Abnormal fracture of 7085 high strength aluminum alloy thick plate joint via friction stir welding

Abstract

7xxx series high-strength aluminum alloy joints by friction stir welding (FSW) generally fracture in the heat affected zone (HAZ), i.e. the lowest hardness zone (LHZ) during tensile test. However, the experiment results of this study indicate that defect-free FSW joints of 7085-T7452 aluminum alloy thick plate fracture in the weld nugget zone (WNZ) instead of the HAZ for the whole joint or the top slice exhibits very low strength and elongation when using a high rotational rate of 600rpm. Both the microstructure characteristics and the microhardness distribution are not main reasons for the abnormal fracture in WNZ for the whole joint and the top slice. WNZ-top presents both the lowest average Taylor factor and the largest area fraction of grains with high strain. Moreover, the significant strain is concentrated in WNZ-top, showing the inhomogeneous deformation, for the whole joint or the top slice with the increase of nominal stress. But no strain concentration is presented in WNZ-middle and WNZ-bottom and the strain increases uniformly in HAZ for the middle and bottom slices. Combined actions of the above three factors lead to the characteristic of preferential intergranular fracture in WNZ for the whole joint and the top slice.