TIG Welding of Nickel-Titanium to 304 Stainless Steel

Shape memory nickel-titanium (NiTi) is attractive for use in solid-state actuators as it exhibits large recoverable stresses, limited by its ultimate shear strength of over 120 ksi (960 MPa), and large recoverable strains, up to 8%. Broad application of NiTi is hindered by the expense, complexity, and lack of reliability in machining and joining it to structural materials. This paper investigates the use of orbital Tungsten Inert Gas (TIG) welding to join NiTi to 304 stainless steel (304 SS), a common structural material that can be readily machined and welded. Tubes of NiTi and 304 SS were joined using a nickel filler to mitigate the formation of brittle intermetallics. Both tubes had a 0.375 in (9.53 mm) outer diameter with wall thicknesses of 0.065 in and 0.075 in (1.7 mm and 1.9 mm) for the 304 SS and NiTi tubes, respectively. Viable joints were created and characterized through X-ray analysis, optical microscopy, hardness mapping, and strength testing. The joints had an average failure torque of 450 in-lb (52 N-m), corresponding to an ultimate shear strength of approximately 50 ksi (350 MPa). This was sufficient to detwin the NiTi in the tubes, which occurs at a shear stress of 16 ksi (110 MPa), and plastically deform the annealed 304 SS tubes. Optical microscopy and hardness mapping revealed a heat-affected zone 0.005 in (125 μm) wide with a maximum hardness of 817 HV. Outside of this heat-affected zone the hardness was not affected, indicating that no large-scale loss of superelastic or shape memory properties arises from TIG welding.