Stainless Steel Investment Casting
Stainless steel precision casting is also called fusion die precision casting. The process in the casting process to minimize or no cutting, is a less cutting or cutting-free casting process, is an excellent technology in the foundry industry. Its application is very wide, not only applicable to various types and alloys of casting, but also produces castings with higher dimensional accuracy and surface quality than other casting methods. At the same time, castings that are resistant to high temperatures and not easily machined can be cast by investment casting. This casting method due to the casting process is not in the ultra-high temperature state, more suitable for casting aerospace, defense and other high-precision industrial components.
The earliest stainless steel precision casting can be traced back to the 1930s and 1940s, the United States was the first to use stainless steel precision casting method for its then leading technology in the turbine blades of the aero-engine casting, the finished product was well received by all parties, which in turn made this method to promote. China’s research and development and use of stainless steel precision casting is after the founding of New China, with the development of China’s defense industry, Liaoning, Guizhou, Beijing, Shaanxi, Hunan and other places have opened a stainless steel precision casting institute or foundry. However, objectively speaking, China’s stainless steel precision casting industry is still in the development stage, not yet able to meet the development needs of China’s aerospace, defense industry and other industries.
Stainless steel precision casting industry is different from the traditional foundry industry, because the added value of stainless steel precision casting products is higher. According to relevant statistics (2013 data), the aerospace equipment and excellent engines in Europe and the United States, precision casting original product value-added nearly accounted for 70% of the total value-added, but China’s share is less than 35%. Although China is now the largest casting volume of countries, but the casting value is still in the upper middle level, can not yet reach the top level. Therefore, the fundamental significance of the development of stainless steel precision casting, one is the fundamental needs of national defense and science and technology, the second is the development of the economy must be the way.
Technology continues to mature, but there is a need for improvement
At present, in stainless steel precision casting, there are already a variety of mature technology and process.
Silica-sol shell process. Silica-sol shell process is commonly used in the more sophisticated internal combustion engine parts casting industry. This method uses a coating that is more stable, does not require a chemical hardening process, and is resistant to high temperatures and has better resistance to deformation. However, the disadvantage of this technology is the poor warming of the wax mold, although it can be improved by adding surfactants, but it will increase the investment to some extent.
Water-glass shell process. Water glass shell process was invented very early, China also introduced the technology from the Soviet Union in the 1950s and 1960s. The technology is low-cost, relatively simple operation, while the requirements for raw materials are not high, in the stainless steel precision casting is more widely used. However, the biggest problem of this technology compared to the silica-sol shell process is the general surface quality of the resulting castings, the dimensional accuracy is lower. However, because of its cost advantage, China is still using this technology in some civil or low precision internal combustion engines. Since the introduction of this technology, our researchers have made significant improvements to it, mainly in terms of improving the shell coating, optimizing the hardener, and using composite shells.
The main improvement of improving the shell coating is to add a certain amount of refractory clay to the back coating of the shell, so that the strength of the shell has been greatly improved and single shell roasting and firing injection has been realized. In the optimization of hardener, the traditional hardener mostly uses ammonium chloride, but this material will release a lot of ammonia and nitrogen oxide gas during the casting process, causing pollution to the atmosphere. Therefore, the hardener is changed to aluminum chloride solution and further changed to aluminum chloride crystals, and the effect of this hardener is similar to that of ammonium chloride.
In the use of composite shell, because the surface quality of the water glass coating shell has certain defects, so many original casting parts are cast in the form of multi-layer mold composite casting, on the one hand, to save costs, on the other hand, also improve the surface quality of the casting.
In addition, in the development of new processes, the current more mature new processes are self-absorbing casting process, foam mold and fusion model shell casting and other processes. However, these processes have obvious advantages and disadvantages in some aspects, the future still need to continue to improve the technology workers.
Multi-technology crossover and integration development trend is obvious
In the future direction of development, stainless steel precision casting technology crossover and integration will become more and more obvious, a new round of science and technology and the direction of the industrial revolution will not rely solely on one or two types of disciplines or a single technology, but multi-disciplinary, multi-technology areas of a high degree of crossover and deep integration. Therefore, stainless steel precision casting should also be integrated with the development of related technologies.
Cross-use with rapid prototyping technology. The cross-use of science and technology is mostly to complement each other. Stainless steel precision casting of the wax mold making process of design and mold manufacturing is more complex and takes a lot of time. The rapid prototyping technology can well compensate for this shortcoming. However, the material limitations, the use of rapid prototyping technology alone will not be able to meet the requirements. Therefore, in recent years, many workers first use polymer technology to obtain prototypes of castings, and then manufacture wax molds, which are put into use in the stainless steel precision casting industry.
For example, light-curing three-dimensional molding technology (SLA) and selective laser sintering technology (SLS) is currently used in conjunction with the more mature technology of investment casting. SLA technology can provide high dimensional accuracy, especially the accuracy of the outer surface of the part; and SLS raw material prices are relatively low, the accuracy is also lower than SLA technology. In the use of these two technologies, attention must be paid to controlling the key combination of rapid prototyping technology and stainless steel precision casting technology, including cost control and comprehensive consideration of casting accuracy issues of the parts. Selecting the right balance is a key issue in the organic combination of rapid prototyping technology and investment casting.
