Investigation On Thixoforming Of SiC_p /Mg Composites And Simulation

Particle reinforced magnesium matrix composite are getting increasing attension in many fields because of their excellent mechanical properties, such as low density, high specific stiffness, high specific strength, good high temperature resistance, good impact resistance, low expansion coefficient and good property of shock absorption. However, the widely application of particle reinforced magnesium matrix composite is restricted because of their difficulty process. Semi-solid metal forming processes can solve the second processing problem of magnesium alloy because of its near net shape, good mechanical properties and less machining.The magnesium matrix composites, AZ91 magnesium alloy reinforced with SiC particles have been respectively fabricated by stirring-melt casting process. And the semi-solid specimens of SiCp/AZ91 composites were prepared by the use of semi-solid isothermal heat treatment. Furthermore, the effects of process parameters on the microstructure and properties and the semi-solid thixotropic behaviors of SiCp/AZ91 composites were studied.The results show that the particles volume fraction and diameter affect the distribution of particles. With the increase of volume fraction or diameter, the distributions of particles are more uniform. During the process of semi-solid isothermal heat treatment, the liquid volume fraction of semi-solid microstructure increase with the temperature, the liuquid volume fraction is increasing and the spheroidize microstruture is more finer from the center of semi-solid blank to the edge, the variation law of SiCp/AZ91 composite with the isothermal prolonged is dendrite combination, massive microstructure, crystal boundary melting separation, spheroidize microstruture, microstruture growth, The deformation mechanism is mainly by solid-phase particle deformation and particle sliding in the process of isothermal compression tests. With the increase of thixotropic temperature or the particle diameters and the reduction of the strain rate or particles volume fraction, the flow stress is increasing.Based on the semi-solid constitutive equation of matrix alloy, the composites semi-solid constirutive equation related to the particle volume fraction and diameter has been established. And then substitution of SiCp/AZ91 composites thixotropic compression data into the constitutive equation, the constitutive equation of SiCp/AZ91 composites with high solid volume fraction was established. Finally, the distribution of effective stress, effective strain and temperature field during the thixotropic extrusion process were analyzed using the constitutive equation of SiCp/AZ91 composites by DEFORM software. The numerical simulation results in agree with the experimental datas, which proved the constitutive equation of SiCp/AZ91 composites is accurate.