Effect of CO 2 laser welding on the shape-memory and corrosion characteristics of TiNi alloys
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INTRODUCTION
SHAPE memory alloys (SMAs) are recognized as functional materials and have many industrial applications based on their shape-memory effect (SME) and superelastic or pseudoelastic (PE) properties.[1,2,3] The TiNi alloy system, which exhibits thermoelastic martensitic transformation, is known as one of the most important SMAs. For engineering applications, the weldability might have great influence on the usage of a material and should be taken into account in the evaluation process. In case of welding SMAs, an additional requirement is to maintain the shape-memory characteristics of the weld. Many research projects have been conducted on joining, in order to develop appropriate welding processes for TiNi alloys. For instance, Nishikawa et al. (1982) utilized resistance butt welding to join a TiNi alloy and indicated that the welded specimen can reach 80 pct of the tensile strength of the base metal (BM) and exhibit a SME.[4] Beyer et al. (1986) studied the microstructures of a resistance buttwelded Ti-50.3 at. pct Ni alloy and found that Ti2Ni precipitates can affect the performance of the SME and weaken the strength of grain boundaries in the weld metal (WM).[5] Hirose et al. (1990) joined TiNi alloys with a CO2 laser and observed the existence of cellular dendrites in the WM and Ti2NiOx oxides in between the dendritic arms.[6] Furthermore, good PE properties were also obtained after cyclic tensile tests with a 4 pct strain amplitude, in which a residual strain of about 0.2 pct was reported after 50 cycles. Schloßmacher et al. (1994) joined Ti-49.3 at. pct Ni with a NdYAG (neodymium-yttrium aluminum garnet) laser and revealed that the differential scanning calorimetry (DSC) and tensile curves of the welded specimens are almost the Y.T. HSU and Y.R. WANG, formerly Graduate Students, Institute of Materials Science and Engineering, National Taiwan University, are now Process Engineers with the Taiwan Semiconductor Manufacturing Company, Hsinchu, Taiwan 300, Republic of China. S.K. WU and C. CHEN, Professors, are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China. Manuscript submitted November 30, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
same as those in the BM.[7] They also pointed out that the decreased fracture strain of the tensile curve in the welded specimen was due to the Ti2Ni precipitated at the grain boundaries. Recently, the same authors found that the PE behavior of a Ti-51.5 at. pct Ni alloy can be maintained after laser welding.[8] From the aforementioned literature survey, it can be seen that laser welding is a promising method for joining TiNi alloys. However, in most of the tensile tests on laser-welded TiNi alloys, conventional tensile specimens were used with a narrow WM in the central part of the specimen.[6] Under this situation, the tensile axis of the welded specimens was normal to the welding direction. Obviously, the WM only occupied a small fraction (⬍10 pct) of the gage length, and, therefore, the st
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