| A356 alloy,a typical aluminum-silicon casting alloy,is widely used in the automotive industry owing to its advantages of good casting fluidity,atmospheric corrosion resistance,and low coefficient of thermal expansion.With the development of automobile lightweight technology,the higher requirements have been placed on the mechanical properties of alu-minum alloy.However,the traditional aluminum alloys exhibit the drawbacks of low strength and elongation.The microstructure,the strength and the hardness of A356 alloy can be enhanced by adding reinforcement particles,especially in-situ nanoparticles to form par-ticle reinforced A356 aluminum matrix composites.Therefore,in this study,with the assis-tance of high-energy ultrasound,the relatively uniform nano ?-Al2O3p/A356 composites with high-strength and high-plasticity were synthesized through the method of exothermic dispersion(XD)combined remelting method,and further processed by squeeze casting.Moreover,the reaction mechanism between Al and nano-SiO2 was explored,and the varities in the microstructure and mechanical properties of the composites as well as the fracture mechanism of the material were systematically studied.The details were as follows:(1)Kinetic studies of isothermal and non-isothermal modes indicated that the in-situ reaction between Al and nano-SiO2 could occur spontaneously.The system could react completely thus generate nano ?-Al2O3 and Si without any other harmful by-products when the holding temperature was 900? and the holding time was 30min.(2)Al-SiO2 compacts were prepared by high-energy ball milling and cold pressing.The nano ?-Al2O3p/Al preforms were prepared by XD method,which were pre-melted and di-luted with the assistance of high-energy ultrasound,and finally,the 0.3vol.%,0.5vol.%and 0.8vol.%nano ?-Al2O3p/A356 composites with spherical ?-Al2O3 particles of 80nm were successfully synthesized.Nano ?-Al2O3 particles had the effect of refining on the eutectic Si phase and the secondary spacing of ?-Al phases in the composites,and the lengths of the eutectic Si phase and secondary spacing decreased first and then increased as the particle volume fraction increased.When the particle volume fraction was 0.5%,the refinement ef-feet of nano ?-Al2O3 particles was optimal.At this time,the longest length of eutectic Si was 18 ?m,and the secondary spacing was reduced to 16-18?m.(3)The nano ?-Al2O3p/A356 composites with the particle content of 0.5vol.%were squeeze-casted.The results showed that the composites became denser with the increase of squeeze pressure.When the pressure reached to 80MPa,the pores and porosities easily caused by gravity casting in the composites disappeared completely,and the Si phase was refined,while the ?-Al dendrites were broken into finer dendrites.However,the distribution of nano ?-Al2O3 particles had no obvious change,indicating that squeeze casting had no effect on the dispersion of particles.After the T6 heat treatment,the eutectic Si in the com-posites was spheroidized and passivated,and the distribution of Si phase became more uni-form.(4)The experiments of room temperature tensile and hardness tests suggested that the mechanical properties of 0.5vol.%nano ?-Al2O3p/A356 composites reached utmost level when the squeeze pressure was 80MPa.The tensile strength,elongation and hardness in the T6 state were 315MPa,6.28%and 131.5HV,which increased by 16.23%,30.83%and 15.04%,respectively,compared with gravity casting,and increased by 25%,62.27%and 7.52%,respectively,in relation to A356 matrix alloy in the other condition same situation.(5)Tensile fracture morphology research of T6 state material indicated that the fracture modes of A356 matrix and nano ?-Al2O3p/A356 composites under gravity casting was mainly brittle fracture.After squeeze casting under 80MPa,the large cleavage steps on the fracture surface of material were refined to quasi-cleavage steps with uniform distribution and small size.On the other hands,the number of tough dimples increased,while the frac-ture mode of both the matrix and composites tended to ductile fracture,as the plasticity was significantly improved. |
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