Study On Preparation And Properties Of A356.2 Al Matrix Composites Reinforced With Silicon Carbide For Automotive Wheel Hubs

With the development of modern science and technology in the aerospace,missile systems,automobile and other industries,the requirements for materials are becoming higher and higher.Especially,the aluminum matrix composites?AMC?are favourable among them due to their high specific strength,high specific modulus,low expansion coefficient and good dimensional stability.The SiC/A356.2 aluminum alloy as the research object were studied in this article.Firstly the surface modification of sub-micron silicon carbide was carried out by heterogeneous precipitation.Next the effect of hydrofluoric acid content,copper ion content,zinc powder addition rate and silicon carbide particle size on the morphology of copper plating on the surface of silicon carbide are systematically studied.Then SiC/A356.2 aluminum matrix composites were prepared by semi-solid stirring process.In order to obtain SiC/A356.2 aluminum matrix composites with excellent comprehensive properties and to provide a theoretical and practical basis for the wide application of A356.2,the effects of silicon carbide copper plating morphology on the static tensile properties,impact properties,hardness,aging hardening and corrosion resistance of the composites were studied.Also the correlation between microstructure and properties of aluminum matrix composites,the mechanism of heterogeneous nucleation and the law of aging hardening are discussed.The results are as follows:In this experiment,hydrofluoric acid pickling increases the surface roughness of silicon carbide.And as the content of hydrofluoric acid increasing,its roughness also increases which is conducive to the adhesion of copper ions.When the addition rate up to 3.2g/s,there appears spontaneous nucleation of dendritic copper ions in the solution.What's more the structure of copper coated with silicon carbide can be successfully prepared when the addition rate is 0.32g/s.The size of silicon carbide particle size also affects the morphology of silicon carbide copper plating.After the silicon carbide with50nm particle size is cleaned by 20%HF solution,the roughness at the edge of silicon carbide is higher than that on the surface of silicon carbide,which makes the copper ion preferentially wrap in the edge of silicon carbide.Because the size of 1?m silicon carbide is too large,copper ion can not be completely encapsulated but the particle size of 500 nm of silicon carbide can be uniformly encapsulated by copper ion after 20%HF pickling.The tensile strength and yield strength of SiC/A356.2 increased firstly and then decreased and reach 365.6MPa and 283.9MPa respectively when added 0.5wt.%SiC,increased by 40.6%and 19.5%compared with the A356.2 without adding SiC.At the same time,the impact energy and impact toughness increased by 40.00%and 47.91%respectively,and their values reached 2.8J and 3.89J/cm² respectively.The tensile fracture mode of A356.2 aluminum matrix composites is a quasi-cleavage fracture.With the addition of silicon carbide,the dimples gradually become shallow and the density of dimples also decreases.And the impact fracture mode of A356.2 aluminum matrix composites is the combination of plastic fracture and brittle fracture.It was found that the addition of silicon carbide can refine the aluminum phase and the silicon phase so that the arrangement of the impact dimples changes from an irregular shape to a more regular strip dimple.With the increase of the amount of silicon,the silicon phase become more and more uniformly dispersed in the?-Al matrix This is due to the fact that the silicon phase tends to two-dimensional nucleation mechanisms,it easy to form coherent or semi-coherent planes on the surface of silicon carbide.Silicon carbide has greater thermal conductivity than silicon,providing a certain degree of nucleation for the silicon phase,silicon phase preferentially form nucleation and growth on silicon carbide.The more equally distributed of silicon carbide,the more equally distributed of the silicon phase.When the silicon phase segregated again when added 0.7wt.%and 1.0wt.%silicon carbide.The addition of silicon carbide shortens the peak aging time of SiC/A356.2aluminum matrix composites and accelerates the aging hardening rate.The aging hardening curve shows that the peak aging of 0.5wt.%SiC/A356.2 aluminum matrix composite is at 180?/4h,and the hardness up to 118.2 HB,increased by 14.8%compared with the 0.5wt.%SiC/A356.2 solution.The aging precipitation order of SiC/A356.2 aluminum matrix composites is Co-clusters?GP zone?pre-?''??''??'??phase?Mg₂Si?.The reinforcement phase is mainly the pre-?'',?''and?'during peak aging and the thermal mismatch stress appears in the matrix with the addition of silicon carbide.It leads to dislocation around silicon carbide which can strengthen A356.2 aluminum matrix composites.It is also found that the Si phase tend to be spheroidized,angle-passivated and small size while the long needle-like,lamellar silicon phase disappears compared with the as-cast A356.2 matrix composite after heat-treated.The pitting corrosion resistance of SiC/A356.2 aluminum matrix composites in3.5%NaCl solution was decreased with the addition of silicon carbide,its mainly due to the shortening of passivation range and the increase of corrosion current density.With the increase of silicon carbide content,the pitting potential gradually moves to negative point.However,when the silicon carbide was added 0.7wt.%and 1.0wt.%,the pitting potential would move to positive point.This because the uneven distribution of silicon carbide agglomerates in the matrix,which makes the interface between the silicon carbide and aluminum in the matrix less.The pitting corrosion mainly occurs at the interface where the silicon carbide and aluminum are combined.The reduction of the interface caused a decrease in pitting corrosion.In the aging process of SiC/A356.2aluminum matrix composite,the pitting resistance of the composite decreases gradually,mainly as the shortening of the passivation zone,the appearance of pitting pits on the corrosion surface and the depth and area of the pitting pits gradually deepen and expand with the aging time increasing.