Interfacial heat transfer and solidfication of Mg and A1 alloys in a single belt casting process

Strips of magnesium (A-Z91 and AM50) and aluminum (AA6111) alloys were cast on a moving mold system devised for simulating single belt casting operations. Different type of substrates were used for the moving mold; copper, low carbon steel, alumina coated steel, nickel coated steel, and zirconia coated steel. Interfacial heat fluxes between the solidifying strip and the moving mold substrate were calculated by the IHCP (Inverse Heat Conduction Problem) technique. The temperature history of the solidifying strip was calculated using the Enthalpy FVM (Finite Volume Method) associated with Scheil's model of solidification. The influence of thermophysical and morphological properties of mold substrates, wetting environment, casting speed, melt superheat, and grain refiner, on strip microstructures and their cooling rates, were thoroughly investigated.Maximum heat fluxes appeared about 0.1~0.2 seconds after the moment of thermal contact. For a copper substrate, these reached up to 7 MW/m 2, whereas those for a steel substrate were ~3 MW/m2. The rate of heat transfer to coated substrates was significantly delayed and reduced because of the low thermal conductivity of coating layers. Maximum cooling rates of aluminum and magnesium strips reached about 200°C/s on copper and about 150°C/s on steel.It was found that the major thermal resistance resided in an interfacial layer separating the strip from the substrate. This interfacial layer presumably corresponded to the shrinkage of solidified skin or the entrainment of a thin air film resulting in numerous air pockets at the bottom surface of strips. Oil spraying onto the mold surface changed the interfacial wettability and dramatically improved the cast surface of the strips. The good quality of strip samples obtained suggests the high potential for the direct casting of these alloys on single belt casters