Feature Article How it’s Made:


Premium Grade Virginia Mullite™ by Steven Ashlock, Director of Technology and Research, Ceramic Engineer, Kyanite Mining Corporation & Austin Scheer, Ceramic Engineer, Kyanite Mining Corporation


Abstract T poured,


he selection of refractory flours and stuccos used in investment casting vary based on the alloy temperature


at which they


are poured, and the hold time of the alloy at the pour temperature. Within a particular refractory mineral category, deposit characteristics, as well as beneficiation techniques can result in different raw material properties. In addition, varying levels of impurities can greatly differentiate mineral sources among suppliers. Contaminants such as iron (Fe2 earths (Na2


have a dramatic effect on the high temperature performance of investment casting shell molds, impacting casting quality and their as-cast dimensions. For example, impurities in the facecoat can lead to mold- metal reactions that effect the surface finish of the casting (increased rework) and the shell removal. Impurities in the backup layers can lead to decreased strength, increased thermal expansion, and increased creep, which again negatively affect the dimensional stability of castings. Understanding the source of the impurity, the location of


O, K2 O3 impurities in the shell, and their


potential impact is crucial to consistent casting quality, yields, finishing labor (rework), and process control. Iron oxide is one of the most problematic impurities in refractories. Iron acts as a flux, causing glassification of the refractory which leads to high temperature creep (shell bulge); ultimately impacting dimensional stability of castings. This paper will focus on the processing of Virginia Mullite™ and how the complex beneficiation process of the ore body produces a consistent raw material with very low iron content. Recent improvements in the purification process have led


20 ❘ February 2020 ®


) and the alkali/alkaline O, CaO, MgO) can


to the production of new Premium Grade Virginia Mullite™


which will also


be discussed in detail. These process changes have enabled a lowering of the iron oxide content from a maximum of 0.75% to less than 0.2%. Testing was conducted to examine the effects of the lower iron content on the physical properties of the shell. Results indicate that a reduction in iron oxide in the shell refractory leads to increased creep resistance in the shell. This produces a more dimensionally stable casting, improved yields, and help reduce finishing labor costs at the cast house.


Introduction Investment casting is the process of creating a complex metal shape by encasing wax in a ceramic refractory shell and removing the wax to create a hollow cavity into which molten metal can be poured. The shell must be able to withstand the high temperatures and pressure of the investment casting process and maintain its constant shape (pattern geometry) in order to produce dimensionally consistent castings. There are many refractory raw materials used in the investment casting industry today; some of these include zircon, alumina, fused silica, and aluminosilicates. Each material has its advantages and disadvantages that must be considered when selecting a mold material. Physical properties, such as maximum usage temperature, creep resistance, strength, thermal expansion, cristobalite formation (health and safety), and the overall relative cost must be considered. Selecting the proper raw material


type is the first step; the second is to compare different sources. The obvious and initial consideration is cost and availability, but the origin of the raw material is of great importance. No two deposits are identical in terms of


purity and performance. The naturally occurring purity of the deposit, the amount and type of impurities, mining techniques, and mineral processing techniques can vastly affect how the material will perform in the shell and ultimately impact the scrap rates and the quality of the casting.


A good example of how the differences in natural geology and processing techniques affect the performance of the material can be seen in the aluminosilicates. As the name suggests, aluminosilicates are minerals made up of alumina and silica in various ratios. Pure silica and pure alumina are both very refractory raw materials but have their drawbacks. Pure silica has a high melting point of 3135°F (1723°C).1 However, impurities in silica act as a flux and drastically reduce the melting point, forming glass. This glass will begin to soften and flow which will affect the dimensional stability of the shell and can cause it to bulge. This can lead to castings that are out of dimensional tolerance. Impurities in a face-coat can result in a glassy phase sticking to the surface of the casting leading to surface defects,


scrap, or increased rework.


Alumina is also a highly refractory material with a melting temperature of 3730°F (2054°C).1


It is one of the


pillars in the world in ceramics for its high temperature properties and is more resistant to fluxing than silica. However, pure alumina performs poorly under thermal shock conditions. This can lead to shell cracking and positive metal defects on the as-cast surface. One way to combat the potential issues of pure silica and/or pure alumina is to mix the two materials together. In Figure 1, you can see the Silica-Alumina phase diagram.2


There is only one


thermodynamically stable intermediate phase in the alumina-silica system:


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