Strengthening mechanism of high-pressure linear friction welded AA7075-T6 joint

Abstract

The high-pressure linear friction welding of AA7075-T6 alloy was performed to investigate the relationship between the microstructures and mechanical properties of the joints. It was found that the joint obtained by the high-pressure linear friction welding of 470 MPa exhibited a complete flat hardness distribution across the welding line by maintaining precipitation strengthening. The precipitates formed in the weld regions of the AA7075-T6 joints are mainly composed of MgZn2 phase and Cr-bearing dispersoids. At the low pressure of 60 MPa, the volume fraction of both MgZn2 phase and Cr-bearing dispersoids at the weld center significantly decreased due to the relatively high welding temperature. Consequently, the softening occurred at the weld interface of the joint fabricated at low pressure even though the equiaxed grains were refined due to the DRX, thus the joint efficiency was approximately 58%. On the other hand, at high pressure (i.e. low welding temperature), the volume fraction of the Cr-bearing dispersoids was maintained even though the volume fraction of the MgZn2 phase was reduced, due to the high thermal stability of the Cr-bearing dispersoids. Therefore, a sound joint with superior mechanical properties, which is equivalent to the base material, was obtained since the effect of the precipitation strengthening in the weld regions was maintained in the high-pressure linear friction welded AA7075-T6 joint.