B. Tam, M. I. Khan, Y. Zhou, University of Waterloo, Waterloo, ON, Canada
Nitinol alloys are an excellent material for various applications not only because they exhibit high strength and high ductility but also because of their pseudoelasticity, shape memory, and biocompatible properties. Currently, Nitinol alloys are being used extensively in medical device applications. As demand for more sophisticated medical equipments grow, alternate processing methods are required. Welding is a key fabrication process used in the manufacture of medical devices. However, Nitinol welding is still not well understood and further research is required before its full potential can be realized in practical applications. The current study details the mechanical properties of micro-resistance spot welding (MRSW) of Nitinol crossed wires. Joint performance was tested using a micro-tensile tester, expressed in joint breaking force and also by micro-hardness tests. Results showed the joint breaking force increased with increasing peak current. Optical microscopy coupled with micro-hardness results showed the grain growth and recrystallization in the weld metal induced local softening.
Summary: A detail investigation on the effects of peak current applied on the microsturcture and mechanical properties of resistance-mircro welded (RMW) crossed 50.7at% Ni-Ti wires was conducted in this study. 410µm nitnol wires were welded at 90 degree of each other using a current-controlled welding schedule. The joint mechanism was discussed by means of optical microscopy, while mechanical performance was assessed using micro-tensile tester. Furthermore, the failure path and fracture surfaces were examined using scanning electron microscopy (SEM). Results revealed a predominately solid-state bonding process. The joint breaking force increased with increasing peak current applied and decreased after reaching the max breaking force. The work hardening effect induced by the original cold wire drawing process was lost due to recrystallization. At higher peak current, surface melting and severe expulsion was observed. Further analysis revealed the fracture modes to be depended on the evolution of recrystallized grains and contaminates, such as oxides retained in the weld metal.
