Fig. 11. The typical microstructures of a diecast oil control valve (a) without carbon-fullerene coating and (b) with carbon-fullerene coating are shown.
remelting solidified layers during the filling process. Tere- fore, when the infusion of the molten metal starts, the mold temperature at the lower part of the cavity is higher if there is no carbon-fullerene coating because the heat is transferred during the filling process. Based on the temperature histo- ries at the lower cavity area, Td2
temperatures for cases with and without a carbon-fullerene coating were the same. However, the temperature of the mold without a carbon-
and Tm2
fullerene coating increased by up to 202F (68C). When no coating was applied, the metal cooled at the lower part of the cavity. With the carbon-fullerene coating, heat insula- tion during infusion and heat transfer during pressurization were achieved. For oil control valves and valve bodies with complicated
oil grooves, foreign matter in the oil grooves is not accept- able. Terefore, when chipping and peeling occur during processing, they should be removed by hand. Ten, thor- ough inspection and cleaning are necessary. By applying carbon-fullerene coating to these parts, the microstructure is improved. Figure 11 shows the cross-section micrographs of a cast component. Te component without carbon-fullerene coating (Fig. 11a) has a chill layer about 30 µm thick. Beyond the chill layer, the size of α-Al grain becomes larger and a clear chill boundary is confirmed. Te diameter of α-Al grain size, found on the surface of
the cross-section of cast parts produced with a carbon-fuller- ene coated mold (Fig. 11b), is the same as Fig. 11a. Tere are no boundaries and the grain sizes show gradual microstruc- tural changes, which drastically reduces chipping and peeling. Additionally, the reduction in blow-hole and galling defects was confirmed. Chipping, peeling and galling are eliminated by obtaining this type of cross-section microstructure. Aluminum diecasters must avoid solidification dur- ing molten metal infusion as much as possible to improve the pressure propagation at the completion of filling. By doing so, due to the physical property of a cavity surface that promotes heat transfer to the mold, a microstructure without boundaries between chill layers and the general internal structure can be obtained. When a carbon-fullerene
February 2014 MODERN CASTING | 61 , the molten metal
coating was applied on the mold surface to achieve the above requirements, it improved the strength of the surface layer. As a result, various effects such as the prevention of galling at mold release and the reduction of peeling during machining can be obtained.
Tis article is based on a paper (13-1243) published in AFS Transactions 2013.
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