On The Formation of Macrosegregations in
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INTRODUCTION
IT is well known that natural convection plays an important role in the structure of large castings and even in small laboratory ingots, tl,2,3j The grain structure taj and macrosegregation 1~,2,3j in the cast products are directly influenced by the magnitude of convection. In binary alloys, primary solids first solidify with a leaner alloy composition (R < 1). This results in enrichment of the alloying element in the remaining liquid compared with the initial composition. The density of the melt may increase or decrease depending on the alloying element. The difference in the density can initiate convection in the melt and results in transfer of the alloying element into the bulk liquid, f5'61The final effect is a large degree of composition variation in the casting known as macrosegregation. Different types of macrosegregations are defined in the literature, tT~ The most common ones are A-, positive, and negative segregates. The A-segregates are channels formed due to an interdendritic flow in the two-phase region (solid/liquid) during the solidification process. They are formed in the same way as freckles, t81 Positive and negative segregates are a deviation from the average composition, where a positive segregation means a higher alloying content and a negative means a lower concentration. They can be formed by many different mechanisms, the most common being interdendritic flow during the solidification process. The flow can be caused by solidification shrinkage or density variations creating a natural convection. Different mechanisms of macrosegregation, such as A, positive, and negative, are discussed in the literature. The authors have developed an experimental apparatus to study the effect of gravity on the formation of macrosegregation. The experimental apparatus will fly in the space shuttle in the near future. A number of numerical H. SHAHANI, Doctor, formerly with the Department of Casting of Metals, Royal Institute of Technology, Stockholm, is with Sandvik Hard Materials, Stockholm, Sweden. G. AMBERG, Doctor, Department of Hydrodynamics, and H. FREDRIKSSON, Professor, Department of Casting and Metals, are with the Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted June 21, 1991. METALLURGICAL TRANSACTIONS A
and experimental investigations have been performed. A portion of these will be presented in this article. In order to study the effect of gravity on convection and macrosegregation, the Pb-Sn system was selected. The main reasons for choosing this system were the wellknown properties of the elements and the low melting points of the alloys which makes them suitable for microgravity investigation. In order to study the effect of both a positive and a negative density coefficient, a Pb-rich and a Sn-rich alloy were studied. In the fLrst alloy, the interdendritic melt has a lower density than the initial melt, while the interdendritic melt in the second alloy has a greater density. These alloys were solidified unidirectionally with the gravity vector pe
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