Influence of Solidification Thermal Parameters on the Columnar-to-Equiaxed Transition of Aluminum-Zinc and Zinc-Aluminum
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THE microstructure of Al-Zn and Zn-Al (ZA) ingot castings is of technological interest because the Al-Zn alloys combine the high wear resistance, heat resistance, and strength with the castability demanded for many aeronautic and automotive industrial applications.[1,2] Also, ZA alloys combine high strength and hardness, good machinability with good bearing properties, and wear resistance often superior to standard bronze alloys. The ZA castings compete with cast iron, bronze, and aluminum because of various property and processing advantages. All ZA alloys offer superior creep resistance and performance at elevated temperatures compared to standard zinc alloys and can be applied where bearing properties are important.[3–8] In the present report, the focus is on the directional solidification of Al- and Zn-based alloys and, more particularly, on the columnar, equiaxed growth, and columnar-to-equiaxed transition (CET). Experimental evidence of the CET has been reported in previous articles,[9–25] and also a number of mechanisms for the phenomenon of the CET have been proposed.[26–47] It A.E. ARES, Research Assistant, and C.E. SCHVEZOV, Professor, are with the Consejo Nacional de Investigaciones Cientı´ ficas y Te´cnicas, (CONICET)/Faculty of Sciences, Misiones State University, (3300) Posadas, Misiones, Argentina. Contact e-mail: schvezov@ fceqyn.unam.edu.ar This article is based on a presentation made in the symposium entitled ‘‘Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,’’ which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee. Article published online May 25, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
has been demonstrated that the CET is influenced by casting size, composition, and constitution of the alloy system, melt superheat, stirring, heat-transfer coefficient at the metal mold interface, inoculation, cooling rate of the mold, temperature gradient, etc.[11] Recently, Mangelinck-Noel et al.[25] obtained results on the CET by X-ray radiography. In recent years, numerical modeling of the CET has been the subject of studies.[26–47] Essentially, it is possible to find two types of models: stochastic models[26,27,28] and deterministic models.[29–47] The combination of multiple factors and the complexity of phenomenon present difficulties for the total understanding and mathematical modeling of the CET. In the present investigation, directional solidification experiments on aluminum-zinc and zinc-aluminum alloys are performed in order to measure the temperature fields during the transition, which are used to derive and analyze the most important parameters such as cooling rate, velocity of the liquidus and solidus fronts, local solidification time, temperature gradients, and recalescence. The evolution of these parameters is followed and contrasted with the grain structure obtained after solidification and with previous research in other alloy syst
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