significant difference in the V,,,,, profiles shown in the lower curves of Figures 86 and 9. In Figure 8b the Ksscw is the lowest along the casting direction, which implies that a diverging feature exists for the planar wave propagating in this direction, but it is the highest in Figure 9, which implies that a focussing feature exists for the planar wave propagating in this direction. In the next section, more illustrations on these two profiles will be given. Guided rods
acoustic
wave
characteristics
in
V,,,,, profile in the lower trace of Figure to a slowness (l/V) curve14, we see that, due to the concave feature near the casting Z direction, the guided longitudinal acoustic waves propagating in If the velocity
9 is converted
Acoustic characterization of metals with columnar grains: C.K. Jen et al.
the OCC Sn-Pb alloy rod will encounter a focussing effecti provided the acoustic wavelength is small compared with the rod diameter. Figures II and 12 show the longitudinal acoustic wave pulseecho measurement for a conventionally fabricated Sn (12.7 mm diameter) and
5.0
l-nT7-l Y'
X'
4.0 4.6 4.4
5.0 r,
, , , , , , , , , , , , , , , , , , ,
Figure 12 Reflected longitudinal wave echoes through a 47 mm long 12.7 mm diameter OCC Sn-7.5% Pb rod. Upper curve: at 5 MHz; lower curve: at 10 MHz
4.2t~“~“““““““~l -100
-50
50
100
@ (&W) Figure 10 Measured VLsscw variations for the OCC Cu sample shown in Figure 3; upper curve: in the X’-Y’ plane; lower curve: in the Z-X plane. Z refers to the casting direction
Figure 13 Reflected 5 MHz shear wave echoes through the (a) Sn rod in Figure 77; (b) Sn-Pb alloy rod in Figure 72
Figure 11 Reflected longitudinal wave echoes through a 47 mm long 12.7mm diameter 99.9% Sn rod made by the conventional fabrication method. Uppercurve: at 5 MHz; lowercurve: at 10 MHz
an OCC cast Sn-PB alloy rod (11 mm diameter) each 47mm long. In these two figures, the upper curves were performed at 5 MHz and the lower curves at 1OMHz. The ultrasonic transducers were broadband transducers of 6.35 mm diameter. The measurement conditions were kept identical for both rods except at lOMHz, and the signals obtained for the OCC Sn-Pb alloy were attenuated 20dB more than that for the conventionally-made
Ultrasonics
1995 Vol 33 No 3
185
Acoustic
characterization
of metals
with
columnar
grains:
Sn by an attenuator. At 5 MHz the ultrasonic signals were very similar for both rods, however, the spurious signals l2 for the OCC Sn-PB alloy are smaller than the conventionally made Sn. At 10MHz the signal for an OCC Sn-Pb alloy is 20dB stronger and the signal-tonoise ratio is also much better. This could be due to the fact that, at lOMHz, the waveforms in the rod are closer to planar than those at 5 MHz and they experience better focussing due to the concave shape of the slowness curve. Figures 13a and 136 are the shear wave pulsee echo measurements at 5 MHz for the above conventionally made Sn and OCC Sn-PB alloy rods respectively. Again, the measurement conditions were kept the same except, in this case, the signals for the conventional tin were attenuated 10 dB more than that for the OCC Sn-Pb alloy by an attenuator. This means that, in contrast to the longitudinal wave along the casting Z direction, shear waves do not propagate well in the OCC Sn-PB alloy. In addition to the lower signal strength, a longitudinal wave ultrasonic pulse was observed in Figure 13b and this echo was caused by the shear to longitudinal mode conversion process induced by the columnar grain structures, shown in Figure 2a along the rod.
C.K. Jen
Acknowledgement The financial support of the Natural Sciences and Engineering Research Council of Canada and the Ontario Centre for Materials Research is gratefully appreciated. References I 2
3
4
5
6 1
8
Conclusions Acoustic characterization measurements of Sn, Pb, Al and Cu metallic alloys with highly oriented long needle grains were presented. Using a 225 MHz line-focusbeam scanning acoustic microscope we have observed the ultrasonic anisotropy of a transversely isotropic symmetry for small grain width samples. The columnar grain structures induce unique acoustic propagation characteristics, such as focussing and diverging, which have been illustrated in the guided longitudinal and shear acoustic wave pulseecho measurements performed for rod geometries. Knowledge of their elastic anisotropy is of importance for ultrasonic non-destructive evaluation of these materials-such as centrifugally cast stainless steels, austenitic stainless steel welds and metallic plates, rods and tubes-produced by OCC methods.
