Preparation And Modification Of Hypereutectic Al-si Alloy Composite Modifier

Hypereutectic aluminum-silicon alloy has the advantages of low thermal expansion coefficient,excellent wear resistance and corrosion resistance,and is widely used in the field of automobile cylinder liners and engine pistons.However,the defects of such alloys before their modification have limited their application.At its root,the higher silicon content in the hypereutectic Al-Si alloy leads to the presence of coarse-grained primary silicon and needle-like eutectic silicon.The angular tip causes stress concentration,splitting the matrix,and reducing the mechanical properties of the alloy.The performance,and uneven dispersion of silicon as a hard phase reduces the machinability of the alloy.Therefore,over the years,the performance improvement of hypereutectic Al-Si alloys has focused on refining primary crystal and eutectic silicon in hypereutectic Al-Si alloys.In this paper,the modification and refinement effect of amorphous SiO₂ on Al-24Si alloy was studied.Three kinds of SiO₂ particles with different sizes and dispersions were prepared by direct ball milling and sol-gel method.Metallographic microscopes（OM）were used to compare the alloy microstructures before and after metamorphism.The results show that the spherical SiO₂ particles with a particle size of 500 nm have the best metamorphism effect on the Al-24Si alloy,and the average size of the primary silicon in the alloy structure is refined from 120μm to less than 60μm.The optimum process parameters of SiO₂ as Al-24Si alloy modifier were summarized:metamorphic temperature 830°C,holding time 10 min,and addition amount 0.6 wt.%.The modification mechanism of SiO₂ was analyzed by means of XRD,EDS and DSC.Finally,it was concluded that SiO₂ particles were chemically reacted with Al matrix after addition of eutectic aluminum-silicon alloy to form Si,which increased the number of heterogeneous cores,thus playing the role of refining the primary silicon.A new composite modifier P-SiO₂-RE was developed on the basis of SiO₂ with a particle size of 500 nm.By designing orthogonal experiments,the effects of the content of each of the three components in the composite modifier on the modification effect were studied,and the optimum addition amount of the three components was obtained:0.06 wt.%P,0.2 wt.%SiO₂,0.3 wt.%RE,the use amount of P modifier is greatly reduced,and the average size of primary silicon in the alloy structure after modification is refined to 20μm or less,and the eutectic silicon is refined into a dot shape by a needle shape,and the effect is better than that of P single component modification.At the same time,the hardness and wear resistance of the alloy and the modified alloy were tested.The results show that the hardness of the alloy after P-SiO₂-RE composite modification is slightly smaller than that of the alloy,and the wear amount is reduced by nearly 60%.,greatly improving the wear resistance of the alloy.