Investigation of temperature and strain rate effects on material flow curve and microstructure of aluminum sheet metal alloys

The use of lightweight materials such as Al and Mg alloys in the vehicles is known as a prominent and long-term solution to the fuel economy problem. Previous studies have shown that the formability of these lightweight materials increases at elevated temperature levels and higher elongation could be obtained when forming is carried out at low strain rates. In this study, a warm hydraulic bulge testing apparatus was developed to investigate the material behavior of Al alloys (A15052 and A16061) at different temperature levels (20-300°C) and strain rates (0.0013-0.013 s^-1). Based on the bulge test data and using existing analytical models based on Hill's theory, the material flow curves were calculated for different testing conditions. The calculation results showed a decrease in the flow stress values with increasing bulging temperature, and larger elongation was obtained when a lower strain rate was used. In addition, microscopy analysis was performed to better understand the effects of the temperature and strain rate on the grain structure that may have caused the observed changes in the material behavior. The analysis results showed that the grain size became larger at higher temperature levels for both Al alloys; thus, leading to the decrease in the flow stress values as observed in the experimental results. However, no significant changes on the grain structure were observed between the specimens bulged at the different strain rates used in this study (0.0013 and 0.013 s^-1).