Study on mechanical performance of laser-welded NiTinol sheet

Abstract

NiTinol, because of its several superior properties like shape memory effect, pseudoelasticity, corrosion resistance and biocompatibility, has become a promising member for various application areas including biomedical, micro-electro-mechanical system and aerospace. Poor machinability, lack of joining techniques and high cost of the material are presently restricting the use of the material in practical applications to its full potential. Laser welding is the majorly used fabrication technique for this material. The effects of welding on the bead geometry, microstructure, new phase formation, phase transformation temperature for laser welding of 2-mm thick NiTinol sheet in bead-on-plate configuration were investigated through this study using Yb-fiber laser in continuous mode of operation. Moreover, the mechanical properties of the joint were assessed by means of tensile tests and cyclic loading–unloading test. The functionality of the cyclic loading–unloading was tested by analysis of the mechanical hysteresis response up to 6% strain levels. Furthermore, the fractured surface was analyzed by scanning electron microscopy. Results revealed that the microstructure varied across the different zones of welding depending on temperature gradient and solidification rate. The microhardness value of the weld-bead was reduced considerably from that of the parent material. Brittle intermetallic compounds of Ti and Ni were formed during welding. It was also found that the welding greatly influenced the phase transformation temperature, overall mechanical behavior and the mode of failure in comparison with parent material.