Porosity Characterization of Cast Al Alloys with X-Ray Computed Tomography andScanning Electron Microscope

Cast Al-Si alloys are widely used in automotive industry to produce structural components, such as engine block and cylinder head, because of the increasing demands in reducing mass for improved fuel efficiency. The fatigue performance of the castings is critical in their application. Porosity is highly detrimental to the fatigue behavior of cast Al-Si alloys. Therefore, accurate measurement of pore sizes is important in order to develop the correlations between porosity and fatigue strength. However, quantification of porosity is challenging and shows large variation depending on the measurement methods, particularly for micro-shrinkage porosity due to the torturous and complex morphology. The conventional metallographic image analysis method in the 2D polished surface often underestimates the actual pore size particularly when the porosity morphology is complex. In this study, two experimental techniques including X-ray Computed Tomography (X-ray CT) and scanning electron microscope (SEM) are adopted to characterize the size and morphology of porosity for cast Al-Si alloys. Samples were taken from high pressure die casting (HPDC) blocks made of A380 alloy. Pore size distribution in the sample was extracted from X-ray CT scan data and analyzed using Generalized Extreme Value Distribution (GEVD) function. The Equivalent Circular Diameter (ECD) 3-sigma values were obtained to represent the pore size at each location in the sample, which was validated using the maximum ECD pore size observed on the fracture surface with SEM. This study provides an effective and reliable approach to quantify the porosity which can be applied to predict fatigue properties based on the pore size.