Comparison between different methods to determine material constants of the ZK60 Mg alloy from hot bulge tests data

ZK60 is an Aluminium-free Magnesium alloy able to achieve high ductility in superplastic conditions. Those properties make the alloy suitable to be investigated for temporary prostheses production by means of the Superplastic Forming (SPF) process. To properly design the manufacturing process by numerical simulations, an accurate material constitutive model is needed. In the present work, bulge tests at 400 degrees C were conducted on a ZK60 Magnesium sheet using different loading conditions, namely two different levels of constant pressure (CP) and jump pressure (JP) between the two pressure levels. The dome height evolution was acquired during each test and used to calibrate the material constants of the Backofen constitutive equation (sigma = C . (epsilon)over dot(m)) using three different methodologies: (i) an analytical approach (ii) an inverse methodology based on a single CP test and (iii) an inverse methodology based on the JP test. The obtained constants were validated by subsequent numerical simulations. Comparing the numerical/experimental dome height curves, it was found that the sets of constants determined using the inverse methodology based on the JP test are able to describe the material superplastic behaviour over a wider span of loading conditions.