High-end precision casting technology development outlook

From Mr. Chen Bing's collection

High-end precision castings or high value-added precision castings, involving aerospace, weapons, industrial gas turbines and other important sectors, gradually varieties, specifications, whether metallurgical or appearance quality are very demanding. In this paper, only a few typical examples of the development history and trends to make a brief introduction.

Defense Industry

 1、Aero-engine turbine blades

 The core part of the modern aviation engine (or jet engine) is mainly composed of three parts: the pressurizer, combustion chamber and turbine (Figure 1). Closely related to the precision casting are the pressure engine after several stages of blades and turbine blades, including static blades (guide blades) and dynamic blades (working blades). Different types and uses of aircraft, the requirements of the engine are not the same, but one thing is common, that is, constantly improve the thrust-to-weight ratio and reduce fuel consumption. To this end, improving thermal efficiency is the key, and the main technical measures to improve thermal efficiency is nothing more than to maximize the pressurization ratio of the pressurizer and the turbine front inlet temperature (Figure 1). The result will inevitably make the whole system temperature rising, the relevant parts, especially the turbine blade working temperature is getting higher and higher. The working temperature of the first-class turbine blades is roughly equivalent to the turbine inlet temperature, and the current turbine inlet temperature of the most advanced aero-engine is as high as an incredible 1700 ℃, exceeding the melting point of nickel-based high-temperature alloys by about 300 ℃.

 According to the basic principles of metallurgy, grain boundaries are weak links at high temperatures, so the grain boundary area of the turbine blade alloy material is bound to get smaller and smaller, from the development of equiaxed crystals (1950S) to the direction of the main stress direction of the oriented columnar crystal parallel to the direction of the main stress (1970S), the blade operating temperature is also increased from 900 to 1000 ℃ to 1000 to 1100 ℃, and casting methods are also correspondingly from the ordinary vacuum melting -casting developed to directional solidification until single crystal casting (Figures 2 and 3).

 Different countries developed single-crystal alloy chemical composition is not the same, but the microstructure is basically the same, that is, nickel-based solid solution γ-phase as a substrate, is a cubic high-temperature strengthened phase γ 'inlaid therein (Figure 4). So far, the single crystal alloy R & D has experienced one, two, three generations, is now entering the fourth, five and even the sixth generation of the research and development stage, is moving towards the use of the temperature of 1150 ℃ goal. Despite decades of unremitting efforts, from the alloy point of view alone, the service temperature has not yet exceeded 1150 ℃. However, the latest aero-engine turbine inlet temperature requirement is as high as 1700℃, a difference of nearly 600℃. How to make up for this 600 ℃ gap? At present, mainly with the help of two technical measures, namely, air cooling inside the blade and the blade exterior thermal barrier coating. In order to continuously improve the air cooling efficiency, the blade internal cooling channel must be tortuous, so that its cooling effect all over the leaf body parts, especially the edge of the intake and exhaust. This results in increasingly elaborate and complex structural shapes for the ceramic cores that form the internal cavities (Figure 5). In view of the poor forging and machining properties of almost all high-temperature alloys, especially with complex internal cavities (cooling channels), precision casting has deservedly become the only molding method for aero-engine turbine blades to date.

 Thermal barrier coating, in short, is a heat-insulating coating, a new technology developed only at the end of the last century. Its basic structure consists of two layers, a ceramic top layer with a small thermal conductivity and an adhesive layer designed to prevent oxidization of the substrate and to bind it tightly (Fig. 6). The former is formed mainly by physical vapor deposition methods such as electron beam or plasma, while the latter is formed by chemical vapor deposition. The overall thermal expansion coefficient of the coating should match that of the substrate material to ensure that it does not crack and flake off at a high temperature of 1700°C. The coating should also be applied to the substrate material. Figure 7 shows a hollow turbine blade with a thermal barrier coating.

