Some observations on the matrix microstructure of aluminum-silicon alloy-graphite particle composites
- PDF / 1,036,710 Bytes
- 3 Pages / 613 x 788.28 pts Page_size
- 87 Downloads / 165 Views
~ , , o i.
Chemical Composition of LM13 Alloy
Elements Silicon Copper Magnesium Nickel Iron Manganese Aluminum
Wt Pct 10 to 12 1 1 1.5 0.8 0.5 balance
elsewhere. E81Briefly, the various steps involved are melting the alloy, creating a vortex by mechanical stirring, addition of preheated graphite particles during stirring, and casting the composite melt into a metallic mold of 18 mm internal diameter. In order to facilitate the wettability of graphite particles by an aluminum alloy, magnesium metal (1 wt pct) in 10-gram pieces was added to the melt prior to the dispersion of graphite particles. The samples were polished using standard metallographic practice, etched with Keller's reagent, and observed in a JEOL 35 CF scanning electron microscope. Typical microstructures of LM13 alloy without graphite particle dispersions are shown in Figure 1. The microstructure consists of primary aluminum dendrites, and the dendrite arm spacing (DAS) is found to be of the order of 20/~m, as seen in Figure l(a). The shape of the eutectic silicon solidified in the interdendritic region (Figure l(a)) and around the dendrite boundaries (Figure 1(b)) appears to be in the form of platelets. A microstructure of LM13 alloy containing 3 wt pct graphite particles is shown in Figure 2. It can be seen that the shape of the silicon solidified between primary aluminum dendrites situated close to the dispersed graphite particle is considerably modified. However, the region surrounding the graphite particle shown in the top of the micrograph (Figure 2) is considerably coarser. Figure 3 shows an SEM micrograph of the microstructure of the matrix solidified between two graphite particles separated by a distance of about 10 p~m. It can be clearly seen that the shape of the silicon between these two graphite particles is significantly modified. It is well known that in diecast hypoeutectic aluminumsilicon alloys, the first phase to solidify is primary aluminum in a dendritic manner. The eutectic silicon is solidified in the interdendritic region and around the dendrite boundary. In the case of LM13-graphite particle composites the suspended graphite particles are pushed by the primary aluminum dendrite to the last freezing eutectic liquid. Since the size of the graphite particles in the present case is more than the dendrite arm spacing of primary aluminum, the entrapment of graphite particles at the interdendritic region can be ruled out. We can now consider two situations of the microstructure of a hypoeutectic aluminum-silicon alloy in the presence of second phase dispersed particles, as shown schematically in Figure 4. In case I (Figure 4) the closest distance between the tip of the dendrite arm and graphite particle is much higher than the average length of the eutectic silicon normally present in diecast alloy without any graphite particle dispersions (Figure l(b)). In case II, this distance is of the order of or less than the length of the eutectic silicon. In the latter case the anisotropic growth of VOLUME 19A, MAY 1988-- 1365
(a)
Fig
Data Loading...
