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 °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.