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Metal Casting Technologies : June 2006
28 METAL Casting Technologies June 2006 TECHNICAL FEATURE conventional materials. MMC's offer considerable promise to automobile engineers. In the automobile sector, Al MMC is used for making cylinder liners, brake drum, cylinder blocks, drive shafts, etc. Wear resistance is an important property provided by MMC. The addition of hard reinforcements improves the wear resistance of the metal. An important application of MMC14 are the brake rotors for the German high speed train ICE-2 made from a particulate reinforced aluminum alloy (supplied by Duralcan developed by Knorr Bremse AG). Another application of aluminum MMC was a selectively reinforced piston for diesel engines by Toyota. It consisted of a fiber preform in the ring groove area to provide improved wear resistance. The preform was infiltrated by the molten metal using squeeze casting process15. The ceramic reinforcements have smaller values of CTE than the metal. The addition of ceramic reinforcements to the metal like aluminium will result in reduction in the CTE value. MMC finds applications in thermal management and electronic packaging areas because coefficient of thermal expansion (CTE) of the MMC can be adjusted by varying the SiC % while maintaining the thermal conductivity2. For electronic packaging applications CTE should be lower. The volume fraction of ceramic particles must be higher then only the CTE will be reduced and comparable to that of alumina or silicon and gallium arsenide. By matching the CTE's of the materials in an electronic assembly, stresses developed between them during power cycling are minimized. Al/SiC MMC provide a match to most ceramic and semiconductor materials. Metals like Cu, Al, Ni-Fe alloys and Cu- W and Cu-Mo blends do not fulfill the required needs of advanced electronic applications for properties like higher thermal conductivity, lower coefficient of thermal expansion, low density and reduced cost. SiC has lower cost, lower coefficient of thermal expansion and a good thermal conductivity. Aluminium matrix composites reinforced with SiC are the materials for electronic packaging17. MMC components are used in the recreational market. They include bicycle tubing, track spikes and lacrosse stick shafts18. Aluminium graphite particle composites are good bearing materials. Gorbunov et al. have reported that graphitic-aluminium alloys have good antifriction and anti-wear properties in wet media: better than bronze and the base alloy. The alloys can be recommended for the production of plain monolithic bearings which can be used under boundary lubrication conditions19. Bruni et al have shown that wear of graphitic aluminium when used as piston liner material is almost negligible and certainly lower than that of a conventional cast iron liner20. Aluminium graphite composites possess lower friction coefficients and Figure 6. Bottom Pouring of Melt Slurry Figure 7. Castings Figure 8a & 8b Distribution of ceramic particles in the matrix (8a) (8b) Product MMC Preparation Method Features Year Ring groove Al2O3 /Al alloy squeeze casting wear resistance Toyota reinforced piston light weight 1983 Cylinder liner Al2O3, CF/Al alloy squeeze casting wear resistance Honda low pressure light weight 1985 Diesel engine SiCw /Al alloy squeeze casting light weight Niigata piston wear resistance 1989 Table 1. Some MMC applications16