Effect of CO
2
Laser Welding on the Shape-Memory and
Corrosion Characteristics of TiNi Alloys
Y.T. HSU, Y.R. WANG, S.K. WU, and C. CHEN
A CO2laser has been employed to join binary Ti50Ni50and Ti49.5Ni50.5shape-memory alloys (SMAs),
with an emphasis on the shape-memory and corrosion characteristics. Experimental results showed that a slightly lowered martensite start (MS) temperature and no deterioration in shape-memory
character of both alloys were found after laser welding. The welded Ti50Ni50, with an increased amount
of B2 phase in the weld metal (WM), had higher strength and considerably lower elongation than the base metal (BM). Potentiodynamic tests revealed the satisfactory performance of laser-welded Ti50Ni50in 1.5 M H2SO4 and 1.5 M HNO3 solutions. However, the WM exhibited a significantly
higher corrosion rate and a less stable passivity than the BM in artificial saliva. On the other hand, the pseudoelastic behavior of the laser weld was investigated only for the Ti49.5Ni50.5alloy, to facilitate
tension cycling at room temperature. The cyclic deformation of Ti49.5Ni50.5indicated that the stress
required to form stress-induced martensite (sm) and the permanent residual strain («p) were higher
after welding at a given number of cycles (N ), which were certainly related to the more inhomogeneous nature of the WM.
I. INTRODUCTION same as those in the BM.[7] They also pointed out that the
decreased fracture strain of the tensile curve in the welded
SHAPE
memory alloys (SMAs) are recognized as func- specimen was due to the Ti2Ni precipitated at the grain
tional materials and have many industrial applications based boundaries. Recently, the same authors found that the PE on their shape-memory effect (SME) and superelastic or behavior of a Ti-51.5 at. pct Ni alloy can be maintained pseudoelastic (PE) properties.[1,2,3]The TiNi alloy system,
after laser welding.[8]
which exhibits thermoelastic martensitic transformation, is From the aforementioned literature survey, it can be seen known as one of the most important SMAs. For engineering that laser welding is a promising method for joining TiNi applications, the weldability might have great influence on alloys. However, in most of the tensile tests on laser-welded the usage of a material and should be taken into account in the TiNi alloys, conventional tensile specimens were used with evaluation process. In case of welding SMAs, an additional a narrow WM in the central part of the specimen.[6]Under requirement is to maintain the shape-memory characteristics this situation, the tensile axis of the welded specimens was of the weld. Many research projects have been conducted normal to the welding direction. Obviously, the WM only on joining,in order to develop appropriate welding processes occupied a small fraction (,10 pct) of the gage length, and, for TiNi alloys. For instance, Nishikawa et al. (1982) utilized therefore, the stress-strain curves of such specimens were resistance butt welding to join a TiNi alloy and indicated difficult to compare with the unwelded ones. To overcome that the welded specimen can reach 80 pct of the tensile this shortcoming, tensile specimens with a high volume frac-strength of the base metal (BM) and exhibit a SME.[4]Beyer
tion (,66 pct) of the WM within the gage length were et al. (1986) studied the microstructures of a resistance butt- prepared for this study. This could be easily achieved by welded Ti-50.3 at. pct Ni alloy and found that Ti2Ni precipi- making the welding direction parallel to the loading axis of
tates can affect the performance of the SME and weaken tensile specimens. Apart from that, the corrosion resistance the strength of grain boundaries in the weld metal (WM).[5]
of the TiNi alloy in various environments is of great impor-Hirose et al. (1990) joined TiNi alloys with a CO2laser and tance for practical applications, yet little is known about
observed the existence of cellular dendrites in the WM and this subject,[9 –12] especially for the welded structures. The Ti2NiOx oxides in between the dendritic arms.[6] Further- purpose of this study was to investigate not only the SME
more, good PE properties were also obtained after cyclic but also the corrosion characteristics of TiNi SMAs after tensile tests with a 4 pct strain amplitude, in which a residual welding. In order to evaluate the corrosion behavior of laser-strain of about 0.2 pct was reported after 50 cycles. Schloß- welded Ti
50Ni50 specimens, potentiodynamic tests in acid
macher et al. (1994) joined Ti-49.3 at. pct Ni with a Nd- and chloride solutions were performed. These results were YAG (neodymium-yttrium aluminum garnet) laser and also compared with those of unwelded specimens for better revealed that the differential scanning calorimetry (DSC) judgement regarding the use of welded Ti
50Ni50 in a
spe-and tensile curves of the welded specimens are almost the cific application.
