Gas forming of an AZ31 magnesium alloy at elevated strain rates

In this work, the gas forming of AZ31 magnesium alloy 0.75-mm-thick sheets at elevated strain rates (fast gas forming) is investigated through an experimental-numerical approach. First, free inflation tests were carried out to find the conditions, in terms of temperature and forming pressure, able to give the best compromise between the alloy formability and the forming time. The analysis was successively moved to a closed die forming application with a stepped geometry case study in order to analyse the real forming process. Both an axisymmetric model of the free inflation test and a 3D model of the closed die forming process were built to correlate the results from free inflation tests (in terms of optimal strain rate values) to the closed die forming test: Numerical simulations were run to find the pressure value to be applied in gas forming tests. Experimental gas forming trials were finally conducted in order to support the approach and to analyse post-forming characteristics of the formed parts. Results showed that very small fillet radii can be reached on a commercial Mg alloy sheet setting very short forming times (few seconds). The choice of the forming temperature and of the corresponding optimal strain rate strongly affects the grain growth and the cavitation phenomena. Even if the alloy is prone to a strong static and dynamic grain growth at elevated temperatures, a small mean grain size value can be reached in the formed component due to the short forming times.