No abstract

Porosity is one of the most common defects to degrade the mechanical properties of aluminum alloys. Prediction of pore size, therefore, is critical to optimize the quality of castings. Moreover, to the design engineer, knowledge of the inherent pore population in a casting is essential to avoid potential fatigue failure of the component. In this work, the size distribution of the porosity was modeled based on the assumptions that the hydrogen pores are nucleated heterogeneously and that the nucleation site distribution is a Gaussian function of hydrogen supersaturation in the melt. The pore growth is simulated as a hydrogen-diffusion-controlled process, which is driven by the hydrogen concentration gradient at the pore liquid interface. Directionally solidified A356 (Al-7Si-0.3Mg) alloy castings were used to evaluate the predictive capability of the proposed model. The cast pore volume fraction and size distributions were measured using X-ray microtomography (XMT). Comparison of the experimental and simulation results showed that good agreement could be obtained in terms of both porosity fraction and size distribution. The model can effectively evaluate the effect of hydrogen content, heterogeneous pore nucleation population, cooling conditions, and degassing time on microporosity formation.

No abstract

Metal cleanliness is one of the most critical parameters affecting microporosity formation in aluminum alloy castings. It is generally acknowledged that oxide inclusions in the melt promote microporosity formation by facilitating pore nucleation. In this study, microporosity formation under different casting conditions, which aimed to manipulate the tendency to form and entrain oxide films in small directionally cast A356 samples was investigated. Castings were prepared with and without the aid of argon gas shielding and with a varying pour surface area. Two alloy variants of A356 were tested in which the main difference was Sr content. Porous disc filtration analysis was used to assess the melt cleanliness and identify the inclusions in the castings. The porosity volume fraction and size distribution were measured using X-ray micro-tomography analysis. The measurements show a clear increment in the volume fraction, number density, and pore size in a manner consistent with an increasing tendency to form and entrain oxide films during casting. By fitting the experimental results with a comprehensive pore formation model, an estimate of the pore nucleation population has been made. The model predicts that increasing the tendency to form oxide films increases both the number of nucleation sites and reduces the supersaturation necessary for pore nucleation in A356 castings. Based on the model predictions, Sr modification impacts both the nucleation kinetics and the pore growth kinetics via grain structure.