Prospect of Aluminum Alloy Welding Technology in Aerospace Industry
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Aluminum alloy not only has high specific strength, specific modulus, fracture toughness, fatigue strength and corrosion stability, but also has good forming process and good weldability, so it has become the most widely used in the aerospace industry. Non-ferrous metal structural materials.
For example, aluminum alloys are the primary structural material for launch vehicles and various spacecraft. The command cabins of the Apollo spacecraft in the United States, the lunar module, the space shuttle oxyhydrogen propellant tank, and the crew cabin are also made of aluminum alloy as a structural material. Aluminum alloys have been widely used as the main structural materials for various large-scale launch vehicles developed in China.
The development and application of aluminum alloy welding technology in aerospace industry is closely related to the development of materials. This paper will briefly review the development of aluminum alloy welding technology in aerospace industry and introduce several aluminum alloy welding technology with great application prospects.
Development of aluminum alloy welding technology
1 LD10CS aluminum alloy welding review
The propellant tank structure materials of some early missiles and long-range launch vehicles mainly used Al-Mg series alloys, especially the LF3 and LF6 anti-rust aluminum used for annealing and semi-cold hardening. Both of these aluminum alloys have excellent weldability.
With the development of aerospace technology, the propellant tank structure materials of the launch vehicle have changed from the use of non-heat-treated rust-proof aluminum to the use of heat-resistant reinforced high-strength aluminum alloy. The LD10CS alloy has been successfully used in a variety of large launch vehicles and solid-state missiles. Because of its excellent ultra-low temperature performance, it has also been applied to the three-stage liquid hydrogen and liquid oxygen propellant tanks.
It should be pointed out that the welding performance of LD10 alloy is poor, the tendency to form hot cracks during welding is large, and it is sensitive to various factors in the welding process. The fracture toughness of welded joints is low, especially when welds exist. In the case of welding defects, low pressure blasting often occurs in the test piece during the hydraulic strength test.
In the early 1970s, in the early days of developing the LD10 alloy rocket propellant tank, there were great difficulties in the welding process. The "two-sided three-layer welding" process (frontal base, cover, and back-sealed sealing) invented in the "three-in-one" research has achieved the design requirements for welded joints. In the LD10 welding production practice, it is concluded that if the elongation of the welded joint zone is not less than 3%, the plasticity of the welded joint can meet the requirements for use. For many years thereafter, the “elongation rate of not less than 3%†has been used as an important acceptance indicator.
For decades, the welding process has been mainly argon arc welding (TIG), including manual argon arc welding and automatic argon arc welding. From the aspect of welding process, in order to reduce the welding residual stress and deformation of the welded structure, the welding heat input amount is usually minimized in the welding process selection. Especially for the heat-treated reinforced aluminum alloy, due to the action of the welding heat process, there is a softening zone in the heat affected zone of the weld, which has better plasticity and lower strength. The welded joint strength coefficient is 0.5 to 0.7.
Why is the LD10CS tank using a two-sided three-layer welding process? Theoretical analysis and practical results show that if this welding method is not used, the plastic joint of LD10CS aluminum alloy will be poorly plasticized, and the weld on the back of the weld will be prone to cracks. In the case of two-sided and three-layer welding, the root and bottom seal welding can eliminate such cracks. At the same time, due to the large heat input, the heat affected zone has different degrees of annealing or overaging, which reduces the hardness and improves the plasticity. The fracture position of the welded tensile specimen is the weld softening zone. Thus, in the structure, the welded joint compensates for the lack of plasticity in the fusion zone by the plasticity and deformation of the softened zone under a complicated stress state. However, after the weld of the tank weld is repaired, low pressure blasting sometimes occurs.
Due to the special requirements of double-sided welding, the application of new techniques of automatic welding and welding (such as vacuum electron beam welding, variable polarity plasma welding, etc.) is limited. This is because the heat input quantity of argon arc welding is larger than that of high energy beam vacuum electron beam welding. At the same time, considering the structural bearing capacity of welded joints, it is difficult to apply new welding technology with relatively concentrated welding heat input, which restricts the new welding technology. Applications.
In welding production, the common defects in aluminum alloy welds are weld pores. Hydrogen is the main cause of pores in the fusion of aluminum and its alloys. The amount of hydrogen in the base metal, the moisture adsorbed by the welding wire and the oxide film on the surface of the base metal, and the moisture in the atmosphere of the arc column are all important sources of hydrogen in the pores of the weld. Aerospace welding workers have made unremitting efforts and efforts to ensure the success of the delivery and launch of aerospace welding products. However, due to a number of factors and conditions, there is still a porosity difference in individual tanks in production.
