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 det
ermined 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 (strai
n) ratio (R) of -1.0, 0.1 and 0.2. Results show that the fatigue life decre
ases by a factor of three in low-cycle fatigue (R = -1.0) and by a factor o
f 100 in high-cycle fatigue (R = 0.1) as solidification cooling rate decrea
ses from similar to 10 to similar to 0.3 K s(-1), as indicated by measureme
nts of the secondary dendrite arm spacings in the ingots. Fatigue cracks in
itiated from porosity in the material solidified at slower cooling rates. W
hen pore size is below a critical size of similar to 80 mu m, as a result o
f increasing the cooling rate, the fatigue cracks initiated from near-surfa
ce eutectic-microconstituent. When present at or near the surface, large ox
ide inclusions initiated fatigue cracks.