Preparation And Thixoforming Of A356 Alloy Semi-Solid Billet

The semi-solid thixoforming is a formed process under pressure in the semi-solid temperature range.It combines the advantages of casting and forging forming methods,achieves near net shape and improves product quality and production efficiency.It has been an attractive material processing method.Common semi-solid processing methods require the preparation of non-dendritic billet.How to shorten the process and save economic costs becomes the current task.In this context,the related research on the preparation and thixoforming of A356 alloy semi-solid billet is carried out.The specific research contents include the influence of hot upsetting on the microstructure of A356 alloy,the microstructure evolution of A356 alloy in semi-solid isothermal treatment,thixoforming of A356 alloy semi-solid billets and heat treatment of semi-solid thixoforming A356 alloy parts.In this thesis,the wrought aluminum alloy was obtained by using the A356 alloy ignots as the raw material.The microstructure observation and phase determination were completed with the method of advanced material analysis and testing techniques.The evolution of the microstructure in the semi-solid isothermal treatment was determined.A356 alloy parts with desirable microstructure and mechanical properties were obtained through semi-solid thixotropic die forging.The microstructure and mechanical properties of A356 alloy were further improved by T6 heat treatment.Advantages and disadvantages of semi-solid thixotropic die forging,liquid die forging and traditional casting process were discussed.The main findings and conclusions are as follows:The hot upsetting above the recrystallization temperature improves the distribution of the original microstructure and achieves the effect of refining the grains.The larger the amount of deformation is,higher degree in elongation the crystal grains will have.The higher the deformation temperature is,the greater possibility of recovery and recrystallization will have.Semi-solid isothermal treatment can transform the primary phase of A356 alloy from dendritic structure to spherical structure.Increasing the isothermal temperature or the holding time can lead to a comlete change of the primary phase from dendrites to spheroidal grain.But the long-term holding time will cause an abnormal change from spheroidal grain to irregularly shaped grains.The higher isothermal temperature promotes the increase in primary grain size and shortens the time for dendritic morphology to granulate.The mechanical properties of semi-solid thixoforming depend on the size and spheroidization of the grains along with the morphology and distribution of the eutectic silicon.The T6 heat treatment significantly improved the yield strength and tensile strength of A356 alloy.The improvement of mechanical properties of thixoformed part with T6 heat treatment is mainly due to the granulation of single crystal silicon and the enhancement effect of acicular precipitate ??? phase.