Abstract: In order to analyze the effect of structure of hypereutectic Al-Si alloy on its machinability, the cutting test to two hypereutectic Al-Si alloys which have the same chemical composition but different structures is made and their cutting mechanism is studied. The results show that, after transformation the size, shape and distribution of primary silicon in hypereutectic Al-Si alloy have a serious effect on the tool life and the machined surface quality of the alloy, the alloy whose primary. Silicon is fine, spread and dull angular has a better machinability. It is considered as the main reason that the fine silicon is easier to be pressed into the matrix and to cooperate with the matrix deformation in the machining of alloy, but the coarse silicon breaks, drops out and displaces usually during the machining, which results in strenuous friction between the tool and the work and various defects in machined surface.

Key words: Hypereutectic Al-Si alloy Structure Machinability Tool wear Surface quality