Solidification of hypereutectic Al-38 Wt Pct Cu alloy in microgravity and in unit gravity
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INTRODUCTION
GRAIN size and morphology are important factors in foundry industries. Fine and equiaxed grain structures exhibit higher strengths at low temperatures, reduced anisotropy, and enhanced high-cycle fatigue properties. The production of an equiaxed zone in a casting requires the existence of small crystallites or nuclei in the bulk liquid during solidification. Three mechanisms for the provision of these nuclei have been proposed:[1] (1) heterogeneous nucleation driven by constitutional supercooling; (2) ‘‘big bang’’ mechanism; and (3) dendrite detachment mechanism. Two grain-multiplication processes can operate: one involves dendrite remelting and the other is associated with fluid motion. It has been shown that dendrite detachment or fragmentation via fluid flow probably accounts for the interior equiaxed structure in some alloy castings.[2,3] Also, Tiller and O’Hara[4] have shown that fluid motion during solidification leads to an increased probability of grain multiplication and refinement by either fragmentation of dendrites and misalignment of the separated segments, or by coagulation and disorientation of dendritic arrays. They showed that a fluid velocity of 0.25 m/s past an array of dendrites could cause bending stresses greater than 1 MPa. A stress of this magnitude could be sufficient to cause deHONG YU, formerly Graduate Student, Metallurgical Sciences Laboratory, Department of Mechanical and Industrial Engineering, University of Manitoba, is an Engineer with Standard Aero Ltd., Winnipeg. K.N. TANDON, Associate Professor, ITC-Advanced Materials Initiative, and J.R. CAHOON, Professor, Metallurgical Sciences Laboratory, are with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6. Manuscript submitted August 5, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
formation of the dendrites, which are at a temperature very near their melting temperature and thus have low strengths, and the local adiabatic heat generated by the process could cause melting or recrystallization. Gravity-driven convective flow can significantly influence the microstructure of cast metals and alloys. The microsegregation and macrosegregation occurring during solidification,[5,6,7] as well as the role of gravity-driven phenomena such as convection and sedimentation[8,9,10] on microstructure, have been studied in various alloys. Scale analyses were used to establish order-of-magnitude forces arising from natural convection during the growth of protein molecules.[11] It was found that the shear stresses arising from natural convection were much too weak to have any effect on the growth characteristics of protein molecules. Recently, Cahoon et al.[12] investigated the role of convective mixing on grain refinement during solidification of Al-Ti-B alloys. They concluded that supernucleant TiB2 particles were responsible for grain refinement and that gravity-induced phenomena played little, if any, role. Thermosolutal convection caused by solute buildup ahead of gro
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