Experimental investigations to find the effect of post weld heat treatment (PWHT) on the microstructure and mechanical properties of FSW dissimilar joints of AA2024-T351 and AA7075-T651

In this present study, a Friction stir weld (FSW) technique is applied to fabricate the FSW joints of aluminium alloy plates of 2024-T351 and 7075-T651 in butt-joint arrangement using a FSW tool of shape taper threaded cylindrical (TTC). The FSW joints are produced using constant FSW parameters of tool rotation speed (TRS) of 650 rpm, tool traverse speed (TTS) of 150 mm/min and tool plunge depth (TPD) of 6.20 mm. Tensile and microstructure specimens are removed from the weld joint in the direction perpendicular to the tool traverse and across the weld region. Initially, each of these specimens are heat treated to solution temperature (400, 420, 440, 460, 480, 500 °C) for 1 h and tested for tensile properties. The heat-treated microstructure samples are examined for the presence of abnormal grain growth (AGG) in the weld nugget zone (WNZ), thermally affected zone (TMAZ), heat affected zone (HAZ) of the weld region using stereo-zoom microscope and scanning electron microscope (SEM). Microstructure specimens subjected to solution heat treatment up to 480 °C are stable and reveals no AGG. However, for 500 °C the microstructures are not stable and the presence of AGG in the weld regions are observed. At 480 °C, the tested specimen shows homogeneity in hardness distribution across the weld regions. Thus, solution temperature of 480 °C is chosen as an optimizing solution temperature. Further, the tensile specimens are subjected to ageing temperature with constant temperature of 180 °C for various durations range 5, 8 and 12 h. Thus, these experimental investigations indicate that the specimen subjected to solution treatment at 480 °C followed by ageing temperature of 180 °C for 12 h show higher ultimate tensile strength (UTS) of 435 MPa and percentage increase of 6.8% compared to the specimens without heat treatment having UTS of 407 MPa.