Effect of Post Weld Heat Treatment on Donor Material Assisted Friction Stir Welding of AA6061-T6 Alloy on Microstructure and Mechanical Properties

In this study, the influence of post weld heat treatment (PWHT) on the microstructure and mechanical properties of a copper (Cu) donor material assisted friction stir welding (FSW) of AA6061-T6 aluminum alloy has been investigated. Cu assisted FSW joints of AA6061-T6 alloy were prepared at an optimized constant tool rotational rate of 1400 rpm and welding speed at 1 mm/s. The Cu donor material of 20% thickness with respect to the workpiece thickness was selected to assist the FSW joining at the plunge stage. FSWed AA6061-T6 samples were prepared using solid solution treatment at 540 degrees C for 1 hour followed by quenching in water at room temperature. It was then artificially aged at 180 degrees C for 6 hours and 24 hours followed by air cooling. The microstructure and tensile fractured surfaces were analyzed using scanning electron microscope and optical microscope. The microstructure depicts an additional grain refinement in the stir zone (SZ) due to the occurrence of recovery and recrystallization with increasing aging time. Vickers micro hardness indicates a softening effect due to the dissolution of hardening precipitates. Hardness recovery is most likely attributed to the uniform distribution of fine hardening precipitates at all aging time levels. The maximum hardness was 92.5 HV at the SZ and the tensile properties were significantly improved by 20 % after solution heat treated at 540 degrees C for 1 hour followed by artificial aging at 180 degrees C for 24 hours.