2、Large Integral Structural Components

A typical aero-engine profile is shown in Figure 8. The casing consists of a number of compartments, often referred to as the "magazine". After the combustion chamber there is generally no casting, while the pressurized part of some of the very complex shape of the magazine is often only through the casting of molding. Boost ratio of the ever-increasing requirements of the pressurized air compressor operating temperature continues to rise, so the molding method of these magazines from aluminum, magnesium alloy sand casting (1950s ~ 1960s development of stainless steel sheet metal - welded to the current titanium alloy or high-temperature alloys precision casting. Figure 9 shows the large titanium alloy magazine produced by PCC, and Figures 10 and 11 show the titanium alloy magazine and wax mold produced by Howmet respectively.

Since titanium alloys have higher specific strength and specific stiffness than other alloys,large thin-walled complex structural components of titanium alloys have also been widely used in large load-bearing structural components of airplanes (Fig. 12) and missile hulls (Fig. 13) since the 1970s.

3、Aluminum alloy precision castings with integral structure

These castings are widely used in racks, frames, bases and radiators for electronic and telecommunication equipment, etc. Figure 14 shows a few typical examples.Aluminum alloy precision castings with integral structure.

  Industrial gas turbines

Reducing greenhouse gas emissions has become a serious challenge for all mankind. The largest amount of carbon dioxide is undoubtedly emitted from thermoelectric (thermal) power stations around the world. Generally, thermal power generation is categorized as coal-fired, oil-fired, or gas-fired, depending on the fuel used. The carbon dioxide emissions per unit of heat produced by these three systems are very different, with relative values of 1.00, 0.76 and 0.53 respectively, which means that carbon dioxide emissions can be reduced by about half if all conventional coal-fired power generation is replaced by gas-fired power generation. As we all know, at present, China's thermal power generation equipment is still mainly coal-fired steam turbines, from the point of view of improving the energy efficiency of power generation equipment, industrialized countries have long been changed to gas turbines fueled by oil or gas. Not only power generation equipment, ship power and some high-power mechanical pumps and pressurizers and other equipment are also powered by gas turbines. Therefore, almost all industrialized countries, gas turbine is one of the main market of precision casting industry, its sales accounted for about one-third of the market share. Unfortunately, due to various reasons, so far China's gas turbine industry is still at a fairly low level of development, including turbine blades, including many key components are still in the development stage, accounting for the proportion of precision casting market share is negligible.

In fact, the working principle and structure of industrial gas turbine is basically the same as that of aviation engine, and the core part is also composed of three parts, such as pressurizer, combustion chamber and turbine (Fig. 15). Therefore, many advanced fine casting technologies used in aviation engines, such as directional solidification, single crystal casting, blade cooling and thermal barrier coating, have been transplanted to gas turbines in large quantities. Like aero-engines, the thermal efficiency of gas turbines is directly related to the inlet temperature of the turbine front. For example, for the traditional type of combined cycle generator set popular in the world, the turbine front inlet temperature is 1100~1300℃, and the thermal efficiency is 43%~48%; while the new type of gas turbine front inlet temperature is 1500℃, and the thermal efficiency is 52%; and the target value of the thermal efficiency of the ultra-high-efficiency gas turbine in the process of research and development is 56%~60%, and the turbine front inlet temperature will be further increased to 1700 ℃.

Although the requirements of industrial gas turbines in terms of thrust-to-weight ratio are not as high as those of aero-engines, the difficulty is the large size. Figure 16 shows the primary and secondary turbine blades of the MS9001FA industrial gas turbine of GE, with lengths of 43cm and 56cm, and masses of 13.5kg and 12.6kg, respectively. Figure 17 shows the fan section of the turbine guide vane, with a length of 60cm and a mass of >60kg. It has also been reported that the design lengths of industrial gas turbine turbine turbine blades have even reached 90cm. Figure 18 shows a 30 cm long single crystal turbine blade successfully developed by Mitsubishi Heavy Industries in Japan in 2011. Figure 19 shows a wax mold in the process of pressing. Figure 20 shows a ceramic core (blank) being trimmed.

  Automobile engine turbocharger

The original purpose of installing a turbocharger in an automobile is to make full use of the exhaust gases from the engine to drive the turbocharger turbine to rotate, which in turn drives the impeller of the coaxial compressor to compress the air and supply it to the engine, thus increasing the engine power and torque to achieve the effect of reducing fuel consumption and making the emission gases cleaner (Figures 21 and 22).

