(1) The experimental scheme and process. The slurry was prepared by lspsf semi-solid pulping process, and the 7075 aluminum alloy wheel parts were formed by rheo squeeze casting with 3000kN vertical semi-solid squeeze casting machine independently developed and manufactured in the laboratory. The overall diameter of the product is 160mm and the thickness is 35mm. Qg-3 metallographic sample cutting machine manufactured by Shanghai metallographic mechanical equipment Co., Ltd. was used to take some 15 × 15 × 15mm heat-treated samples from the parts, and the corresponding steel seal number was printed on the side of the samples. In order to determine the temperature range of solution treatment, the clean and unoxidized alloy powder was drilled by drilling machine in the core of the part for DSC thermal analysis. According to the DSC endothermic peak value, several temperatures were selected for solution treatment for 2 hours, and then metallographic analysis was carried out.
After polishing, the samples were corroded by Keller reagent, and the microstructure was observed by xjp-6a electron microscope. The Brinell hardness of the sample surface was tested. After metallographic analysis and solution analysis, the best temperature and time were determined. The best aging temperature and time were obtained by grouping experiment, metallographic analysis and hardness analysis. According to the best combination of solid solution and aging heat treatment process, wire cutting is used in the core of the part, and a round bar with diameter of 15mm is selected for heat treatment. After heat treatment, the standard tensile specimens were processed according to the national standard. The tensile test and SEM scanning of fracture surface were carried out to analyze the results.
(2) DSC analysis of the sample. As shown in the figure is the DSC thermal analysis curve of 7075 aluminum alloy semi-solid rheo squeeze casting parts. It can be seen from the figure that there is an obvious endothermic peak near 491 ° of the workpiece, which indicates that phase transformation occurs near this temperature. The endothermic peak near 491 ° is the melting temperature of non-equilibrium solidification eutectic structure on the grain boundary of the workpiece.
According to the initial temperature 491 ℃ of the endothermic peak of DSC curve, the experimental temperature of solid solution treatment was set as 460 ℃, 475 ℃, 490 ℃, 505 ℃, 520 ℃ and 535 ℃ respectively. During the experiment, all samples were marked with steel seal according to the temperature, and were kept for 2 hours at 15 ℃ intervals. Then the samples were transferred to water for quenching within 10 s. The water temperature was 26 ~ 28 ℃ and the air humidity was 71%. The whole experiment was carried out continuously.