Research Of Microstructure And Properties Of Semi-solid 7A04 Aluminum Alloy With Al-5Ti-1B Grain Refiner

Al-Zn-Mg-Cu alloys(7xxx)are widely used in aerospace and automotive industries due to their excellent comprehensive properties,such as low density,high strength and ductility.However,these alloys are hard to manufacture products with high mechanical properties by conventional casting because of their high tendency of hot tearing.The semisolid slurry preparation,rheo-formed tensile samples and heat treatment of 7A04 aluminum alloys have been studied.This is fundamental for industrial application of rheoforming of wrought aluminum alloy in the industry.In this paper,the effects of Al-5Ti-1B grain refiner addition on the microstructure and secondary solidification of 7A04 aluminum alloy semisolid slurry,the uniformity of microstructure in the radial direction of crucible were investigated.The results indicate that the primary a-Al particles and a2 particles of 7A04 aluminum alloy semisolid slurry could become fine and globular by addition of Al-5Ti-1B,but for as particles,there was little change with theirs average particle diameter and shape factor,the phases of 7A04 aluminum alloy semisolid slurry were the same.The microstructure of 7A04 aluminum alloy semisolid slurry could be obtained with average particle diameter of 71 ?m,29?m and shape factor of 0.49,0.45 for the primary a-Al particles and a2 particles,respectively,by addition of 0.1wt%Al-5Ti-1B grain refiner at 630 ?.The effect of grain refining was slightly improved when the amount of Al-5Ti-1B grain refiner is higher than 0.1%.The uniformity of microstructure in the radial direction of 7A04 aluminum alloy semisolid slurry was enhanced by adding Al-5Ti-1B,and the coefficient of variation Cv was 2.8%at 635?.The rheo-diecast samples of 7A04 aluminum alloy were produced with Al-5T-1B grain refiner,the effects of pour temperature,amount of Al-5Ti-1B and injection velocity on microstructure,porosity and mechanical properties of rheo-diecast samples were studied.The results show that the microstructure was refined by adding Al-5Ti-1B grain refiner and the effect of grain refining was significant at 635?.With the increase of the addition of Al-5Ti-1B,the effects of Al-5Ti-1B on the grain refining,porosity and mechanical properties of rheo-diecast samples of 7A04 aluminum alloy were not significant.The shape factor and size of particles of rheo-diecast samples was slightly improved with the increase in injection velocity,at 700? and 660?,respectively.Higher injection velocities result in smaller size and better shape factor of the particles in the rheo-diecast parts at 635?,but there was little change for a2 particles during secondary solidification.The optimal porosity,density,tensile strength and elongation of rheo-diecast parts were achieved at four circles of velocity control valve with the same pour temperature.The porostiy of rheo-diecast samples was reduced with the decrease in injection velocity,and the density and mechanical properties were enhanced.The rheo-diecast parts of 7A04 aluminum alloy with Al-5T-1B grain refiner were heat treated,the effects of rheo-diecast temperature and amount of Al-5T-1B on porosity and mechanical properties of samples after heat treatment were investigated.The results indicated that the levels of Al-5T-1B grain refiner did not have striking effect on porosity and mechanical properties of rheo-diecast samples after heat treatment.After heat treatment,the porosity was slightly increased and the density decreased.Both the tensile strength and elongation were greatly enhanced,and reached their maximum level,i.e.,391 MPa and 2.5%,respectively,compared to the as-cast samples.Nevertheless,the mechanical properties of rheo-diecast samples were much lower than forged parts due to porosity.