Study of mechanical behaviour and microsegregation in interdendritic region of a single-pass plasma arc welding of thick IN625 plate

Abstract

The ingenuity of the work lies in joining of 6.18 mm thick IN625 plate with single-pass plasma arc welding without any weld groove preparation and without any filler wire. Two welding speeds (100, 120 mm/min) and welding currents ranging from 115 to 145 A at 5 A intervals were used. The weld joint metallurgy was characterised using field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, etc., to establish a connection with the mechanical performance. The micrograph of the fusion zone exhibited a variety of dendritic structures in terms of size and shape, leading to non-uniform true strains across the weld cross-section. The specimen with the smallest fusion zone area attained a maximum joint strength of 831.8 \(\pm\) 12 MPa, which is 96% of the BM strength. The Laves phase formation and the interdendritic microsegregation were particularly noticeable within the fusion zone and contributed to the joint strength reduction. The fusion zone exhibited the lowest microhardness compared to other zones due to the supersaturation of strengthening elements, namely Nb and Mo in the γ-matrix along with Laves phase developed within the weldment. The partition coefficients in the dendritic core of the Nb and Mo elements were less than the one that leads to microsegregation in the interdendritic region. The fracture surface cross-section view showed that the micro-cracks were initiated during the tensile deformation process in the Laves phase. The fracture surface consisted of shallow dimples, which were attributed to low ductile features.