Stir Casting

In casting of metal matrix composites, the reinforcement is incorporated as loose particles or whiskers into the molten metal matrix. Because most metal reinforced systems exhibit poor wetting, the mechanical force produced by stirring is required to combine the phases. In stir casting, shown schematically in Fig. 9.7, the particulate/whisker/short fiber reinforcement is mechanically mixed into a molten metal bath. A heated crucible is used to maintain the molten matrix at the desired temperature, while a motor drives a mixing impeller that is submerged in the molten matrix. The reinforcement is slowly poured into molten matrix at a controlled rate to insure a smooth and continuous feed. As the impeller rotates, it generates a vortex that draws the reinforcement particles into the melt from the surface. The impeller is designed to create high shear to strip adsorbed gases from the reinforcement surfaces. Shearing also helps to cover the reinforcement with molten matrix, promoting reinforcement

Cost (Log Scale)

Fig. 9.6. Performance/Cost Trade-offs for MMCs2

Cost (Log Scale)

Fig. 9.6. Performance/Cost Trade-offs for MMCs2

Reinforcement

Reinforcement

Fig. 9.7. Stir Casting

wetting. Proper mixing techniques and impeller design are required to produce adequate melt circulation and a homogeneous reinforcement distribution. The use of an inert atmosphere or a vacuum is essential to avoid the entrapment of gases.4 A vacuum prevents the uptake of gases; eliminates the gas boundary layer at the melt surface; and eases the conditions for particle entry into the melt. Difficulties can include segregation/settling of the secondary phases in the matrix, particle agglomeration, particle fracture during stirring, and excessive interfacial reactions.

Good particle wetting is important. Adding particles, and especially fibers, to the melt increases the viscosity. In general, ceramic particles are not wetted by the metallic matrix.5 Wetting can be enhanced by coating the reinforcement with a wettable metal which serves three purposes: (1) protects the reinforcement during handling, (2) improves wetting, and (3) reduces particle agglomeration. For example, the addition of magnesium to molten aluminum has been used to improve the wettability of alumina, and silicon carbide particles have been coated with graphite to improve their weattability in A356 aluminum.5 Effective wetting also becomes more difficult as the particle size decreases due to increases in surface energy, greater difficulty in dispersing due to increased surface area, and a greater tendency to agglomerate.

Microstructural inhomogeneities can occur due to particle agglomeration and sedimentation in the melt. Redistribution, as a result of the reinforcements being pushed by an advancing solidification front, can also cause segregation. After mixing, reinforcement segregation can occur due to both gravity effects and solidification effects. The reinforcement, when it encounters a moving liquid/solid interface, may either be engulfed in the metal or it may be pushed by the interface into areas that solidify last, such as interdendritic regions. Since stir casting usually involves prolonged liquid-reinforcement contact, substantial interfacial reactions can result that degrades the composite properties and also increases the viscosity of the melt, making casting more difficult. In SiCp/Al, the formation of Al4C3 and silicon can be extensive. The rate of reaction can be reduced, and even become zero, if the melt is silicon-rich, either by prior alloying or as a result of the reaction.6 Therefore, stir casting of SiCp/Al is well suited to high silicon content aluminum casting alloys but not to most wrought alloys.

Unreinforced liquid metals generally have viscosities in the range of 0.1-1.0 P.7 Adding particles to a liquid metal increases the apparent viscosity because the particles interact with the liquid metal, and each other, resulting in more resistance to shear. Typical values are in the range of 10-20 P for aluminum reinforced with 15 volume percent SiCp. Since viscosity is a function of reinforcement percentage, shape and size, an increase in volume fraction, or a decrease in size, will increase the viscosity of the slurry, often limiting the reinforcement level to about 30 volume percent.

Porosity in cast parts usually results from gas entrapment during mixing, hydrogen evolution, and/or shrinkage during solidification. Preheating the reinforcement before mixing can help in removing moisture and trapped air between the particles. During casting, porosity can be reduced by: (1) casting in a vacuum, (2) bubbling inert gas through the melt, (3) casting under pressure, and (4) deformation processing after casting to close the porosity. It has been observed that porosity in cast composites increases almost linearly with particle content.

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