Abstract: An aluminium based alloy has a weight percentage composition of from 5 to 15% silicon, 0 to 0.25% magnesium, 0 to 0.25% titanium, 0.2 to 0.65% manganese, 0.1 to 0.6% iron, 1 to 4% copper, 0 to 3% zinc, less than 0.01% in total of silicon modifiers (with less than 0.007% strontium), less than 0.05% tin, less than 0.2% in total of other transition or rare earth metals (with less than 0.05% chromium), less than 0.5% in total other elements, and a balance of aluminium. The limits for iron and manganese are constrained such that the amount of iron present in the alloy is 0.4 to 1.6 times the manganese content and the alloy has a sludge factor (SF), calculated as SF=(1×wt % Fe)+(2×wt % Mn), of from 0.8 to 1.6. A casting made from the alloy has enhanced fracture resistance relative to casting of the same product made of a conventional HPDC alloy when compared in the as cast or same heat treated state.