Process optimization and thermodynamic modeling of weld zone in nickel powder-added dissimilar pulsed-laser NiTi/ASS joint

Abstract

Dissimilar joining of NiTi alloys to stainless steel has attracted abundant consideration. In this research, pure nickel powder diluted in PVA adhesive was added to the joint area with different concentrations to eliminate or minimize the brittle intermetallic compounds. Also, thermodynamic concepts of phase stability employing the quaternary CALPHAD method were used to calculate the weld zone's phase stability. The results indicated that laser welding combined with 30% nickel powder addition did not have a noteworthy effect on detracting the content of Fe2Ti in the weld zone. In contrast, adding nickel powder with a concentration of 50% reformed the chemical composition and changed the Fe2Ti primary solidifying phase to more ductile phases of Ni3Ti and γ. The results of equilibrium thermodynamic calculations also predicted that Ni3Ti, γ, and B2-(Fe,Ni)Ti phases are possible to form in this condition. In laser welding with 70% nickel powder, unmixed nickel areas and gas porosities were observed in the weld zone. The superlative mechanical properties were obtained for the samples with 50% nickel powder addition. The microhardness results revealed a diminution of the weld zone's average hardness from 667 HV (without nickel powder addition) to 325 HV (50% nickel powder addition). Evaluation of joint strength revealed the twofold increase in 50% nickel powder added joints compared with conventional samples; the fracture strength and fracture strain reached 310 MPa and 2.9%, respectively. Therefore, Ni powder addition is an effective process for improving the mechanical properties of dissimilar joints of these two alloys.