To reduce the total weight of the aircraft, while maintaining or improving its operational characteristics, is one way to improve any modern passenger aircraft. This would allow, for example, to reduce fuel consumption in flight or to place additional equipment on the plane. The design of the aircraft can be facilitated, in particular, through the use of new materials, which are searched almost continuously for the needs of aircraft construction. For example, the use of parts made of titanium aluminide in aircraft turbojet engines can significantly reduce the weight of the power plant. Scientists fromNITU “MISiS” , in partnership with whom this material was written, has been actively working since 2010 as part of the federal target program.
What are you talking about?
The development of modern civilian passenger aviation is largely determined by economic factors: airplanes must be inexpensive, carry a lot of passengers, consume little fuel and have a low maintenance cost. Thus, turbofan dual-circuit engines, which today are installed on almost all civilian passenger and cargo aircraft, make it possible to reduce fuel consumption. Of course, civilian turbofan engines cannot provide a fast speed set and access, for example, to supersonic flight speed, but they consume less fuel and produce less noise than jet engines of combat aircraft. In fact, the Soviet designer Arkhip Lyulka became the father of modern double-circuit aircraft engines in April 1941
A turbofan dual-circuit engine with a large diameter fan (turbofan engine) consists of two parts. One of them is the inner contour. It consists of a compressor zone, a combustion chamber, one or more turbines and a nozzle. In flight, the air is drawn in and compressed slightly by the fan – the largest and very first propeller in flight. Then part of this air enters the compressor and is compressed even more, after which it enters the combustion chamber, where it is mixed with fuel. After fuel combustion, hot gases escape from the combustion chamber and rotate the turbine. The latter is a heat-resistant propeller rigidly mounted on the shaft. By this shaft, the turbine is directly or through a reducer connected to compressors and a fan at the engine inlet. After the turbine, the gas stream enters the nozzle and flows out of it,
The second part of the engine — the external circuit — is often a guide vane, duct, and, in some cases, its own ring nozzle. During the flight, part of the air slightly compressed by the fan, which did not fall into the internal circuit, enters the guide apparatus, where it is braked. Due to braking, the pressure in the air stream rises. Then the compressed air enters the duct, and then into the nozzle and forms the remainder of the draft. In modern turbofan engines of civilian aircraft, the main part of the thrust, contrary to the opinion of people who are far from aviation, is formed not by the internal circuit, but by the fan and the external circuit – their share in the total thrust of the power plant can account for up to 80 percent or more. Unlike turbojet engines of combat aircraft, where most of the thrust is created by just the inner contour.
The fan, compressor, turbine in an aircraft engine are propellers with blades of a special shape, which allow compressing the incoming air or converting the linear movement of the air flow into rotational. Some of these elements operate in the zone of very high temperatures. For example, the temperature in the turbine zone can reach 1.8 thousand Kelvin. For this reason, the same turbine should be made of heat-resistant, but at the same time light alloys. In modern engines, compressor blades and turbines are made of nickel alloys, and existing casting technologies make it possible to create such elements hollow while maintaining general strength and temperature stability. This reduces the weight of parts made of nickel alloys.
One of the most common alloys based on titanium aluminide is TNM-B1. Starting from 2010, a whole series of research and development work has been carried out at NUST “MISiS” in order to improve the quality of castings from TNM-B1 intermetallic alloy and to obtain a Russian analogue alloy based on titanium aluminide. These works were carried out jointly with the Ufa State Aviation Technical University. The research was conducted by a group of scientists led by Professor Vladimir Belov, director of the Engineering Center “Foundry Technologies and Materials” of NUST “MISiS”. Work on the production of cast parts from Ti-Al intermetallic carried out on the basis of the UEC UMPO enterprise provided for the use of imported TNM-B1 alloy based on titanium with a mass fraction of Al 28.6 ± 0.7 percent, Nb – 9.2 ± 0 5, Mo – 2.3 ± 0.5 and B – 0,
In general, the use of titanium aluminide can reduce the mass of compressor blades and turbines by an average of half compared to traditional nickel-based alloys. Moreover, such a material has the best strength indicators in the so-called average temperature range (from 600 to 950 degrees Celsius), in which, for example, a low-pressure turbine operates in an aircraft engine.
Well, before, there was nothing like that?
The manufacture of cast parts for aircraft engines from alloys based on titanium aluminide, although it is a relatively new development for Russia, is already being implemented in the production of imported serial engines. For example, low pressure turbine blades are made from titanium aluminide for GEnx turbofan engines of the American company General Electric and PW1100G from Pratt & Whitney. The first ones have been mass-produced since 2011 and are installed on Boeing 747-8 and Boeing 787 Dreamliner passenger planes, and the second – since 2016 and are put on the Airbus A320neo, Bombardier CSeries and Embraer E-Jet E2. In addition, the PW1100G is planned to be used on the promising Japanese Mitsubishi Regional Jet and the Russian MS-21 (in one of the delivery options; in the other, the aircraft will be equipped with Russian PD-14,
The main developer of the technology for the preparation of Ti-Al-based intermetallic alloys in Russia is the All-Russian Institute of Aviation Materials (VIAM). It developed alloys VTI-1, VTI-2 and VTI-4. The production of VTI-1 and VTI-4 alloys is commercially conducted at the Chepetsk Mechanical Plant, and work on the development of the production of ingots from intermetallic titanium alloys at this enterprise has been carried out since 2012, accompanied by production from VIAM. Smelting is performed by triple vacuum-arc remelting (melting occurs in vacuum with the creation of high temperature using an electric arc). However, since NUST MISIS began working with TNM-B1 back in 2010, all developments using this alloy were carried out by this institute. According to Vladimir Belov,
In particular, the specialists of NUST “MISiS” developed special casting molds allowing to take into account the shrinkage of the material as it hardens. The fact is that TNM-B1 and other similar alloys based on titanium aluminide have practically zero ductility. This means that in conditions when other materials can be deformed under the influence of external forces, this one simply collapses.
