Study On The Solidification Structures Of Hypereutectic Al-Si Alloy

Hypereutectic Al-Si alloy possesses low density, low thermal expansion coefficient, good wear resistance and dimensional stability. Therefore it has a promising prospect as automotive parts such as cylinder liner. However, hypereutectic Al-Si alloy castings always includes coarse primary silicon and serious primary Si segregation, thus its mechanical properties has been greatly reduced. This paper studies the effects of cooling rate and casting temperature on the solidification structures of Al-15Si, Al-17Si, Al-20Si, A390alloys. Meanwhile, the effects of P modifier and Cr addition on solidification structure of Al-20Si alloy are also discussed in this paper. Combined with cooling rate obtained by in-site temperature measurement and microstructure, the influences of different content of Si, Mg and Cu addition, P modifier, Cr addition on macrosegregation, morphology and size of Primary Si are investigated. The conclusions are as followings:(1) In terms of Al-15Si and Al-17Si alloys, comparing the samples casted at800℃,850℃and900℃, the macrosegregation of primary silicon of the sample casted at850℃are the mildest. While for Al-20Si alloy, the macrosegregation of primary silicon are obvious with all of the three casting temperature; the casting temperature have little effect on size and morphology of primary silicon particle in Al-XSi alloys.(2) With the increase of the distance from the tip of wedge-shaped castings, the cooling rate between mushy zone decreases; Cooling rate at the same height decreasing with increasing casting temperature; The size of Primary Si follows the rule that it remains constant at some range of cooling rate, otherwise it decrease as increasing cooling rate.(3) The morphologies of primary Si are plate-like and octahedral-like with corners in Al-15Si alloy and Al-17Si alloy; Complex morphologies appear in Al-20Si alloy such as bulky five star-like.(4) With the addition of Mg and Cu elements, the phenomenon of macrosegregation and size of Primary Si reduced in A390alloy.(5) The phenomenon of macrosegregation of Primary Si markedly reduced in Al-20Si alloy with the addition of P modifier and Cr element. P modifier results in five star-like Primary Si disappearing in the microstructures. New phase CrSi2appeared in the Al-20Si alloy with the addition of Cr element and five star-like Primary Si disappeares in the microstructure. The modification of Primary Si resulted from Cr addition is not as good as that results from P modifier.