Preparing Via Liquid Phase Reaction Sintering And Thixoforming Of Semi-Solid Al-Si Alloy

Al-Si alloys are widely used in aviation,aerospace and automotive industries because of their good castability,low density,high conductivity and thermal conductivity,high specific strength and stiffness,easy recovery,excellent corrosion resistance and machinability.In the field of industrial production,the semi-solid forming technology is often used for near-net forming of Al-Si alloy.The key is to prepare semi-solid alloy with equiaxed solid grains uniformly dispersed in the liquid phase.However,the traditional semi-solid alloy preparation methods have many problems,such as complex process,cumbersome process,high energy consumption,and the semi-solid alloy preparation also often has problems such as uneven microstructure,residual dendrite and deformed structure.The existence of these problems is not conducive to saving energy and resources,but also will increase the difficulty of semi-solid forming and reduce the quality of formed parts.Based on the liquid phase sintering technology of powder metallurgy,combined with the special microstructure requirements of semisolid forming for billets,a new method of preparing semi-solid Al-Si alloy materials by powder liquid phase reactive sintering was proposed in this paper.In this method,the liquid phase is generated through the alloying reaction between Al and Si powder particles,and the liquid phase flows and wraps the remaining spherical solid Al particles to form a semi-solid microstructure.After the semi-solid structure is formed in the process of liquid-phase reaction sintering,the semi-solid thixoforming of the billet can be directly carried out.The semi-solid alloy is formed by direct reaction of nearly spherical elemental powder as raw material,which eliminates the complex process,ensures the nearly spherical solid grains,prevents the occurrence of dendrites and residual deformed structures,and has the characteristics of simple process,low energy consumption,uniform semi-solid microstructure,and easy to realize industrial production.Focusing on the new method of preparing semi-solid alloy by powder liquid phase reaction sintering,the preparation and thixoforming properties of semi-solid Al alloy were systematically studied with Al and Si element powders as the initial materials.By means of mixing powder and cold pressing test of mixed powder,the mixing and pressing characteristics of element powder were investigated,and the optimum mixing powder and pressing process parameters were obtained;The liquid phase reaction sintering of green billet was carried out,the formation and evolution mechanism of semi-solid microstructure was studied,and the evolution model of semi-solid microstructure was established;Through the analysis of reaction thermodynamics and kinetics,the reaction mechanism of Al-Si binary system was clarified,the kinetic equation of the evolution of liquid phase production was obtained,and the control theory of semi-solid microstructure was established;Through the above research,the preparation process of semi-solid Al-Si alloy blank and the control principle and process method of semi-solid microstructure were established.The results show that the cold-pressed green compact with 97%density can be obtained after 50h powder mixing and 500MPa cold pressing;The formation of liquid phase promotes the evolution of semi-solid microstructure,and its content increases with the increase of reaction temperature,time and Si content,which can bedescribedbyy_（6%）=14.7exp（-25487.4/RT）t^0.35and y_（8%）=95.5exp（-36154.2/RT）t^0.4.The strength of Al-Si alloy was effectively improved by adding Cu.The strengthening mechanism and mechanism were systematically analyzed,and the strengthening mechanism model was established;The preparation process of high strength Al-Si alloy was determined by mixing Al,Si and Cu powder,cold pressing and liquid phase reaction sintering experiments;Through thermodynamic and kinetic analysis,the reaction mechanism of Al-Si-Cu ternary system was clarified,and the kinetic equation of liquid phase production and evolution was obtained,and the control theory of semi-solid microstructure was established.The results show that high strength Al alloy can be prepared by liquid phase reaction sintering,and the kinetic equation of liquid phase production is y=9.74exp（-23004.8/RT）t^0.25;Orowan strengthening,which contributes more than 80%of the yield strength,is the most important strengthening mechanism,which is mainly due to nanoα-Cu particle has a strong pinning effect on dislocation,and makes its YS and US reach 257MPa and 420MPa respectively.Isothermal compression tests were carried out on semi-solid Al-Si alloy billets,their rheological behavior and influencing factors were studied,a flow stress model was established,and the critical liquid content for semi-solid rheological behavior of billets was explored.The rules and mechanisms of microstructure evolution and liquid phase flow during thixotropic compression were analyzed through microstructure observation,the rules and influencing factors of semi-solid microstructure instability were explored,and the microstructure evolution model during thixotropic compression was established.Based on the above research,the rheological behavior and microstructure evolution rules and mechanisms of semi-solid billets are clarified,which provides a theoretical basis and experimental support for the formulation of semi-solid forming process plan and the prediction of the microstructure of formed parts.The results show that when the content of liquid phase（f _L）≥0.138,the liquid phase can coordinate the stable rheology of Al grains,prevent the rheological instability,and obtain uniform deformed microstructure.The constitutive equation can be described byσ=0)^0.001+1310/（-0.03）^-0.021?^0.057（1-1.51）)^0.805.The semi-solid thixoforming experiment of Al-Si alloy cup was carried out,and the optimal parameters and experimental scheme of semi-solid forming were determined.Through the thixoforming process test of cup shaped parts and the research on the microstructure and properties of the parts,the influence of semi-solid billet preparation and thixoforming process conditions on the microstructure and properties of the final parts was clarified.The thixoforming experiment shows that when 0.141≤f _L≤0.167,the formed parts with uniform microstructure can be prepared by direct thixoforming.Under the strengthening effect of fine grains and Si particles,the formed parts with f L=0.167 have the best mechanical properties,with YS,US and EL of 115MPa,220MPa and 0.381respectively.In this paper,a new process for preparing semi-solid Al-Si alloy billets by powder liquid phase reactive sintering and thixoforming was established.The reaction principle and reaction kinetics of Al-Si system,the formation and evolution mechanism of semi-solid microstructure,thixotropic deformation behavior and mechanism and other basic theoretical issues are clarified.The basic laws of thixotropic forming process,microstructure and property evolution of semi-solid Al-Si alloy are revealed,which lays a theoretical and technical foundation for the application of this new process.