From SPIRAL to MAKS

Metal Materials for the Advanced Aerospace Systems

Sergeev K.N., Bulgakova S.G.
The trends of application of some new metal materials developed recently by VIAM for the Aerospace Systems are considered. The materials are partially mastered by the Russian industry and the metallurgical base is technologically ready for their manufacture. Many of specified materials are investigated in the NPO MOLNIYA and the part of them had been tested in the BURAN Orbital Spaceship’s structure.

The materials widely used in the aviation and space industries, including those tested in the BURAN Orbiter as well as advanced metal materials are planned for application in the design of the MAKS Multipurpose Aerospace System. Serial materials:

• 1163, D16ch, 1201 aluminum alloys;
• VT23, VT20 titanic alloys;
• VKS210 and VKS170SH high-strength structural steels;
• VNS49, VNS25, 12KH18N10T stainless steels;
• EI696, EP742, SHS6У, VSH98 high-temperature steels and alloys, niobium alloys.

Required increased level of characteristics of traditional serial materials can be achieved by optimization of general alloying element content and thermo-mechanical and thermal processing modes.

However, to meet all working requirements presenting for high-velocity aerospace flying vehicles it is necessary to develop and apply new high-strength, reduced-density, heat-resistant and corrosion-resistant materials.

Brief information about several new alloys, the prospective ones from the point of weight efficiency of the system, is shown below.

The 1460 aluminum-lithium high-strength welded alloy developed specially for tank of cryogenic and hydrogen fuel is of significant interest. The 1460 alloy differs from the 1201 (2219 – the USA) alloy used for that purposes by reduced density (on 9%) and considerably higher strength (Table 1).

Tank developments for aerospace systems in Europe and the USA are absolutely oriented on aluminum-lithium alloys. Large volume of researches on the 1460 alloy’s properties and welded joints was worked out. The production technology for semi-finished items and details is carried out. This technology shows alloy prospects.

The estimation of 1460 alloy main characteristics at cryogenic temperatures (table 2) was performed. The data indicated testifies to alloy serviceability in a wide range of temperatures. It is necessary to mention that speed of propagation of fatigue crack speed on extruded semi-finished items is practically the same at low temperatures.

All types of welding may be used for the 1460 alloy. It opens wide technological possibilities for application of this material. For welded joint in ‘hardening + artificial aging + welding’ conditions (without extra technological techniques) the ultimate strength for automatic welding is more than 0,55 out of ultimate strength of parent metal (Table 3).

The 1460 alloy is also well welded with the 1201 alloy thus providing combined constructions where advantages of both materials are presented.

Nowadays VIAM developed the 1461 alloy based on 1460 alloy. It has advanced plasticity properties at slight strength reduction (Table 4).

The 1461 alloy in combination with 1460 alloy is supposed for massive high-loaded welded assemblies of the tank structure .

For non-welded structures of airframe’s members operating in the temperature range –130…+150°С it is possible to use aluminum-lithium alloys - 1140, 1441, 1450, 1451. They are possessed of reduced density and higher strength-to-weight ratio. The characteristics of these alloys are shown in table 5.

For the load-bearing members of the airframe it is better to utilize high-strength titanic alloys: the VT23K alloy for longerons, beams, docking units and VT23L and VT20 - braces.

Alloys were produced by the industry and well tested in the BURAN Orbiter.

The VT23 alloy was used in the following temperature range [-130...+ 350°С] with strength level σ = 110...125 kg/mm2. It and was chosen in due time for aerospace system as a material having the best strength-plasticity ratio at normal and reduced temperatures.

Notes: FCS testing (fatigue crack speed) of sheets and panels was conducted with stress intensity factor of range ΔК = 60 kg/mm^3/2, for plates ΔК = 70 kg/mm^3/2

The VT23K alloy has reserves on strength and reliability, which can be realized in advanced development at optimization of thermo-mechanical and thermal processing technology.

In addition to the mentioned titanic alloys, it is suggested to use the VT18U heat-resistant titanic alloy in the orbital plane’s members with operating temperature up to 600°С. These members are honeycomb soldered-structures and holders of hanging junctions. The alloy was previously used in engines.

The partial replacement of the details made of the 12Kh18Kh10T-type heat-resistant steels with the VT18U titanic alloy will allow to reduce mass of the assemblies (Table 6) due to reduced density at high strength. Heat-resistant nickel alloys (Table 7) tested in turbine disks and the last stages of the gas turbine engines is supposed to be used in hot parts of the orbital plane. Such experience was gained in the BURAN Orbiter: fasteners for wing panel joints were made of EP742-ID alloy. As table 7 shows, EK79ID and EK152-ID alloys have increased strength at normal and as well as at high temperatures.

The structural elements of complicated form made of nickel alloys are labor consuming for mechanical processing. The details of brackets can be cast from the ShS26U and ShS32 alloys. Indicated alloys have increased cyclic strength and heat-resistance in comparison with the ShS6U alloy (Table 8).

To realize all potential alloys characteristics it is desirable to introduce special casting method. The equipped crystallization casting and high-gradient oriented crystallization casting allow to ensure required mechanical properties and detail structure. These alloys also have increased values of short-time strength and better plasticity characteristics for 20…1150°С-temperature range (Table 9).

The VN3, VN4 and VN9 niobium alloys with special protection covering are supposed to be used in hot joints with temperatures higher than 1050°С. The VN4 alloy is prospective for details and assemblies operated at temperatures up to 1500°С and VN9 – at temperatures up to 2000°С.

These alloys were not used previously in structures. It is needed additional study of their properties. The further development directed to extent niobium alloy application at temperatures higher than 1200°С should be concentrated on creation of coverings of multi-layered composite systems as well as on technology providing required temperatures and reliability of protective coating.

The conclusions

Each new phase of aerospace system development increases importance of applied materials. In spite of rapid development of composite, non-metal and other materials, the metal materials in the nearest decade will remain main structural materials for creation of airframes of the orbital planes. It is a task for designers, technologists to use materials with certain characteristics for production of particular details. Simultaneously with structure development, it is necessary to improve technology of semifinished item production from new and serial materials on purpose to reduce dispersion of mechanical properties. Many from suggested materials can be successfully used in different industries.

References

1. A.G. Bratukhin, N.F. Anyushkin and other. Titanic alloys application for aviation constructions // Titan. - 1996. - № 1. p.p. 77-81.
2. Super-plastic aluminum alloys // NPO MOLNIYA review, 1990.
3. R.E. Shalin New materials and technologies – prospects of development and creation of aviation techniques // Aviation techniques and technology. -1995. - № 1. p.p. 13-20.
4. R.E. Shalin Further development of aerospace materials // Works collections of the 1st International Aerospace Conference. - Moscow: RIA, 1994. - volume 5. – p.p. 4-22.