Microstructure and mechanical properties of friction stir welding joint of in-situ ZrB2/AA7085 composites

Abstract

Friction stir welding (FSW) has been widely used to join Al-Zn-Mg-Cu alloys. As the mechanical properties of the heat affected zone (HAZ) of Al-Zn-Mg-Cu alloys joints are affected by the additional welding thermal cycle, which always contributes to a weak HAZ. In this paper, the in-situ ZrB2 particle was used to optimize the mechanical properties of the HAZ of Al-Zn-Mg-Cu alloy joint. The effects of mass fraction of ZrB2 particle on the microstructure and tensile properties of in-situ ZrB2/AA7085 composites FSW joints were studied. The results showed that the coarsening of precipitates contributed to the softening of HAZ and declined the strength of AA7085 FSW joint to approximately 294.3 MPa. Small amount of in-situ ZrB2 (1 wt%) provided an additional strength of HAZ and increased the strength of the FSW joint to approximately 367.2 MPa. However, when the content of ZrB2 increased to 3 wt% and 5 wt%, many ZrB2-rich band structures were formed in stir zone. The brittle nature of the ZrB2-rich band structure resulted in the crack initiation and propagation in stir zone. It declined the strength of ZrB2 (3 wt% and 5 wt%)/AA7085 FSW joint to 308.3 MPa and 275.5 MPa, respectively.