Hot deformation behavior of coarse-grained 5052 aluminum alloy

Abstract

There has been world-wide research in both the academic institutes and leading industrial establishments in the understanding of deformation behavior and forming limits of sheet metals, specially in aluminum alloys. However, most research work focuses on cold forming of metal sheets rather than at elevated temperatures. Recently, researchers have demonstrated that some coarse-grained aluminum alloys exhibit superplastic or superplastic-like behavior. Their findings are significant and worthwhile as they may lead to the possibility of superplastic forming of conventional aluminum alloys. More work is therefore needed to further investigate the hot deformation behavior and forming properties of conventional coarse-grained aluminum alloys under multi-axial stress states. In this dissertation, the hot forming behavior of commercial coarse-grained aluminum 5052 alloy under different stress states was examined for the first time. A maximum elongation of 194 % was observed at an initial strain-rate of 2.0x10-1S-1 and at a testing temperature of 873 K, with a maximum m value of 0.24. The relatively high optimal strain rate was of significant practical value. After bulge forming of the alloy using elliptical dies with various aspect ratios, an experimental hot forming limit curve was constructed. The shape of the curve was significantly different from that obtained in conventional sheet metal forming at room temperature. The cavitation behavior of the aluminum 5052 alloy deformed by hot uniaxial and biaxial tension was also investigated for the first time. Comparisons between the deformation and cavitation behavior of fine-grained aluminum 5083 alloy were also made. The maximum elongation and the strain rate sensitivity value of the aluminum 5083 alloy in uniaxial tension were much larger than that in aluminum 5052 alloy. By investigating the fracture mechanism, a nearly neck-free fracture surface was observed in aluminum 5083 alloy, but a significant localized necking occurred in aluminum 5052 alloy. The cavitation growth rate of aluminum 5083 alloy in hot deformation was significantly greater than that of aluminum 5052 alloy. It was believed that the hot formability of aluminum 5052 alloy was not only correlated with its cavitation behavior, but also related to its fracture phenomenon and the strain rate sensitivity value. An instability model based on the M-K model and the experimental cavitation behavior was successfully developed to predict the limit strains of the alloy. Good agreement between the theoretical and experimental limit strains was achieved.