Aluminum-lithium alloy: the choice of new materials for modern aircraft
Aluminum-lithium alloy material is an advanced lightweight structural material that has developed rapidly in aerospace materials in recent years. It has low density, high modulus of elasticity, high specific strength and specific stiffness, good fatigue performance, corrosion resistance and good welding performance. Excellent overall performance. Replacing the conventional high-strength aluminum alloy can reduce the structural quality by 10%-20%, and the stiffness is increased by 15%-20%. Therefore, it has broad application prospects in the aerospace field.

Although aluminum-lithium alloys have shown broad application prospects in the aerospace field. However, due to its higher cost than ordinary aluminum alloy, poor plasticity at room temperature, high yield ratio, obvious anisotropy, and easy cracking in cold working, it is difficult to form. At present, only simple parts can be formed, and it is difficult to manufacture complex parts. , thus limiting its application in structural components. In recent years, the development and forming technology of foreign aluminum-lithium alloys has become increasingly mature, not only in military aircraft and spacecraft, but also in the use of aluminum-lithium alloys for civil aircraft, such as the outer tank of the space shuttle "Endeavour". Airbus A330/340/380 and other series of aircraft. In China, due to the aluminum-lithium alloy casting process, the sheet rolling extrusion technology is immature, and the development of new aluminum-lithium alloys is relatively backward. Currently, there are only a few applications in some types of spacecraft.

1. Development status of advanced aluminum-lithium alloy

According to the historical process and composition characteristics of the development of aluminum-lithium alloy, it can be divided into three stages.

The first phase is the initial development phase, which spans from the 1950s to the early 1960s. Its main representative is the 2020 alloy successfully researched by American Alcoa Company in 1957, and applied to the wing skin and the horizontal stabilizer surface of the Navy RA-5C military early warning aircraft, which achieved a weight reduction effect of 6%. The former Soviet Union successfully developed the BAII23 alloy in the 1960s. However, these two alloys have low ductility, high notch sensitivity, and difficult processing and production, which cannot meet the requirements of aviation production and performance, and have not been further applied.

In the mid-1960s, under the pressure of the energy crisis, aluminum-lithium alloys were re-emphasized and entered the stage of rapid development, the second stage. During this period, aluminum-lithium alloys have been rapidly developed and comprehensively studied. The representative alloys are: 1420 alloy developed by the former Soviet Union, 2090 alloy of Alcoa Company of the United States, 8090 and 8091 alloy of Alcan Company of the United Kingdom. These alloys have the advantages of low density and high modulus of elasticity, which can be used to replace the aerospace parts 2xxx and 7xxx aluminum alloys. For example, the former Soviet Union used 1420 alloy components on the MiG-29, Su-35 and other long-range missile warhead shells. Although the second-generation aluminum-lithium alloy has achieved remarkable research and application results, due to the existence of severe anisotropy, low plastic toughness, severe heat exposure, and loss of toughness, most alloys are not solderable, making it difficult to 7xxx aluminum alloy competition.

In the late 1980s, the third generation of high-strength weldable aluminum-lithium alloys represented by the US Weldalite 049 series alloys were successively developed and successfully applied in aerospace and other fields. At present, the new third-generation aluminum-lithium alloys are developing in the direction of super strong, super tough, ultra-low density, among which high-strength weldable alloys and low-isotropic alloys are studied. In addition, an aluminum-lithium metal matrix composite with isotropic, granule or whisker SiC ceramics as a reinforcement has been developed, which has a modulus of elasticity of 130 GPa, making it a strong competitor for other composite materials in the aerospace industry. .

2. Application and development trend of aluminum-lithium alloy in aerospace

According to statistics, for every 1kg of structural weight reduction, more than 10 times economic benefits can be obtained, so the aluminum lithium-lithium alloy with lower density is widely concerned by the aerospace industry. Aluminum-lithium alloys have replaced conventional high-strength aluminum alloys on many aerospace components. Among them, the application of the United States is developing very fast, especially in the aerospace industry. Lockheed Martin used the 8090 aluminum-lithium alloy to build the payload bay of the Hercules launch vehicle, losing 182kg.

In 1994, in order to solve the problem of overweight in the outer tank of the Endeavour space shuttle, Lockheed Martin and Reynolds Metal developed a new 2195 material to replace the previous 2219 alloy. The alloy is 5% lighter than the 2219 alloy and its strength is 30% higher than the latter. The overall welded structure tank made by 2195 reduces the weight by 3405kg, including 1907kg of liquid hydrogen tank, 736kg of liquid oxygen tank, and direct economic benefit of nearly 75 million US dollars. Therefore, it is called SuperLight Weight Tank. ). Russia has also been a leader in the research, production and application of aluminum-lithium alloys. To improve the load capacity, the spacecraft's external fuel tanks are made of aluminum-lithium alloy. The "Energy" carrier rocket's cryogenic tank is used. Made of 1460 aluminum-lithium alloy.