In terms of welding materials, foreign-made welding plates are used, and the hydrogen content of the base metal is less than 2×10-7. However, there is no requirement for hydrogen content in the technical conditions of domestic aluminum alloy sheet manufacturing.
2 Overview of aluminum alloy 2219 and aluminum-lithium alloy welding
The outstanding features of 2219 high-strength aluminum alloy are good welding performance. From -253°C to +200°C, it has good mechanical properties, stress corrosion resistance, low sensitivity to welding hot cracks, good weld joint plasticity and low temperature toughness. . In the United States, it has been used as the main structural material of propellant tanks. The US Saturn V No. I tank has adopted 2219 aluminum alloy. The former Soviet Union used a large number of 1201 (equivalent to 2219) aluminum alloys in both the Energy and Snowstorm space shuttles.
The S147 aluminum alloy developed in China is similar to the 2219 aluminum alloy. The tendency to form weld cracks is low, but the sensitivity of generating pores is strong. Especially the fusion zone and dense micropores are the main defects affecting the performance of welded joints.
With the development of aerospace technology, higher requirements have been placed on the strength and weight reduction of aluminum alloys. Al-Li alloys have developed rapidly in recent decades. Because each addition of 1% Li can reduce the mass of aluminum alloy by 3%, the modulus of elasticity by 6%, and the modulus of elasticity by 9%. Compared with the 2024 and 7075 alloys commonly used in aircraft products, the density is higher. The drop is 7% to 11%, and the modulus of elasticity is increased by 12% to 18%. Compared with the widely used Duraluminum (hard aluminum) Д16 (2024) alloy, the former Soviet Union's 1420 alloy has a 12% decrease in density, a 6% to 8% increase in modulus of elasticity, good corrosion resistance and low fatigue crack growth rate. The strength, yield strength and elongation are similar, and the weldability is good.
The former Soviet Aeronautical Materials Research Institute (ВИÐÐœ) И.Ð.ФридлÑндер and others invented the Al-Mg-Li 1420 alloy in the 1960s, and studied the welding of the alloy. In the 1970s, the welding research on the alloy has achieved results. They believe that the alloy argon arc welding can use AMг6, AMг6T and 1557 welding wire, and the strength coefficient of the welded joint reaches 0.7 or more. Pre-weld and post-weld heat treatment have a great influence on the strength of welded joints. The strength of welded joints in quenching state is 78.5 MPa lower than that in quenching and artificial aging conditions. After welding, quenching and artificial aging can make the strength coefficient of welded joints. It reached 0.9-1.0. In 1980, the 1420 alloy was used to make the welded fuselage, fuel tank and cockpit of the MiG-29 supersonic fighter, which significantly reduced the weight of the aircraft by 24%. To date, 1420 alloy has been successfully used for more than 30 years and is widely used in military, civil aircraft and rockets.
In the 1980s, Russia developed a high-strength, high-modulus 1460 (Al-Cu-Li) alloy. This alloy is strengthened by the addition of Sc element, which changes the grain and subgrain structure and increases the tensile strength by 30-50. MPa, the welding performance is significantly improved. The 1460 alloy welding process is basically the same as that of the 1420 alloy. It can be welded with 1201 (Al-Cu-Mn) alloy wire, or stellite (Sc) can be added to the wire. After comparing various components, it is recommended to apply CB-1207 or CB-1217 welding wire. The composition of this wire is to add Cu, Sc, Zr, Ti, etc. on the basis of ALCu. The specific composition needs further understanding. The application of such a wire can significantly reduce the thermal crack sensitivity of the weld. The strength of the argon arc welded joint is greater than 250 MPa, the strength coefficient of the welded joint is greater than 0.5, and the strength and hardness of the welded joint after welding are increased. This kind of welding wire can ensure joints without cracks and fine grain structure, and reasonable selection of welding process and pre-weld preparation can obtain welded joints without pores.
The outer tank of the US Discoverer Space Shuttle uses 2195 (Al-Cu-Li-Mg) high-strength aluminum-lithium alloy instead of the 2219 alloy that was originally used for 25-40 years. The newly designed tank SLWT (Super Light Weight Tank) is 5% lighter than the original tank, ie 3 405 kg, of which LH2 box weight loss is 1 907 kg, LO2 box weight loss 736 kg, the weight loss between the boxes is 341 kg, and the other weight loss is 422 kg. A 1 kg payload can be added for every 1 kg of mass reduction, which increases the payload of 3 405 kg. The United States produced a total of 120 SLWTs and completed all space flight plans.
