Propagation of short fatigue cracks in permanent and semi-solid mold 357 aluminum alloy

The high cycle fatigue strengths of aluminum–silicon–magnesium 357 alloy prepared by permanent molding and semi-solid molding are measured. Results show that precipitation hardened permanent mold (PM) and semi-solid mold (SSM) materials have fatigue strengths at 107 cycles of 82 MPa and 113 MPa respectively at R = −1, and of 47 MPa and 78 MPa respectively at R = 0.1. For all tested specimens, the propagation of short cracks occupies a significant portion of the fatigue life. Observation of many short cracks using the replication technique indicates that grain boundaries are important microstructural barriers to short crack propagation in these materials. It is proposed that the fatigue strength at 107 cycles of the studied materials is the stress amplitude under which a short crack will be stopped by the first grain boundary encountered. In the absence of defects bigger than the grain size, the average fatigue strength is inversely proportional to the square root of the average grain size.