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

Abstract

Post weld heat treated AA6061-T6 alloy resulted from the application of a Cu donor stir assisted (CDSA) friction stir welding (FSW) material was examined for crystal structure and mechanical properties. CDSA FSW samples were tested at a constant tool rotational speed of 1400 rpm and a welding translational speed of 1 mm/s. CDSA samples of 20% and 60% thickness of the AA6061-T6 base alloy were selected to assist the FSW joining at the plunge stage. The FSW AA6061-T6 samples were solid solution treated at 540 °C for one hour, followed by quenching in water at room temperature. The samples were then artificially aged at 180 °C for 6 hours, respectively, followed by air cooling. The samples were tested for microstructure, crystal structure, chemical composition, and mechanical properties using optical microscopy, scanning electron microscopy, X-ray diffraction, and nanoindentation. The microstructure shows the additional grain refinement in the stir zone (SZ) due to recovery and recrystallization with increasing aging time. Examination of the chemical contents of the FSW AA6061-T6 alloy samples using scanning electron microscopy with energy dispersive spectroscopy (EDS) revealed Al (parent material) as the predominant element, while Cu (CDSA) was minimally present as expected. XRD results of the CDSA FSW samples depicted crystal orientations similar to the orientations of the AA6061-T6 alloy. Nanoindentation tests revealed softening effects due to the dissolution of hardening precipitates at the SZ. The hardness of the base metal (BM), left and right regions, is reported as ~ 6.5 GPa, whereas at the SZ, the hardness is ~ 5.5 GPa at a depth of indentation of 4.7 µm.