Y.T. HSU and Y.R. WANG, formerly Graduate Students, Institute of II. EXPERIMENTAL PROCEDURE Materials Science and Engineering, National Taiwan University, are now
Process Engineers with the Taiwan Semiconductor Manufacturing Com- The TiNi SMAs with chemical compositions (at. pct) of pany, Hsinchu, Taiwan 300, Republic of China. S.K. WU and C. CHEN, Ti
50Ni50 and Ti49.5Ni50.5 were used in the experiment. The Professors, are with the Institute of Materials Science and Engineering,
latter alloy was only used to study the PE behavior of
laser-National Taiwan University, Taipei, Taiwan 106, Republic of China.
(Af) being slightly lower than room temperature. High-purity
zones (HAZs). Hence, the tensile results of laser welds elements of Ti and Ni were melted and cast in a vacuum
represent the mechanical behavior of both the WM and arc remelting furnace to obtain ingots of about 100 g. The
the HAZ, which are the two distinct regions associated weight losses were measured to be less than 0.01 pct after
with welding. In addition,cyclic loading-unloading experi-melting. Ingots were then hot rolled with intermediate
ments, with a maximum strain of 3 pct and up to 50 cycles, annealing at 850 8C into strips of roughly 2 mm in thickness.
were performed on a Ti49.5Ni50.5SMA at 25 8C. The
selec-All strips were subjected to a final annealing of 800 8C/1 h
tion of such an alloy facilitated the comparison of PE to eliminate residual stresses from rolling.
properties between unwelded and welded specimens at For the laser welding experiments, a commercially
avail-room temperature. able laser (Rofin-Sinar 850) was employed. This laser
gener-The potentiodynamic behavior of Ti50Ni50 specimens
ates a continuous-wave CO2 laser beam at a 10.6 mm with and without welding was studied in nitrogen-purged
wavelength, with an output power of up to 5 kW.
Bead-on-1.5 M H2SO4(25 8C), 1.5 M HNO3(25 8C), and artificial
plate welds were made without filler-metal additions, by
saliva (37 8C) solutions. The composition of artificial proper selection of processing parameters, to obtain
full-saliva (each gram) contained 0.844 mg NaCl, 1.2 mg KCl, penetration welds. Table I lists the laser welding variables
0.146 mg CaCl2, 0.052 mg MgCl2 ? 6H2O, 0.34 mg
utilized in the investigation. The average bead width on
K2HPO4, 60 mg C6H14O6, 2 mg C8H8O3, and 3.5 mg
the top and bottom surfaces was 2 mm. After welding, no
ROCH2CH2OH. The electrochemical cell was a
conven-postweld heat treatments were made on these welds. The
tional three-electrode cell consisting of a working elec-welded specimens were then characterized by means of
trode, a saturated calomel reference electrode (SCE), and metallographic examinations,microhardness measurements,
a platinum counter electrode. The specimens, which were and X-ray diffraction (XRD) with Cu Karadiation. connected to the working electrode, were polished with
The changes in martensitic transformation temperatures of
up to 1000-grit SiC abrasive paper. The exposed surface Ti50Ni50and Ti49.5Ni50.5after welding were measured using area of specimens to the solution was 10 3 10 mm2 for
DSC. Samples of the BM and the WM were cut from laser
the BM and 10 3 2 mm2for the WM. The potentiodynamic
welds to obtain a mass of 15 mg for the DSC measurement,
curves were determined with a Princeton Applied Research which was carried out by using a DUPONT* 2000 thermal
273 potentiostat and associated computer software for
cor-*DUPONT is a trademark of E.I. DuPont de Nemours, Wilmington, DE. rosion measurements. A scan rate of 300 mV/min was
used in the range of 2250, 1 1900 mV (SCE) for the analyzer with a controlled cooling/heating rate of 10 8C/min. H
