What metallic materials are used in aerospace applications?

Since the Wright Brothers invented the airplane in 1903, the renewal of aircraft materials has shown a high rate of change and transformation. From the initial proportion of up to 47% of the wood-based to today’s, various metal materials continue to emerge and be used.

In the development of aviation manufacturing, over a hundred years of history, materials, and aircraft have been driven by each other to continue to develop and improve.

After years of development, metal materials still occupy a prominent position in this field and are still the most widely used materials in aerospace.

Metallic materials, strength, hardness, and use of temperature and other properties are better than polymer materials, its toughness is far above theof ceramic and other inorganic non-metallic materials, and, it does not absorb moisture, does not age, dimensional stability, can be conductive and thermal conductivity, and another excellent performance is by no means comparable to other materials.

Today, high-performance metal materials play a crucial role in developing aerospace technology, including aluminum alloys, titanium alloys, magnesium alloys, ultra-high-strength steels, high-temperature alloys, intermetallic compounds, and metal magnetic materials.

Specific heat treatment processes need to be implemented for particular aerospace components to meet the comprehensive performance requirements of aerospace components. The following briefly describes the application of several typical metal materials in aerospace.

Aluminum alloy

Compared with other materials, aluminum alloy has outstanding advantages such as high specific modulus/strength, good corrosion resistance, good processing performance, low cost, etc.

Therefore, it is still regarded as the best structural material for aerospace applications despite competition from new materials such as titanium alloys and composite materials. Based on its good overall performance, it is also the most used metal structural material in the aerospace industry.

For example, a recent NASA study reported that although composites have higher unidirectional mechanical properties, specific strength, and specific stiffness, Al-Li alloys are better suited than composites for constructing the Orion crew capsule because of better multidirectional impact resistance.

Lockheed Missile Aviation Corporation for NASA research survey satellite SEASAT-A for a variety of sensors, transmitters, and antennas, such as the installation of the fixtures mass 1189kg, of which 404kg for the aluminum alloy material (accounting for 34% of the total mass);

The aerospace structure known as the Centaur standard cover has a length of 17.89M, a diameter of 4.27M, and a mass of 2973kg, of which 1905kg is made of aluminum alloy (accounting for 64% of the total weight).

Because of aluminum alloy’s enduring comprehensive performance advantages, governments regard it as a strategic material. Its development planning, guidance, and support, the end of the 20th century, the U.S. Department of Energy’s Aluminum Industry Technology Guidelines Program, Japan launched the Super Aluminum Program aimed at further reducing the energy consumption of aluminum production through technological innovation and improving the quality of aluminum, to make it have a more substantial adaptive capacity and higher value of use.

Aluminum alloys are mainly used as load-bearing structures in aerospace structures, including hull structures, load-bearing wall panels, beams, instrumentation mounting frames, fuel tanks, etc.

The alloys currently used are concentrated in the 2XXX, 7XXX series, and Al-Li alloys. They are used in a wide variety of forms and sizes.

For example, strategic missile shells, large aircraft, such as the whole wall plate is often up to tens of meters long, thickness varies from several millimeters to hundreds of millimeters, mostly 2XXX and 7XXX forgings; and skin, instrumentation framework, fuel storage tanks are primarily thin-walled components, mostly 2XXX and Al-Li alloy manufacturing.

The main bearing beams of large aircraft are mainly used as forgings, and the materials are concentrated in 7XXX alloy; the complicated shapes of spacesuits are also mostly made of aluminum alloy, and the limbs are made of 6061 alloys, and the waist with 7075 alloys is required for higher strength.

Titanium Alloy

Titanium alloy and aluminum, magnesium, steel, and other metal materials, compared with the following advantages: high specific strength value, good corrosion resistance, good fatigue resistance, thermal conductivity is minimal, the coefficient of linear expansion is small, the mechanical properties of high and low temperatures is excellent, generally speaking, can be used for a long time in 350 ~ 450 ℃ below, low temperature can be used to -196 ℃.

However, some shortcomings exist, such as lower specific modulus, poor wear resistance, more complex manufacturing processes, and higher material costs.

The amount of titanium alloy is also growing rapidly in the aviation industry.

Aero-engine compressor blades, magazine, and body main bearing components are commonly manufactured titanium alloys.

F22 fighter is widely used in the manufacture of titanium alloy main bearing structure of the typical representative of the initial design of the amount of titanium alloy for 15.9%, and into the engineering manufacturing and development stage, the proportion of titanium alloy has increased to 41%;

Among them, the wing structure of titanium accounted for 42%, and the rear fuselage section was as high as 55%. F22 uses titanium alloy types, mainly Ti-62222 and Tc4; the application form is primarily forging and castings.