In the aviation field, many advanced fighters and civil aircraft have chosen aluminum-lithium alloys. In 1988, Lockheed Martin Combat Aircraft Systems, Aircraft Systems, and Reynolds Metals jointly developed a plan to develop the 2197 alloy application – using its slabs to make the fighter bulkhead deck. In 1996, the US Air Force F-16 aircraft began to use the alloy slab to make the rear deck and other components.

In addition to the United States, other countries, such as Russia, the United Kingdom, and France, are actively promoting the use of aluminum-lithium alloys in aerospace vehicles: 25% of Westland EH101 helicopters are made of 8090 alloy. Its total mass dropped by about 15%; France's third-generation Rafele fighter plane plans to use aluminum-lithium alloy to make its structural frame; Russia in Jacques-36, Su-27, Su-36, MiG-29, The MiG-33 and other fighters have a large number of parts made of aluminum-lithium alloy.

In terms of civil aircraft, Airbus Industries' A330, A340 and A380 passenger aircraft use aluminum-lithium alloys. Among them, A330 and A340 each have about 3t of aluminum-lithium alloy for fuselage structure, purlins and other components. At present, the new A350 passenger aircraft uses the new 2198 aluminum-lithium alloy on the fuselage skin for the first time. The US Boeing 747, 777, and McDonnell Douglas series use aluminum-lithium alloys, and their use includes fuel tanks, bulkheads, wing skins, leading edges, and trailing edges. The Bombardier C-Series aircraft fuselage will also all adopt a new aluminum-lithium alloy.

3. Advanced manufacturing technology and development trend of aluminum-lithium alloy

Superplastic forming and diffusion bonding technology

Superplastic forming and superplastic forming/diffusion joining technology (SPF and SPF/DB) are processes that use the superplasticity of materials to form a thin-walled part that is complex in shape and difficult to process, using blow molding, bulging, etc. A special forming method with almost no surplus, low cost and high efficiency. Al-Li alloys, like other superplastic materials, can be obtained by alloying or mechanical heat treatment to obtain uniform, fine, equiaxed crystals to produce superplastic properties. The SPF study of aluminum-lithium alloys began in 1980. At the 1982 Farnborough International Air Show, the British superplastic forming metal company demonstrated the superplasticity of aluminum-lithium alloys and its superplastic F parts for the first time.

The American Weldalite 049 alloy has different superplasticity, and it is solution treated at 507 °C without back pressure. The elongation rate can reach 829% at 4×10-3 strain rate. This strain rate is significantly higher than the strain rate of other aluminum alloys, which is important for solving the problem of low speed of the superplastic process. Russia has processed many aircraft parts for the 1420 using the SPF process, some of which are 1200mm x 600mm.

The National Aerospace Materials and Technology Research Institute and the Beijing Aviation Manufacturing Engineering Research Institute have carried out a lot of pioneering work on the SPF and SPF/DB combination processes of aluminum-lithium alloys, and have achieved many results. At present, the superplastic forming of aluminum-lithium alloy is being developed from the secondary bearing member to the main bearing member, and the combination process from a single superplastic forming to a superplastic forming/diffusion joint makes the processing cost of the aluminum-lithium alloy lower. The structure is more integrated and lighter in quality.

Spin Forming

Spinning technology is an advanced process that combines the characteristics of forging, extrusion, drawing and bending with less cutting. Shear spinning is a new spinning technology developed on the basis of traditional spinning technology in recent years. It does not change the outer diameter of the blank and changes its thickness to realize the spinning method of manufacturing various axially symmetric thin-walled parts such as cones. Conical thinning and spinning). The forming method is characterized in that the rotating wheel is less stressed, the half cone angle and the wall thickness influence each other, the material flows smoothly, the surface roughness is good and the forming precision is high, and the forming, stretching and spinning are relatively easy to form. s material. Many of the Al-Li alloy components on the spacecraft are hollow rotor-shell structures, which are particularly suitable for processing by spinning. The more typical part is the dome cover of the launch vehicle cryogenic tank.

The dome of the US Hercules launch vehicle was made by spinning three Weldalite 049 plates with a diameter of 0.65 m and a thickness of 10.7 mm. One of the middle parts is welded by variable polarity plasma arc welding (VPPA). After 343 ° C / 4 h to remove stress, all the blanks are heated by flame to maintain 317 ° C; after forming, 505 ° C / 0.5 h solution treatment, Water quenching; after 177 ° C / 18 h artificial aging, the tensile strength at room temperature is about 600 MPa, and increased to 700 MPa at -196 ° C, and has good fracture toughness. The outer tank dome of the Space Shuttle Endeavour also uses the same spinning technology and uses advanced shear spinning technology in the barrel section of the outer tank.

Roll Forging

Al-Li alloys, in particular, Weldalite series alloys and 1420 alloys have good forging properties, and die forgings made therefrom do not crack, which has been confirmed by more than 150 forgings. Therefore, its application to the aerospace industry has broad prospects. Roll forging is a new type of near-net forming technology developed in recent years. The material is plastically deformed under the action of a pair of counter-rotating molds to obtain the plastic forming process of the required forging or forging blank. There are two important areas in the development of roll forging.

