Itsuo Ohnaka1, Kouji Nishii2 and Tatsuichi Fukusako1
1Department of Metallurgical Engineering, Faculty of Engineering, Osaka University, Suita
In order to make clear the formation mechanism of surface segregation of Al-4.5%Cu ingot cast in a steel mold, the solidifying ingot surface was observed directly. Further the effect of cooling and heating the solidifying ingot surface on the segregation was examined with SEM, EPMA and metallographic techniques. The experiment revealed that the solidifying ingot surface can be covered with a liquid layer, the liquid phase cannot move in the dendrite network of higher fraction solid than about 0.85, and weak water spray cooling of the surface can cause a heavy surface segregation. The latter phenomenon, which is important in the DC casting, seems to be caused by destruction of the balance between aluminostatic pressure and surface tension of oxide film supporting the surface liquid layer. This destruction is due to boiling of water on the surface. It is concluded that the formation of surface segregation can be explained by the squeeze out of interdendritic solute-enriched liquid into the surface layer by aluminostatic pressure, the growth of the α phase towards the mold face in the liquid layer, and the movement of the liquid phase on the surface into the interior of the ingot by solidification shrinkage.
Please do not copy without permission.