186
Ultrasonics
1995
Vol
33 No 3
et al.
9
10 II
12
13
14
Pelseneer, J.P. and Louis, G. Ultrasonic testing of austenitic steel castings and welds Brir / Non-Des@ Test (1974) 16 1077113 Yoneyama, H., Shibata, S. and Kishigami, M. Ultrasonic testing of austenitic stainless steel welds: false indications and the cause of their occurrence NDT Int (1978) II 3-8 Kupperman, D.S., Reimann, K.J. and Fiore, N.F. Role of microstructure in ultrasonic inspectability of austenitic stainless steel welds Mart Ecu/ (1978) 36 70-74 Kupperman, D.S. and Reimann, K.J. Ultrasonic wave propagation and anisotropy in austenitic stainless steel weld metal l/TEE Trcttt.s Sottics and Ulrrason (1980) 27 7-I 5 Ahmed, S. and Thompson, R.B. Attenuation of ultrasonic waves in cubic metals having elongated. oriented grains Non&str Te\/ EraI (1992) 8-9 525-53 I Ohno, A. Continuous casting of single crystal ingots by the O.C.C. Process J Meto/.s (1986)38 14-I 6 Ohno, A. and Soda, H. Development and application of OCC technology Proc F. W&thrrq Itt~rrtta/iotttrl S~ntposiuttt Ott Solid~fic~cttiott Procc~ss, J.E. Lait and I.V. Samarasekera (eds) Pergamon Press, New York (1990) 215-228 Soda, H., Ichinose, A., Motoyasu, G., Ohno, A. and McLean, A. A new fabrication method and its process variables for cored materials C’W Meuh (1992) 5 955102 Shimizu, T., Yamazaki, H., Soda, H. and McLean, A. Development of Al alloy welding rod for hard facing applications Proc, 32 Annual C‘on/crrrrc~~o/’ Me~ulluryi.~t.v, Drrelopnwnt.s and Applicuriotts of C’wtmtk.s uttd NC,\, Mrtrtl ,41/o~,sQuebec. Canada, 29 Aug-2 Sept (1993) 541-549 MES-OCC innovated products, Advanced materials and products catalog. Mitsui engineering & Shipbuilding Co. Ltd (1992) Kushibiki, J. and Chubachi, N. Material characterization by line focus beam acoustic microscope IEEE Trutzs .SONICSUllruson (1985) 32 1X9-212 Jen, C.K., Neron, C., Miri, A., Soda, H., Ohno, A. and McLean, A. Fabrication and characterization of continuously cast clad buffer rods J. Acoust Sot Am (1992) 91 356553570 Jen. C.K.. Wane. Z.. Nicolle. A.. Bussiere. J.F.. Adler. E.L. and Abei K. Acoust% waveguiding rods of graded’velocity profiles Ultrasonics (1992) 30 91-94 B.A. Auld Acoustic Fields and Waves in Solids Vols I & II, Wiley (1973)