2SO4and HNO3solutions and in the range of 21200,
Heats of transformation (DH ) were automatically calculated 11900 mV (SCE) for the artificial saliva. from the areas under DSC peaks using the software package
provided by DUPONT. The shape memory (shape recovery)
III. RESULTS AND DISCUSSION
was examined by a bending test with 1 3 1 3 40 mm3
specimens. The specimens were deformed to an angle ofui A. Metallography and Hardness
in liquid nitrogen and then heated to various temperatures
up to 125 8C. The initially deformed angleuiwas found to Figure 2(a) is a typical surface macrostructure of the
laser-welded specimen, in which columnar grains growing toward recover touf, which was temperature dependent. The shape
recovery is defined as (ui2uf)/ui, and the detailed measure- the weld centerline are obvious.Since the liquid/solid
bound-ary of the trailing portion of a tear-drop weld pool is straight, ment of the shape-recovery characteristics has been
pre-sented elsewhere.[13] the grains are aligned in a direction close to the normal of
the weld centerline for welding at high speeds. Figure 2(b) The specimens for tensile testing were conducted on a
MTS machine at room temperature, with a strain rate of shows the optical photograph near the fusion boundary of the weld, with evidenceof epitaxial growth, i.e., grain growth 1 3 1024s21. The loading axis was aligned parallel to the
welding direction in the tensile specimens, and the strain initiating from the HAZ at the fusion boundary and proceed-ing toward the weld centerline. It should be pointed out that was measured by an extensometer with a gage length of
12.5 mm. Figure 1 shows the dimension of the tensile no grain growth in the HAZ was found, owing to the low heat input (#100 J/mm) of the process and the use of a fully specimens, in which the WM consists of nearly two-thirds
the volume fraction of the material within the gage length. annealed (strain-free) material prior to laser welding. The microhardness profile across the fusion zone of the laser This configuration is similar to a lamellar composite, with
(a)
Fig. 2—Metallographs showing (a) the surface macrostructure of laser-welded specimens and (b) the microstructure near the fusion boundary.
(b)
Fig. 4—DSC results of (a) the base metal and (b) the weld metal of Ti50Ni50SMA.
Table II. Shape Memory Recovery of Ti50Ni50Alloy with
and without Laser Welding Shape Recovery (Pct)
Temperature The Base Metal The Weld Metal
40 8C 0 pct 0 pct
75 8C 37 pct 40 pct
95 8C 95 pct 93 pct
125 8C 97 pct 98 pct
Fig. 3—Microhardness profile across the fusion zone of a laser weld.
B2 ÖB198 martensitic transformation with a martensite start (MS) temperature of 38 8C (Figure 4(a)), while the WM
shows an MStemperature of 32 8C (Figure 4(b)). By
compari-structures between the fusion zone and the HAZ (or the BM)
son of Figures 4(a) and (b), it is found that transformation existed, no apparent changes in hardness were disclosed.
peaks and MStemperature are all smaller in the WM. This
The hardness (Hv) of the fusion zone (or the WM) was
might come from the fact that the interstitial atoms, in partic-approximately235 Hv, resembling that in the BM or the HAZ
ular, oxygen, were introduced into the WM during welding. of laser-welded specimens. The results of metallographic
The content of oxygen in the BM and the WM was found examinations and hardness measurements suggested that the
to be in the neighborhood of 300 and 500 ppm, respectively. HAZ could be considered the same as the BM in the
pres-For the shape-recovery test, the results of Ti50Ni50specimens
ent study.
