Mechanical and superelastic properties of laser welded Ti-Ni shape-memory alloys produced by powder metallurgy

Abstract

Tensile test showed that the strength of the welded joints of powder metallurgy Ti–51%atNi shape-memory alloys (SMAs) decreases after disk-laser welding due to the presence of pores and columnar structure in the fusion zone. There was a reduction in the superelastic behavior of the welded joints compared to the base metal. Superelastic properties of specimen LW1 (with welding speed of 6 m/min and laser power input of 3 kW) were close to the base metal after loading and unloading at room temperature. Welded specimens, which were subjected to solution treatment at 1000 °C followed by aging at 500 °C showed an increment in superelasticity properties. Superelasticity improved significantly after heat treatment in all the welded joints except LW2 (with welding speed of 7.5 m/min and laser power input of 3 kW), because it contained high amounts of porosity in the fusion zone. LW1 exhibited the best properties after heat treatment with superelasticity strain increasing up to 7%, which was higher than the base metal. Differential scanning calorimetry (DSC) investigations showed that heat-treated weld seams had similar phase transformation peaks of martensite, austenite, and R phases to the base metal, which may be due to the recovering of Ti3Ni4 precipitates.