H. Soda?, Y.S.
of metals with
Liul, C. Neron”,
A. Ohnotgand
A. McLean+
* IMI, National Research Council, Boucherville, Quebec, Canada J4B 6Y4 ‘Department of Metallurgical & Materials Science, University of Toronto, Toronto, Ontario, Canada M5S lA4 : Department of Electrical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7 BDepartment of Metallurgical Engineering, Chiba institute of Technology, Narashino, Chiba-ken, Japan Received
7 July
1994; revised
18 January
1995
Acoustic characterization measurements of Sn, Pb, Al and Cu metallic alloys with highly oriented columnargrainsare presented. Using a 225 MHz line-focus-beam scanning acoustic microscope we have observed the ultrasonic anisotropy of a transversely isotropic symmetry for small grain width (-acoustic wavelength) samples. The long needle grain structures induce unique acoustic anisotropic characteristics, such as focussing and diverging, which have been illustrated in the guided longitudinal and shear acoustic wave pulse+cho measurements performed for rod geometry. The understanding of elastic anisotropy is of importance for non-destructive testing of such materials. Keywords: scopy
acoustic
characterization;
Polycrystalline metals are composed of numerous grains and each grain has its regular and single crystalline structure. These grains are, in general, randomly oriented and elastically anisotropic. However, in certain structures---such as nuclear power plants-highly oriented long needle-like grains appear in centrifugally cast stainless steels and austenitic stainless steel weldslm4. Such columnar grains introduce a different elastic anisotropy from those randomly oriented grains. The reported theoretical and experimental investigations1-4 have emphasized the effects of anisotropy on the ultrasonic testing of the austenitic stainless steel welds. The lack of information on anisotropy could, for instance, lead to false indications of defect locations’. In order to improve the ultrasonic non-destructive evaluation of metals having such elongated grain structures a fundamental understanding of the ultrasonic velocity anisotropy, which is the objective of this paper, is essential. Recently, Ahmed and Thompson’ have also theoretically studied the ultrasonic scattering loss mechanisms for the columnar grain configurations. In this work we use a newly invented Ohno continuous Casting (OCC) process6-’ to produce metallic tin (Sn), lead (Pb), aluminium (Al) and copper (Cu) alloy samples of columnar grain structures. Many copper and aluminium alloys with the columnar grains produced by this technology have been used commercially by the audio and video, and the automobile industries respectively’.
0041-624X/95/$09.50 c,: 1995 - Elsevier Science B.V. All rights reserved SSDI 0041-624X(95)00029-1
metallic
alloys;
grains;
acoustic
micro-
Other OCC-process produced metallic alloys are also utilized as heat exchanger tubes and pipings etc”. Thus, knowledge of the elastic anisotropy of metals with columnar grain structures is also of importance in order to perform properly the ultrasonic non-destructive evaluation on the products manufactured by the OCC process. A line-focus-beam scanning acoustic microscope’ ’ is used here to measure the ultrasonic anisotropy. Guided longitudinal and shear-type acoustic waves are applied to study the ultrasonic focussing or diverging effect of the waves propagating in these alloys, which have columnar grains.
Sample preparation and characterization using optical micrographs The columnar grain metal samples used for our experimental measurements were fabricated in the cylindrical rod shape by the above mentioned OCC process. In addition to the unique long needle grain structures, this process is capable of producing other novel cast features such as plate, net and near net-shape7, clad rod geometries’, extremely clean cast surfaces free from witness marks and good internal structures free from segregation and shrinkage cavities. The key characteristic that differentiates the OCC process from conventional continuous casting techniques is the delivery of molten
Ultrasonics
1995 Vol 33 No 3
181
Acoustic
characterization
of metals
with
columnar
grains:
C.K. Jen et al.