with and without laser welding are given in Table II. Obvi-ously, the WM of laser-welded specimens showed no deterio-B. The DSC and Shape-Recovery Tests ration in shape recovery. This feature, together with a minor change in MStemperature, confirmed that the laser welding
Figure 4 shows the DSC results of a Ti50Ni50SMA with
comprised of B2 and B198 mixed phases at room tempera-ture. Nevertheless, the welded specimen had more B2 phase than the unwelded specimen at room temperature. It has been reported that the B2 phase has a higher strength than the B198 phase.[16]Therefore, a higher tensile strength
of the laser weld than that of the BM is expected in the stress-strain curves. Figure 6 also reveals that the fracture (b)
strain of the BM (15 pct) is higher than the recoverable
Fig. 5—XRD results of (a) the base metal and (b) the weld metal of strain (7 to 8 pct) of the shape-memory recovery in a
Ti50Ni50specimens. Ti
50Ni50 SMA, but the fracture strain of the laser weld
is considerably lowered (7 pct). The deterioration of the fracture strain in a laser-welded Ti50Ni50 SMA could be
attributed to (1) the segregation of solute and impurity C. The XRD Analysis
elements during solidification; (2) the coarse-grained and dendritic structures in the WM; and (3) the increase in B2 Figure 5 presents the XRD results of Ti50Ni50specimens
before and after laser welding. It also reveals the existence of phase of specimens after welding. Figure 7 shows tensile-fractured fractographs of the unwelded and laser-welded B2 and B198 peaks in these specimens at room temperature.
However, the relative amount of B2 in the BM (Figure 5(a)) specimens. Distinct fracture modes are observed in these specimens, i.e., a predominant dimple fracture (Figure is less than that in the WM (Figure 5(b)). This coincides
with the DSC results, as indicated in Figure 4, in which the 7(a)) for the unwelded specimen and a brittle appearance (Figure 7(b)) for the welded specimen in the WM region. BM has more B2 that has been transformed to B198 than
does the WM at room temperature. It is important to note It is noted that the HAZ (which is basically the same as the BM in this study, as discussed in Section III–A) in the that an abnormally high intensity of B2200 is observed in
Figure 5(b), which is different from JCDPS diffraction data welded specimen has a similar fracture appearance, as shown in Figure 7(a). In addition, the weld centerline, for nickel titanium, which has the highest intensity of
B2110.[14]This effect is due to the developmentof a solidifica- which is a narrow region with the most severe segregation
in the welding, still can be seen in Figure 7(b). Although tion texture associated with the welding process. The surface
of the specimen analyzed by X-rays (Figure 5(b)) was per- the laser-welded specimen consisted of only 66 pct WM in the gage section, the tensile properties of the entire WM pendicular to the growth direction of the crystals during
solidification. It is known that the ^100& directions are the specimen could be estimated. As shown in Figure 1, the WM is sandwiched by two strips of BM in the gage section preferred growth directions for cubic metals.[15]The Ti
50Ni50
SMA is a B2 (ordered bcc) structure, and, thus, it is quite of tensile specimens, resembling a lamellar composite under isostrain conditions of loading. The tensile strength reasonable to have ^100&B2 preferred growth orientations,
which cause the abnormally high intensity of B2200in Fig- of the composite (scom) can be calculated by the rule of
mixtures: ure 5(b).
(a)
Fig. 7—Tensile-fractured appearance of (a) unwelded and (b) laser-welded Ti50Ni50specimens.
scom50.66sWM10.34sBM
(b)
wheresWMandsBMare the tensile strengths of the WM and Fig. 8—Stress-strain curves of the cyclic deformation with N 5 1, 5, 10,
BM, respectively. Taking sBM 5 650 MPa (point A) and and 50 for (a) unwelded and (b) laser-welded Ti49.5Ni50.5specimens at
scom 5740 Mpa (point B) from the stress-strain curves in room temperature.
