Dendrite coherency during equiaxed solidification in binary aluminum alloys

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I.

INTRODUCTION

EQUIAXED dendritic growth is a frequently observed phenomenon during the solidification process in an undercooled alloy. Equiaxed solidification usually provides the most favorable structure in many castings and therefore has long been of interest to materials scientists. Several models t~-6] have been proposed. In order to study grain refinement by nucleants, Maxwell and Hellawell t~] proposed a model to analyze equiaxed crystal growth in the early stage of solidification. Hunt [2] proposed a theoretical approach to predict the volume fraction of equiaxed grains and the fraction liquid within the semisolid dendritic grains by means of a steady-state model and a heat-flow equation. Dustin and Kurz t3] first used the concept of the internal volume fraction of solid to describe equiaxed dendritic grains. Their model can predict the temperature-time evolution during equiaxed dendritic solidification and also some interesting features, such as grain number and recalescence. Rappaz and Thrvoz ~4,5] have developed a detailed solutal diffusion model for equiaxed dendritic solidification. They proposed that the internal volume fraction of solid is a function of the supersaturation of the melt and the Prclet number of the dendritic grain. This result is important in the analysis of equiaxed dendritic growth and dendrite coherency. In the early stage of equiaxed dendritic solidification, the dendritic crystals are separate and can move freely. When the developing dendrites start to impinge on one another, the dendritic network becomes coherent. At this point, the system behaves less as a liquid and begins to resemble a solid. There is a liquid-solid transition of properties, such as shear strength, thermal conductivity, and contraction, and mass feeding also ceases. For consistency, we will call this stage the "dendrite coherency" point. The temperature, time, and fraction solid at this GUOCAI CHAI, Ph.D., and LENNART BA,CKERUD, Professor, are with the Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-10691 Stockholm, Sweden. TONE ROLLAND, Doctoral Candidate, and LARS ARNBERG, Professor, are with the Department of Metallurgy, Norwegian Institute of Technology, N-7034 Trondheim, Norway. Manuscript submitted April 25, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A

point are called coherency temperature, coherency time, and coherency fraction solid, respectively. When the dendrites become coherent, a number of processes start to take place simultaneously in the semisolid region, and casting defects such as macrosegregation, shrinkage, porosity, and hot tearing start to develop. To understand where dendrites become coherent is therefore important to control the formation of solidification structure. Dendritic coherency has been studied by utilizing the fact that the strength of the solidifying material increases sharply at the dendrite coherency point, t7-9] The change of strength during this stage can be determined by isothermal [7,8] or continuous shear stress measurement. [

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