Estimation of air gap and heat transfer coefficient at different faces of Al and Al-Si castings solidifying in permanent mould

In the casting processes, the heat transfer coefficient at the metal/mould interface is an important controlling factor for the solidification rate an d the resulting structure and mechanical properties. Several factors intera ct to determine its value, among which are the type of metal/alloy, the mou ld material and surface conditions, the mould and pouring temperatures, cas ting configuration, and the type of gases at the interfacial air gap formed . It is also time dependent. In this work, the air gap formation was comput ed using a numerical model of solidification, taking into consideration the shrinkage and expansion of the metal and mould, gas film formation, and th e metallostatic pressure. The variation of the air gap formation and heat t ransfer coefficient at the metal mould interface are studied at the top, bo ttom, and side surfaces of Al and Al - Si castings in a permanent mould in the form of a simple rectangular parallelepiped. The results show that the air gap formation and the heat transfer coefficient are different for the d ifferent casting surfaces. The bottom surface where the metallostatic press ure makes for good contact between the metal and the mould exhibits the hig hest heat transfer coefficient. For the sidewalls, the air gap was found to depend on the casting thickness as the larger the thickness the larger the air gap. The air gap and heat transfer coefficient also depend on the surf ace roughness of the mould, the alloy type, and the melt superheat. The air gap is relatively large for low values of melt superheat. The better the s urface finish, the higher the heat transfer coefficient in the first few se conds after pouring. For Al - Si alloys, the heat transfer coefficient incr eases with increasing Si content.