Application of Various Refractory Materials in Investment Casting

In the investment casting process, there are a series of strict requirements for refractory materials. These materials must not only have sufficient refractoriness to withstand high-temperature environments but also exhibit excellent thermochemical stability to ensure that chemical changes do not occur at high temperatures. At the same time, the thermal expansion coefficient must be controlled within an appropriate range, and the particle size must be moderate enough to meet the needs of the casting process. In addition, affordable prices, abundant resources, and non-toxicity are also indispensable considerations. Among the many refractory materials, zircon and Al2O3-SiO2 materials are widely favored due to their excellent performance.

In the investment casting process, the particle size distribution of zircon powder is a key parameter. Its particle size directly affects the casting effect and product quality. Therefore, choosing the right zircon powder particle size is crucial to ensure the smooth progress of the casting process and the performance of the final product.

(1) Zircon powder

Zircon powder, a tetragonal island structure orthosilicate mineral, has a molecular formula of ZrO2·SiO2 or ZrSiO4. In the theoretical composition, ZrO2 accounts for 67.1wt% and SiO2 accounts for 32.9wt%. The density ranges from 3.9-4.9g/cm3, and the Mohs hardness reaches 7-8, showing a small thermal expansion coefficient and a large thermal conductivity. This makes zircon powder cool quickly during the cooling process and has good thermal shock stability. Pure zircon is colorless and transparent crystalline, while impurities often appear in yellow, orange, red, brown and other colors. It is worth noting that zircon is the only compound in the ZrO2-SiO2 binary system. At high temperatures, it can decompose into highly active amorphous SiO2, which can react chemically with certain elements in the molten metal, thus potentially causing casting defects. For specific requirements on the chemical composition and particle size distribution of zircon for investment casting, please refer to the zircon standard.

(2) Kaolinite clinker

Kaolinite, with the molecular formula of Al2O3·2SiO2·2H2O, has a density of 2.6g/cm3 and a melting temperature range of 1750-1785℃. In its natural state, kaolinite is in the form of raw material, while the clinker used in investment casting is usually calcined and crushed at high temperatures, and its main components include mullite, cristobalite or glass phase.

In the investment casting process, bauxite sand (powder) plays an important role. Its chemical composition and grading code are crucial to ensure casting quality and performance. Specifically, the chemical composition and properties of bauxite sand (powder), as well as its applicable grading code, are key factors affecting the casting effect. By understanding this information, we can better select and utilize bauxite sand (powder) to meet different casting needs.

(3) Bauxite clinker

Bauxite ore with an Al2O3 content of 48wt% or more and a low ferric oxide content after high-temperature calcination is often referred to as bauxite clinker in the field of refractory materials. This clinker is mainly composed of aluminum hydroxide and hydrated aluminum silicate, and is an indispensable raw material for investment casting.

Bauxite clinker plays a vital role in investment casting, and its particle size composition has a direct impact on the quality and efficiency of the casting process.

The bauxite clinker used in investment casting is mainly divided into two categories. One is the clinker made directly from natural bauxite ore by high-temperature calcination, and its key components include mullite and corundum. The other is the clinker rich in mullite obtained by mixing bauxite and clay in a specific proportion and calcining it. This clinker is also often called bauxite synthetic material. In addition, according to the Chinese national standard GB/T12215-1990, bauxite clinker is divided into three different grades: 85, 80 and 70 according to its Al2O3 content.

(4) Fused corundum

Fused corundum is divided into white corundum and brown corundum, and fused white corundum is commonly used in investment casting. White corundum is made by high-temperature melting in an electric arc furnace, followed by cooling and crystallization, and then crushing and screening. Its Mohs hardness is as high as 9, the thermal expansion coefficient is 86×10-7℃-1, the melting point reaches 2030℃, and the density is 4.0gcm-3. This material has excellent chemical stability, can resist acid and alkali corrosion, and remains chemically stable in molten metal.

In the investment casting process, fused white corundum plays a vital role. Its chemical composition has a direct impact on the casting quality. Therefore, understanding the chemical composition of fused white corundum is of great significance for optimizing the casting process and improving product quality.

In the investment casting process, the particle size distribution of white corundum is also a key factor. White corundum of different particle sizes will affect the quality and efficiency of casting. Therefore, the reasonable selection of the particle size of white corundum is crucial for optimizing the casting process and improving product quality.

RS Refractory Bricks Factory

As a high-performance investment casting refractory material, although relatively expensive, its application is particularly critical in certain circumstances, such as the molding of alloy steel, high-temperature alloy, and magnesium alloy castings. Compared with the black pitting problem on the casting surface that may occur when using fused quartz and zircon sand as the shell surface material, the introduction of fused corundum can effectively avoid such defects. In addition, the chemical composition and particle size distribution of fused white corundum are specified in detail in standard HB5347-18-986, providing strong support for the optimization of investment casting process.

On the other hand, as an important casting material, the preparation process of fused quartz involves the rapid cooling of natural high-purity SiO2 after high-temperature melting in an electric furnace. The obtained fused quartz has excellent thermal shock stability and dimensional accuracy improvement. Its low thermal expansion coefficient and low density not only help prevent cracking and deformation during shell dewaxing and roasting, but also reduce labor intensity. However, it is worth noting that molten quartz will transform into cristobalite at high temperature, and the volume change during the cristobalite crystal transformation may cause a large number of cracks in the shell, thereby affecting the shell strength and the difficulty of shell stripping and cleaning.