metal into a heated mould, the temperature of which is held just above the solidification temperature of the metal to be casth.‘. In conventional continuous casting systems. the mould is water cooled; thus, crystals nucleate on the mould surface and grow towards the inner part of the casting and the direction of crystal growth is perpendicular to the casting direction. In the OCC process, there is no nucleation on the mould surface due to the external heat applied to the mould, and the heat is only extracted through the strand being cast: as a result, the crystal growth is parallel to the casting direction. This gives rise to the change in solidification morphology, which significantly influences metallurgical and mechanical properties of the solidified materials. Due to unidirectional solidified grain structures, such metals have good workability and resistivity to corrosion’“. Fi(prtxs IN, lb. I’tr, 30 and 3h show the optical micrographs of Al-45%Cu, Sn-7.5%Pb and 99.99%Cu samples cast by the OCC technique in the Z-X and X’- Y’ planes, where Z is the casting direction. The X, Y and X’, Y’ axes are orthogonal and in the same plane, which is perpendicular to the Z axis. F~JUWS4 and 5 show the optical images of a 99.9%Sn and 99.9%Cu produced by the conventional fabrication technique respectively in the X’-Y’ plane. Note that it is very difficult to produce small Al-45%Cu alloy rods by the conventional fabrication
Figure 2 Optical micrographs of an OCC S-7.5% Pb sample: (a) Z-X plane; (b) X’-Y’ plane. Z is the casting direction
technique”. As expected, the random oriented grains can be seen from Figurrs 4 and 5 and no additional micrograph is necessary to illustrate such a structure. As for OCC samples with columnar grains we use two planes (X-Z and X’-Y’) to show the grain features. In the Z-X plane, as shown in Figures lm-34 the preferred and highly oriented grain structure can be observed. Cross-sections of the cast metal rods in the X’-Y’ plane are given in FigJ1iw.s lb-3h. For the Al-45%Cu and Sn-7S%Pb the dendrite microstructures are also visible. Acoustic acoustic
Figure 1 Optical micrographs of an OCC AI-45% Cu sample: (a) Z-X plane; (b) X-Y plane. Z is the casting direction
182
Ultrasonics
1995
Vol 33 No 3
characterization waves
using
leaky
In order to investigate the acoustic anisotropy, a 225 MHz line-focus-beam scanning acoustic microscope (LFBSAM) is used to evaluate the leaky surface acoustic waves (LSAW), I&w, and/or leaky surface skimming compressional wave (LSSCW) velocity, I/Lsscw, variations’ ‘.i2. At this frequency, the compressional wave wavelength in water is 6.6pm. The spatial resolution of the LFBSAM depends on the sample, however and is around lOOOpm, by a few hundred micrometres, due to the defocussing mechanism of the qz) measurement’i. Since LSAW and LSSCW have predominantly shear and longitudinal wave components respectively, I&,, and V,,,,, profiles can be regarded as those of shear and
Acoustic
characterization
of metals
with
columnar
grains:
C.K. Jen
et al.
longitudinal wave velocity l3 . Because the microscope lens has been designed in such a way that it is not able to measure a leaky wave velocity of less than 2100m s-r, to slow velocity materials, only V,,,,, measurements are shown. Figures 6 and 7 show the VtSScWvariations in the X’-Y’ plane for 99.9% tin with an average grain size of around 130 pm, and 99.9% copper with an average grain
Figure 5 Optical micrographs of a 99.9% conventional fabrication method
Cu sample
made
by a
3.5
3.1 -100
Figure 3 Optical micrographs of an OCC Cu sample: plane; (b) X-Y plane. Z is the casting direction
Figure 4 Optrcal micrographs of a 99.9% conventional fabrication method
Sn sample
(a)
made
100
-50
Figure 6 Sn sample
Measured V,,,,, shown in Figure
variations
in the x’-Y’
plane for the
Figure 7 Cu sample
Measured VLsscw variations shown in Figure 5
in the X’-Y’
plane for the
4
Z-X
by a
size of around 10 pm, made by the conventional fabrication technique. We can see the near isotropic velocity profiles that have also been observed for the Z-X plane. For the OCC samples Figures 8a, 8b and 9 show the VLSAW and variations of the Al-45%Cu and the vLSScW VLSSCW of the Sn-7S%Pb respectively. The upper curve is for the X’-Y’ and the lower is for the Z-X plane. The casting direction is along the Z-axis. We can see that, in
Ultrasonics
1995
Vol 33 No 3
183
Acoustic
characterization
of metals with columnar
grains: C.K. Jen et
the with a grain of around 15 pm behave like a transversely material and show the anisotropic features in the Z-X plane for both V,,,, and I&,,. For the Sn-7.5%Pb sample, width
-100
2.8 r-1
0’
1:: LL-__.d -100
-50
(dDe, W,,;
@’ 3.0,,
2.9
0
-50
, ,I,
3.sc 100
50
100
.‘..,,
4
50
100
(degrees)
.,,,,,,,.,,*,
3.4
O
,,I,,
, ,,,,
I,
1
3.3 I ‘,
il
3.2 3.1
t* -100
-50 @
(digrees)
Figure 9 Measured VLsscw variations for the OCC S-7.5% Pb sample shown in Figure 2; upper curve: in the X’-Y’ plane; lower curve in the Z-X plane. Z refers to the casting direction
-100 a
-100
100
-50 @
(dOegrees;O
0’
(ckgrees;
100
-50 O
6m2m 6.0 I,
w
