Metal-mold heat transfer and solidification of magnesium alloys in belt casting processes

A high speed strip casting simulator has been designed to simulate the casting of magnesium sheet alloys (AM50 and AZ91) on a single belt horizontal caster. Using Inverse Heat Transfer Analysis, temperature-time data from thermocouples inserted in the copper or steel bar substrates were used to deduce instantaneous heat fluxes between the solidifying strip and the moving mold substrates. Maximum, or peak, heat fluxes were registered downstream of the impact region for metal delivery onto the moving chill mold; for a copper substrate moving at 0.7m/s, peak heat fluxes were recorded after 0.3 seconds of contact, whereas those for a yttria stabilized zirconia coated steel substrate were registered following 2.5 seconds of metal-mold contact. The microstructures of the thin strips (3-4mm thick, 1m long, and 40mm wide) were characterized in terms of SDAS (Secondary Dendrite Arm Spacings) as well as cooling rates. It was found that the major thermal resistance to heat flow from the strip to the substrate resided in an interfacial layer separating the strip from the substrate. This interfacial layer presumably corresponded to the entrainment of a thin gas film, which was manifested in the texture of the bottom surface which was seen to contain numerous small air pockets. Methods to resolve these issues, and to improve thermal contact, are discussed. Results to date suggest that the peaks in the maximum heat fluxes corresponded to maxima in the release of the alloy's latent heat of crystallization during their transformation from the liquid to solid crystalline state.