Effect of Ni addition on the microstructural evolution and lap-shear performance of AISI 630 martensitic stainless steel resistance spot welds

Abstract

Martensitic stainless steels exhibit poor resistance spot weldability, with high susceptibility to interfacial failure due to the formation of brittle martensite in the fusion zone (FZ). To alter the composition of the FZ and improve the lap-shear performance, Ni interlayer was employed during the resistance spot welding (RSW) of 2-mm-thick 630 martensitic stainless steel (MSS). The microstructures of the joints with and without Ni addition were characterized using optical microscopy and EBSD analysis. Calculation of phase diagrams (CALPHAD) technique was used to analyze the phase evolution in the joints as a function of temperature. The FZ of the joint without Ni addition consisted of predominantly martensite with small amount of dispersed retained austenite. Under lap-shear loading, the crack propagated through the martensitic FZ, leading to interfacial failure (IF), irrespective of the welding time or current. The joints could only reach a peak load of 8.26 kN and energy absorption of 6.83 J. The Ni addition changed the solidification mode from ferritic-austenitic to fully austenitic mode and significantly lowered the Ms. temperature (from 108 to −188.7 ° C). Accordingly, the Ni-added joints exhibited fully austenitic microstructure and failed largely through pullout mode with significantly improved energy absorption. They exhibited a peak load of 13.27 kN and energy absorption of 27.22 J.