Investigation of formability, microstructures and post-forming mechanical properties of heat-treatable aluminum alloys subjected to pre-aged hardening warm forming

Currently, the hot forming of aluminum alloys has garnered significant attention for overcoming poor formability at room temperature (RT). However, the current hot forming technique is facing problems such as timeconsuming procedures and high economic cost. To improve forming efficiency, we herein propose a novel forming technique for heat-treatable aluminum alloys, termed pre-aged hardening warm forming (PHF) process. An Al-Zn-Mg-Cu alloy is used to investigate the flow behavior, drawing formability, and post-forming mechanical properties during the aforementioned process. Uniaxial tensile tests and Erichsen tests are conducted to evaluate the plastic flow behavior and drawability, respectively. Stamping tests are performed on a cap beam component to verify the engineering feasibility of the forming technique. Optical microscopy (OM) and transmission electron microscopy (TEM) are performed to characterize the microstructure evolution during the PHF process. The tensile test results show that the pre-aged alloys exhibit true fracture elongation similar to the fracture elongation of the O-temper alloy at RT, and that the Erichsen values of the pre-aged alloys are 5%-16% higher than that of the O-temper alloy at 200 degrees C. The results of the stamping tests for the cap beam component indicate that the optimal final strength of the stamped component achieves sigma/sigma 0.2 = 566 MPa/500 MPa, exceeding the strength of the AA7075-T6 alloy. OM observations show that the stamping process does not significantly affect the grain sizes of the pre-aged alloys. TEM results show that numerous thermostable GPII zones exist in the pre-aged alloys, and that the stamping procedure transforms the GPII zones to dispersed eta ' phases and results in the entanglement of numerous dislocations. The dual effects of phase transformation and plastic deformation during the PHF process contribute to the desirable mechanical properties of the stamped components.

Automated summary

The PHF process improves the formability of aluminum alloys, with stamping tests confirming the feasibility of the technique on cap beam components. Optical and transmission electron microscopy observations show that the grain sizes of pre-aged alloys are not significantly affected by the stamping process in the PHF process.