Analyses on formability and flow stress of an Al-Cu-Mn alloy sheet under biaxial stress at cryogenic temperatures

To study the formability and flow stress of an Al-Cu-Mn alloy sheet under biaxial stress state at various cryogenic temperatures, a geometric model of the bulged dome and a calculation method of the flow stress curves were newly proposed. Based on a novel testing device equipped with digital image correlation (DIC) measurements, the strain, displacement, bulging height, punch force and wall thickness were obtained during the rigid punch bulging at RT, -140 degrees C and -160 degrees C. The evaluation indexes regarding the expansion ratio of surface area and average deviation of thickness were proposed to analyze the effects of cryogenic temperatures on the formability and deformation uniformity of the Al-Cu-Mn alloy sheet. The expansion ratio of surface area at -160 degrees C is approximately 2.3 times greater than that of the RT. The average deviations of thickness are decreased by 9.1% and 37.0% at -140 degrees C and -160 degrees C with an identical bugle height, respectively. Based on the geometrical and stress models, the flow stress curves of the Al-Cu-Mn alloy sheet are determined under biaxial stress state at RT, -140 degrees C and -160 degrees C. The validity of calculated flow stress curves is illustrated by the cryogenic deep drawing simulations combined with the experiments. Furthermore, potential applications of the calculated flow stress curves are suggested to accurately analyze the deformation behavior and formability of the Al-Cu-Mn alloy sheet at cryogenic temperatures.