Typically, passenger car diesel engines emit exhaust gases at a maximum temperature of about 850°C, while gasoline engines can reach up to 1050°C. Therefore, turbines are usually precision cast from nickel-based high-temperature alloys such as 713C or MarM. The patented anti-gravity casting (vacuum casting) technology of Hitchiner Manufacturing Co., Inc. in the U.S.A. (Fig. 23) is considered to be the most efficient method for producing such products. The front compressor impellers, on the other hand, do not operate at high temperatures and are usually made from aluminum alloy plaster-type precision casting. Supercharger impellers and turbines are generally small in size, but have very high rotational speeds, up to 250,000 rpm or more, resulting in very stringent quality requirements for these two impellers. Although there are also a number of domestic manufacturers are producing these two impellers, but the production in the total demand does not account for a large proportion.

In recent years, the use of small engines with turbochargers to replace naturally aspirated large engines has become a popular trend in Europe due to the growing calls for reduced fuel consumption and improved urban air quality, and with 2011 as the base year, the installation rate of superchargers will double or more by 2016, accounting for as much as 76% of the total. However, due to the European countries automotive generators are mainly diesel engines, only the installation of turbocharger is not enough to meet the constantly updated EU vehicle emissions standards, but must also be equipped with both intake and exhaust systems to minimize PM2.5 and NOx and other harmful emissions. Many parts in the system, most of the material is stainless steel, with complex geometry, thin wall and dimensional accuracy requirements of higher characteristics, is very suitable for molding with precision casting method, which makes the automotive precision castings accounted for the rapid expansion of the market share from 5% in 2004 to 16% in 2011, during the same period of time in Japan, from 24% jumped to 43%. Although the automobile supercharger intake and exhaust system precision castings, from a general point of view in the precision casting market can only be counted as a mid-range products, but because of the market demand, for the development of China's precision casting industry provides a rare opportunity. Into the 21st century, China's Yangtze River Delta, Bohai Sea and other areas of many precision casting enterprises, relying on the reform and opening up to provide policy and geographic advantages, lose no time to seize the opportunity to make this part of the precision casting products in these areas have settled, and quickly developed into a new product market, at the same time, no matter whether from the management of the enterprise to process technology and equipment, but also on the basis of the original to enhance a step. It is worth mentioning that the domestic automobile engines, although passenger cars are still dominated by gasoline engines, but freight cars are dominated by diesel engines. With the gradual improvement of China's air quality requirements, turbocharger and its supporting equipment is bound to quickly form a new domestic market, will be the development of China's precision casting industry and then add a new wave of development vitality.

Reducing the density of the rotating parts of the supercharger has a positive effect on both engine efficiency and transient response. Titanium-aluminum alloy (TiAl) specific gravity of 4.2, and nickel-based alloy (713 C) specific gravity of 7.9, so if the former instead of the latter, the mass will be reduced by nearly half (47%), thereby reducing the turbine's inertia, shortening the response time of the torque increase, so that the turbocharger's transient response characteristics can be improved. According to Japan's Daido Casting Co. (Daido Casting Co.), using a combination of suspension melting and vacuum casting has successfully cast titanium-aluminum alloy turbocharger turbine, and put into mass production. Figure 24 for the principle of suspension melting (left) and melting pool (right), Figure 25 for the suspension melting - vacuum suction casting device schematic.

   Titanium aluminum alloy precision casting

Titanium-aluminum alloy is an intermetallic compound with low density, good high-temperature mechanical properties and oxidation resistance, which can be used to replace certain high-temperature alloys and other high-temperature-resistant materials in aerospace, industrial gas turbine and automotive industries. Mechanical properties test results prove that it has high creep resistance and excellent fatigue resistance at 760 ℃. But such materials are brittle, room temperature elongation of less than 2%, impact resistance and crack extension performance is poor, so the die casting and become the best means of forming such materials. Since the nineties of last century, more and more is used instead of nickel-based high temperature alloys to manufacture industrial gas turbine blades (Figure 26a), the whole casting turbine (Figure 26b), aero-engine blades (Figure 26c), automotive internal combustion engine cylinder inlet and exhaust valves and other products.