Fusion welding of nickel–titanium and 304 stainless steel tubes: Part I: laser welding

Abstract

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 NiTi-based 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 μm with a maximum hardness of 929 HV, while a possible heat-affected zone in NiTi is limited between 1 and 2 μm over most of the weld.