Microstructure dependence of fatigue life for A356.2

Fatigue life of a cast aluminum alloy (A356.2) was measured in specimens removed from ingots with a gradient of microstructures. The variation of microstructures in an ingot, quantified by the secondary dendrite arm spacing (DAS), was controlled by varying the cooling rate along the ingot height during solidification. Low- and high-cycle, completely reversal fatigue tests were conducted under both axial and reciprocating bending loading conditions. Experimental results indicated that secondary dendrite arm spacing was closely related to the total fatigue life which decreased at least three times as DAS increased from 15 μm to 50 μm. Fatigue cracks initiated from near-surface eutectic regions in microstructures with fine DAS, whereas porosity became the dominate crack initiation site for microstructures with large DAS. Secondary cracks interacted with the dominant fatigue crack under both low- and high-cycle fatigue.