Research On Complex Optimization Of 20%Mg₂Si/Al-12Si Composite By Modification And T6 Treatment

20%Mg2Si/Al-12 Si composite is a potential kind of material with multiple promising applications e.g. aerospace, automobile manufacturing and military equipment, since the high specific strength and stiffness, excellent wear-resisting property, and low thermal expansion coefficient. However, the adverse effects on mechanical properties are mainly triggered by the morphology of primary Mg2 Si phases in the composite matrix with a shape of arborization, as well as the agglomeration of eutectic Al-Si. In order to further optimize the strength and toughness, a suitable method is vital significant to be carried out based on the controlling of microstructures. However, most research focus on modification or heat treatment, and a complex optimization process needs to be systematically studied.In this thesis, Al-12 Si matrix was reinforced by Mg2 Si in-situ particles, and the preparation of 20%Mg2Si/Al-12 Si composite was obtained by gravity casting. At present work, the complex optimizing process, correlating 0.1%P modification and T6 treatment, has been utilized to promote strength and toughness. Experiments have been conducted in three aspects:(1) The different microsturctures obtained by 0.1%P modification were given to compare with the raw composite ingot; furthermore, the mechanism of morphology variations of T6 treatment had been explored by means of Optical Microscope(OM) and thermodynamics analysis;(2) The hardness variation caused by T6 aging treatment was further investigated correlated to the metallography;(3) Relationship between tensile performance value of the ingots after T6 aging treatment and the fracture surface morphology observed by SEM was discussed based on fractography.Evidences from the research of 0.1%P addition not only show the morphology change of Mg2 Si dendritic crystal, which transformed to polygon shaped phase, but also indicate a refinement effect on Mg2 Si particles, simultaneously with the coarsening of Al-Si eutectic phases. Subsequent solution treatment results in successively broke, granulate, homogenize and coarsening again at the eutectic phases. Meanwhile,the primary phases would be slightly blunted with 540℃×6h treatment, and matrix would be over-burnt with 560℃×12h treatment.Hardness of unmodified and 0.1%P-modified composite were 89 HBW and 105 HBW, respectively.The hardness curves of composites existed peak points during solution treatment. The peaks of unmodified and modified composites were both emerged at 6h, and the hardness values were 102 HBW and 119 HBW, respectively. Hardness curves during aging treatment are divided into three stages: under-aged stage, peak aging stage and over-aged stage with two peak points. The peaks of unmodified and modified composites revealed at 14 h and16h, and the hardness values were 134 HBW and 148 HBW, respectively.Tensile tests were carried out based on 20%Mg2Si/Al-12 Si ingot and T6 treatment samples. The tensile strength and elongation of the as-cast sample were 108 MPa and 0.83%. After being modified, the corresponding values improved by 21 MPa and 0.38%; after T6 treatment, the corresponding values improved by 57 MPa and 1.10%; and the values would improve by 98 MPa and 1.20% after modification and T6 complex optimization, indicating that complex optimization process of modification and T6 can largely improve the strength and ductility at the same time, and revealed a synergistic effect on mechanical propertyAnalysis of fractography shows that the fracture mechanism of as-cast composite is mainly brittle rupture. The Mg2 Si with smooth appearance have apparent river patterns on it, which are classic cleavage fracture features. The clear appearances indicate the transgranular fractures in Mg2 Si phases. After modification and T6 treatment, the ups and downs on the fracture surfaces reduce, and more ductile fracture dimples emerge.Microstructure observation and mechanical properties tests indicate that the optimum T6 treatment parameters of unmodified and modified composite are 540℃×6h+175℃×14h and 540℃×6h+175℃×16h, respectively.