Investigation on variations of microstructures and mechanical properties along thickness direction of friction stir processed AA2014 aluminum alloy via ultra-rapid cooling

Abstract Fine-grained AA2014 aluminum alloy was innovatively achieved by cold source assisted friction stir processing (CSA-FSP) under different cooling conditions. The investigation detailedly addresses the effects of cooling capacity induced by liquid CO2 on microstructural evolution and mechanical properties of the stirred zone (SZ) along thickness direction. The results indicated that FSP with liquid CO2 cooling can provide fine grain structures in the upper region of SZ due to higher strain rate, higher cooling efficiency, lower deformation temperature and shorter high temperature residence time. Besides the dominant CDRX and GDRX mechanisms in the entire SZ, the additional grain refinement mechanism of DDRX in the surface region while PSN in the bottom region was partially involved. The strength and microhardness of CSA-FSP 2014 aluminum alloy were enhanced as the cooling rate increased. The ultimate tensile strength in the upper region of SZ cooled with liquid CO2 achieved 519.7 MPa with a high elongation of 9.4%, which was 110.66% and 118.6% higher than that of the homogenized base alloy (246.7 MPa and 4.3%) due to the fine grain strengthening and second phase strengthening. The refined and retained second phase particles also can further enhance the grain refinement and dislocation accumulation effect, thus achieving an excellent combination of strength and plasticity. Meanwhile, the contribution of various strengthening mechanisms to the overall strength of CSA-FSP 2014 aluminum alloy was qualitatively evaluated