In addition, casting from an alloy based on titanium aluminide is very demanding on the choice of material for the manufacture of the mold. The fact is that of all the substances used in the TNM-B1 alloy and the like, titanium is the most active, interacting with different materials. Because of this, the result is a casting with an unsuitable outer layer that interacted with the mold. The worse the materials are selected, the thicker this layer, the higher the consumption of material during casting. The fact is that after casting, the surface layer must be removed, in particular by grinding. Considering that the smallest wall thickness of the blades, for example, of a high-pressure compressor, reaches one millimeter, the removal of an unsuitable surface layer should actually be jewelry. An additional difficulty in choosing the material for the mold is creating
In the course of research and development, scientists of NUST “MISiS” developed a technology that made it possible to produce castings for the blades of a high-pressure compressor (HPC) and a low-pressure turbine (HPH) for the PD-14 engine from the imported TNM-B1 alloy. In particular, scientists have developed a method for casting blades, selected material for the manufacture of a mold and determined the parameters of smelting and pouring of imported alloy. Some time after the completion of the work, several Western countries imposed sanctions against Russia, as a result of which, in particular, it became impossible to purchase TNM-B1 alloy abroad and there was a need for import substitution of materials. In this regard, NITU “MISiS” began to develop a technology for producing an alloy – an analogue of TNM.
In particular, scientists determined the order of interaction of alloy components during the alloying process, investigated several directions for conducting vacuum-arc skull molding in a copper water-cooled crucible in a furnace with a non-consumable electrode in an argon atmosphere. When skull melting, the melt is in contact with the solid phase of a material of the same chemical composition; this avoids contact with the structural elements of the furnace. After refinement of the alloy structure, it became possible to obtain cast products from it. Graphite was chosen as the material of the mold. Studies have shown that pouring the resulting alloys provides minimal interaction of the liquid melt with the mold surface. The amount of contamination of the surface layer does not exceed 15 micrometers, which corresponds to the quality of pouring into a ceramic mold with a protective coating.
At the same time, the manufacture of graphite molds for casting requires fewer steps and, on the whole, simplifies and speeds up production. In particular, such molds are made of graphite on numerically controlled machines – this increases the speed and accuracy of manufacture. Moreover, the developed design of molds for casting the blades of a high-pressure compressor and a low-pressure turbine allows several dozen parts to be cast simultaneously, and the scale of simultaneous casting can be increased.
And what kind of science is it?
As part of the first stage of the work, it was necessary to develop a technology for the serial casting of parts from titanium aluminide according to investment casting. This technology involves the manufacture of the so-called master model, which is used to produce molds. Then this model is simply smelted from the mold as it is calcined. In the case of the TNM-B1 alloy, conventional chemically neutral water-soluble refractory mixtures were used. The trick was that the final casting mold would predictably collapse in the right places when the casting cools. Scientists from NUST “MISiS” succeeded, including using computer modeling, to develop such molds and select materials for molds that allowed free shrinkage of titanium aluminide castings – for various protruding elements, for example, shelves at the base of blades,
Finally, a certain difficulty was the development of a technology for producing alloys based on titanium aluminide. The fact is that even to repeat the same TNM-B1 alloy, it is not enough to know, even if very accurately, its composition. Due to the high chemical activity of the elements, it was important to observe the sequence of their introduction in the preparation of the alloy. Violation of this sequence will lead either to the production of an alloy with other than expected properties, or even to the waste of materials without obtaining a stable alloy. Scientists were able to develop a method for producing an alloy based on titanium aluminide and select such alloying additives that allow one to obtain an intermetallic cast alloy with improved technological characteristics.
Who needs this?
At the end of September 2017, the Russian Cabinet of Ministers approved a development strategy for the export of civilian aviation products: engines, on-board equipment and instruments. This program is designed until 2025. It involves the entry of Russian products into new markets, increasing its recognition among customers and the formation of demand, as well as building an extensive system of after-sales service and service. To increase demand, for example, for Russian engines, they must have characteristics that are at least not inferior to foreign power plants. And the manufacturing technology of parts from titanium aluminide can allow this to be achieved. It is possible that this will help maintain a competitive level of prices for aircraft engines, since alloy for their parts will be produced in Russia, and not from abroad.
Studies on the casting of the blades of a high-pressure compressor were completed at NUST “MISiS” in 2013, and the blades of a low-pressure turbine were completed in 2015. In 2016, the developers presented Russian technologies for producing an alloy based on titanium aluminide and manufacturing molds from graphite. Parts of an aircraft engine made of an intermetallic alloy are obtained on average two times lighter than similar parts of a nickel alloy. Even while maintaining the original engine design, which used a compressor and a nickel-based alloy turbine, the use of new vanes made of titanium aluminide will significantly reduce its mass. This, in turn, will lead to the improvement of several indicators of the power plant at once, including thrust-to-weight ratio (the ratio of engine thrust to its mass) and specific fuel consumption on cruising mode.
The serial development of NUST “MISiS” in the field of producing alloys based on titanium aluminide and casting various parts from them in Russia has not yet been applied. However, the United Engine Corporation, which develops and produces power plants for almost all Russian aviation equipment, is interested in using them today. Compressor and turbine blades (researchers from NUST “MISiS” plan to develop a manufacturing technology for high pressure turbine blades and blades from intermetallic alloys) can be used in promising samples of Russian aircraft engines.