Numerical/experimental investigation of the production of thin magnesium alloy components via superplastic forming

摘要

Magnesium (Mg) alloys are increasingly gaining attention from industry since they appear suitable for a variety of applications, spanning from medical, sports, and automotive to aerospace. Beside the possibility of reducing the fuel consumption and green-house gas emissions in transportation sector due to the lightweighting, they are attractive also for biomedical prostheses being not only biocompatible and resorbable, but also very similar to the human bone in terms of mechanical characteristics. Due to the low formability at room temperature of these alloys, nowadays the possibility of producing complex shaped components is almost limited to the adoption of casting process. However, even through the high pressure die casting process, extremely thin parts cannot be easily produced while the sheet metal forming seems more suitable. However, the adoption of elevated working temperature is necessary. In the present study, SuperPlastic Forming (SPF) is proposed for producing a very thin Mg part having a hemispherical shape. In particular, the attention was focused on the AZ31B Mg alloy which, as proven by literature, shows an emphasized superplastic behavior at 450°C. Both a numerical analysis and experimental trials were conducted and detailed in this work. In order to correctly model the investigated alloy, free inflation tests at 450 °C were carried out setting different pressure levels in a range able to guarantee strain rate values between 10~(-4) and 10~(-1) s~(-1), in which both Grain Boundary Sliding (GBS) and Dislocation Climb (DC) creep can be observed. Both the strain and the micro structure were investigated by analyzing specimens formed by means of free inflation tests considered to better reproduce the strain condition of the industrial forming processes than tensile tests. In such a way, parameters of a material model from literature, able to take into account the influences of both the grain coarsening and the stress value on the strain rate, were det