Tensile Strength and Pseudo-elasticity of YAG Laser Spot Melted Ti-Ni Shape Memory Alloy Wires

Abstract

In order to investigate the applicability of laser micro welding to the fabrication of medical devices, Ti-Ni shape memory alloy wires with pseudo-elasticity were micro spot melted by using YAG laser. By changing the melting parameters, such as laser power or pulse duration, the evolution in microstructure, tensile strength and pseudo-elasticity was investigated. Melted metal width decreased with decreasing the input energy and pulse duration. The microstructure of melted areas was cellular dendrite structures and that of heat affected areas was grown cell structure with a cell size of about 3∼10 μm. Tensile strength of spot melted wires was 30% lower than that of base metal, and the fracture occurred in melted areas with a brittle fracture surface. Although melted areas and heat affected areas did not show a pseudo-elasticity due to the larger grain size, minimizing the width of melted metal improved the overall pseudo-elasticity of spot melted samples. For the sample wires that were prepared by spot melting of cold rolled wires and by the subsequent heat treatment for shape memorizing, the pseudo-elasticity was improved because heat affected zone recrystallized to form fine grains.