The Effect of the Natural Convection on the Transition from Columnar to Equiaxed Crystals
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THE EFF"ECT OF THE NATURAL CONVECTION ON THE TRANSITION FROM COLUMNAR TO EQUIAXED CRYSTALS: HASSE FREDRIKSSON, Department of casting of Metals, The Royal Institute of Technology, 100 44 Stockholm,
Sweden
ABSTRACT During dendritic solidification of metal alloys there normally occur temperature gradients in the liquid ahead of the solidification front. The gradient causes a natural convection ahead of the front. The natural convection causes a heat transfer from the interior of the liquid, which gives the possibility for crystals to grow ahead of the solidification front. The growth of these crystals is determined by the cooling rate and the kinetics of the solidification process. The paper will deal with the theory behind the growth of free crystals ahead of a solidification front. Some experimental results will be given. The effect of gravity on the growth process will. be discussed. INTRODUCTION The cast structure is mainly very important for the quality of an ingot and the formation of the different crystal zones in an ingot has been extensively discussed through the years. Particularly much attention has been paid to the transition from the columnar to the equiaxed crystal zone. It has been shown that this transition is influenced by the constitution of the alloy system, the composition of the alloy, the cooling rate of the mould, by innoculation, by stirring and by the convection (1-7). During the last twenty years a lot of work has been done by Cole and associates (6-7) who especially pointed out the importance of the natural convection for the transition. In these early works by Cole and coworkers the influence of the natural convection on the nucleation of free crystals was pointed out to be the most important parameter. However, in a recent work by Fredriksson and Olsson (8) it was shown that the most important parameter for the transition from columnar to equiaxed crystals was the growth of free crystals. The growth of those crystals was deterimined by the natural convection. Both M Johnson et.al. (9) and H. Fredriksson et.al. (10) have reported that the transition from columnar to equiaxed crystals are influenced by the gravity. In this paper the model derived by Fredriksson and Olsson (8) will be used in order to predict the influence of the gravity on the transition from columnar to equiaxed crystals. The calculations will be performed on an experimental geometry used in the experiments performed by Fredriksson and Hillert (11). This geometry has been chosen depending on that we have proposed to do experiments'similar to those in space. In this paper Fredriksson and Hillert's (11) experimental results
620 will be used for a comparison of the theoretical model. Their experimental results have been evaluated in a different way than before. Therefore we will in this paper give a short summary of those experimental results. Experimental observation. In the experiments performed by Fredriksson and Hillert a U-shaped construction between two flat insulating brick walls was used. The heat extraction t
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