Two millimetres thick Al–Mg–Si (6061Al-T6) alloy plates were friction stir welded at various welding conditions. Under a low rotation rate of 400 rev min–1 with rapid water cooling, the softening zone in the joint disappeared and a nanostructure with an average grain size of 80 nm was obtained in the stir zone (SZ). Therefore, a weld with equal strength to the parent metal (PM) was successfully achieved with the fracture occurring in the PM. Further, the average microhardness and ultimate tensile strength (UTS) of the SZs increased with the decreasing rotation rate and increasing cooling speed. The average microhardness and UTS of the SZ with nanostructure reached up to 134 HV and 505 MPa, respectively; though the initial strengthened precipitates disappeared. This work provides an effective strategy of achieving high property joints and enhancing the mechanical properties of precipitation-hardened Al alloys.
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