Solidification processing of Al-Al 2 O 3 composite using turbine stirrer
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I.
INTRODUCTION
The major concerns in solidification processing of metal-matrix composites (MMCs) containing ceramic particles are: (1) wetting between molten alloy and the ceramic dispersoids,[1,2] (2) segregation of dispersoids during its mixing into the melt[3] and solidification,[4] and (3) porosity.[5] While mixing relatively coarser alumina particles (;70 mm) in a molten, aluminum-magnesium alloy with the help of a flat blade stirrer, Ghosh and Ray have correlated the particle content and porosity in the cast composite ingot with the process variables during mixing.[6] The most significant process variables identified by earlier investigators for mixing solid particles with liquid are as follows: (1) stirrer diameter characterized by dimensionless parameter d/D, where d is the diameter of the stirrer and D is the diameter of the container at the surface of the liquid; (2) position of the stirrer inside the liquid characterized by dimensionless parameter h/H, where h is the height from the bottom of the container at which the stirrer is placed, and H is the depth of liquid in the container; and (3) stirring speed.[7] Ghosh and Ray included temperature T of the melt as an additional variable in the context of melt-particle mixing also to include the mixing of particles in a semisolid alloy at a temperature below that of the liquidus of the alloy.[6] J.A. AL-JARRAH, Research Scholar, Department of Mechanical and Industrial Engineering, S. RAY, Professor, Metallurgical and Materials Engineering, and P.K. GHOSH, Project Scientist (Professor), Welding Research Laboratory, are with the University of Roorkee, Roorkee, 247 667, India. Manuscript submitted August 27, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
In an earlier study, it has been observed that the overall particle content in the cast ingot is maximum at optimum values of d/D, h/H, and the stirring speed, which, apart from the design of the stirrer, decide the radial and axial flow patterns developing in the molten alloy and their strength.[8] El-Kaddah and Chang observed that there is a critical stirring speed below which the lift force in the axial flow is not able to overcome the settling of particles.[9] Thus, one understands broadly the importance of flow behavior and variables of stirring in controlling the particle content in the slurry and in the ingot cast from this slurry. But the maximum porosity at the same values of d/D, h/H, and the stirring speed where particle content is maximum has been observed in cast composites containing particles not wetted by the molten matrix alloy. Porosity in an alloy adversely affects mechanical properties like ultimate tensile strength nonlinearly, but in a particle reinforced composite containing the same alloy as the matrix, the ultimate tensile strength sp reduces linearly with an increase in volume percent of porosity P, as[10]
sP 5 1 2 aP s0
[1]
where s0 is the ultimate tensile strength of the matrix alloy alone, and a is a constant called weakening factor that depends on the matrix alloy and the
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