Fusion welding of nickel-titanium and 304 stainless steel tubes: Part II: tungsten inert gas welding

Shape memory nickel-titanium is attractive for lightweight actuators as it can generate large blocking stresses and high recovery strains through solid-state operation. A key challenge is the integration of the nickel-titanium components into systems; this alloy is difficult and expensive to machine and challenging to weld to itself and other materials. In this research, we join nickel-titanium and 304 stainless steel tubes of 9.53 mm (0.375 in) in diameter through tungsten inert gas welding. By joining nickel-titanium to a common structural material that is easily machined and readily welded to other materials, the system integration challenges are greatly reduced. The joints prepared in this study were subjected to optical microscopic inspection, hardness mapping, energy dispersive X-ray spectroscopy, mechanical testing, and failure surface analysis via scanning electron microscopy. The affected zone from welding is approximately 125 mu m (0.005 in) wide including partially mixed zones with a maximum hardness of 817 HV and a possible heat-affected zone of 1-2 mu m (39-79 mu in) wide. The maximum average ultimate torsional strength is 415 MPa (60.2 x 10(3) lbf/in(2)). Implementation of this joining method is demonstrated in the construction of a solid-state torsional actuator that can lift a weight of 2.3 kg (5 lb) to a distance of 610 mm (24 in). The laser and TIG welding processes are compared.