Figure 6, a tensile strength of the WM (sWM) of 786 MPa
could be obtained accordingly.However, a tensile elongation of the WM in the neighborhood of 7 pct, similar to that of the composite, would be expected. This was supported by the fractographic observation of laser-welded specimens, in which the WM exhibited a brittle fracture mode while the BM revealed a ductile fracture mode. Apparently, the frac-ture strain of the composite (or the laser weld) was limited by the tensile ductility of the WM. Since the BM had a better ductility, the fracture strain of the WM was reached prior to that of the BM in the tensile test of laser-welded specimens.
E. Cyclic Loading-Unloading Test
The DSC results of a Ti49.5Ni50.5 SMA indicated that the
MStemperatures were approximately 220 8C and 225 8C
for the BM and the WM, respectively. The shape-recovery test at 25 8C also disclosed no deterioration of this alloy
Fig. 9—Schematic illustration of sm, E1, and E2 in a typical loading-after welding (96 pct of the WM vs 98 pct of the BM). unloading curve.
Therefore, the cyclic deformation of the alloy at room tem-perature with 3 pct strain amplitude could exhibit the PE behavior. Figure 8 displays such a test of Ti49.5Ni50.5
speci-mens before and after welding for N (number of cycles) 5 PE parameters of the stress-strain curve, such as sm, E1,
and E2, are defined and illustrated in Figure 9 to facilitate
1, 5, 10, and 50, in which the permanent (or irreversible)
the moresmis needed for SIM. Figure 10(b) is the plot of
«pvs N, in which «p increases rapidly at the beginning as
N increases and then remains nearly constant after 20 cycles. The magnitude of «p was approximately 0.09 pct for the
BM and 0.19 pct for the WM after 50 cycles. The increased «p of the WM might come from the fact that the more
(a) inhomogeneous characteristics, in terms of grain structures,
solidification substructures, segregation, etc., inherently adhere to the WM. The higher «p(the less recoverable strain)
would also induce more work hardening, which is supported by the evidence that the slope of the PE portion becomes steeper as N increases (Figure 8). Figure 10(c) shows that the efficiency of energy storage increases with increasing N for both the BM and the WM at low cycles and reaches a steady state at high cycles. This tendency is similar to the «pvs N curves, as shown in Figure 10(b). It has been
demon-strated that the cold-rolled (work-hardened) TiNi SMA can improve itshvalue.[17]Therefore, the improvedhvalue at
low cycles could be related to the effect of cyclic (work) hardening, which became saturated as N further increased. Clearly, the smaller the E1 and the larger the E2 values of
the WM, as compared to those of the BM at all cycles (Figure 8), are responsible for the improvedhvalue of the WM. However, the exact mechanism that causes thehvalue of the WM to be higher than that of the BM after cycling still needs to be uncovered.
(b) F. Potentiodynamic Behavior
Potentiodynamic curves for the BM and the WM of a Ti50Ni50SMA exposed to 1.5 M H2SO4and HNO3solutions
at 25 8C are presented in Figures 11(a) and (b), respectively. All curves in the figures display no active-to-passive transi-tion peaks, indicating that the surface layers of both welded and unwelded specimens were passivated spontaneously in these solutions. The BM and the WM had nearly the same passive region in either H2SO4or HNO3; however, the
pas-sive current density of the Ti50Ni50SMA tended to increase
after welding. For instance, the passive current density increased from 25mA/cm2in the BM to 42mA/cm2in the
WM in H2SO4and from 19mA/cm2in the BM to 33 mA/
cm2in the WM in HNO
3. Thus, the polarization behavior
of the WM in these solutions was characterized by a small increase (less than 20 mA/cm2) in current density relative
to the BM, particularly at potentials of the stable passive range. It is apparent that the inhomogeneous nature of the WM might cause a less stable passive layer, as can be
con-(c) cluded from a slightly higher passive current density in the
solution. The potentiodynamic curves in the 1.5 M H2SO4 Fig. 10—Unwelded and laser-welded Ti49.5Ni50.5 specimens after 3 pct and HNO3solutions revealed that the welded specimens did cyclic deformation at 25 8C for (a)sm, (b) «p, and (c)hvs Nplots.
