Effect Of Oxide Complex Modification On Hypereutectic Al-Si Alloy

Hypereutectic aluminum-silicon alloys are widely used in the automotive industry,such as the manufacture of pistons and engine shells,due to their advantages of good wear resistance,low coefficient of thermal expansion,and low specific gravity.However,there are a large amount of five-lobed star-shaped,plate-shaped primary crystal silicon and long needle-shaped eutectic silicon in the unmodified hypereutectic aluminum-silicon alloy structure,which damage the Al alloy matrix to a certain extent and weakened the overall mechanics of the alloy The performance above restricts the industrial application of hypereutectic aluminum-silicon alloy.Therefore,scientific researchers have studied for many years to adopt reasonable means to modify and refine the silicon phase structure of the hypereutectic aluminum-silicon alloy to improve the performance of the alloy.In this paper,the metallographic microscope(OM),X-ray diffractometer(XRD),scanning electron microscope(SEM),hardness tester and wear friction test machine and other test methods were used to study the effects of hollow SiO₂ and Al(NO₃)₃to modification and refinement of primary silicon in 24wt.%Si alloy and the influence of casting process conditions on the hardness and wear resistance of the alloy.The results show that the modifier of SiO₂@PS spheres as the hollow SiO₂precursor has a significant refinement effect on the primary silicon in the Al-24Si alloy.Orthogonal experimental study on the modification and refinement of Al-24wt.%Si alloy by using 5?m and 10?m SiO₂@PS spheres prepared by the template method found that SiO₂@PS spheres with a particle size of 5?m can reduce Al-24wt.%Si alloy,which is refined primary silicon from 110?m to less than 50?m;Meanwhile,SiO₂@PS spheres with a particle size of 10?m can refine the primary crystal silicon of the Al-24 wt.%Si alloy to below 55?m.The modification effect of both is equivalent.The experiment determined that the best modification process parameters of SiO₂@PS sphere for Al-24 wt.%Si alloy are:modification temperature 830?,holding time 15 min,addition amount 0.6wt.%.According to the analysis,When SiO₂@PS spherical particles were added to the hypereutectic Al-Si alloy melt and reacted with the Al matrix to generate Si and in-situ Al₂O₃.Al₂O₃ and Si have a cubic structure and have a small lattice mismatch of about 3%.The Al₂O₃ particles generated in situ and the aluminum alloy matrix have good wettability with Al alloy melt and good dispersibility.Therefore,both Si and Al₂O₃generated in situ can become the core of the primary silicon nucleation,increasing the number of silicon phase nucleation cores,thereby playing the role of refining the silicon phase.That is,the in-situ Al₂O₃ modification of the silicon phase in the hypereutectic aluminum-silicon alloy conform to the heterogeneous nucleation mechanism.In order to further optimize the modification and refinement effect on the silicon phase,a new complex modification agent SiO₂@PS-Al(NO₃)₃ was designed and studied in this paper.And the best addition amount of modification agent was:0.5wt.%SiO₂@PS sphere+0.04 wt.%Al(NO₃)₃.After complex modification,the average size of primary silicon in the alloy structure is refined from 110?m to below 32?m.It shows that the compex modification effect is better than the single SiO₂@PS sphere modification effect.Through friction and wear experiments,it was found that the wear resistance of the alloy was significantly improved by SiO₂@PS-Al(NO₃)₃complex modification.Compared with the unmodified alloy,the wear quality was reduced about 35%.