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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 µm (0.005 in) wide including partially mixed zones with a maximum hardness of 817 HV and a possible heat-affected zone of 1–2 µm (39–79 µin) wide. The maximum average ultimate torsional strength is 415 MPa (60.2 x 103 lbf/in2). 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.

Due to their large blocking stresses, high recovery strains, and solid-state operation, nickel-titanium actuators can offer substantial weight and space savings relative to traditional electric or hydraulic systems. A challenge surrounding NiTibased actuators is integration of the NiTi components into a given system; this alloy is difficult and expensive to machine and challenging to weld to itself and structural materials. In this research, we join NiTi and 304 stainless steel tubes of 9.53 mm (0.375 in) in diameter through laser welding to create joints with weld depths up to 1.65 mm (0.065 in). By joining NiTi to a common structural material that is easily machined and readily welded to other materials, system integration is greatly improved. The joints prepared in this study were characterized through optical microscopy, hardness mapping, energy dispersive X-ray spectroscopy, mechanical testing, and analysis of the resulting fracture surfaces. The average ultimate shear strength of these joints is 429 MPa (62.2 10 3 lbf/in 2 ) and the resulting fusion zone has a maximum width of 21.9 mm with a maximum hardness of 929 HV, while a possible heat-affected zone in NiTi is limited between 1 and 2 mm over most of the weld.