Three-dimensional visualization and quantification of microporosity in aluminum castings by X-ray micro-computed tomography

Porosity is a major issue in solidification processing of metallic materials. In this work, wedge die casting experiments were designed to investigate the effect of cooling rate on microporosity in an aluminum alloy A356. Microstructure information including dendrites and porosity were measured and observed by optical microscopy and X-ray micro-computed tomography(XMCT). The effects of cooling rate on secondary dendrite arm spacing(SDAS) and porosity were discussed. The relationship between SDAS and cooling rate was established and validated using a mathematical model. Three-dimensional(3-D)porosity information, including porosity percentage, pore volume, and pore number, was determined by XMCT. With the cooling rate decreasing from a lower to a higher position of the wedge die, the observed pore number decreases, the porosity percentage increases, and the equivalent pore radius increases.Sphericity of the pores was discussed as an empirical criterion to distinguish the types of porosity. For different cooling rates, the larger the equivalent pore radius is, the lower the sphericity of the pores. This research suggests that XMCT is a useful tool to provide critical 3-D porosity information for integrated computational materials engineering(ICME) design and process optimization of solidification products.