Study On The Modification Process And Direct Chill Casting Of Hypereutectic Al-Mg₂Si In-situ Composite

As a new kind of pseudo-binary eutectic composite,Al-Mg2Si in-situ composite is an important lightweight material in automotive and aerospace with the advantage of high specific strength,high interfacial strength,excellent wear resistance and abundant production resources.In normal as-cast hypereutectic Al-Mg2Si composite,dendritic primary Mg2Si with the sharp edges and corners and plate-like eutectic Mg2Si structure break the continuity of Al matrix seriously and lead to poor mechanical properities.In this paper,the effects of modification process of P and Sr on the microstructure and properties of Al-16wt%Mg2Si in-situ composite were studied.The optimal parameters of modification process were obtained and applied to direct chill casting.The effect of casting speed on the microstructure and properties of Al-16wt%Mg2Si in-situ composite was also studied.The following conclusions can be drawn:(1)As a modifier P can improve the microstructure and properties of Al-16wt%Mg2Si in-situ composite effectively.In this study,the optimal P modification parameters are:the P content is 0.05wt%and modification temperature is 830?.With the optimal P modification parameters,the primary Mg2Si exhibits homogeneous distribution,which the average size(12.11?m)decreases 53%than that in unmodified composite(25.71?m).The eutectic Mg2Si has a dot-like morphology.With 0.05wt%P addition at 830?,the hardness of the composite increases from 75.52HV to 91.90HV.The conductivity is 32.1%IACS,and the friction coefficient and wear rate are 0.236 and 1.88×10-5g·m-1,respectively.(2)Sr also can improve the microstructure and properties of Al-16wt%Mg2Si in-situ composite,but the modification effect is weaker than P.In this study,the optimal Sr modification parameters are:the Sr content is 0.01wt%and modification temperature is 730?.With the optimal Sr modification parameters,the average size of primary Mg2Si(18.68?m)decreases 27%than that in unmodified composite(25.71?m).With 0.01wt.%Sr addition at 730?,the hardness of the composite increases from 75.52HV to 83.25HV.The conductivity is 29.6%IACS,and the friction coefficient and wear rate are 0.277 and 2.15×10-5g·m-1,respectively.(3)With the increasing of cooling rate,the primary and eutectic Mg2Si are refined further in modified Al-16wt%Mg2Si in-situ composite.The hardness and conductivity also have further improvement.(4)In direct-chill casting billet of Al-16wt%Mg2Si in-situ composite,with the increasing of casting speed,the size of primary Mg2Si decreases gradually.The morphology of eutectic Mg2Si transforms from fibrous to dot-like shape.The hardness,conductivity and tensile properties are improved.When the casting speed increases to 300 mm/min,the average size of primary Mg2Si decreases to 10.40?m.The conductivity and hardness increase to 34.1%IACS and 107.50V,respectively.The highest tensile strength and elongation values were found to be 241MPa and 4.52%for Al-16wt%Mg2Si composite at the speed of 3 00mm/min,respectively.