The tank of 2195-T8 alloy is welded with 4043 welding wire and variable polarity plasma arc welding (VPPA). VPPA has high arc temperatures, high arc voltages, and more concentrated heat. The key to VPPA welding 2195-T8 aluminum-lithium alloy is the back protection of the weld. The aluminum-lithium alloy contains active Li element, such as poor back protection during welding and easy oxidation. The Marshall Flight Center has developed a stainless steel "protective box" with a length of 229 mm, a width of 25.4 mm and a height of 152 mm. The "protective box" travels with the welding torch during welding, so that the oxygen in the weld area is less than 0.5%. In addition, a stainless steel tube with a diameter of 51 mm and a length of 229 mm was mounted on the back of the workpiece, and the welding torch was moved during welding to effectively protect the back weld. If the two protection devices are used at the same time, the effect is better.
Promising process technology
1 Variable polarity plasma arc welding technology (VPPA)
In 1978, the NASA Space Agency Marshall Aerospace Center decided to replace the tungsten argon arc welding process with the TIG plasma arc welding technology to weld the space shuttle outer tank. The outer space of the space shuttle is 2219 aluminum alloy. A total of 6400 m welds are welded. After 100% X-ray inspection, no internal defects are found. The weld quality is significantly improved compared with TIG multilayer welding.
Variable polarity plasma welding technology is used for aluminum alloy welding. The thickness of single-pass welding aluminum alloy can reach 25.4 mm. The process feature is that there is a penetrating small hole in the center of the welding pool during the welding process, and the vertical welding process is usually adopted in the actual production, which is beneficial to the front forming of the weld and the hydrogen in the molten pool. Escape, reducing porosity defects. It is therefore called "zero defect welding".
During the "Eighth Five-Year Plan" period, based on the introduction of a variable polarity plasma welding system from a foreign company, the welding process tests of LF6 and LD10 aluminum alloy plates (thickness 3 mm, 6 mm, 10 mm) were carried out.
During the “Ninth Five-Year Plan†period, the research on variable polarity plasma welding technology was carried out jointly with Harbin Institute of Technology, and a prototype of variable polarity plasma welding equipment was developed, and LF6 and LD10 aluminum alloy sheets (thickness 3 mm, 5 mm, 12 mm) were carried out. The welding process test completed the welding of the simulating parts of the tank with the longitudinal joint and the annular joint, which solved the problem of arc-punching and arc-filling and filling and the end-to-end joint of the weld during the circumferential seam welding. The welding simulation passed. The hydraulic test has pushed the engineering application of variable polarity plasma welding technology a big step forward.
With the application of 2219 aluminum alloy and 2195 aluminum lithium alloy, variable polarity plasma welding technology has broad application prospects in the future large thickness tank welding production.
2 partial vacuum electron beam welding technology
Since the vacuum electron beam welding process is to weld the workpiece to be welded in a vacuum environment, a high quality weld can be obtained. At the same time, the high energy density of the electron beam makes the weld be narrow, the aspect ratio is large, the welding stress and deformation are small, and it has been widely used in various fields of industry, especially the national defense industry.
However, for some large-scale components such as the launch vehicle tank casing, if a vacuum electron beam welding process is employed, a large vacuum chamber is required, which has a volume of up to several hundred cubic meters, and the electron beam welding equipment is expensive. In order to solve this problem, foreign countries began to design and apply partial vacuum electron beam welding equipment. Instead of putting the workpiece to be welded into the vacuum chamber as a whole, a vacuum environment was established in the weld to complete the welding.
The former Soviet Union applied partial vacuum electron beam welding technology to the welding of different types and sizes of rocket fuel tank casings. There are 7 types of welds in the longitudinal joints, butt joints and flange joints of the casing. Partial vacuum electron beam welding process is applied to seams, butt joints and flanges. In the early 1990s, it was used for the φ2.5 m diameter shell circumferential seam welding. The longitudinal slot of the energy rocket storage box was partially vacuum electron beam welding, the wall thickness was 42 mm, and the partial seal was magnetic fluid seal and rubber ring seal. And other technologies.
In the future thick-wall structure of 2219 aluminum alloy and 2195 aluminum-lithium alloy spacecraft, especially for flange ring seam welding production with high welding residual stress and deformation requirements, the application of partial vacuum electron beam welding technology has improved welding quality. Extremely important.
