Effect Of SiO₂ Addition Amount And Casting Processing Parameters On Microstructure And Properties Of ZL102 Alloy

Cast Al-Si alloy has excellent casting performance and corrosion resistance,which is the most widely used cast Al-Si alloy at present.However,for Al-Si alloys containing more than 6%Si,needle-like eutectic Si is commonly found,and coarse primary Si is often found in eutectic and hypereutectic Al-Si alloys.The microstructural characteristics will seriously affect the mechanical properties of Al-Si alloy and limit its application.SiO₂ is a common and cheap material.The addition of SiO₂ to Al-Si alloys can effectively change the microstructures and improve the mechanical properties of the materials.However,little research has been done on the effects of morphology,size and distribution of eutectic and primary Si by using SiO₂as modifier or refiner.Therefore,this study focuses on the effect of SiO₂ addition amount and casting process parameters on the structure and mechanical properties of ZL102 Al-Si alloy.The research have important reference significance and certain application value for the popularization and application of high performance Al-Si casting alloy.In this study,the alloy was prepared by gravity casting method.The microstructure and phase composition were analyzed by optical microscopy,scanning electron microscopy and X-ray diffractometry.The experimental results show that SiO₂ reacts with Al at 860°C to form?-Al₂O₃ as the non-uniform nucleation core of primary Si,which improves the nucleation rate of primary Si and refines the size of primary Si.The aluminum-rich state is formed at the interface between the primary Si and the melt.When the melt temperature is lowered to the eutectic point,Si and?-Al form a eutectic structure in a bridging manner,so that?-Al and eutectic Si are simultaneously refined.The size of eutectic Si decreased first and then increased with the addition of SiO₂ at 860°C for 1 h.The addition of SiO₂ was 2 wt.%,which was the best for Si phase refinement.The length of Si is 6.5um,and the aspect ratio is 3.8,the tensile strength of the alloy is 236MPa,and the elongation is 6.94%.Compared with ZL102 alloy,the length and aspect ratio of eutectic Si are reduced by 28.6%and16.8%,respectively.Strength and elongation increased by 40.9%and 66.83%,respectively.By changing the holding temperature,holding time,pouring temperature and stirring frequency,the influence of casting process parameters on the alloy structure was investigated.It was found that increasing the holding temperature,prolonging the holding time and stirring the melt at a certain frequency can uniform the composition and the temperature of the whole alloy melt.The?-Al dendritic structure is eliminated,but the interval time of stirring is too long to lose the homogenization effect,and the stirring will causes the aggregation of the Al₂O₃ particles,thereby losing the refining effect on the Si phase.?ZL102+2 wt.%SiO₂?alloy can change the morphology of eutectic Si in ZL102 alloy from needle like to fine-grained and short-rod when melt at860°C for 6h,and?-Al transforms from dendritic to plate-like,eliminating the dendritic structure and refines the size of primary Si and eutectic Si.At this time,the tensile strength of the alloy reaches 246.6 MPa,the elongation reaches 9.72%,and the fracture mode is dimple fracture.Pouring at high temperatures reduces the cooling rate and supercooling of the melt,resulting in the formation of a large amount of coarse?-Al dendritic structure.Due to the limitation of?-Al dendrites,the eutectic Si size is refined.