Investment Casting(lost-wax)

Present Situation of Investment Casting

Investment casting is an advanced molding process, which can produce high-quality castings. It is mainly used in the fields of national defense industry, aerospace and traditional civil industry. In recent years, investment casting has developed rapidly, and China as a large manufacturing country, is particularly interested in it.

During the pouring process of investment casting, the shell surface layer is in direct contact with the casting surface, which has a great impact on the surface quality of the final casting. Therefore, the choice of the surface material of the shell is particularly important. At present, the surface refractory materials commonly used in investment casting include fused corundum, Zircon sand, fused silica, etc. Zircon sand is widely used in investment casting because of its high fire resistance, small and uniform coefficient of thermal expansion, good thermochemical stability and many other excellent properties. However, the reserves of Zircon sand in the world are small, mainly produced in Australia and South Africa, China mainly depends on imports, and the supply market of Zircon sand is unstable, not mentioning the price fluctuates greatly. In addition, the latest research from the United States, Europe and other countries shows that Zircon sand contains radioactive elements, which is harmful to health.

Fused Ceramic Sand Can Be Used as Surface Material in Investment Casting

Fused ceramic sand is a kind of artificial sand independently developed in China. According to statistics, the annual output is relatively large. At present, it is widely used by manufacturers at home and abroad. The main properties of Fused ceramic sand are: high refractoriness, small specific gravity, low thermal expansion rate, good fluidity and filling property, excellent collapsibility, and the price is lower than that of zircon sand. The popularization and application of fused ceramic sand in investment casting surface material can not only make full use of China's own resources, but also save a lot of costs for foundry enterprises.

Specific function of Fused Ceramic Sand in Investment Casting

The selection requirements of surface materials in investment casting process and the influence of different surface materials on shell properties and final casting quality. A domestic manufacturer has made the following experiment to analyze the comparison of fused ceramic sand and zircon sand in investment casting. By comparing the physical and chemical properties of fused ceramic sand and common Zircon sand, and combining the characteristics of Fused ceramic sand, the surface preparation process of the shell in the experiment was formulated (divided into three groups, group A was sprayed with Zircon sand, group B was sprayed with two layers of Fused ceramic sand, and group C was sprayed with one layer of Fused ceramic sand). The shell was baked at 1050 ℃ for two hours, and the 304 stainless steel pump cover was poured at 1570 ℃, Then, the macro and micro analysis of the shell was carried out by means of scanning electron microscope and crystal phase microscope, and the surface quality of the pump cover cast with Fused ceramic sand and Zircon sand as surface sand was compared and analyzed:

1. Fused ceramic sand meets the selection requirements of surface material. The thermal expansion rate of fused ceramic sand is close to that of Zircon sand, so it is easy to obtain good casting quality; the specific gravity is much smaller than that of Zircon sand. Under the same volume condition, the amount of fused ceramic sand is small, which can save costs. In addition, from the accumulation model, it can be clearly seen that when Zircon sand is used as surface sand, there are both point contact and surface contact between sand particles, while when Fused ceramic sand is used as surface sand, the sand particles are all point contact, and the permeability of mold shell is better than that of Zircon sand mold shell, which is conducive to the final shell cleaning.

2. In the preparation process of investment casting shell, the characteristics of three kinds of shell (sodium silicate, composite, silica sol) were compared. In this experiment, the high-quality silica sol shell was selected, and the surface sand was fused ceramic sand and zircon sand. The stainless steel pump cover is poured, in which the roasting temperature is 1050 ℃, the time is 2h, and the casting temperature is 1570 ℃.

3. When the surface layer is sprayed with two layers of fused ceramic sand, the mold shell has high strength and good sintering density, which can effectively prevent the immersion of molten metal. In addition, the mold shell has good collapsibility and is easy to be cleaned. When pouring stainless steel liquid, there is no obvious interface reaction, and the surface quality of the casting is good, without obvious defects such as sand sticking and burr. The surface quality is similar to or better than that of the casting poured with Zircon sand as surface sand. When one layer of fused ceramic sand is sprayed on the surface layer, the surface layer of the mold shell is prone to the problem of uneven coverage of the surface sand. The uneven surface sand on the casting surface leads to the occurrence of sand sticking and other defects.

We found that fused ceramic sand, if used well, can meet the selection of surface materials, has good application prospects, and can save costs compared with zircon sand, which provides an effective choice for investment casting manufacturers.