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Six-millimeter-thick 6061Al-T651 plates were friction stir welded with varied welding parameters and tool dimensions. The low hardness zones (LHZs), determined by constructing the hardness distribution maps around the heat-affected zones, changed the location and inclination with the welding conditions, but the hardness values of the LHZs were mainly dependent on the welding speed. The thermal cycles experienced by the LHZs had approximately the same peak temperature of 360°C to 370°C with varied durations that were governed by the welding speed. The microstructure of the LHZs was characterized by a low density of b¢ precipitates, which tended to reduce with increasing the welding speed. The fracture of the welds occurred along the LHZs, and the tensile strength of the welds increased with increasing the welding speed and was independent of the tool dimension and the rotation rate. A heat source zone-isothermal dissolution layer model was proposed to explain the observed effects.

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