Casting

Application of precision casting in aerospace field

Investment casting plays an increasingly important role in the aerospace field due to its high dimensional accuracy and the ability to cast complex thin-walled parts. With the increasing complexity and integration of the structure and size of castings in the aerospace field, higher requirements are put forward compared to investment precision manufacturing. According to actual needs, it is particularly important to develop corresponding precision casting molding raw materials and key heat treatment equipment, control and ensure the overall dimensional accuracy of large precision casting structural parts, meet design requirements, and shorten the development cycle.

The target requirements of the new generation of military aircraft in the United States: reduce the quality by 50%, reduce the number of fuselage fasteners by more than 80%, reduce the batch production cost by more than 25%, and shorten the manufacturing cycle by 1/3 to 1/2. In addition to the extensive use of new materials, a large number of large, thin-walled, complex, integral, high-quality precision castings are also required. It can be seen that the demand for parts in the aerospace field in the future will be lighter in weight, higher in specific strength, reduced in quantity, larger in volume, more complex in structure, and at the same time reduce production costs and production cycles.

In order to achieve the above goals, the one-shot investment casting process has been widely used. Taking an engine STME in the United States as an example, the engine widely adopts the one-step precision casting process, which reduces the number of combustion chamber parts by more than 50, and reduces the number of welding by about 90, and the manufacturing cost is significantly reduced. At present, investment casting technology is mainly used in the manufacture of aluminum alloys, magnesium alloys, titanium alloys, superalloys and other parts in the aerospace field. Precision casting manufacturer Yufeng Metal focuses on the research and production of aluminum alloys, and combined with SLA printing technology, can better produce suitable products for the aerospace field.

Aluminum alloy has the advantages of low density, high specific strength, good corrosion resistance, easy forming and low cost, so it has been widely used in the aerospace field. Aluminum alloy precision casting technology refers to the casting of large, thin-walled, complex, integral, high-performance aluminum alloy components without or with a small margin. The outline size is generally about 1500mm, and the wall thickness is generally about 3mm. The performance of the casting basically reaches the performance of medium deformation alloy Indicator, no or less machining allowance.

Developed countries such as Europe and the United States have widely adopted integral precision casting technology, and have developed various large-scale thin-walled integral structure aluminum alloy precision castings, which are used in key parts such as advanced combat aircraft and airframes, missile engines and missile bodies, and replace some aluminum alloy forgings and sheet metal. Aluminum alloy investment casting generally uses gypsum or porcelain shells. The rotary injection method is widely used to refine metals, and elements such as Ti, B, Zr are added to refine grains and improve casting properties. Finally, investment casting mainly adopts anti-gravity casting to improve the filling ability of the molten metal, enhance the feeding effect of the molten metal on the casting, and obtain a stable liquid flow. Anti-gravity casting can be divided into low pressure casting, vacuum suction casting and differential pressure (positive pressure or negative pressure) casting according to the introduction method of filling pressure.

In the world, most of the precision foundries use the high-pressure steam dewaxing method, which can easily and quickly remove the wax in the casting ceramic mold shell. This article aims to help readers understand this technology and explore current trends and future developments.

Investment Casting, Dewaxing or Lost Wax, these are the same techniques described in different technical terms, i.e. the oldest primitive form of craftsmanship in human history.

Recap History

5000 years ago, the casting technology of ancient Egypt and Mesopotamia has been able to make copper bodies in the technology of removing the clay mold from the beeswax mold. The material for the original model was made of beeswax and charcoal-fueled fire, which required three steps:

1. The beeswax mold flows out after liquefaction;

2. The planting shell made of dots is sintered;

3. The molten bronze ball is poured into the mold cavity from the upper runner.

Not only in the Middle East, but even in the Chinese Han Dynasty with the Aztecs of Mexico and today's Fengning and Nigeria, this crafting process is well known and perfecting such methods for making works of art, tools and weapons.

