Investigation on strengthening of 6A02 aluminum alloy sheet in hot forming-quenching integrated process with warm forming-dies

New manufacturing technology for light-weight structure materials has been used to meet the requirements for weight reduction and safety performance. In this paper, the hot forming-quenching integrated process of 6A02 aluminum alloy sheet at different forming-dies temperatures ranging from 50 °C to 350 °C was investigated. Vickers hardness test and uniaxial tensile test were carried out to reveal the strengthening behavior. Microstructure evolution was observed to clarify the strengthening mechanism with electron backscattering diffraction (EBSD), transmission electron microscopy (TEM) and scanning electron microscope (SEM) methods. Results show that the Vickers hardness and tensile strength decreased with increasing forming-dies temperature. No obvious decrease in strength appeared until the forming-dies temperature reached 250 °C. The corresponding Vickers hardness was increased to 100.6 HV from original 73 HV in rolled condition, and the tensile and yield strengths were 303.8 MPa and 257.1 MPa, respectively. When the temperature of forming-dies rose to 350 °C, the tensile and yield strengths dropped to 270.1 MPa and 213.7 MPa, respectively. The strengthening phase in Al matrix was the dispersed needle-shaped β″ precipitates with size about 10−50 nm. Transformation of the rod-shaped β′ precipitates with 150–300 nm in size at the grain boundaries and in grain interiors led to the decrease in strength. The forming-dies temperature should not be higher than 250 °C for the integrated forming of 6A02 aluminum alloy sheet to obtain enough strengthening effect.