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Metal Casting Technologies : June 2006
26 METAL Casting Technologies June 2006 TECHNICAL FEATURE were added in the vortex and the melt was stirred to get homogeneous slurry. Magnesium was added to increase the wettability of SiC particles with aluminium. The melt was stirred at about 400 rpm and cast in preheated metallic mould. The base matrix was cast in same condition for the comparison of properties. Strip fluidity test was done on MMC and base metal for comparison of fluidity. The mould consisted of thin rectangular channels. In the second case aluminium alloy / Al2O3 composite was prepared. The alloy composition is 4.4 wt. % Cu, 1.5 wt. % Mg, 0.6 wt % Mn, and bal. Al. Bottom pouring arrangement was used in this case. A tapered hole was machined in the base of the graphite crucible and a tapered graphite stopper was fabricated to fit this hole. Internal threads were machined in the base of the stopper and an L shaped metal rod was screwed in it. Figure 5a & 5b shows the graphite stopper and crucible. The graphite stopper was inserted in the crucible base and the L shaped steel rod was tied to the furnace body by a metallic wire. After stirring was over, the stopper was removed from the bottom and the melt particle slurry was bottom poured into a preheated metallic mould. Figure 6 shows the melt being bottom poured. 3. TESTING AND RESULTS Figure 7 shows the castings. Tensile samples were machined from the castings. Base aluminium alloy showed tensile strength of 70 MPa whereas the Al/SiC MMC showed value of 76 MPa. Hardness tests were carried out on MMC and base metal. The Al/SiC MMC showed hardness value of 26 compared to 20 of base metal. The CTE value of MMC is 23 and that of aluminium is 25.1 μm/m-°C. If large volume % of SiC is added in aluminium, CTE value will be reduced to a great extent. The silicon carbide % can be adjusted to vary the coefficient of thermal expansion of the composite. With reduced CTE MMC's find applications in thermal management and electronic packaging areas. To observe the distribution of particles in matrix a sample was sectioned from the casting. Optical microscope was used to observe the distribution of SiC particles in the aluminium alloy matrix. Figure 8a & 8b shows the distribution of SiC & Al2O3 in the aluminium matrix. It can be seen that SiC particles are uniformly distributed in aluminium alloy matrix. Fluidity tests showed MMC has reduced fluidity compared to aluminium. Addition of SiC particles made the melt viscous and reduced its fluidity. Aluminium showed fluidity length of 14 inches and MMC showed fluidity length of 9.3 inches. Figure 9 shows the fluidity castings. For wear tests, specimens were machined in the form of pins. A coarse SiC paper was used as the abrasive medium. Pins were cleaned with acetone and dried. Wear is determined from the weight loss. Base matrix showed wear volume loss of 0.08 mm3 compared to 0.01 mm3 for the Al/Al2O3 MMC. Addition of SiC particles reduced the fluidity, reduced the CTE value and increased the hardness and strength of the composite. Al alloy displayed more weight loss compared to Al/Al2O3 MMC. This is due to the high hardness of the MMC material. Thus it can be concluded addition of ceramic particles improved wear properties of the material. It can be concluded that MMC possesses improved properties. 4. APPLICATIONS With advancing engineering technology, newer materials are required with better properties. Engineers are always searching materials with improved properties. MMC possess advantages like lower density, improved strength, stiffness and increased wear resistance. MMC produced by casting method has lower processing cost. Aluminium matrix composites can provide better properties in some applications compared to Figure 4. MMC Casting Set up (5a) (5b) Figure 5a & 5b Crucible & Stopper