3 gas pulse TIG and MIG welding technology
In the aerospace industry, the TIG and MIG processes are widely used in aluminum alloy welding, and the shielding gas is argon gas and helium gas, of which argon gas is used more. For TIG welding, there are two processes of AC argon arc welding and DC positive arc welding. Compared with argon (Ar), helium (He) has a high maximum ionization energy, and the arc voltage is higher when other conditions and parameters are the same. Therefore, the arc welding arc temperature is high, the welding heat input is large, and the energy density is higher. Compared with the argon arc welding, the penetration depth is large, and the welding defects, especially the welding pores, are small.
According to the data, since the DC positive arc welding does not have the effect of the argon arc welding cathode atomization to remove the oxide film, the degree of damage of the oxide film depends on the length of the arc, so the DC positive arc welding uses short arc welding to remove the oxide film. This makes it difficult to fill the wire during welding. In addition to the constraints of equipment and other factors, DC positive arc welding has not been widely applied.
In order to take advantage of the high heat of helium arc and avoid the disadvantages caused by pure helium, foreign welding of aluminum alloy by gas pulse Ar+He TIG and MIG welding technology can greatly reduce welding pores.
Drawing on the experience of foreign countries, gas pulse TIG welding technology research has been started in recent years. Preliminary experiments show that the use of gas pulse (Ar+He) TIG welding process to weld S147 aluminum alloy has obvious effect on suppressing welding pores. The 7 mm plate can be penetrated at one time without opening the groove, and the surface gloss is the same as that of argon arc welding to avoid darkening of the surface of the DC positive arc welding. Welding processability and operability are also the same as argon arc welding, and the arc length is not particularly limited. This will have great application value for future models that will use S147 aluminum alloy and 2195 aluminum lithium alloy which are sensitive to pores.
4 friction stir welding technology
The aerospace industrial aircraft structure uses a large number of aluminum alloys, and some materials have to be riveted due to poor weldability. Friction stir welding, invented in 1991 by the British Welding Institute (TWI), provides a new way of thinking about the connection of such materials. Since this method is a solid phase welding, it is particularly suitable for use in non-ferrous metals with poor fusion weldability. Welding defects associated with melting, such as hot cracks and pores, are not produced relative to the fusion welding method. However, due to method limitations, its application is limited to workpieces of simple structure.
The principle of friction stir welding is that, by the heat generated by the friction, the metal around the special-shaped finger of the stirring head which is rotated at a high speed is rapidly heated, and a very thin thermoplastic metal layer is formed. As the mixing head moves, a weld of friction stir welding is formed. At present, the aluminum alloys that have been successfully studied for friction stir welding include: 2000 series (Al-Cu), 5000 series (Al-Mg), 6000 series (Al-Mg-Si), 7000 series (Al-Zn), 8000 series (Al-Li). In 1998, Pope's Space Defense Laboratory used this technology for welding certain parts of the rocket. Currently, ESAB is manufacturing friction stir welders for commercial applications. It is planned to be installed in TWI in 2002 to weld workpieces measuring 8 m × 5 m. The thickness of workpieces that can be welded is expected to be ～1.5 to 18 mm. Some universities and research institutes in China have also started research work in this area. It is reasonable to believe that the most promising application of friction stir welding technology in China will be the aerospace industry.
5 welding repair technology
Welding repair of aluminum alloy structural parts is an inevitable problem encountered in the production and use of spacecraft. In welding production, due to accidental factors in materials, structure, equipment, process and environmental conditions, weld defects exceeding the standard are found in the weld after welding, which requires repair welding. Although the traditional manual TIG welding method is simple and easy to operate, due to the large amount of local welding heat input, grain growth may occur, local toughness may decrease, and large residual stress may be caused at the repaired portion, which often becomes a low pressure. The source of the blasting. On the other hand, in the future, the carrier can be reused. After repeated use, defects such as cracks may occur locally in some components, and welding repair is required. At this time, the outside of the carrier is covered with a heat insulating material, which is extremely strict with temperature rise. It is required to adopt a welding process with a concentrated heat transfer and a small amount.
