Characteristics and fracture morphology of Ti-Ni type shape memory alloy and its laser weld joint.

Mechanical properties and fracture behaviour of Ti-50.7at%Ni shape memory alloy and its laser weld joint have been investigated.The base metal aged at 673K for 3.6ks exhibited super elasticity and higher tensile strength than the solution base metal. However, the aged material showed less fracture strain and more brittle fracture surface than the solution material.Butt welding of 3mm thick Ti-Ni alloy was performed using 10kW CO2 laser. The laser welded specimens showed a successful shape memory effect and super elasticity. These properties were identical with those of the base metal.However, the tensile strength and the fracture strain of the laser welded specimen were less than those of the base metal. The fracture occurred at the columar crystal boundary at the center of the weld metal. The boundary was perpendicular to the direction of the loading and, moreover, oxide inclusions crystallized at the boundary. These facts seem to cause low strength of the weld joint.The fracture strain of the welded specimen, nevertheless, was more than 6% which is the maximum recoverable elongation in polycrystal Ti-Ni base metal. Laser weld is, therefore, applicable to the joining of Ti-Ni shape memory alloy when the alloy is used as a functional material.