Until the end of the 19th century, in the field of American dentistry, the flame molding process experienced the first technological leap because investment casting technology was first used for the production of dental crowns and dental inlays. After about 40 years, investment casting technology has become a new development direction because traditional manufacturing methods can no longer meet the requirements of the military. After 1945, the flame casting process was continuously developed and adjusted, and applied to the mass production of series products. Good Slow Through Manufacturing technology is always applied to high-precision and complex components to achieve minimum workpiece rework. Materials include high melting point alloys such as titanium alloys and alloy steels for precision molds. Therefore, today's precision casting is most widely used in the manufacture of gore ball joints, medical artificial replacement crystals made of Hegehe alloy, and parts production in the automotive, racing and aerospace industries.

As you can see, this is only the beginning of a bigger development in the booming investment casting industry: the latest buzzwords in investment casting today are "directional solidification" and "single crystal solidification" techniques. Castings with only a few parallel columnar crystals are obtained by the structural form of the Mak and the condensed connection material in a special heat treatment process.

The best ideal castings are cast from a single grain with little or no grain boundaries, resulting in higher mechanical and high temperature properties. Applying this technology to many aircraft engines and gas turbine turbine blades used to generate electricity could allow engines and gas turbines to operate at unprecedented levels of efficiency. While the first single-piece castings were only the size of a thumb-and-cap, the growing demand for single-piece castings—the largest single-piece castings today are about two feet long—is unclear as to the precursor to this exciting development.

High pressure steam dewaxing

Whether it's a fertility goddess made of bronze 5,000 years ago or an aviation jet engine that uses superalloys to make compressors today, they are still in principle the same process: the mold is made in wax, and then the wax mold is painted with The type of ceramic material is not, and finally the wax mold is removed as easily as possible by the method of melting the mites. From the flash burning method, the soaking method to remove the mites, the microwave method, the convection type heating furnace (oven) to the most popular high-pressure swallow steam dewaxing furnace in the world, all kinds of different mite mold technologies are used. appeared.

The following will focus on the introduction of high-pressure swallow steam dewaxing furnace.

The principle of the high-pressure steam dewaxing furnace has been discovered a long time ago, dating back to the French Denis Paz who invented the steam engine and the skiff in 1674. He realized in practice that the boiling point of water is directly related to environmental pressure. If you increase the pressure, it will cause the boiling point of water to increase, at 10 bar pressure, the boiling point of water is 183C. The high-pressure steam dewaxing furnace uses this principle to obtain higher energy density.

Until one day, a precision casting practitioner had a good idea to use a steam pressure steel furnace to remove the wax from his mold by accident. This technique is simple and effective. Such a "yan steam engine" only contains a few parts, and due to the high energy of water vapor, it is perfectly suitable for rapidly melting wax molds

Crafting Process

The first is to have a base steam generator, which is a boiler with half of the water added, which is heated by electrical components or gas-fired burners. As soon as the proper pressure and temperature are reached, the port valve is opened for a 260 m/s, 183C saturated base steam flow rate close to the speed of sound. Legend has it that it can accommodate two modular second boilers. To improve efficiency, it is necessary to preheat the empty liner boiler. The base steam generator through the party is designed to be a boiler with an inner liner, that is, the actual pressure is beneficial.

After the pressure is increased, high-energy water vapor builds up on the mold in the furnace and transfers its heat to the wax mold immediately through the mold shell. The surface of the wax mold melts, the wax leaks out of the mold shell, and part of the wax penetrates into the mold shell. Since all of this happens very quickly, the mold shell does not develop any cracks due to expansion. If a process parameter is incorrect, or the pressure is over-protected (or turbulent), the high-pressure steam dewaxing furnace will also cause the rejection rate to rise sharply.

After 10-20 minutes, the boiler pressure is reduced by opening another valve. Ideally, when it reaches ambient pressure, a specific door can be opened by a latch to remove the fully dewaxed mold.

However, the following factors still need to be considered: it is not during the casting process that the liquid is completely cast to the highest strength of the mold, but rather, it is the very inconspicuous process of dewaxing (because it is almost invisible), the most The wet strength of the vexing ceramic formwork, which determines casting success to total failure.

