Effect of post-weld heat treatments on microstructure and mechanical properties of friction welded alloy 718 joints

Abstract

The effect of post-weld heat treatments on the microstructure and mechanical properties of friction welded joints of alloy 718 was studied in the present work. Alloy 718 rods were friction welded with two prior heat treatments – solution treatment and solution treatment and aging. Solution treatment was done at 995°C for 1h. Aging was done at 720°C for 8h followed by furnace cooling to 620°C and holding at 620°C for 8h followed by air cooling. After friction welding, the joint samples were subjected to two types of post-weld heat treatments – direct aging (aging after welding, the same aging treatment mentioned above) and solution treatment and aging. Electron back scattered diffraction technique and transmission electron microscopy were used to study the development of microstructure. Hardness and tensile properties of the weld joints were evaluated. In the as-welded condition, samples welded with prior solution treatment and aging condition exhibited lower hardness at the weld zone and inferior tensile properties compared to the base material due to the dissolution of strengthening precipitates in the weld zone. On the other hand, formation of fine grains due to dynamic recrystallization led to higher hardness at the weld zone compared to the base material welded with prior solution treatment condition. Solution treatment and aging post-weld heat treatment resulted in an abnormal grain growth in the weld zone and thermomechanically affected zone. Owing to the formation of strengthening precipitates, solution treatment and aging post-weld heat treatment resulted in a significant increase in tensile strength of joint samples compared to that of as-welded friction weld joints. However, solution treatment and aging post-weld heat treatment done on friction weld joint samples with prior solution treatment or solution treatment and aging heat treatment condition resulted in inferior tensile properties compared to those of samples subjected to direct aging post-weld heat treatment. This may be attributed to grain coarsening that occurred during the post-weld solution treatment. Therefore, direct aging after welding is the recommended post-weld heat treatment for friction welded alloy 718 joints as compared to solution treatment and aging after welding.