Excerpt from the Complete Casting Handbook, Second Edition by John Campbell
Inclusions of molding sand are perhaps the most common extrinsic inclusion, but whose mechanism of entrainment is probably rather complicated. It is not easy to envisage how minute sand grains could penetrate the surface of a liquid metal against the repulsive action of surface tension and the presence of an oxide film acting as a mechanical barrier. The penetration of the liquid surface would require the grain to be fired at the surface at high velocity, like a bullet. However, such a dramatic mechanism is unlikely to occur in reality. Although the following description appears complicated at first sight, a sand entrainment mechanism can occur easily, involving little energy, as described next.
In a well-designed filling system for a sand casting, the liquid metal entirely fills the system, and its hydrostatic pressure acts against the walls of the channel to gently support the mold, holding sand grains in place. Thus, the mold surface becomes hot. If bonded with a resin binder, the binder will often first soften, then harden and strengthen as volatiles are lost.
The sand grains in contact with the metal will finally have their binder degraded (pyrolysed) to the point at which only carbon remains. This carbon becomes hard and rigid like coke, forming strong mechanical links between the grains. The carbon layer remaining on the grains has a high refractoriness. This layer continues to protect the grains from oxidation because at this late stage most of the local oxygen has been consumed to form carbon monoxide. The carbon forms a non-wetted interface with the metal, thus, enhancing protection from penetration and erosion.
The situation is different if the filling system is poorly designed, allowing a mix of air to accompany the melt. This problem commonly accompanies the use of filling systems that have over-sized cross-section areas, and remain unfilled with metal. In such systems, the melt can ricochet backwards and forwards in the channel. The mechanical impact, akin to the cavitation effect on a ship's propeller, is a factor that assists erosion.
Other factors are also important. The contact of the melt with the wall of the mold heats the sand surface. As the melt bounces back from the wall, air is drawn through the mold surface, and on the return bounce, the air flow is reversed. In this way, air is pumped backwards and forwards though the heated sand, and so burns away the binder. The burning action is intensified akin to the air pumped by bellows in the blacksmith's forge. When the carbon is finally burned off the surface of the grains, the sand is no longer bonded to its fellow grains in the mold.
Furthermore, the oxide on the surface of the melt can react with the freshly exposed silica grain surface, and thus, adhere to them. If the melt ricochets off the sand surface again, the oxidized liquid surface peeling away from the mold is adhered with sand grains. As the surface folds over, the grains are enfolded into an envelope of oxide film (Figure 2.15). Under the microscope, such oxide films can be seen enfolding clusters of sand inclusions. Similarly, sand inclusions are often found on the inner surfaces of bubbles.
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