Research On Semi-solid Thixoforming Of 7075 Aluminum Alloy Composites Reinforced With Nano-sized SiC Particles

Aluminum matrix composites(AMCs) hold the advantage of high strength, good wear resistance and excellent thermal conductivity. AMCs have been widely used in aviation, aerospace and civilian areas. Also nano-sized particles reinforced aluminum matrix composites achieve excellent higher strength and better toughness. Thixoforming technology can avoid turbulence and hot crack during metal casting process. In this paper, a new method combined the method of preparing nano-sized particles reinforced AMCs and aluminum semi-solid thixoforming technology is used to manufacture a typical deep cylinder part made of nano-sized particles reaiforced AMCs. It will promote the application of thixoforming technology on the forming area of nano-sized particles reinforced AMCs structural components.Ultrasonic-assisted semi-solid mechanical stirring method was used to prepare 7075 aluminum matrix composites reinforced with nano-sized Si C particles. The microstructure of composites consists of typical spheroidal grains and the average grain size was 30μm. The nano-sized Si C particles were uniformly distributed inside the grains and gathered at grain boundaries. In the semi-solid isothermal treatment experiment, the size and shape coefficient of the grain were depended on reheating temperature and holding time. When the reheating temperature was 590℃ and 600℃, the semi-solid billet could achieve smaller grain size and better shape coefficient. If the holding time was longer than 20 min, the solid phase in the semi-solid billet of composite grew remarkably and the rosette microstructure was found in the semi-solid billet of composite.Numerical simulation of thixoforming process of 7075 aluminum matrix composites reinforced with nano-sized Si C particles was performed by using Deform-2D finite element software. The results of numerical simulation showed that during the thixoforming process the equivalent stress decreased with the increase of the billet temperature. The formed part held similar equivalent strain value at different locations. Raising the die temperature and the punch speed can effectively reduce the heat loss of the billet.The thixoforming experiments of 7075 aluminum composites reinforced with nano-sized Si C particles were completed. The results showed that the microstructure of the thixoformed cylinder parts was quite different at different locations. The grains were severely stretched along the extrusion direction and the deformation of grains of sidewall region was relatively small in the direction perpendicular to the extrusion direction. The distribution of nano-sized Si C particles in the matrix did not change during the experiment of thixforming. The distribution of Si C particles at grain boundary was higher the distribution in the grain. Compared with the original 7075 aluminum, the yield strength of the components had reached 245 MPa and had an improvement of 53%, the ultimate tensile strength reached 313 MPa and increased by 2.3%. The tensile strength of sidewall region was higher than that of the bottom area. The fracture mode of sidewall region was mainly ductile fracture. After T6 heat treatment, the segregation of copper was resolved. The ultimate tensile strength of the components had reached 552 MPa and increased 12.7% compared with the raw materials. After T6 heat treatment, the elongation of the components was 8%, close to 7075-T6 aluminum alloy. The wear resistance of the composites had not been significantly improved at room temperature under atmospheric environment. The wear form of the composite was mainly abrasive wear.