Microstructural characterisation of a dissimilar weld of alloy D9 and AISI 316M stainless steel produced using a 2.5 kW CW CO 2 laser

The dissimilar weld, a fuel clad tube (D9 alloy) and an end plug made of AISI 316M forms a part of the fuel pin in the 500 MW Indian Prototype Fast Breeder Reactor (PFBR). The present study aimed at the development of a crack resistant microstructure of the dissimilar weld by optimisation of 2.5 kW CO 2 laser welding parameters. The D9 alloy, because of its low Cr eq /Ni eq ratio (close to 1), solidified first as primary austenite which makes its welds particularly susceptible to solidification cracking. In this study, the primary mode of solidification in the fusion zone was modulated by controlling the relative degree of fusion of the fuel clad tube and the end plug. A defect free weld with predominantly a primary ferrite mode of solidification was obtained by optimising laser parameters and displacing the focused laser beam towards the end plug side to achieve a greater contribution of end plug material to the fusion zone. The formation of a crater and associated defects at the termination site of laser welding were eliminated successfully by ramping the laser power during welding.