An induction heating process with coil design and solutions avoiding coarsening phenomena of Al-6 Pct Si-3 Pct Cu-0.3 Pc
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I. INTRODUCTION
FOR the thixoforming process, both the coexisting solidus-liquidus phase and the reheating conditions to obtain the globular microstructure are very important. In a study on the induction heating process of semisolid materials (SSM), Sebus and Henneberger[1] verified that the design of the induction coil had an important influence on the induction heating process. Matsuura and Kitamura[2] suggested that it is very important not to destroy the crystallization matrix of semisolid metals due to the change of the size of spherical dendrite grains during reheating. In a study on the induction heating of semisolid aluminum alloys, Midson et al.[3] proposed that a coil design was necessary for uniform induction heating. Rudnev and co-workers[4–7] showed that the required temperature distribution along the end of the slug depends on the frequency, the coil and slug geometry (including the slug-to-coil air gap and coil overhang), the material properties of the slug, its emissivity, coil refractory, power density, and cycle time. Hirt and co-workers[8,9] and Garat and co-workers[10–13] have both repeatedly emphasized that it is important to prevent the coarsening for thixoforming, because, generally, an average grain size below 100 mm in the reheated state will be sufficient to ensure a homogeneous material flow of the semisolid alloy and good dimensional stability during die filling. Kang et al.[14] experimentally proposed the optimal reheating conditions for thixoforging and the semisolid die casting processes using SSM (A356 alloys) with d 3 l 5 39 3 85 (mm) and d 3 l 5 76 3 60 (mm). Jung and Kang[15] proposed and manufactured an optimal coil design to reduce the temperature gradient of H.K. JUNG, Research Assistant, and C.G. KANG, Director, are with the School of Mechanical Engineering, Engineering Research Center for Net Shape and Die Manufacturing, Pusan National University, Pusan 609735, Korea. Manuscript submitted December 21, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
the billet and to obtain a globular microstructure for thixoforging and semisolid die casting processes by using A356 (ALTHIX) alloy with d 3 l 5 76 3 90 (mm) and a 60 Hz frequency for the induction heating system. Moreover, they demonstrated the suitability of the coil design by performing reheating experiments. The effects of reheating time, billet size, and holding temperature on microstructures for the semisolid die casting and thixoforging of Al-6 Pct Si-3 Pct Cu-0.3 Pct Mg alloy (ALTHIX 86S) have not been reported. Therefore, the objective of this study is to determine the effects of the reheating conditions on the globularization of microstructures. The reheating time, the holding temperature, the holding time, the adiabatic material size, and the capacity of the induction heating system were considered as parameters of the globularization of microstructure. Particular interest is focused on the solutions avoiding the coarsening phenomena. Moreover, as the first part of our investigation, the optimal coil design to reduce
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