Research On Complex Modification On Hypereutectic Al-Si Alloy By Novel AlP-Al₂O₃ Master Alloy

Hypereutectic Al-Si alloy is an important cast aluminum alloy.Due to its low density,high specific strength,good wear resistance and excellent casting performance,it is widely used in automobile manufacturing,aerospace,marine,and chemical industry.However,under the traditional casting conditions,hypereutectic Al-Si alloy has coarse plate-like primary silicon and long needle-like eutectic silicon,which severely separates the alloy matrix and significantly degrade the mechanical properties of the alloy.Therefore,the alloy must be modified before use.In this paper,a novel AIP-Al2O3 master alloy was prepared as the modifier of the Si phase in hypereutectic Al-Si alloy.The modification effects of the modifier and traditional refiner AlP on primary and eutectic Si of Al-30Si were compared,and the modification mechanism of the modifier was explained theoretically.The mechanical properties and thermal expansion coefficient of the modified Al-30Si alloy were experimented,the effects of the morphology and size of Si phase on the properties of the alloy were investigated.The results show that a large number of γ-Al2O3 particles are dispersed in the matrix of the novel AIP-Al2O3 master alloy prepared in this experiment,and its average size is 1.41 μm.The modifier has a significant effect on the modification of the primary Si of the Al-30Si alloy.When the modifier is added in an amount of 0.5 wt%,the size of primary Si is refined from 256 m to 27 m,and the refining efficiency is as high as 89%,which is more remarkable than the effect of AlP modifier.Moreover,the modifier can simultaneously modify the long needle-like eutectic Si in the alloy into a fibrous,short rod and granular shape,and the eutectic silicon size is reduced from 31.7 m to 2.5 m,with a decrease of 92%.At the same time,the γ-Al2O3 in the modifier can form a double-layer composite structure with AlP,which improves the nucleation efficiency of AlP and produces more significant modification effect on the primary Si.The AlP-Al2O3 master alloy can inhibit the formation of porosities in the alloy while modify the Si phase of the Al-30Si alloy.The porosity of the Al-30Si alloy with 0.5 wt.%APO is reduced from 2.51%to 0.81%.With the refinement and modification of the Si phase,the number of silicon particles in the alloy increases,and the grain boundaries and phase boundaries increase,which can hinder the dislocation motion and improve the mechanical properties of the alloy.With 0.5wt.%APO addition,the hardness,ultimate tensile strength and elongation of the alloy were increased from 54.6 HV,24 MPa,and 0.05%to 113 HV,116 MPa,and 0.97%,respectively.The unmodified alloy has a lower coefficient of thermal expansion due to higher porosity,which is shown as 19.2×10-6/℃.With 0.5 wt.%APO addition,the alloy has a lower thermal expansion coefficient of the alloy due to the refinement and modification of the Si phase,which is shown as 19.2×10-6/℃.In the theoretical model for calculating the thermal expansion coefficient,the calculation results of the Turner model agree well with the theoretical values.