Simple Solutions That Work! Issue 7

The modern Aluminum caster finds an increasing amount of options when looking at refractory products in the cast shop. Many casters are running multiple alloys at different times, or the same time in different parts of the shop. Even given just one alloy, there also are variables of treatment and process steps required in casting, in which just one type of refractory will not be sufficient. Given this, it is good to review the types of refractories available, the properties that they have and some of the differences and applications that each one could be used in. Before we discuss these materials and applications, let’s review the general terminology for the properties of ceramics: Coefficient of Thermal Expansion (CTE): Thermal expansion is the tendency of material to change in shape, area, and volume in response to a change in temperature. As far as refractory use is concerned, ceramic materials expand when heated, but this expansion occurs at different rates for different materials. When designing parts that fit together, or parts that nest together (a ceramic sleeve inside of a metal tube, for example), the user must know what the different expansion rates are for each material, so that when in operation at maximum temperature, there are not any conflicting mechanical interferences or tolerance issues between parts. Thermal Conductivity: Thermal conductivity is the property of a material to conduct heat. For nearly all casting house applications with molten metal, understanding this property is critical to success of the ceramic components used. Heat transfer occurs at a lower rate across ceramic materials of low thermal conductivity than across ceramic materials of high thermal conductivity. Because of this, ceramic materials with high thermal conductivity are used in heat sink or heat transfer applications and ceramic materials of low thermal conductivity are used as thermal insulation. Ceramics generally fall into two categories: insulating (low thermally conductive ceramics) or conductive (high thermally conductive ceramics). As you can imagine, ceramics used for resistant heaters need to be thermally conductive to be effective in their use. Materials such as silicon carbides fall into the thermally conductive category. In addition, ceramics used for insulation or long-term metal holding need to be in the low thermally conductive category to be effective, and a ceramic like fused silica is a good insulating ceramic. PHILIP GEERS Molten Metal Market Manager BLASCH PRECISION CERAMICS ARTICLE TAKEAWAYS: 1. General terminology for properties of ceramics 2. Ceramic materials used in aluminum casting and properties CERAMICMATERIAL SELECTION INMOLTEN ALUMINUMAPPLICATIONS: 20 Figure 1. Thermal shock test being performed on a ceramic test bar