(b) (a)
(c)
Fig. 11—Potentiodynamic curves of Ti50Ni50specimens with and without laser welding in the solutions of (a) 1.5 M H2SO4at 25 8C, (b) 1.5 M HNO3at
25 8C, and (c) artificial saliva at 37 8C.
as compared to the unwelded Ti50Ni50 SMA. However, a solidified structure. As a result, welding not only reduced
the passive range but also enhanced the anodic dissolution distinct polarization behavior between the WM and the BM
in artificial saliva at 37 8C is observed, as shown in Figure of the Ti50Ni50SMA by increasing the passive current density
from 22.3mA/cm2in the BM to 169mA/cm2in the WM in
11(c), in which the corrosion potential (Ecorr) of the WM
was shifted toward a more noble potential of 258 mV, in artificial saliva.
The corrosion rate (micrometers/year), determined by contrast to that of 2495 mV of the BM. The passivity of
the TiNi SMA has been reported to be unstable in chloride- Tafel extrapolation methods to obtain the corrosion current density (Icorr) from potentiodynamics curves, is given in
containing media and is also affected by the presence of an
intermetallic phase, which provides initiation sites for the Table III. A lower Icorr value is thought to indicate a better
resistance to general corrosion. The corrosion rate is related breakdown of passive films.[19] Apparently, the WM has a
Corrosion rate (mm/year) 5. Potentiodyanamic tests revealed the satisfactory perfor-mance of the Ti50Ni50SMA after welding in 1.5 M H2SO4
and HNO3 solutions. In contrast, the WM exhibited a
53.27(Icorr,mA/cm
2) ? (Equivalent weight, g)
(Density, g/cm3) ? (Surface area, cm2) significantlyhigher corrosion rate and a less stable
passiv-ity (a higher passive current denspassiv-ity with a reduced pas-The calculated equivalent weight and the measured den- sive range) than the BM in artificial saliva. Accordingly, sity of the Ti50Ni50SMA are 26.65 g and 6.47 g/cm3, respec- the use of a welded Ti
50Ni50SMA in saliva or like
envi-tively. The surface area of specimens exposed to the solution ronments should be more carefully evaluated. was 1 cm2for the BM and 0.2 cm2for the WM, as mentioned
in the experiment. As can be seen in Table III, the corrosion
rates of the unwelded specimens in H2SO4, HNO3, and artifi- ACKNOWLEDGMENTS
cial saliva solutions are considered to be quite low, in the The authors gratefully acknowledge the support of the range of 8 to 17mm/year. In case of the welded specimens, Republic of China National Science Council (Contract Nos. the corrosion rates were higher than those of the unwelded NSC86-2216-E-002-013 and NSC87-2216-E-002-030) specimens in these solutions. The increase in corrosion rates
of the alloy after welding was relatively small in the 1.5 M REFERENCES H2SO4and HNO3solutions, but quite substantial in artificial
1. Engineering Aspects of Shape Memory Alloys, T.W. Duerig, K.N.
saliva. The high corrosion rate of the WM (274.4mm/year),
Melton, D. Stockel, and C.M. Wayman, eds., Butterworth-Heinemann
associated with an increased passive current density and Ltd., London, 1990.
a reduced passive range in artificial saliva (Figure 11(c)), 2. SMST-94 Proc. 1st Intl. Conf. on Shape Memory and Superelastic strongly suggested that a laser-welded Ti50Ni50SMA might Technologies, A.R. Pelton, D. Hodgson, and T.W. Duerig eds., MIAS,
Monterey, CA, 1994.
not be suitable for use in saliva or like environments.
3. SMST-97 Proc. 2nd Intl. Conf. on Shape Memory and Superelastic
Technologies, A.R. Pelton, D. Hodgson, S. Russell, and T.W. Duerig eds., MIAS, Monterey, CA, 1997.
