Strength mismatch mechanism on friction stir welding joint of 7075 aluminum alloy

The precipitated phases in the WNZ, TMAZ, HAZ and BM of the friction stir welding（FSW） joint were observed using the transmission electron microscopy（TEM） and the lattice fringe spacing of the precipitated phases was measured. Combined with X-ray diffraction（XRD）, the types of precipitated phases among the joint were confirmed and then the strength mismatch mechanism was revealed.The results show the precipitated phases of 7075 aluminum alloy FSW joint mainly consist of MgZn;, AlCuMg and Al;CuMg. The microzone of the joint experienced different thermal cycles, the types and sizes of precipitated phases are different and the strengthening effect is different. The strengthening effect of the AlCuMg and Al;CuMg are better than that of MgZn;. The precipitated phase in the WNZ mainly includes AlCuMg and Al;CuMg, as well as the grain size is fine, the microhardness in this zone is pretty high. The number of precipitated phase AlCuMg and Al;CuMg is smaller in the TMAZ and the MgZn;is relatively more, which lead the microhardness decrease. The number of precipitated phase MgZn;is relative larger in the HAZ, as well as the grain coarsening, the microhardness in this zone is lowest of the joint. At the same time, there are the precipitate free zones（PFZ） among the 7075 aluminum alloy FSW joint, which decreases the microhardness of the whole joint to some extent.