F22’s engine compartment frame uses the Wyman-Gordon company to provide monolithic Ti-6Al-4V forgings; the main wing beam is cut from titanium forgings.

In the flight missile (flight body), in addition to the primary use of low-density materials, but also the use of temperature, alternating stress sensitivity is minimal material; in addition to the application of good fatigue toughness of the material, but also selected in the ultra-low temperature has superior mechanical properties of the material.

For example, titanium NASA-satellite gear system coat, ribbed plate with 320kg single piece lining of the total weight of 1T mercury sealing capsule made of Ti-5Al-2.5SN alloy;

Apollo program spacecraft’s two-person module and hermetic chamber wing beams and ribs are also made of Ti-5Al-2.5SN alloy. The lining is made of pure titanium;

The jacket of the ELDO-Europe 1 rocket is made of the alloy Ti-13V-11cr-3Al; the high-pressure tanks or fuel reservoirs are preferably made of the alloy Ti-6Al-4V;

Ablestar (Apollo) rockets with the reservoir, after the nozzle from the 13 forged Ti-6Al-4V (with low oxygen content) alloy plate composition, and after welding and made into a reservoir. The accumulator is used to store the oxidation catalyst of the power fuel. Ti-6Al-4V alloy has been widely used in the ascent section of the rocket motor casing.

High-temperature alloys

In aerospace, high-temperature alloys are mainly iron-based, nickel-based, and cobalt-based alloys of three kinds, of which nickel-based high-temperature alloys are most widely used, such as GH1040, GH2028A, GH4169, GH4141, GH4586, etc., commonly used as space engine turbine disk and blade materials.

Although in recent years, the development of intermetallic compounds has been faster, the application of higher calls, nickel-based high-temperature alloys are still forming good, high reliability, and other outstanding performance to become the first choice of materials for aircraft engine turbine discs and turbine blades.

For example, PW company for the F22 manufacturing F119 engine is still using reliable iNcoNEL718 manufacturing turbine disk and turbine blade, and through the additional single-crystal, water-cooled and spraying thermal barrier coating technology to improve the temperature before the turbine.

In the aerospace power facilities, nickel-based high-temperature alloys have also achieved a wide range of applications. Recently just ended the ground test of the U.S. X51A super-combustion ram engine that a large number of iNcoNEL625 alloy to manufacture the inlet wall plate and other airflow channel components.

NASA used in the engine in the closed-loop fuel system allows multiple engine components using iNcoNEL625 nickel-based alloy manufacturing without having to use special high-temperature materials, which is the X51 compared to the X43 more practical and reliable pursuit of one of the key measures.

In addition, in order to meet the needs of future hypersonic spacecraft applications, high-temperature alloys will be used as the main material to manufacture the 800 ~ 1100 ℃ high-temperature bearing materials.

For example, X43A on the large size of the horizontal tail and vertical tail are used Haynes230 nickel-based high-temperature alloy welded structure; X43A in the rudder, cowl localized large number of Hayness188 alloy, in the wing and tail position used Haynes230 alloy to manufacture the precise shape of the parts; in the Netherlands and Russia and other countries to develop the In the Netherlands and Russia and other countries co-developed the “DELFLT” space re-entry test vehicle thermal protection system is also mainly used PM1000 nickel-based high-temperature alloys.

Ultra-high strength steel

Ultra-high-strength steel has a high tensile strength, and maintain sufficient toughness, than the strength (strength to density ratio) and flexural strength ratio (σS / σb) high, and has good weldability and formability. Ultra-high-strength steel can be divided into low-alloy, medium-alloy and high-alloy ultra-high-strength steel 3 categories.

Ultrahigh-strength steel in the rocket is mainly used for engine shell, engine nozzle and all levels of booster. Especially as a large solid fuel propellant rocket engine shell, because the work of several atmospheric gas pressure, the requirements of the material not only has high strength, high toughness, but also to have good molding and weldability.

Currently used for solid rocket motor shell of ultra-high-strength steel are D6Ac, 406, 18Ni, 300M, 35NcD16, etc., of which D6Ac steel has been successfully used in the U.S. Space Shuttle solid booster rocket shell.

Ultra-high-strength steel high-pressure gas cylinder is an important part of the aerospace engineering, due to the long-term nature of the cylinder workload and ultra-high-strength steel higher environmental sensitivity, its stress corrosion threshold value has become the most important performance of the security of gas cylinders. In addition, ultra-high strength steel is also used in aircraft landing gear.

