Effect of Postweld Heat Treatment on the Microstructure and Cyclic Deformation Behavior of Laser-Welded NiTi-Shape Memor
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UCTION
NICKEL titanium (NiTi) is emerging as one of the most popular materials in applications as biomedical implants and microelectromechanical systems because of its unique properties such as shape memory effect (SME), super-elasticity (SE), and biocompatibility.[1,2] However, because of the low formability and expensive manufacturing cost, the use of NiTi in small-sized and complicated-shaped components has long been a problem for product designers and mechanical engineers. The critical problem for the fabrication of NiTi microcomponents by conventional joining methods is the remarkable degradation in the mechanical and functional properties at the weld zone (WZ) because of high heat input during welding.[3] Laser welding technology is proposed for welding NiTi parts, especially wires, because of its ability to concentrate the beam energy onto a small area and, hence, to minimize the heat input and to reduce the size of the heat-affected zone (HAZ).[4–12] However, laserwelded NiTi parts cannot retain fully the SME and SE because of the segregation of solute and impurity element during solidification or the presence of incoherency of microstructure in the WZ, HAZ, and the adjacent base metal (BM).[13–15] As reported by Nishida et al.,[16] different Ni-rich precipitates, including Ni4Ti3,
Ni3Ti2, and Ni3Ti, might appear, depending on the aging temperature and time. Among the precipitates, metastable Ni4Ti3, which usually appears at a shorter aging time and lower temperature, is known to be the most important secondary phase that affects the SME and SE of NiTi.[17–22] Consequently, it is interesting to investigate whether the stress–strain behavior of the NiTi weldment can be improved by the precipitation of Ni4Ti3 particles via postweld heat treatment (PWHT). For long-term use of NiTi as in microdevices or components, it is important to study the cyclic deformation behavior of the joint after laser welding. Numerous researchers have investigated the cyclic deformation behavior of single-crystal and polycrystalline NiTi alloys at different manufacturing or heattreatment conditions. It is reported in the literature that the characteristics of the cyclic deformation behavior in NiTi alloy are changed with the precipitation of Ni4Ti3.[23–27] However, no attempt has been made to explore the effect of PWHT on the cyclic deformation behavior of laser-welded NiTi wires. The aim of this study is to investigate the effect of PWHT on the microstructure and the cyclic stress–strain hysteresis of laser-welded shape-memory NiTi wires.
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EXPERIMENTAL DETAILS
A. Materials and Sample Preparation C.W. CHAN, Ph.D. Student, H.C. MAN, Professor and Department Head, and T.M. YUE, Professor, are with the Laser Processing Group, Advanced Manufacturing Technology Research Centre, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong, P.R. China. Contact e-mail: [email protected] Manuscript submitted July 25, 2011. Article published online January 19, 2012 1956—VOLUME 43A, JUNE 2012
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