Phenomena of Formation of Gas Induced Shrinkage Porosity in Pressure Die-Cast Mg-Alloys

Abstract

It is usually believed that formation of gas and shrinkage porosity has different dependencies on the process conditions, and consequently, the gas and shrinkage pores are usually regarded as independent microstructural features. In this contribution, it is shown that in high-pressure die-castings, gas pores can lead to the formation of shrinkage porosity under specific conditions. This is because the air/gas in the gas pores is an efficient heat-insulating medium. Therefore, presence of gas porosity can retard heat transfer in the liquid melt as compared to similar regions without gas porosity. Consequently, the local solidification rate is lower in such regions, leading to shrinkage porosity formation. Two and three-dimensional (3D) microstructural observations using digital image analysis and numerical simulation for heat transfer support such a hypothesis. Finite element (FE)-based simulations have also been performed on the 3D microstructures to reveal the variations of local stresses caused by gas induced shrinkage porosity.