Research On Preparation And Thixoextrusion Of Semi-solid Billet Of Hot Extruded 7075 Aluminum Alloy

In this paper,Semi-Solid Isothermal Treatment of Hot-Extrude Aluminum Alloy(SSITHEAA)method was exployed to prepare semi-solid billets.Based on this method,the preparation of semi-solid billets of hot extruded 7075 aluminum alloy and thixoforming extrusion process were studied.This article focused on the recrystallization phenomenon during the isothermal processing of billets firstly,the recrystallization law during the temperature increase of billets was studied.This study found that as the temperature of the billet increases,the extent of grains’ recrystallization gradually increases,the crystal orientation shifts from biased towards [1 0 1] to disorder,and the texture also shifts from R to a variety of textures,and the intensity of the texture gradually decreases.By analyzing the microstructure morphology,microstructure size,grain roundness and EDS spectra of semi-solid billets with different parameters,the effect of holding temperature and holding time on the microstructure of 7075 aluminum alloy semi-solid billet prepared by SSITHEAA method was studied.The results showed that the parameters for the best preparation of semi-solid billets were kept at 600°C for 10 minutes.The DEFORM-3D simulation software was used to simulate the semi-solid thixotropic extrusion process of deep cavity cylinders.The effects of billets’ temperature,mold temperature,and forming speed on the forming process of the cylinder were studied.The results show that increasing temperatures of semisolid billets and mold are beneficial to reduce the deformation resistance of the cylindrical part with deep cavity during forming;Increasing the mold temperature and the speed of top mold can help to reduce the heat loss during the forming process of the cylindrical part with deep cavity,so these are favorable for the filling of the aluminum alloy semi-solid billet in the mold.The thixoextrusion experiment of deep cavity cylinders was conducted on a hydraulic machine.Through the microstructure analysis of the deep cavity cylinders which formed under different parameters and the EDS spectrum analysis of some samples,the influence of different forming parameters on the microstructure of deep-cavity cylinders was studied.The research results show that the short holding time and the low isothermal temperature of the billet led to severe deformation.In the interior of the grains,Al was mainly present,however,Cu,Fe,and Mn segregated to varying degrees in the grain boundaries.In this paper,the effect of T6 heat treatment on the mechanical properties of cylindrical parts with deep cavity was studied by orthogonal experiments.It was found that the optimal parameters of T6 heat treatment were solid solution at 465℃ for 16 h and aging at 150℃ for 16 h.The tensile strength of the cylinder wall was 315.23 MPa before the heat treatment and was elevated to 553.60 MPa after heat treatment,which was 75% higher than that before the heat treatment,and the elongation was increased from 1.9% to 10.3%.It can be seen that the heat treatment has a significant effect on the comprehensive mechanical properties of the cylindrical part with deep cavity.The optimal parameters of thixoforming the cylinder with deep cavity are that the billet is kept at 600°C for 10 minutes,the tensile strength after heat treatment is 573.57 MPa,and the elongation is 13.4%.The analysis of the tensile fracture shows that the fracture of the cylindrical part with deep cavity is characterized by ductile fracture.