The nucleation and growth of microporosity in aluminum-7% silicon foundry alloy

Small cavities, called micropores, are common defects found in aluminum castings. Mathematical modeling of microporosity is difficult due to a lack of chemical parameters and a limited understanding of the mechanisms of pore formation and development.; The objective of the present work is to obtain a better understanding of the nucleation and growth of microporosity in Al-7%Si (A356) foundry alloy. This consisted of the measurement of the surface tension of A356 alloy, a study of the factors that affect the final amount of porosity in castings, and the measurement of the evolution of porosity during solidification.; The measurement of the surface tension of pure aluminum and A356 alloy was performed under vacuum and hydrogen atmospheres. The surface tension of A356 alloy under vacuum at 630°C was 0.889 N/m. The addition of hydrogen did not significantly alter this value. The addition of strontium decreased the surface tension by about 5%. It was concluded that the effect of strontium on the surface tension was not the cause of the increase in porosity observed in strontium modified castings.; The effects of hydrogen content, local solidification time and strontium modification on microporosity were studied. Two critical fraction solids were associated with the formation of microporosity. The first, the critical fraction solid for pore growth, is the point at which preexisting baseline porosity begins to grow due to hydrogen evolution. It varied with hydrogen content and strontium modification. The second, the critical fraction solid for nucleation, is the point at which shrinkage type porosity nucleates. It did not vary with hydrogen content or strontium modification, and remained constant at approximately 75–80% solid.; The measurement of microporosity was performed using image analysis. A new method for filtering and sorting the data, based on nearest neighbor cluster analysis, was developed.; A mathematical model based on the current experimental results was created. The model is capable of predicting the evolution of the percentage porosity during solidification, the final percentage porosity and the maximum pore size.