Temperature variation and influence on local mechanical properties assessed by nanoindentation in AA6061-AA7075 dissimilar FSW

The present study aims to elucidate the temperature advancement along and transverse to the weld direction during dissimilar friction stir welding (FSW) between AA6061-T6 and AA7075-T651 with two different tools. Temperature measurements by thermocouples near surface and inside the plates exhibited faster rise in temperature for the threaded with intermittent flat faces pin. Experimental measurements and finite element simulation shows asymmetric temperature distribution in-and-near the stir zone (SZ) with high temperature towards retreating side (AA7075) whereas at distant, temperature is slightly higher on advancing side (AA6061) owing to its higher thermal diffusivity. Additionally, asymmetricity is also noted under shoulder influence region, as well along the weld line with higher temperature towards the trailing side. The electron back scatter diffraction analysis shows regions experienced high temperature yielded high angle grain boundaries. Higher modulus and hardness for AA6061 in the recrystallized region is noted from nanoindentation tests as grain refinement along with the intense strain and dislocation pileup at grain boundaries prevailed the other strengthening mechanism. Recrystallized AA7075 within SZ demonstrated reduction in hardness with respect to base metal due to the dissolution of strengthening precipitates.