7XXX aluminum alloy is an age-strengthened super high-strength aluminum alloy, which is an important aviation structural material. Aviation aluminum alloy components put forward higher requirements on the corresponding forming technology. Aluminum hot stamping process was proposed to overcome the defects of cold stamping. In the hot stamping process, aluminum blank is first heated to solution temperature for a certain period of time. Then, it will be quickly transferred to the tool for forming and quenching at the same time in cold dies. With the obtained state of supersaturated solid solution, artificial aging treatment will be conducted to improve the strength of the material. Hot stamping has great potential in forming complex thin-walled structural parts of aviation aluminum alloy components. Artificial aging treatment after forming is an important step to obtain excellent mechanical properties and stress corrosion resistance matching of aviation aluminum alloy components. In this paper, the effect and strengthening mechanism of the secondary aging treatment process on mechanical and stress corrosion resistance of hot stamping 7B04 aluminum alloy were studied. Hot stamping and aging treatment of 7B04 aluminum alloy sheet were performed by hot stamping equipment and resistance furnace. Characterization and analysis of properties and microstructure of hot stamped 7B04 aluminum alloy after secondary aging treatment were conducted by hardness, tensile, electrical conductivity tests and transmission electron microscope (TEM). Results showed that after hot stamping, the average hardness, yield strength and tensile strength of the 7B04 aluminum sheet were HV 149, 362 MPa and 544 MPa, respectively. Two-step aging heat treatment significantly improved hardness, strength and electrical conductivity of hot stamping 7B04 aluminum alloy parts. The secondary aging temperature had a greater impact on performance than secondary aging time. Hardness and strength decreased and electrical conductivity increased with the increase of over-aging. When the secondary aging time was 5 h, as the secondary aging temperature increased from 145 to 175 ℃, the yield strength decreased from 510 to 457 MPa, and the tensile strength decreased from 572 to 525 MPa. The strength decreased most when forming temperature increased from 165 to175 ℃. Under the condition of the secondary aging temperature of 155 ℃, as the secondary aging time increased from 1 to 3 h, the yield strength and tensile strength of the components increased slightly and showed a slight downward trend when the aging time was further increased to 11 h. The hardness change law of hot stamping 7B04 aluminum alloy was consistent with strength. When the secondary aging time was 5 h, as the secondary aging temperature increased from 145 to 175 ℃, the hardness decreased from HV 197 to HV 179. Under the condition of the secondary aging temperature of 155 ℃, as the secondary aging time increased from 1 to 3 h, the yield strength and tensile strength of the components increased from HV 195 to HV 198 and decreased to HV 175 as the aging time further increased to 11 h. The conductivity of the 7B04 aluminum alloy after hot stamping was only 0.27%IACS. When secondary aging time was 5 h, the conductivity of the aluminum alloy increased from 0.316%IACS to 0.362%IACS as the secondary aging temperature increased in the range of 145~175 ℃. When secondary temperature was 155 ℃, conductivity of the 7B04 aluminum alloy started with a slight drop. The conductivity gradually rose to 0.359%IACS as the aging time further increased. The best strengthening effect was achieved under the ageing condition of 115 ℃/5 h+145 ℃/5 h. The best combination of hardness, strength and electrical conductivity was achieved under the aging condition of 175 ℃/5 h+155 ℃/5 h. Hot stamping process of 7B04 aluminum alloy was an integrated process of rapid non-isothermal plastic deformation and quenching. According to measurement, the cooling rate of die quenching during 7B04 aluminum alloy hot stamping was 200 ℃·s-1. The high cooling rate could not cause a large amount of supersaturated solid solution to precipitated out during die quenching process. The increase of dislocation density caused by hot stamping affected the subsequent aging heat treatment. The strengthening effect of aging heat treatment process parameters on the performance of hot stamping 7B04 aluminum alloy was mainly affected by the precipitate phase and dislocation state. There were many precipitated phases from the supersaturated solid solution in both intragranular and the grain boundaries in hot stamping aluminum alloy after two-stage aging treatment. The precipitates had a pinning effect on the dislocations, hindering the movement of the dislocations, thereby improving the strength and hardness of the material. The intragranular precipitation had a greater impact on the strength of the alloy, while the grain boundary precipitation had a greater impact on stress corrosion resistance. As the increase of aging temperature and time, the conductivity of the material increased because of the further coarsening and aggregation of η phase. © 2022, Youke Publishing Co., Ltd. All right reserved.
