Analysis and prevention of cracks in laser-welded joint of TiNi shape memory alloy and stainless steel

Dissimilar metal joints of TiNi shape memory alloy wire and stainless steel wire were welded by laser welding method.The cracks feature and fracture surface morphology of joints were examined by using scanning electron microscopy(SEM) and confocal laser scanning microscope(CLSM).The mechanism of crack formation were analyzed, and some measurenents were taken to control the welding cracks.The results showed that the micro-cracks usually emerged in the center of the weld zone and fusion zone of TiNi alloy side.The existence of a large number of brittle compounds in the weld was internal cause of cracks, and the joint subjected to tensile stress was the necessary condition of cracks.The cracking susceptibility can be improved to a certain extent by adding Ni interlayer, Co interlayer, changing the laser beam position, applying an axial force to weld zone and optimizing the laser welding parameters.Adding metal interlayer was a more effective method.The tensile strength reached 372 MPa and 347 MPa respectively by using Ni and Co interlayer, and the joint strength increased by 98.9% and 85.6% respectively, compared with the joint without metal interlayer.