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The effects of on-line solution, off-line solution and aging heat treatment on microstructure and hardness of AZ91D die-casting alloy were investigated.Brinell hardness of die casting AZ91D alloy increases through on-line solution and off-line aging treatment but decreases after off-line solution treatment.By means of X-ray diffractometer, optical microscopy, differential thermal analysis, scanning electron microscope and X-ray energy dispersive spectroscopy, it is found that the microstructure of on-line solution and off-line aging AZ91D magnesium alloy die casting is similar to that of die casting AZ91D alloy that consist of α-Mg and β-Al12Mgl7.The precipitation of Al element is prevented by on-line solution during solidification so that the effect of solid solution strengthening is enhanced.The β-Al12Mgl 7 phases dissolve in substructure and then precipitate along grain boundary after off-line aging treatment, and lead to microstructure refinement of AZ91D alloy so the effect of precipitation hardening is enhanced.The β-All 2Mg 17 phases dissolve in substructure after off-line solution treatment and lead to grain boundary strengthening phase is reduced significantly and the hardness of die casting AZ91D is reduced.