In 1995, the Cambridge Welding Institute of the United Kingdom invented the friction plug welding technology. Loma and the NASA Marshall Flight Center conducted a repair welding process. In 2000, it was used for external tank welding repair. This is a new welding repair technique. In the position of the weld defect, a wedge-shaped hole is drilled, and a wedge-shaped rotary plug similar in shape to the hole is inserted into the hole. At the high-speed rotation, the complete wedge plug and the surface of the hole are heated by friction. And to achieve welding. Welding parameters include the diameter of the plug, the speed of rotation, the applied pressure, and the displacement of the plug. It is different from welding repair. It needs to be repeatedly ground and filled before the defect is removed. The welding repair is 20% higher than the normal TIG welding repair strength, which improves the mechanical properties of the repaired joint and is not easy to produce welding defects. This repair process can also greatly reduce repair time and cost.
In addition, some people have proposed the idea of ​​laser repair welding. The difficulty of laser welding of aluminum alloy is that the aluminum alloy has a very high initial surface reflectance (more than 90%) on the CO2 laser beam (wavelength 10.6μm), and the reflectivity of the YAG laser beam (wavelength is 1.06μm) is close to 80. %. Moreover, the aluminum alloy laser beam is also prone to generate pores. These issues are all subject to in-depth research work.
6 Welding process and welding structure safety assessment technology
Due to the particularity of aerospace products, we attach great importance to product quality and reliability. With the development of welding technology, new requirements for welding quality and reliability of aerospace products are constantly being put forward. In actual production, the quality of the welding process depends not only on whether it can complete the welding of the structure it is concerned, but also whether it has a relatively stable ability to make the welding quality meet the product acceptance criteria. The “weldability†concept answers the question of whether welding can be achieved; in the 1990s, the “welding process margin†concept proposed by aerospace welding workers answered the question of whether a welding process could meet the welding quality standards. In other words, the concept of "welding process margin" is the basis for welding process qualification. For example, the ability to ensure welding quality can be evaluated according to the evaluation method of welding process margin, which is divided into “qualified processâ€, “restricted process†and “disabled processâ€. Of course, the evaluation of a particular process still requires the necessary experimental work. First, identify the key factors that affect the quality of the weld, and then comprehensively evaluate these factors.
Due to the current technical level and production conditions, the non-destructive testing of the weld by welding alone cannot fully assess the overall performance of the welded joint. In actual production, at present, only aluminum alloy welds are tested for defects such as pores, inclusions, cracks, and incomplete penetration, and it is difficult to achieve 100% inspection. Especially for fillet welds, it is difficult to carry out effective inspection. Even for the stomatal defects commonly found in aluminum alloy welding, the resolution of X-rays can only detect pores of 0.2 mm or more at present, and the micro-pores that have a great influence on the plasticity of the joints cannot be fully judged. In short, the welding process is still a direct factor in determining the quality of welding, and it is necessary to scientifically assess the quality of the welding process in production.
For the reliability assessment of welded structures, the safety assessment technology for welded structures has been continuously developed in the past 20 years. Only the concept of the "combined use" principle is introduced here. The “comprehensive use†principle is for the “perfect†principle. In the early stage of the development of the welded structure, it is required that the structure should not have any defects during the manufacture and use, that is, the structure should be perfect, otherwise it will be repaired or scrapped; later Edgar Fuchs, former director of the British Welding Research Institute, proved through a large number of experiments. : In the aluminum alloy welded joints, even if there is a certain degree of pores, the influence on the strength of the joint may be negligible, and the unnecessary re-shear welding may cause an increase in local residual stress and an unfavorable change in the microstructure, resulting in performance. Reduced. Based on this research, the British Welding Institute first proposed the concept of “combination with useâ€. After the emergence and widespread application of fracture mechanics, this concept has become one of the central topics in the long-term study of welded structures. It has gradually evolved into a principle and has a clear definition. Standards for the “comprehensive use†principle applied to the design, manufacture and acceptance of welded structures have been established in some countries.
In the "combined use" evaluation standard, three parameters of load, crack-like defects and fracture toughness are required, and the safety assessment method can be roughly divided into fracture mechanics method and structural test method.
Conclusion
Aluminum alloy is one of the main structural materials for aerospace products. With the development of materials technology, the aluminum alloy family has grown. In the United States and Russia, 2219, 1201, 1420 aluminum alloys have been widely used, and 2195 aluminum alloy has also begun to be applied. In China, the application prospects of S147 and 2195 in future aerospace models cannot be ignored. Manned spaceflight and reusable spacecraft put forward higher requirements for the reliability of welded structures. With the advent of this process, the application of new welding technology in aerospace process welding production will surely achieve rapid development. Welding automation and high quality and reliability assurance capabilities will be the basic requirements for welding technology in the 21st century. In particular, aluminum alloy plate and thick plate welding technology will become one of the hotspots of research and promotion of aerospace welding workers in recent years.