>
!f
5.8 5.6
b
-100
-50 @
(dkpes)
50
100
Figure 8 Measured (a) VLsnw and (b) VLsscw variations for the OCC Al-4596 Cu sample shown in Figure I; upper curve: in theX’-Y’ plane; lower curve: in theZ-Xplane. Z refers to the casting direction
184
Ultrasonics 1995 Vol 33 No 3
which is still small compared with the spatial resolution of the LFBSAM, the V,,,, profile is somewhat similar to a transversely isotropic material but with a minor texture. For the OCC copper sample, since the grain width (-hundreds of pm) is comparable with the spatial resolution of the LFBSAM, we start to see clearly the both in the X’-Y’ and anisotropic V,,,,. characteristics Z-X planes shown respectively in the upper and lower curves of Figure IO. Thus, the grain width is an important parameter for the consideration of ultrasonic nondestructive evaluation of metals with columnar grains. Note that if the grain width is much larger than the spatial resolution of the LFBSAM, then the elastic anisotropy of each single grain can be probed. In addition, Al and Cu are cubic, and Sn is tetragonal, and there is a significant difference in the V,,,,, profiles shown in the lower curves of Figures 86 and 9. In Figure 8b the Ksscw is the lowest along the casting direction, which implies that a diverging feature exists for the planar wave propagating in this direction, but it is the highest in Figure 9, which implies that a focussing feature exists for the planar wave propagating in this direction. In the next section, more illustrations on these two profiles will be given. Guided rods
acoustic
wave
characteristics
in
V,,,,, profile in the lower trace of Figure to a slowness (l/V) curve14, we see that, due to the concave feature near the casting Z direction, the guided longitudinal acoustic waves propagating in If the velocity
9 is converted
Acoustic characterization of metals with columnar grains: C.K. Jen et al.
the OCC Sn-Pb alloy rod will encounter a focussing effecti provided the acoustic wavelength is small compared with the rod diameter. Figures II and 12 show the longitudinal acoustic wave pulseecho measurement for a conventionally fabricated Sn (12.7 mm diameter) and
5.0
l-nT7-l Y'
X'
4.0 4.6 4.4
5.0 r,
, , , , , , , , , , , , , , , , , , ,
Figure 12 Reflected longitudinal wave echoes through a 47 mm long 12.7 mm diameter OCC Sn-7.5% Pb rod. Upper curve: at 5 MHz; lower curve: at 10 MHz
4.2t~“~“““““““~l -100
-50
50
100
@ (&W) Figure 10 Measured VLsscw variations for the OCC Cu sample shown in Figure 3; upper curve: in the X’-Y’ plane; lower curve: in the Z-X plane. Z refers to the casting direction
Figure 13 Reflected 5 MHz shear wave echoes through the (a) Sn rod in Figure 77; (b) Sn-Pb alloy rod in Figure 72
Figure 11 Reflected longitudinal wave echoes through a 47 mm long 12.7mm diameter 99.9% Sn rod made by the conventional fabrication method. Uppercurve: at 5 MHz; lowercurve: at 10 MHz
an OCC cast Sn-PB alloy rod (11 mm diameter) each 47mm long. In these two figures, the upper curves were performed at 5 MHz and the lower curves at 1OMHz. The ultrasonic transducers were broadband transducers of 6.35 mm diameter. The measurement conditions were kept identical for both rods except at lOMHz, and the signals obtained for the OCC Sn-Pb alloy were attenuated 20dB more than that for the conventionally-made
Ultrasonics
1995 Vol 33 No 3
185
Acoustic
characterization
of metals
with
columnar
grains:
Sn by an attenuator. At 5 MHz the ultrasonic signals were very similar for both rods, however, the spurious signals l2 for the OCC Sn-PB alloy are smaller than the conventionally made Sn. At 10MHz the signal for an OCC Sn-Pb alloy is 20dB stronger and the signal-tonoise ratio is also much better. This could be due to the fact that, at lOMHz, the waveforms in the rod are closer to planar than those at 5 MHz and they experience better focussing due to the concave shape of the slowness curve. Figures 13a and 136 are the shear wave pulsee echo measurements at 5 MHz for the above conventionally made Sn and OCC Sn-PB alloy rods respectively. Again, the measurement conditions were kept the same except, in this case, the signals for the conventional tin were attenuated 10 dB more than that for the OCC Sn-Pb alloy by an attenuator. This means that, in contrast to the longitudinal wave along the casting Z direction, shear waves do not propagate well in the OCC Sn-PB alloy. In addition to the lower signal strength, a longitudinal wave ultrasonic pulse was observed in Figure 13b and this echo was caused by the shear to longitudinal mode conversion process induced by the columnar grain structures, shown in Figure 2a along the rod.
