Microstructure and tensile-shear behavior of dissimilar resistance spot welds between 2304 duplex stainless steel and Inconel 718 superalloy

Abstract

Dissimilar joints between duplex stainless steels and nickel-based superalloys are of high significance in some industries. In this research, 2304 duplex stainless steel and Inconel 718 superalloy were welded by the resistance spot welding technique. The studies showed that the fusion zone microstructure was composed of a dendritic structure related to the grains of nickel-based solid solution ( γ-phase) and γ′ islands. The heat-affected zone microstructure of 2304 duplex stainless steel includes the austenite phase with a slight volume fraction and the ferrite phase with a high volume fraction. The austenite phase was formed with two morphologies of Widmanstätten and grain boundary allotriomorphs. The heat-affected zone microstructure of Inconel 718 superalloy also contained γ phase (nickel-based solid solution), annealing twins, and some precipitates. The results of the tensile-shear test revealed that tensile-shear strength (i.e. peak load) and fracture energy were increased by increasing the welding current from 1 to 2 kA and the welding time from 1 to 2 s. However, they were reduced by increasing the welding current from 2 to 4 kA and the welding time from 2 to 4 s. This was caused by the surface splash between the electrodes and the sheets. In the above-mentioned range, an increase in the welding current and time may also intensify the surface splash. It was also found that when the welding time was constant (i.e. 2 s), the spot weld obtained at the welding current of 1 kA failed by interfacial fracture mode. All spot welds obtained at the welding currents of 2, 3, and 4 kA failed by pull-out fracture mode. On the other hand, all spot welds associated with the welding times of 1, 2, 3, and 4 s failed by PF mode when the welding current was constant (i.e. 3 kA).