Q We use chills to improve the properties in critical areas of our 206 aluminum alloy castings. Is it possible to over chill, and what are the side effects of doing so?Background: Castings produced from premium aluminum alloys, such as A206 and B356 aluminum, are capable of high static and dynamic mechanical properties. Those properties can vary significantly based on the casting soundness and the solidification rate of the casting section. The solidification rate has a significant effect on the final mechanical properties and can be controlled by molding media, chills and riser placement. Proper gating to produce non-turbulent metal flow and the effective risering of the casting to feed solidification shrinkage are crucial in applications requiring high mechanical properties. The use of chills can improve casting soundness and fatigue strength when combined with the correct risering techniques. Chills are metallic inserts molded into the sand surface to promote high solidification rates (Fig. 1). They are usually made from iron, aluminum or copper and can be machined or cast. The type of chill used depends on ease of manufacture and the desired thermal effects of the chill. Its effectiveness depends on size, conductivity, thermal capacity thermal capacity: see heat capacity. and the thermal transfer See thermal wax transfer printer and direct thermal printer. across the molten metal/chill interface. Chilling may or may not improve the soundness of a casting when measured by standard non-destructive testing techniques like radiography radiography: see X ray. or dye penetrant inspection Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). , but the influence on microstructure mi·cro·struc·ture n. The structure of an organism or object as revealed through microscopic examination. microstructure Noun a structure on a microscopic scale, such as that of a metal or a cell and mechanical properties can be significant. [FIGURE 1 OMITTED] Recommendation: The variation between chilled and unchilled mechanical properties tends to be greater in alloys that have long freezing ranges, such as A206, because chilling can be used to improve directional solidification Directional solidification is a series of measures applied to control the feeding of castings. As most metals and alloys solidify, changing from the liquid state to the solid state they will undergo an appreciable volume contraction. and therefore feeding of the alloy. But aluminum alloys that are difficult to feed and have long freezing ranges can be over chilled to the point where gating systems cannot feed the developing shrinkage. If a casting is chilled in an area that is used as a feed path, the chilling may inhibit proper feeding and result in shrinkage microporosity leading to casting defects and reduced mechanical properties. In these cases, higher mechanical properties can be attained by reducing the solidification rate (less chilling) and improving the casting feeding. Information in this column was supplied by the AFS A distributed file system for large, widely dispersed Unix and Windows networks from Transarc Corporation, now part of IBM. It is noted for its ease of administration and expandability and stems from Carnegie-Mellon's Andrew File System. AFS - Andrew File System Aluminum Div. 2. |
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