Microstructure and mechanical properties of friction welded alloy 718

The present work deals with the effects of pre- and post-weld heat treatments on microstructure and mechanical properties of alloy 718 friction weld joints. Prior to friction welding, alloy 718 rods were subjected to two types of heat treatments - solution treatment (ST) and solution treatment followed by aging (STA). In the as-welded condition, samples welded with prior ST condition exhibited higher hardness in the weld zone compared to the base material in ST condition and this is attributed to grain refinement due to dynamic recrystallization. Samples welded with prior STA condition exhibited decreased hardness in the weld zone and room temperature tensile properties in relation to the base material in STA condition. It is due to dissolution of γ″ (Ni3Nb) phase as the temperature measured at weld zone was found to be 1118 °C. In order to retrieve the hardness and room temperature tensile properties, friction weld joints were subjected to direct aging (DA) treatment. A significant increase in hardness was observed in the weld zone after direct aging for both samples welded in ST and STA conditions. This is attributed to the grain refinement due to dynamic recrystallization and formation of precipitates by the DA treatment.