The influence of grain size and strain rate effects on formability of aluminium alloy sheet at high-speed forming

In order to investigate the coupling influence of grain size and strain rate on formability, a series of limit dome height (LDH) tests were conducted at four different speeds ranging from 0.012 m/s to 11.04 m/s via stamping and electro-magnetic forming process. Aluminium sheets A5052 H32 with 0.5 mm thickness were used for experiments and annealed in different conditions to obtain different micro-structures (T/D = 6.99-27.17). The strain data of FLD were compared with the limit dome height measurement to clarify the influence of various grain sizes on formability. Furthermore, the fracture surface and microstructure of tested samples were observed by SEM and OM to understand the damage mode. The results show that the influence of decreased grain size at high-speed forming not only enhances material strength but also increases the probability of grain boundary fracture that could impair the formability. This new finding can be used in predicting the occurrence of fracture and as a reference for product design and processing simulation analysis.