Effect of solidification cooling rate on the fatigue life of A356.2-T6 cast aluminium alloy

The effect of the cooling rate during solidification on the fatigue life of a cast aluminium alloy (A356.2-T6) is examined. The fatigue lives were determined for specimens removed from ingots with a gradient in cooling rates along their heights. Low- and high-cycle fatigue tests were conducted under both axial loading and reciprocating-bending conditions at a stress (strain) ratio (R) of -1.0, 0.1 and 0.2. Results show that the fatigue life decreases by a factor of three in low-cycle fatigue (R = -1.0) and by a factor of 100 in high-cycle fatigue (R = 0.1) as solidification cooling rate decreases from ∼10 to ∼0.3 Ks^-1, as indicated by measurements of the secondary dendrite arm spacings in the ingots. Fatigue cracks initiated from porosity in the material solidified at slower cooling rates. When pore size is below a critical size of ∼80 μm, as a result of increasing the cooling rate, the fatigue cracks initiated from near-surface eutectic-microconstituent. When present at or near the surface, large oxide inclusions initiated fatigue cracks.