Study on High-Temperature Mechanical Properties and Forming Limit Diagram of 7075 Aluminum Alloy Sheet in Hot Stamping

To improve the formability of aluminum alloy sheets and avoid springback, a newly developed forming technique, HFQ (heat-forming-quenching), has attracted attention in the automobile industry. In this work, a high-temperature tensile test and the Nakazima hemispherical punch method were used to simulate the HFQ process to study the high-temperature flow behavior and formability of 7075-T4 aluminum alloy sheet. The results demonstrated that elongation to fracture was positively correlated with temperature, changing from 15% in the quenching state to 26.1% at 440 degrees C. Strain rate sensitivity was found to vary according to the strain rate due to the coupled competition between temperature and the strain rate. It was concluded that the necking to shear fracture transition of 7075 based on HFQ occurred at 358 degrees C. In addition, the forming limit of the alloy sheet decreased with the increasing waiting time. The temperature field distribution of the alloy sheet for different waiting times at a forming velocity of 10 mm/s was obtained by thermomechanical coupled simulations. Different modes of rupture indicated different thermomechanical behaviors due to the different initial temperatures under hot stamping.