Effect of solidification velocity and hydrogen content on porosity in directionally solidified A356 castings

Abstract

Micro-focus X-ray technology was utilized to evaluate the influence of solidification velocity and hydrogen content on the volume fraction, number density and sizes of pores in the directionally solidified A356 castings. The results indicate that hydrogen content has a significant influence on porosity formation. When hydrogen content is low, few small irregular-shape pores were observed indicating the dominate impact of solidification shrinkage. While in high hydrogen specimens, many large spherical pores were found. The pore size distribution also shows dual populations. The group of large pores is formed in the liquid far from the solidification front. The group of small pores is formed near the solidification front. When hydrogen content is high, increasing solidification velocity (from 0.1mm/s to 0.2mm/s) not only decreases volume fraction of porosity but also significantly reduces the maximum pore sizes of large pores.