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The refractory materials in the shell-making process of investment casting

The refractory materials used in the shell making process of investment casting are generally divided into two types (excluding ceramic cores): one is the refractory powder used to prepare the slurry, and the other is the sanding material used to increase the strength of the shell. The weight of refractory materials should account for more than 90% of the total weight of the shell (after drying), so refractory materials have a significant impact on the quality of the shell.

Refractory materials for shell making should ensure that the shell has sufficient strength at room temperature and high temperature, does not deform at high temperatures, has good breathability, good thermal shock stability, good chemical stability, and good shell shedding properties. Therefore, refractory materials for shell making must have good high temperature resistance, thermochemical stability, small expansion coefficient, appropriate particle size, and be conducive to the stability of coatings. In addition, the author believes that in the production process, refractory materials should also have long-term quality stability, which means having reliable raw material suppliers. In most cases, foundries do not have the ability to distinguish the quality of refractory materials, and often encounter a situation where one batch is good and another is bad, resulting in repeated product quality problems. It is still uncertain which aspect of the problem it is, wasting a lot of manpower and material resources. Therefore, a qualified supplier is quite important.

Under normal circumstances, refractory materials used in silica sol shell precision casting include zircon sand powder, mullite sand powder, white corundum sand powder, etc. Their refractoriness and expansion coefficients are shown in the table below.

Generally speaking, the higher the melting point of refractory materials, the higher their refractoriness. However, due to the multiphase system composed of multiple different minerals, refractory materials contain a certain amount of impurities, and the temperature of the liquid phase in the system often differs greatly from the melting point. Therefore, although the refractoriness of refractory materials is higher than the pouring temperature, the matrix and impurities of refractory materials will form low melting point compounds. At the same time, refractory materials will also be affected by the impact of high-temperature molten steel and the erosion of oxides, which may cause the shell to soften or produce some chemical reactions. Therefore, in investment casting, refractoriness is only of reference value.

As for other physical and chemical properties of refractory materials and their transformations during the process, many books have introduced them, which are not within the author's expertise. Therefore, this article will only discuss the problems encountered in the production process and their impact on the quality of the shell.

Zirconia sand powder, as a surface layer shell material, is directly subjected to thermal shock from high-temperature metal liquid, thus its quality directly affects the surface quality of the product. In the practical technology of precision casting of Yamaya Yangshu, zircon sand powder is divided into two categories: ceramic grade and ordinary grade.

Even if the ZrO2 content reaches 65.9%, it still belongs to the ordinary grade. When producing thick and large parts, using ordinary grade zircon sand powder will deteriorate the quality of castings, especially when the purity of zircon sand powder is high.

The purity and particle size of zircon powder directly affect the surface quality of the product. As mentioned earlier, the particle size and distribution of the powder can affect the flowability of the coating. In our production operation, the formulated coating either has good flowability but no coating properties. The coating quickly flows out of the module surface in a few seconds, saving only a thin layer of adhesion, and even the module color is clearly visible. Another situation is that there is no flowability, and the slurry control operation is particularly difficult. The module surface is full of wrinkles (see the figure below).

In the Lu Gong live broadcast room, I have introduced to you more than once a simple method for distinguishing the quality of zirconium powder and molybdenum powder - precipitation method, as shown in the following figure. This method will not be further discussed in this article.

In the actual production process, there is another question that we need to consider and research. Is it really appropriate to use only one coating when there are many types of castings and large differences in size? Different product types have different slurry control methods. For example, for large products, it is not as convenient to control slurry return as for small products. There are also some products such as narrow channel impellers, which have different requirements for the fluidity and coating properties of the coating. So, what kind of powder should we use to match what kind of slurry to make the surface of the product better? The author cannot provide specific reference data on this point, and only relies on the research and testing of casting workers based on their own product characteristics.

Molay sand powder, it is necessary to clarify that the Molay sand powder we currently use is not the real Molay sand powder. As you can see from the standard, it is called calcined kaolin, which refers to the formation of certain Molay phases in kaolin refractory materials after calcination. Mo Lai Xiang is the key to ensuring the quality of the shell and the key core of the Mo Lai sand we use.

Recently, the most talked about topic in Lu's live broadcast room may be Mo Lai Sha powder. The simple detection method for Mo Lai powder is the same as the zirconium powder mentioned earlier. For Mo Lai Sha, you can compare the following pictures:

The above pictures are all of the currently used mullite sand in the market. You can distinguish which one is better or worse, and then let's take a look at the quality issues of the poured product:

The above pictures are all stainless steel products with this problem on the surface. Based on experience, it is highly likely that the problem is caused by the need to carry the pot during melting, and some problems are caused by the need to carry the pot during shell making. However, after production verification, most of the problems are caused by refractory materials.

In fact, regarding refractory materials, the author is not a professional practitioner and can only introduce them based on on-site usage. Regarding deeper issues such as what kind of material can produce how many phases at what temperature and for how long, and how different phases of material will affect the shell, we casting practitioners still need to conduct in-depth research. Currently, the author has not found relevant information and data.

However, from the perspective of users, the quality of refractory materials on the market varies greatly. It is important for everyone to keep their eyes open and ensure that the materials they use can meet their product production needs. When product quality issues are frequent, there is also an additional perspective to analyze the problem. In addition, there are currently some non mainstream materials or substitute materials for zirconia sand on the market. When using them, everyone should pay attention to the characteristics of their own products.