Intermetallic compounds

Intermetallic compounds because of its superiority to high-temperature alloys, heat resistance, high specific strength, high specific life, high thermal conductivity and high oxidation resistance, as well as superior to the toughness of ceramic materials and good hot workability and has attracted widespread attention. Intermetallic compounds that can be used as high-temperature structural materials include Ni3Ti, NiAl, Fe3Al, FeAl, Ti3Al, TiAl and so on.

Currently, Ti3Al (α2) and TiAl (γ) intermetallic compounds are of most interest to NASA. They are characterized by high-temperature performance; maximum operating temperature of 816 ° C and 982 ° C, respectively; light weight, its density and titanium-based alloys comparable to the nickel-based high-temperature alloys, only 1/2; there is a high degree of rigidity, good oxidation resistance and corrosion resistance.

They are the manufacture of advanced spacecraft shell wall plate and engine high-temperature components of the ideal material, is expected to meet the NASP medium-temperature (300 ~ 1000 ℃) structure of the conditions of use, can be manufactured in the space shuttle fuselage and wing wall plate.

At present, intermetallic compounds successfully used in aviation engine turbine guide vanes, such as aircraft gas turbine engine heated parts of the material used is nickel-based superalloys.

However, due to the material exists higher than 800 ℃ when the oxidation resistance is insufficient, the strength of the decline and other problems, therefore, the development of new materials to replace nickel-based superalloys.

In addition, there are many intermetallic compounds, the strength of which increases or remains unchanged with increasing temperature in a certain temperature range. For example, single-phase Ni3Al has anomalous temperature-strength characteristics and is a good high-temperature structural material.

Ni3Al commercial alloy grade has been developed in the United States IC-50, IC-218 and IC-221M, etc. IC-221M alloy has been selected by U.S. companies as a substitute for Ni-based high-temperature alloys for the manufacture of diesel engine supercharger, in order to improve its fatigue life and reduce costs.

Magnesium alloy

Magnesium and magnesium alloys have some attractive properties: low density, excellent thermal conductivity, electrical conductivity and electromagnetic shielding properties; high strength, specific stiffness and vibration damping properties; excellent processing performance, such as good casting performance, machinability, as well as in the protection of the atmosphere has good welding performance.

But magnesium alloy has in the atmosphere is not corrosion-resistant defects. Generally speaking, the long-term working temperature of magnesium alloy does not exceed 150 ℃.

Magnesium alloy in the spacecraft structure also has certain application value, especially can be made into a complex shape of large castings, has been used as a domestic spacecraft cabin floor, support beams, such as communication satellite antenna, satellite corner frame parts and missile rudder, etc., magnesium alloy in the application of the aircraft as shown in Figure 1.

However, due to the poor corrosion resistance of magnesium alloy, in product design, manufacturing, use, storage and other aspects are brought about by many inconveniences. And the total performance compared with aluminum alloy is not particularly superior.

Fig.1 B-36 Bomber (shaded area is the part of magnesium material)

Fig.1 B-36 Bombardment Aircraft

Metal magnetic materials

Magnetic materials is a wide range of uses, a wide range of basic functional materials, metal magnetic materials have excellent magnetic properties, magnetic stability and mechanical properties and other comprehensive performance, is widely used in spacecraft in a variety of power equipment, electronic devices and navigation, accelerometers and other devices.

Soft magnetic materials such as pure iron, Fe-Si, Fe-Co, Fe-Ni, Fe-Al, etc. are widely used in spacecraft, such as engine servo mechanism of various relays and electromagnetic valve, telemetry system signal generator and exchanger, control system in the power supply transformer, voltage regulator and magnetic amplifier, as well as satellites in a wide range of applications in various types of magnetic core bar.

Hard magnetic materials such as AlNiCo, FeCrCo, SmCo, NdFeB, etc. are used in missiles, satellites and other spacecraft torque motors, moving coil sensors, displacement sensors, accelerometers, magnetrons, as well as flowmeters, traveling wave tubes, permanent magnet motors, hysteresis motors, gyroscopes and so on.

Conclusion

At present, the aerospace with high-performance metal materials, although faced with the challenge of advanced composite materials, but through the heat treatment process of existing metal materials and the development of better performance of the further improvement of new materials, as well as the use of advanced manufacturing technology, high-performance metal materials will be the future of the aerospace structure materials and functional materials is very important.