Investigation of heat transfer coefficient at casting mold interface during solidification

In the casting processes, the heat transfer coefficient at the metal/moldinterface is an important factor for the solidification rate which controlsmicro structure and mechanical properties in the casting. The heat transfercoefficient at the metal/mold interface is time dependent and affected byseveral parameters such as the alloy type, the mold material and surfaceroughness, the mold and pouring temperatures, casting configuration,shrinkage of the metal, expansion of the mold and formation of air gap atthe interface. The purpose of this investigation is to study variations in themetal-mold interface heat transfer coefficient with respect to the moldsurface roughness and time, and to clarify the interfacial heat transfermechanisms.Time-temperature relationships were measured with the copper and hotworking steel (H13) chills having five various surface roughness values inupwards solidifying castings of eutectic Al-Si alloy against the chills. Toevaluate the effect of surface roughness on metal-mold interface heattransfer coefficients measured temperatures were used. In order to clarifythe effect of chills on macro and microstructures, the casting surfacecontacted to the chill and vertical section of the casting were examined.The formation of casting surface deformation was determined from theprofiles of casting and the chill surfaces.The relationship between metal-mold heat transfer coefficient and moldsurface roughness was identified in terms of 24, 60, 280, 400 and 2400grid emery papers.It is concluded that:1- The heat transfer coefficient for 60, 280 and 400 grid emery papersincreased with increasing grid number.2- The heat transfer coefficient for both copper and H13 steel moldshaving 24 grid emery paper was obtained higher than 60, 280 and 400grid emery papers because of the penetration of the liquid metal intothe cavities resulted from 24 grid emery paper.3- 2400 grid emery paper produced the lowest heat transfer coefficientfor copper chill while the highest heat transfer coefficient for H13steel chill, and the formation of relatively high density gas cavitieson both casting surfaces were observed.4- Solidification in castings against copper and H13 steel chills started atpre-dendritic points contacted to the chill surface.5- Dendrites on the castings against H13 steel chill were directional fromthe casting surface although the castings against copper chill this wasmeasured as approximately 200 µm from a distance of the interface.6- Primary silicon crystals were formed of the surface on the castingscontacted to H13 steel chill.7- Concave or convex deformation on the surfaces of eutectic Al-Sicastings contacted the chill was observed to be non-existent.