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Metal Casting Technologies : September 2006
52 www.metals.rala.com.au Krishnan et al. (19) found that the contact angle between aluminium and graphite particulates is high about 160° when molten aluminium is at temperature below 1080°C. It is not easy to add graphite particulates into the melt. It is not easy to introduce graphite particulates into molten aluminium. Badia et al. succeeded in adding nickel coated particle ceramics and graphite by gas stream injection and vortex methods (20, 21, 22). Graphite dispersions impart gall resistance to the metal. Melt fluidity is the measure of the distance a metal can flow before being stopped by solidification (23).The addition of particles into the melt makes the melt viscous and affects its fluidity (24).There are different tests for fluidity. There are two types of fluidity tests that are widely used for measuring fluidity; one is the fluidity spiral test (25) and the other is the Ragone test (26), or vacuum fluidity test. The other fluidity test is strip fluidity in which the mould has rectangular strips. The spiral test is related to ability of liquid metal to flow than to their mould filling ability. The sum of the lengths of these strips is indication of casting fluidity (27). EXPERIMENT - ALUMINIUM GRAPHITE MMC The matrix material used in the study was pure aluminium. The aluminium matrix is reinforced by graphite particles. The average particle size of graphite is 50 μm. Figure 1 shows the micrograph of graphite particles. The preparation of the MMC was carried out by using melt route. Addition of graphite is difficult due its poor wettability with aluminium. The contact angle between aluminium and graphite particulates is high about 160° when molten aluminium is at temperature below 1080°C (19). Addition of graphite particles in the melt can lead to rejection of the particles. Graphite particles were added in the vortex but it lead to the rejection of the particles. This was due to the non wetting behaviour and high feed rate of particles addition. Figure 2 shows the rejected graphite above the melt. Graphite particles were coated with copper before introducing in the melt. The copper coating method has been reported elsewhere (28). Copper can be displaced from the copper bearing salts by zinc by cementation method (29, 30). CuSO4+Zn-->ZnSO4+Cu The cleaned aluminium ingot pieces were added in a graphite crucible and melting was done in an electrical resistance furnace. Temperature was set to 8000C. An electrical resistance furnace with temperature controlling device was used for melting. A graphite stirrer having four rectangular blades was fabricated. Graphite was used as the stirrer material because the melt doesn't stick to graphite due to poor wettability. To avoid any absorption of moisture and splashing of metal the graphite stirrer was preheated by oxyacetylene flame before insertion in the melt. When the ingot was melted, the graphite stirrer was inserted in the melt and the melt was stirred to create a vortex. 6 wt % graphite particles were added in the vortex. The MMC slurry was poured in a preheated fluidity mould. The mould consists of four thin rectangular channels and the sum of the lengths of these strips is an indication of casting fluidity. The rest of the melt was poured in another mould. This will be used to fabricate an Al/Gr MMC bush. The casting was cut and polished with different SiC papers. The casting sample was seen under an optical microscope to observe the distribution of graphite particles. Figure 3 shows TECHNICAL FEATURE Figure 1. Figure 2. Figure 3. Figure 4.