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Metal Casting Technologies : June 2006
METAL Casting Technologies June 2006 24 Figure 1. Melt processing route Figure 2. SEM of SiC particles Figure 3a & 3b, Fabricated Graphite Stirrer (3a) (3b) TECHNICAL FEATURE PM method is expensive and involves larger number of steps3. Melt processing involves introduction of ceramic particles into the melt obtaining a suitable dispersion through stirring followed by solidification of the melt in a preheated metallic mould. Figure 1 shows the general steps of melt processing route. In compocasting4 particles are introduced into the semi solid melt slurry, stirred and then cast. The high viscous slurry prevents the particles from floating, settling or agglomerating. Foundry processes can be used to fabricate large number of components with near net shape, lower cost & high production rates5. Centrifugal casting is another method of producing MMC. Aluminium melt with graphite particles will result in graphite segregation in the inner periphery of the casting due to density difference between graphite and aluminium. The graphite which is segregated in the inner casting periphery would act as a good solid lubricant for bearing application6. MMC can also be made by infiltrating molten metal into a preform. Infiltration can be carried out under vacuum or pressure. In the melt route there is a problem of poor wettability of ceramic particles with molten aluminium and there is segregation of the particles due to density difference between the ceramic particles and aluminium7. The composites prepared by liquid state method shows good bonding between the ceramic and the metal when reactive elements, such as Mg, Ca, Ti, or Zr are added to induce wettability8. Melt fluidity is the measure of the distance a metal can flow before being stopped by solidification9. There are many standard tests for measuring melt fluidity. The common fluidity tests used are the vacuum fluidity and the spiral tests. The vacuum method10 measures the length the metal flow inside a narrow channel when sucked from a crucible by using a vacuum pump. In the spiral fluidity test, the melt is poured into a spiral shaped mould with a small cross-sectional area. The addition of particles into the melt makes the melt viscous and affects its fluidity11. Casting temperature, volume fraction/size of reinforcement and the interface reaction between the particulate reinforcement and the matrix are parameters that influence melt fluidity. The fluidity of aluminum alloys containing suspended particles decreases with a decrease in melt temperature12. MMC's containing hard particles offer resistance to wear. Parts fabricated from MMC give higher abrasive resistance and a longer service life. Surappa et. al.13 found an improvement in the wear resistance of Al-Si alloys when reinforced with 5 wt% Al2O3 particles. 2. EXPERIMENTAL WORK The matrix materials used in the study was aluminium/aluminium alloy. SiC and Al2O3 particles are used as reinforcements. SiC is wear resistant and has high hardness. The average particle size of SiC is 40μm and that of Al2O3 is 50μm. Figure 2 shows the SiC particles. The preparation of the MMC (5 wt % SiC & 10 wt % Al2O3) was carried out by using melt route. The aluminium ingot pieces were cleaned and then charged in a graphite crucible and superheated to temperature of 80000C. An electrical resistance furnace with temperature controlling device was used for melting. A graphite stirrer was fabricated for stirring the melt. The stirrer has two rows of rectangular blades. Each row of the stirrer has 4 blades. The lower blade length is more than the upper. The blades of the stirrer are at an angle of 900 to each other. Figure 3a & 3b shows the two views of fabricated graphite stirrer. The graphite stirrer was preheated by oxyacetylene flame before insertion in the melt. After the alloy in the graphite crucible was melted, the preheated graphite stirrer was lowered into the melt slowly. The motor was put on and stirring was started to create a vortex. Figure 4 shows the casting set up. Silicon carbide particles preheated at 80000C