Creep-fatigue behavior of a friction stir welding 7050-T7451 aluminum alloy: Microstructure evolution and microscopic damage mechanisms

Abstract

Creep-fatigue behavior of a friction stir welding (FSW) Al-Zn-Mg-Cu alloy at different temperatures was investigated. The results show that creep damage is much higher than fatigue damage when a holding time is introduced at the peak load. After creep-fatigue, η’ and η phases are reprecipitated in the weld nugget zone (WNZ) and coarsen with increasing temperature. When the creep-fatigue temperature is below 175 °C, the predominant deformation mechanism is dislocation slip in the heat affected zone (HAZ). The Hall-Petch relationship prevails and thus the fine-grained WNZ is strengthened. The voids and microcracks nucleate at the interiors of coarse second phase particles or interfaces between the matrix and second phases. The FSW joints are fractured at the HAZ, characterized by the transgranular ductile fracture mode. Grain boundary ledges are observed in the WNZ at the creep-fatigue temperature of 200°C, suggesting the occurrence of grain boundary sliding. The plastic deformation of FSW joints is governed by GB-mediated deformation mechanism in the WNZ, indicating the inverse Hall-Petch behavior. Intergranular voids or microcracks initiate from grain boundaries with low values of the Schmid factor and geometric compatibility factor. The fracture mode of FSW joints is dominated by the intergranular fracture in the WNZ.