Effects of hydrogen content and solidification time on porosity formation in LFCs of A356 alloy— a semi-empirical model

摘要

Criterion functions are widely used in computer simulation of casting process to predict the shrinkage formation in various alloys, including aluminum alloys. However, the application of the standard criterion functions, such as Niyama and others, to aluminum alloys is limited to those with low hydrogen content, since the effect of hydrogen content has not yet been modeled. Using stepped lost foam castings (LFCs) of A356 with different hydrogen contents, new relationships between pore size (D in micron) and local solidification time (t{sub}f in minutes) was developed: D = A(t{sub}f){sup}n where n and A are parameters depending on hydrogen content. Using this relationship, a unique semi-empirical model was set up to describe the growth of pores in LFC, for different hydrogen contents. Based on this model, a new criterion function, CR, that includes the contributions of interdendritic feeding mechanism and gas pore formation was proposed to predict the porosity formation of LFCs with different hydrogen levels, shapes and sizes. An evaluation of the new criterion function was carried out with an extensive data analysis of the castings, which turned out to be an excellent correlation between the prediction of this new criterion function and the experimental data. It is possible to extend this criterion function to other alloys in which the influence of gas pores resulting from hydrogen precipitating during solidification cannot be ignored, provided the correlation of pore size to thermal parameters and hydrogen content is available.