Fatigue crack growth behavior and mechanism of AlSi7Mg alloy prepared by laser powder bed fusion

Laser powder bed fusion (LPBF) fabricated AlSi7Mg alloy possesses refined microstructure that provide enhanced strength and toughness, yet its fatigue crack growth (FCG) behavior during service remains insufficiently understood. This study employs multi-scale characterization methods to investigate the FCG behavior and underlying mechanisms in LPBFed AlSi7Mg alloy with crack propagation directions both parallel and perpendicular to the building direction (BD). The results demonstrate pronounced anisotropy in the FCG response of LPBFed AlSi7Mg alloy. Under cyclic loading, crack growth with the propagation direction perpendicular to the BD remains governed by microstructural constraints. In contrast, specimens with crack propagation parallel to the BD exhibit a transition in crack growth behavior from microstructural to microstructural dominance. This shift in the governing mechanism leads to a corresponding change in crack growth rates. Furthermore, the observed anisotropy primarily originates from the distinct dislocation density evolution between the two specimen orientations. In specimens with crack growth perpendicular to the BD, high-density dislocation tangles form a barrier network during early propagation stages. With continued cycling, dislocations accumulate at grain boundaries, intensifying local stress concentrations and ultimately accelerating crack advancement. Conversely, in specimens with crack growth parallel to the BD, dislocations initially shear through Si precipitates, creating low-resistance paths. During later stages, however, the formation of dislocation walls promotes crack tip blunting, driving the transition from microstructure-controlled to plasticity-dominated crack growth mechanism.