Utility of micro-indentation technique for characterization of the constitutive behavior of skin and interior microstructures of die-cast magnesium alloys

There has been increasing thrust lately on the development of lightweight cast magnesium alloy components for structural automotive and other applications. The microstructure of the high-pressure die-cast Mg alloys usually contains a fine-grained “skin” having a microstructure significantly different from that of the bulk material. Characterization of the local constitutive behavior of the skin microstructure is of interest as it can affect the overall mechanical response of the component. However, the standard mechanical tests on the macro-specimens are not useful for characterization of the local stress–strain response of the skin microstructure. In this contribution, we present a novel methodology based on a combination of micro-indentation experiments and three-dimensional (3D) finite elements based simulations that permits computation of the local stress–strain (constitutive) behavior of the skin and the interior microstructures at the length scales of 100 μm in a cast high-pressure die-cast AM60 Mg-alloy. The methodology involves development of a numerical solution to the inverse problem. The computed constitutive equations are then utilized to simulate the effect of skin thickness on the overall global mechanical response of the alloy under uniaxial compression.