How Excess Pressure Works

The higher the pressure, the better. This simple principle is applied to the high pressure steam dewaxing furnace process, which means that the steel furnace that produces the steam should be large enough to easily be able to generate the supersaturated steam process. As a rule of thumb, the volume of the two boilers, that is, the volume of the steam pressure generator and the reaction autoclave that actually loads the de-mite mold, should be about 1.3:1. Only in this way can it be ensured that the empty furnace is filled with steam, that is, the injection time of swallow steam is about 4 seconds under optimal conditions. Every time the module uses the Xiao Wax Furnace to lie, because the cold surface of the mold shell will condense the base steam into water, more energy is required.

Therefore ideally, the pressure in the steam generator should be greater than 10 bar and the temperature should reach 183C to ensure that the maximum pressure can fill the high pressure steam dewaxing furnace in less than 10 seconds. Only at this temperature can saturated water vapour be guaranteed, which means an optimal air and water vapour mixing balance flow. In the worst case, when these values drop, it will lead to the occurrence of mold pattern and slow down the process of dewaxing, wasting energy.

Symmetrical steam inlet

Not only the steam injection rate has a huge impact on dewaxing results and reject rates. However, the importance of the way the steam is imported is also an aspect that is often underestimated. Usually there is only one base gas inlet, which makes the gas flow spread quite asymmetrical. Therefore, the sill shell is placed on a "box base" that is in direct contact with Yanqi. Therefore, the ideal state is an autoclave with high steam inlets on both sides, so that the steam can be injected quickly and steadily. In fact, this method is very good, and the use of aerodynamics can greatly reduce the scrap rate.

Optimized steam injection method

Another secret behind the working principle of the latest generation of steam autoclaves is the application for optimum steam injection. In the standard process so far, the door is locked at the beginning of the process. For example, the reaction autoclave has 1 bar of standard air and the pressure generator is filled with 8 bar of water vapor. By opening the transition valve, these 8 bar steam are injected into the high pressure high pressure steam dewaxing furnace, mixed with 1 bar of air, the pressure is reduced accordingly, and finally an effective steam pressure of 7 bar is achieved.

So-called "retarded shut-off valves" are currently used to avoid this reduction in efficiency. A special control device allows a 0.1-1 second lag in the closing of the exhaust valve. This way, steam injection pushes the cool air out of the valve outlet before closing and building up pressure. The benefit of this switch is the obvious trick: the saturated steam is no longer diluted, but the mould shell gets a "true" 8 bar pressure, which equates to a 12% increase in efficiency.

Perfect the Decompression Process

However, steam pressure strength and injection speed are not the only guarantees for the success of the dewaxing process, because even an incorrect decompression process can cause great damage to the hull. In this process, we are still dealing with an unsintered ceramic shell, and the unsintered formwork cannot withstand any drastic processing. If the pressure is suddenly released, the high velocity air flow between the adhesion force of the residual wax and the penetrating wax can lead to peeling of the mold shell and the creation of micro-strikes, which can cause disastrous results in the subsequent casting process. Therefore, it is necessary to understand the mold shell as the well-known "raw egg", and adopt a slow depressurization process, that is, the pressure of the mold shell is reduced uniformly and slowly. The following figures clearly show what the pressure reduction process is: when one liter of saturated base steam of 10 bar corresponds to 1000 liters of atmospheric base steam, when the base steam is normally released into the atmosphere - it will emit a loud whistling noise and broken hiss, so must be controlled.

An ordinary "unintelligent" valve cannot be used for this purpose, because as a digital system only accepts "open" or "close" commands, the intermediate position is impossible to control. In this case, a so-called "smart valve" is needed, which is a smart valve that can independently regulate pressure relief. The valve is opened and the pressure is relieved according to whether there is overpressure within the set time. If the pressure drops too fast, it can reduce the pressure relief outlet and vice versa. This monitoring is adjusted in full-second increments, thus guaranteeing a freely selectable pressure drop pattern along a perfect quasi-iterative manner.