X-Ray Microtomographic Characterization of Porosity in Aluminum Alloy A356

The presence of microporosity can, in certain circumstances, lead to a significant reduction in the mechanical properties of cast aluminum alloys. The size of the microporosity is especially crucial to the performance of castings in fatigue applications. The aim of this study is to investigate the effect of cooling rate and degassing time on the formation and evolution of microporosity in aluminum alloy A356 (Al-7Si-0.3Mg) castings using traditional metallography and X-ray microtomography (XMT) methods. The final results clearly confirm that increasing the cooling rate and degassing time yield lower microporosity within the microstructure. The XMT analysis enabled the calculation of the volume fraction and size distribution of microporosity. The XMT analysis revealed a bimodel distribution of pores with one population of small pores and a second population of large pores. The large population has been attributed to hydrogen gas solubility, whereas the smaller population is proposed to be linked to the localized entrapment of liquid metal occurring at the end of mass feeding.