The first is to realize volume distribution and preforming in the production of long-axis forgings, reduce the forming load, form a precision forging composite production line, and produce complex forgings in large quantities with less investment. The second is precision roll forging technology, including cold fine roll technology. There are good development prospects in the precision forming of plate-like parts, such as the advantages of blade forming and variable-section leaf springs. In recent years, the two directions of roll forging have been successfully applied to annular forgings of aluminum-lithium alloys and sheet metal parts with ribs. Such as the "Y" frame and docking ring of the outer tank of the "Endeavour" spacecraft.

Welding Technology (Welding)

Welding is one of the main processes for manufacturing aluminum-lithium alloy aerospace products such as tanks and warhead shells. The former Soviet Union studied the welding of 1420 alloy for more than 10 years. From the welding process, welding, welding performance and post-weld heat treatment, in-depth theoretical research and discussion. In the 1980s, research on the weldability of 1460 high-strength alloys was also carried out. The 1460 alloy using tungsten hydrogen arc welding (GTAW) and vacuum electron beam welding (EB) has been successfully used to manufacture the "Energy" carrier rocket tank.

The welding of aluminum-lithium alloys in the United States, Europe and other countries began in the early 1980s. Unlike Russia, the United States pays special attention to the study of welding cracks. The welding methods used in the United States mainly include GTAW, EB, VPPA (polar arc plasma arc welding), etc., and the space shuttle outer tank made of Weldalite 049 alloy is welded by VPPA method. Alcoa adopts EB welding to 12.7 mm thick 2090 alloy plate. Welding, penetration rate of 100%.

In recent years, two new welding technologies: friction stir welding and laser welding technology have also begun to be applied to the research of aluminum-lithium alloy manufacturing. American Lockheed Martin used friction stir welding to weld 2.3-8.5m thick 2195AI-Li alloy and 2219 alloy sheet, and found that the joint strength can be increased by 15% - 26%, and the weld fracture toughness is increased by 30%. The plasticity is doubled and the weld bead structure is extremely small. After more than 20 years of efforts, Airbus has used laser welding technology to manufacture double-beam "T" structural parts for large passenger aircraft, and successfully applied to passenger aircraft fuselage panels such as A330, A340, and A380.

New heat treatment process technology

The main advantages of aluminum-lithium alloys are low density, high specific modulus, strong corrosion resistance, etc. The overall performance is superior to conventional high-strength aluminum alloys. However, in the variable amplitude fatigue test based on compressive stress, this advantage of aluminum-lithium alloy no longer exists. The main reason is that the peak strength material has short-transverse plasticity and fracture toughness, and the anisotropy is serious. Artificial aging Before the peak performance can be achieved by applying a certain amount of cold working before, the expansion speed is significantly accelerated when the fatigue crack is at a fine microscopic level. In order to improve the fatigue and fracture toughness of aluminum-lithium alloys, NASA has done a lot of research work on the new 2195 aluminum-lithium alloy, and developed two-stage, three-stage and five-stage heat treatment processes to break the room temperature of 2195 alloy. Toughness and fatigue performance are improved by nearly 30%, while strength is comparable to traditional aging.

At present, while researching and developing new aluminum alloys in China, a lot of research has been done on the production process. The new heat treatment process (T74, T73) greatly improved the fracture toughness and stress corrosion cracking resistance of 7xxx alloy, and further studied the heat treatment process of 7xxx alloy, such as 7075-T76 for L-1011 wing extrusion siding , 7075-T736 is used for landing gear parts, window frames and hydraulic system components. However, the current research work on aluminum-lithium alloys is still in its infancy, and the basic research is relatively weak, and there is still a distance from the application. The heat treatment of aluminum-lithium alloy should be based on the heat treatment of aluminum alloy, combined with new foreign technology and new methods, to carry out basic research of the system, in order to realize the industrial application of the heat treatment process of aluminum-lithium alloy at an early date.

(1) As an important structural material for aerospace, aluminum-lithium alloys have received extensive attention from Western countries. Today, the third-generation aluminum-lithium alloys have been used in the manufacture of large-scale commercial aircraft and become an important trend in the development of future models. However, at present, the new aluminum-lithium alloy mainly relies on foreign suppliers, which is not only costly, but also cannot be supported by related key technologies such as sheet metal and heat treatment. Therefore, independent development and development of new high-strength, high-damage-capacitance aluminum-lithium alloy is China's aluminum. The important direction of the future development of lithium alloys. In addition, aluminum-lithium alloys and composite materials are important choices for future civil aircraft. How to improve their weight-loss benefits, strength and damage tolerance is a major challenge in the development of new alloys.

(2) On the basis of the mature technology of casting and rolling, the aluminum-lithium alloy has been continuously expanded, and new technologies such as superplastic forming, spinning and roll forging have been continuously innovated, and significant application results have been achieved. However, room temperature forming ability is still difficult due to its own performance limitations. Aluminum lithium alloy

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