Effect of grain refinement on the fluidity of two commercial Al-Si foundry alloys
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I.
INTRODUCTION
DURINGsolidification of a solute-rich aluminum foundry alloy, equiaxed dendrites start to grow into the undercooled liquid. In the first stages, these dendrites can grow relatively independently of each other, but after a certain range of growth, depending on dendrite growth rate and grain density, the primary dendrite tips impinge onto the neighboring dendrites. This is identified as the dendrite coherency point, the point at which a more or less continuous three-dimensional dendrite network is formed in the semisolid material. Previous work ~ 57 has shown that several parameters affect the point where the dendrite network is formed, and dendrite coherency is expected to have significant influence on the castability of these alloys,fr] The castability is an important feature of all aluminum foundry alloys, restricting the applicability of the alloys for casting purposes. Castability is a wide concept including characteristics such as form-filling ability, fluidity, hot-tearing susceptibility, and feeding behavior. One of the important factors that may limit the quality of a casting is the ability of the liquid alloy to flow and fill the mold pattem as cooling and solidification progress. Fluidity is empirically defined as the length the metal flows in a channel with a small cross-sectional area, while solidifying. Fluid flow is also important with respect to feeding, i.e., material transport to compensate for shrinkage. Therefore, some relationship between fluidity and feeding is expected since both depend on the ability of the melt to flow during solidification.t7,8] The solidification mode, morphology, and rate and the grain density are important, determining the size
A.K. DAHLE, Doctoral Student, and L. ARNBERG, Professor, are with the Department of Metallurgy, Norwegian Institute of Technology, N7034 Trondheim, Norway. P.A. TONDEL, Research Scientist, is with Elkem Aluminium, N-8651 Mosjoen, Norway. C.J. PARADIES, Senior Project Engineer, is with Department of Materials, EPF Lausanne, CH1026 Lausanne, Switzerland. Manuscript submitted January 9, 1995. METALLURGICAL AND MATERIALSTRANSACTIONS A
and irregularity of the developing solid, which, combined with the strength and physical properties of the material, define the point of flow inhibition in these channels. Two well-known test methods to investigate the fluidity are the spiral test, often in sand molds, and the vacuum-fluidity test, in glass tubes. Many other parameters have been identified to affect the fluidity and feeding capability of aluminum alloys. Among these are melt superheat, temperature gradients, latent heat, alloy composition, eutectic modification, and grain refinement.t7 151The formal definition of fluidity is the inverse of viscosity, but it has come to reach another meaning in casting terminology. In fact, casting fluidity is independent of the viscosity of the liquid, according to the work by Bastien et al.t~3] This indicates that interdendritic fluid flow is of low significance in the fluidity test and that the lengt
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