In-situ Al₂O₃ Induced Refinement And Modification Of Primary Si Particles In Hypereutectic Al-Si Alloy

The hypereutectic aluminum-silicon alloy has become an excellent engineering material because of its advantages such as high specific strength and specific modulus,good wear resistance and machinability,and excellent thermal stability.In the field of automobile manufacturing,hypereutectic aluminum-silicon alloy is an important material for manufacturing engine parts such as pistons and cylinder liners.However,the unmodified silicon in this alloy often exists as coarse five-petal star-shaped primary crystal silicon and long needle-shaped eutectic silicon.These silicon phases split the alloy matrix and reduce the hardness,wear resistance and thermal stability of the alloy.Therefore,the structure of the hypereutectic aluminum-silicon alloy needs to be modified to refine the size of the silicon phase and improve the morphology of the silicon phase to improve the performance of the alloy.The microstructures and microhardness of the samples were characterized by the metallographic microscope?OM?,X-ray diffractometer?XRD?,scanning electron microscope?SEM?and hardness tester in this paper.The modifier Al₂O₃ were produced by in-situ aluminothermic reaction of TiO2 or CuO with aluminum alloy melts.The modification and refinement of primary crystal silicon in Al-24 wt.% Si alloy by in-situ Al₂O₃ and the influence of casting process on modification and hardness of alloy were studied.The results show that in modified Al-24 wt.% Si alloy by in-situ Al₂O₃,the size of primary silicon was refined from unmodified 100?m to less than 30?m.And the microstructure of the modified alloy was more uniform,in which edges of primary silicon were passivated.Compared with the reaction between TiO2 and Al,CuO can react with Al melt more quickly and completely,and the effect of refinement of primary silicon was better.The orthogonal test methodwas used to research the casting process of in-situ Al₂O₃ modification on Al-24 wt.%Si alloy by CuO aluminothermic reaction.The optimal metamorphic process parameters were determined as follows: holding temperature 900?,holding time15 min,CuO addition amount 3.5wt.%,by which the primary silicon size can be refined to 25?m.The mechanism of modification was preliminarily discussed.Both Al₂O₃ and Si have a cubic structure,between which there is a small lattice mismatch of about 3%,indicating that Al₂O₃ can be used as a heterogeneous core of primary crystal silicon.Furthermore,the in-situ Al₂O₃ particles generated in melt have good wettability with the aluminum alloy matrix.In addition,the in-situ Al₂O₃ particles are well dispersed in the aluminum alloy melt.Therefore,the primary silicon phase is easy to nucleate and grow on the Al₂O₃ substrate,thereby improving the number and efficiency of primary silicon nucleation.That is,in-situ Al₂O₃ increases the number of heterogeneous cores of primary silicon,and the modification of primary silicon in hypereutectic aluminum-silicon alloys conforms to the heterogeneous nucleation mechanism.The microhardness test results of the eutectic structure in the modified Al-24 wt.% Si alloy showed that the better the refinement effect of the alloy structure,the higher the hardness of the alloy.And the maximum hardness value was118.5HV1,with an increase of nearly 50%.In addition,the Al₂O₃ and Al2 Cu generated by in-situ reaction have a certain contribution to increase the hardness of the alloy.