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 K s−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.