Inherent friction stir welding-induced precipitate evolution and its detrimental role on fatigue damage and premature fatigue failure of dissimilar high-strength aluminum alloys joints

Friction Stir Welding (FSW) joint is dominated by fatigue failure during performance. Despite massive research on evaluating its fatigue life, its fundamental mechanism of fatigue damage keeps poorly understood. This work proposed an investigation integrating low cycle fatigue test, and microscopic analysis on precipitates evolution under thermal cycle and dislocations interaction during cyclic loading to unravel the precipitate evolution-driven fatigue damage mechanisms of dissimilar AA7050-AA2024 FSW joint. Results demonstrate the precipitates evolve from the fine and coherent Guinier-Preston-Bagaryatsky (GPB) zones to the coarse and incoherent Al 2 CuMg precipitates in Heat Affected Zone (HAZ) under FSW thermal cycle. This microscopic precipitate evolution promotes dislocation bypassing precipitates and cross slip under cyclic loading, lowering fatigue damage capacity and intensifying irreversible fatigue damage. The correlation among FSW thermal cycle, precipitate evolution, and fatigue damage was built, thereby providing a new insight on intrinsic poor fatigue performance of joints and a guideline for targeted fatigue performance optimization strategy.