Microstructure evolution in the weld metal region of a Ni-based Inconel 718 superalloy produced by tungsten inert gas welding

The microstructure evolution of the weld metal(WM) region in a Ni-based 718 superalloy is discussed.The superalloy sheets were welded using the tungsten inert gas process at different heat inputs.The precipitates and dendrites in the WM of each joint were analyzed by optical microscopy,scanning electron microscopy,and energy dispersive spectroscopy(EDS).Statistics on the dendrite arm spacing and precipitates were obtained from the metallographs.Using the alloy composition determined by EDS and the phase diagram obtained using ThermoCalc,the equilibrium distribution coefficient of Nb,the temperature range of solid/liquid coexistence,and the distribution and morphology of Laves phase in different subregion of the WM were analyzed.There is lamellar segregation,regional segregation,and microsegregation in the WM.As the heat input increases,the arm spacing and the microsegregation increase.At a fixed heat input,the microsegregation rate is smallest in the crater,but with the macrosegregation seriously.