Microstructure-strength relationship in friction-stir welded 6061-T6 aluminum alloy

In this work, a structural response of 6061-T6 aluminum alloy to friction-stir welding (FSW) was studied in wide range of processing conditions in order to establish a better foundation for the microstructure-strength relationship in produced welds. In contrast to the widely-accepted conception of the FSW-induced precipitation behavior, the welding temperature was found to be often below the dissolution threshold and thus no particle dissolution took place. Moreover, the peak temperature never exceeded 500 degrees C, and therefore the particles never dissolved completely. Considering a relatively low cooling rate measured in the stir zone, the dissolved precipitates were suggested to partially re-precipitate as solute clusters during weld cooling cycle thus imparting a substantial hardening effect. Accordingly, the precipitation coarsening was deduced to be the major softening mechanism. Due to the extreme sensitivity of this process to a duration of the weld thermal cycle, the welding speed was surmised to be the key factor controlling weld strength.