Cross-fertilization with computer technology. Stainless steel precision casting process of program design and optimization work is a more labor-intensive and time-consuming work. In recent years, with the continuous development of computer technology, many industries that require a large number of calculations and precision calculations have introduced computer technology and developed a variety of corresponding calculation software, such as ProCAST, Auto CAD, AFSolid, Anycasting and many other software. These software can calculate or simulate the design and casting process of stainless steel precision casting, through the way of data calculation to achieve program optimization, the development of stainless steel precision casting has played a good role in promoting.
Tsinghua University in the conventional general simulation software development has been in the forefront of the world, but also for the aero-engine parts casting simulation software, TiAi alloy turbine blades and other high-end casting in the computer simulation software has been developed; Harbin Institute of Technology has been the space station, rockets and other important structures in the aviation casting laboratory software simulation casting research work; Huazhong University of Science and Technology is for Medium and high end of the melt casting developed the China Casting CAE system, with a wider application, is also China’s current more leading computer technology and melt casting cross-use of the model.
However, in the current use of the process, we should also pay attention to the applicability of computer software modeling, the material itself thermal parameters and other issues, a good solution to these problems can greatly shorten the development of stainless steel precision casting time.
Stainless steel precision casting process.
1、Moulds are made according to different shapes of products. The mold is divided into upper and lower concave mold way, through turning, planing, milling, etching, EDM and other integrated processes to complete. The shape and size of the concave pit is consistent with the product half. Because the wax mold is mainly used for industrial wax pressing use, so the aluminum alloy material with low melting point, low hardness, low requirement, cheaper and lighter weight is chosen to make the mold.
2、Use the aluminum alloy mold to produce a large number of industrial wax solid core models. Under normal circumstances an industrial wax solid core model can only correspond to a blank product.
3、Finishing of the wax mold periphery, deburring and gluing multiple single wax molds on the pre-prepared mold head (also called group tree), which is also the industrial wax solid core model produced by the wax mold.
4, will have been fixed in the die head of multiple wax mold coated with industrial glue and evenly sprayed on the first layer of fine sand (a kind of refractory sand, high temperature resistance, usually with silica sand). This sand particles are very small and fine, so as to ensure that the final blank surface is as smooth as possible.
5, in the set room temperature (or constant temperature) to spray the first layer of fine sand wax mold natural air dry, but can not affect the internal wax mold shape changes, the natural air dry time depends on the complexity of the product itself internal, the general air dry time of the first fine castings in about 5-8 hours or so.
6, after the first sand spraying and natural air dry, the surface of the wax mold continue on the industrial glue (silica sol), and spray the second layer of sand, the second layer of sand particle size than the previous first layer of sand to the large, to the coarse. After spraying the second layer of sand is also set at a constant temperature so that the wax mold natural air dry.
7, after the second sand spraying and natural air dry, in turn, the third sandblasting, the fourth sandblasting, the fifth sandblasting and other processes. Requirements: – According to the product surface requirements, the size of the volume, weight and other corresponding adjustment sandblasting times. In general, the number of blasting is 3-7 times. The sand size of each sand blasting is different, usually the sand of the latter process is coarser than the sand of the former process, and the length of air-drying time is also different. Generally, the production cycle of a complete wax mold sanding is about 3~4 days.
8, will have completed the sandblasting process of the wax mold before the baking process, and then evenly coated with a layer of white industrial emulsion (silicone paste), in order to play a bonding and curing sand, as well as sealing the role of wax mold, for the latter baking process to prepare. At the same time, after the baking process, it can also improve the brittleness of the sand, so that it is easy to break the sand layer and take out the blank.
9、Baking process will be fixed in the mold head, and complete the sandblasting air-drying process of the wax mold into the metal sealed special oven heating (commonly used is burning kerosene steam oven). Because the melting point of industrial wax is not high, the temperature is about 150゜, the wax mold melted by the heat to form wax along the sprue out, this process is the wax removal. After the wax is removed, the wax mold is just an empty sand shell. The key to precision casting is to use this empty sand shell. (Generally this wax can be used many times, but these wax must be re-filtered, otherwise the unclean wax will affect the surface quality of the blank, such as: surface sand holes, pockmarks, and also affect the shrinkage rate of the precision casting products).
10、Bake the sand shell in order to make the sand shell more solid and stable after dewaxing, before pouring into the stainless steel water, must bake the sand shell, usually in a very high temperature (temperature of about 1000゜) furnace baking.
11、The stainless steel water that has been dissolved at high temperature into a liquid state is poured into the sand shell after waxing, and the liquid stainless steel water is filled with the space previously formed by the wax mold until it is completely filled, including the middle part of the mold head.
12, because the boiler of dissolved stainless steel will be mixed with different components of the material, the factory must detect the material percentage. Then according to the proportion needed to adjust the release, such as adding those aspects of the elements to achieve the desired effect.
13, liquid stainless steel water cooled and solidified, with the help of mechanical tools or manpower to crack the outermost sand shell, revealing a solid stainless steel product that is the shape of the original wax mold, which is the final desired blank. Then will be cut one by one, separated again by rough grinding becomes a single blank parts
14, inspection of the blank: the surface has a sand eye, air holes in the blank must be filled with argon arc welding, serious when the scrap cleaning back to the furnace.
15、Cleaning blanks: The blanks that have passed the inspection must go through the cleaning process.
16、Processing of other processes until the finished product.