Fig. 2a. The conventionally cast composite featured silicon carbide in large clusters among the interdendritic regions.
hinder the other’s movement to produce neutral buoyancy. Te presence of graph- ite also makes the aluminum silicon carbide alloy more machinable and wear resistant, with a lower density. During the solidification of a
discontinuously reinforced metal matrix composite, the solidifying alloy interacts with the particulate rein- forcement to either push the particles into the last freezing, interdendritic regions or engulf them in the solidified dendrites. Te phenomenon of particle pushing is a hindrance to achieving a uniform dispersion of reinforcements in cast MMCs. In order to improve the dispersion of micro-size particles, the
Fig. 2b. The ablation composite sample exhibited more evenly distribut- ed silicon carbide and graphite particles with a higher volume fraction.
solidification rate must be high enough to promote engulfment of the particles or refine the dendritic microstructure. Ablation has the potential to increase the solidification rate and provide the required dispersion of the particulate reinforcement in the finished casting. In the process, a core package is made using a proprietary binder. Shortly after the alloy is poured, a liquid or liquid/gas mix- ture is directed to the core surface. Te ablation media dissociates the binder and erodes the silica sand cores, enabling the ablation media solvent to come in direct contact with the liquid alloy, removing the latent heat of solidification with high cooling rates.
As the ablation solvent progressively
moves from one end of the casting to the other, it results in rapid unidirectional solidification. Heavy sections where solidification is slower typically show higher cooling rates when ablated. Tese characteristics of ablation are expected to lead to a more refined microstructure, especially in the case of the interdendritic eutectic, and a uniform distribution of reinforcement particles. A comparison of relative properties of monolithic A356 alloys produced via sand, permanent mold, squeeze and ablation casting are shown in Table 1. In the study, hybrid aluminum-
silicon-carbide graphite composites, known as 10S4G, were convention- ally cast and produced via ablation to study the effect each method has on the matrix microstructure and dispersion of the reinforcement in the composite, as well as the resulting mechanical properties.
Ablation Put to the Test
Te study was conducted with an 8 x 16-in. (20.32 x 40.64-cm) plate-shaped pattern, suitable for both conventional sand casting and ablation. Te section thickness of the plate casting is 2.5 in. (6.35 cm) on the left and 1.5 in. (3.81 cm) on the right. Te total casting weight was 25.6 lbs. (11.6 kg). Te same riser and gating system,
Fig. 1b. A solidified steering knuckle is shown after the sand cores have been removed by the ablation solvent.
which was designed with the aid of solidification modeling software for the conventional sand process, was used for both molds. It is possible the increased cooling rates and thermal gradients
December 2011 MODERN CASTING | 27
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