Settling of multisized clusters of alumina particles in liquid aluminum
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I. INTRODUCTION
SETTLING of ceramic particles in metal matrix composites (MMC) made by liquid state processing occurs during fabrication, remelting, or casting operations. This results in a nonuniform distribution of the ceramic particles in the metal matrix and consequently degrades the mechanical properties of the MMC. Clustering of the particles is a related problem, making the particles settle more quickly. Therefore, the particles may be unevenly distributed macroscopically (denuded regions due to settling) or microscopically (clusters of particles). Consequently, the mechanical properties will be nonuniform.[1] In some instances, subsequent extrusion or rolling may homogenize the microstructure. However, it is clear that a good understanding of the settling process is important to optimize the processing and properties of MMCs produced by melt stirring. The settling characteristics of silicon carbide particles in liquid aluminum have been investigated earlier in our research group.[2,3] For coarse particles (90 mm), it was found that the settling rate was much slower than predicted by Stokes’ law because of the high volume fraction of particles studied (up to 0.25).[2] This slow settling is known as hindered settling, wherein the countercurrent flow of liquid impedes the fall of the particles. In the previous study,[2] it was found that there were some strong similarities between the settling of particles in aqueous and metallic systems. In a second study, with finer silicon carbide particles (14 mm), the settling rate was much greater than expected from hindered settling of individual particles, due to the formation of clusters.[3] The review of the settling literature in aqueous and MMC systems in those two articles will not be repeated here. In the present work, with alumina particles in aluminum, there were striking differences in the settling behavior compared to the silicon carbide-aluminum system. As will be C. TIAN, formerly Post-Doctoral Fellow with the Department of Materials Science and Engineering, McMaster University, is with the Division of Manufacturing Technology, CSIRO, Preston, Victoria 3072, Australia. G.A. IRONS, DOFASCO/NSERC Industrial Research Chair in Process Metallurgy, and D.S. WILKINSON, Professor of Materials Science and Engineering, are with McMaster University, Hamilton, ON, Canada L8S 4L7 Manuscript submitted November 7, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
shown, the differences are attributed to a wider size distribution of the clusters. The experimental findings are reported and a model for the settling of multisized clusters is developed to account for the findings. II. EXPERIMENTAL The sedimentation experiments were carried out in a claygraphite crucible (254-mm i.d. and 475-mm height) placed in an electrical resistance furnace. Initially, 25 kg of aluminum (99.99 pct Al, supplied by Alcan International, Kingston, ON) was melted in the crucible and held at 700 8C. Alumina powder (10.5 6 3.2 mm, Fujimi Corp., Elmhurst, IL) was stirred into the melt with a graph
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