The effect of tool rotation speed on the structure of the friction stir weld of Al-1.7mass percent Cu-1.0mass percent Mg-0.8mass percent Si alloy T4511 extrusions has been investigated. The increase of a rotation speed enlarged the grain size in the stir zone. A postweld heat treatment at 195 deg C for 8 hours hardly increased the hardness in the stir zone welded at the rotation speed of 1000rpm and raised the density of the coarse precipitates, Al_5Cu_6Mg_2 phase. At the rotation speed of 1500rpm, however, the postweld heat treatment produced the fine precipitates which had been already dissolved during the friction stir welding, and increased the hardness of the stir zone to the hardness of the base metal T6511.
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