The heat treatment response of semisolid metal high pressure die cast Al–7Si–Mg alloys A356
and F357 was studied and compared. It was found that the heat treatment behaviour of alloy F357
is influenced markedly by the stability of the Mg containing π phase. This phase, which dissolves
in alloy A356 during solution treatment, persists in F357 and decreases the amount of magnesium
in solid solution. This is the likely origin of the decrease in the aging response of the F357 alloy. The tensile properties (yield strength and ultimate tensile strength) of the alloys correlate much better with the Mg concentration in solid solution than with the bulk Mg content of the alloys. The recently developed shortened T6 heat treatment cycles for rheocast A356 were tested on alloy F357. The optimum artificial aging treatment was determined to be 180uC for 4 h, regardless of the prior natural aging period.