Improving local and global mechanical properties of friction stir welded thick AA7075-T6 joints by optimizing pin-tip profile. Issue 5 (February 2017) - Mao, Yuqing
Abstract Four tools with taper (TA), triangular (TB), three-grooves (TC), and square (TD) pin-tip profiles were designed to friction stir weld (FSW) thick AA7075-T6 alloy plates aiming at improving the local and overall mechanical properties of the joints. The microstructure evolution, hardness, and mechanical properties of the FSW joints were studied by scanning election microscope (SEM), X-ray diffractometer (XRD), and transmission electron microscope (TEM). The results show that equiaxed grains in the bottom nugget zone (BNZ) obtained by using TC are noticeably refined compared to in the case of other tools. Also, finer and more dispersive precipitates are distributed in this BNZ, and higher contents of η phase and dislocations are observed. A similar trend is found in the middle of weld. The bottom slices welded by employing TC show superior strength and ductility with a highest ultimate tensile strength of 388 MPa, yield strength of 315 MPa, and elongation of 7.9 %, which are in agreement with the hardness profiles. This is owing to ultrafine grains, more high-angle grain boundaries, more dispersive and higher contents of precipitates, and optimum hardness distribution. Accordingly, the global mechanical properties of the overall joints prepared by using TC significantly improve.