4. M. Nishikawa, H. Tanaka, M. Kohda, T. Naganra, and K. Watanabe: IV. CONCLUSIONS
J. Phys., 1982, vol. 43, Suppl. 12, pp. C4: 839-44.
5. J. Beyer, P.A. Besselink, and J.H. Cindenhorins: Proc. Intl. Symp. on
1. The DSC results indicated that the transformation temper- Shape Memory Alloys, Y. Chu, T.Y. Hsu, and T. Ko, eds., Guilin, China, China Academic Publishers, Beijing, 1986, pp. 492-97.
atures of the Ti50Ni50and Ti49.5Ni50.5SMAs were slightly
6. A. Hirose, M. Uchihara, T. Araki, K. Honda, and M. Kondoh:J. Jpn. depressed after welding. The minor change in MStemper- Inst., Met., 1990, vol. 54, pp. 262-69.
ature and no deterioration in the shape-memory character 7. P. Schloßmacher, T. Haas, and A. Schußler: SMST-94 Proc. 1st Intl. of the WM suggested that laser welding is suitable for Conf. on Shape Memory and Superelastic Technologies, A.R. Pelton, D. Hodgson, and T.W. Duerig eds., MIAS, Monterey, CA, 1994, pp.
joining TiNi SMAs.
85-90.
2. The XRD results of the Ti50Ni50 SMA confirmed the 8. P. Schloßmacher, T. Haas, and A. Schußler: SMST-97 Proc. 2nd Intl.
presence of more B2 phase in the WM than that in the Conf. on Shape Memory and Superelastic Technologies, A.R. Pelton, BM at room temperature. An abnormally high intensity D. Hodgson, S. Russell, and T.W. Duerig eds., MIAS, Monterey, CA, of B2200 in the WM was also observed, owing to the 1994, pp. 137-42.
9. J.-P. Randin:J. Biomed. Mater. Res., 1988, vol. 22, pp. 649-66.
existence of the ^100& preferred growth directions of B2
10. G. Rondelli, B. Vicentini, and A. Cigada:Corr. Sci., 1990, vol. 30,
during solidification.
pp. 805-12.
3. The laser weld had a higher strength but significantly 11. Y. Oshiki and S. Miyazaki: Corr. Eng., 1991, vol. 40, pp. 1009-25. lower ductility than the unwelded Ti50Ni50. The improved 12. D. Starosvetsky, O. Khaselev, and J. Yahalom:Corrosion, 1998, vol.
54, pp. 524-30.
strength of laser welds was associated with the increased
13. H.C. Lin and S.K. Wu:Scripta Metall. Mater, 1992, vol. 26. pp. 59-62.
amount of B2 in the weld, while the decreased fracture
14. JCPDS-ICDD, 1601 Park Lane, Swarthmore, PA 19081, 1994.
strain was attributed to coarse-grained dendriticstructures 15. R.E. Reed-Hill and R. Abbaschian: Physical Metallurgy Principles,
in the WM. 3rd ed., PWS-KENT Publishing Company, Boston, MA, 1992, pp.
4. The cyclic deformation of the Ti49.5Ni50.5 SMA at room 444-46.
16. K. Otsuka and K. Shimizu:Intl. Met. Rev., 1986, vol. 31, pp. 93-114.
temperature showed thatsm(the stress required for SIM)
17. H.C. Lin and S.K. Wu:Acta Metall. Mater., 1994, vol. 42, pp. 1623-30.
and «p(the permanent residual strain) were higher after 18. S. Miyazaki, Y. Ohmi, K. Otsuka, and Y. Suzuki: J. Phys., 1982, vol.
welding at a given number of cycles. The highersmand 43, Suppl. 12, pp. C4: 255-60.
«pvalues of the laser weld were certainly related to the 19. R.S. Dutta, K. Madangopal, H.S. Gadiyar, and S. Banerjee:Br. Corr. J., 1993, vol. 28, (3), pp. 217-21.
