A TEM study of precipitation and related microstructures in friction-stir-welded 6061 aluminium

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Residual microstructures, including dynamic recrystallization and grain growth structures and a wide range of precipitation phenomena associated with a friction-stir-weld in a thin 6061-T6 aluminium plate have been systematically investigated utilizing light metallography and transmission electron microscopy. In this rather remarkable process, a hard steel head pin rotating at 400 r.p.m. was advanced into a solid 6061-aluminium plate at a traverse velocity of approximately 2 mm s−1 to produce a solid-phase weld in its trailing side. Maximum work-piece temperatures did not exceed 425°C and there was no melt evidence. Dynamic recrystallization associated with the solid-state plastic flow therefore seems to provide the process mechanism. Weld zone hardnesses averaged roughly 55 Vickers hardness number (VHN) in contrast to the base plate or work-piece hardness of 110 VHN. Precipitation microstructures ranged from continuous to discontinuous coherent zones (∼2 nm thick) coincident with {1 0 0} planes, semicoherent and non-coherent needles and plates characteristic of Widmanstatten structures coincident with {1 1 0} planes, and a range of homogeneous precipitate particles often intermixed with these microstructures in the effective heat-affected zone (HAZ′) connecting the friction-stir-weld zone with the unaltered work piece microstructures.

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