C.K. Jen
Acknowledgement The financial support of the Natural Sciences and Engineering Research Council of Canada and the Ontario Centre for Materials Research is gratefully appreciated. References I 2
3
4
5
6 1
8
Conclusions Acoustic characterization measurements of Sn, Pb, Al and Cu metallic alloys with highly oriented long needle grains were presented. Using a 225 MHz line-focusbeam scanning acoustic microscope we have observed the ultrasonic anisotropy of a transversely isotropic symmetry for small grain width samples. The columnar grain structures induce unique acoustic propagation characteristics, such as focussing and diverging, which have been illustrated in the guided longitudinal and shear acoustic wave pulseecho measurements performed for rod geometries. Knowledge of their elastic anisotropy is of importance for ultrasonic non-destructive evaluation of these materials-such as centrifugally cast stainless steels, austenitic stainless steel welds and metallic plates, rods and tubes-produced by OCC methods.
186
Ultrasonics
1995
Vol
33 No 3
et al.
9
10 II
12
13
14
Pelseneer, J.P. and Louis, G. Ultrasonic testing of austenitic steel castings and welds Brir / Non-Des@ Test (1974) 16 1077113 Yoneyama, H., Shibata, S. and Kishigami, M. Ultrasonic testing of austenitic stainless steel welds: false indications and the cause of their occurrence NDT Int (1978) II 3-8 Kupperman, D.S., Reimann, K.J. and Fiore, N.F. Role of microstructure in ultrasonic inspectability of austenitic stainless steel welds Mart Ecu/ (1978) 36 70-74 Kupperman, D.S. and Reimann, K.J. Ultrasonic wave propagation and anisotropy in austenitic stainless steel weld metal l/TEE Trcttt.s Sottics and Ulrrason (1980) 27 7-I 5 Ahmed, S. and Thompson, R.B. Attenuation of ultrasonic waves in cubic metals having elongated. oriented grains Non&str Te\/ EraI (1992) 8-9 525-53 I Ohno, A. Continuous casting of single crystal ingots by the O.C.C. Process J Meto/.s (1986)38 14-I 6 Ohno, A. and Soda, H. Development and application of OCC technology Proc F. W&thrrq Itt~rrtta/iotttrl S~ntposiuttt Ott Solid~fic~cttiott Procc~ss, J.E. Lait and I.V. Samarasekera (eds) Pergamon Press, New York (1990) 215-228 Soda, H., Ichinose, A., Motoyasu, G., Ohno, A. and McLean, A. A new fabrication method and its process variables for cored materials C’W Meuh (1992) 5 955102 Shimizu, T., Yamazaki, H., Soda, H. and McLean, A. Development of Al alloy welding rod for hard facing applications Proc, 32 Annual C‘on/crrrrc~~o/’ Me~ulluryi.~t.v, Drrelopnwnt.s and Applicuriotts of C’wtmtk.s uttd NC,\, Mrtrtl ,41/o~,sQuebec. Canada, 29 Aug-2 Sept (1993) 541-549 MES-OCC innovated products, Advanced materials and products catalog. Mitsui engineering & Shipbuilding Co. Ltd (1992) Kushibiki, J. and Chubachi, N. Material characterization by line focus beam acoustic microscope IEEE Trutzs .SONICSUllruson (1985) 32 1X9-212 Jen, C.K., Neron, C., Miri, A., Soda, H., Ohno, A. and McLean, A. Fabrication and characterization of continuously cast clad buffer rods J. Acoust Sot Am (1992) 91 356553570 Jen. C.K.. Wane. Z.. Nicolle. A.. Bussiere. J.F.. Adler. E.L. and Abei K. Acoust% waveguiding rods of graded’velocity profiles Ultrasonics (1992) 30 91-94 B.A. Auld Acoustic Fields and Waves in Solids Vols I & II, Wiley (1973)