2 1 century industrial aluminum alloy welding technology prospect.
The development of welding technology in industrial aluminum alloy is briefly reviewed, and the application of aluminum alloy in spacecraft at home and abroad is summarized and analyzed. The latest development and application prospect of aluminum alloy welding technology are introduced, including variable polarity plasma welding, local vacuum electron beam welding, gas pulse welding, friction stir welding, welding repair technology, welding process margin and welding structure safety assessment technology.
Keywords: aluminum alloy; Welding; astrospace
1
Aluminum alloy not only has high specific strength, specific modulus, fracture toughness, fatigue strength and corrosion resistance stability, but also has good forming technology and weldability, so it has become the most widely used non-ferrous metal structural material in aerospace industry.
For example, aluminum alloy is the main structural material of launch vehicles and various spacecraft. The command module, lunar module, hydrogen-oxygen propellant tank and crew module of American Apollo spacecraft also use aluminum alloy as structural materials. Aluminum alloy is also widely used as the main structural material of various large launch vehicles developed in China.
The development and application of aerospace industrial aluminum alloy welding technology is closely related to the development of materials. This paper will briefly review the development of industrial aluminum alloy welding technology, and introduce several aluminum alloy welding technologies with great application prospects.
2 Development of aluminum alloy welding technology
2. Summary of welding of1LD10cs aluminum alloy.
Some early missiles and long-range launch vehicles mainly used al? Mg series alloys, especially LF3 and LF6 rust-proof aluminum in annealed and semi-cold-worked hardened state, are widely used. Both aluminum alloys have excellent weldability [1]. ?
With the development of space technology, the structural material of propellant tank of launch vehicle has changed from rust-proof aluminum strengthened by non-heat treatment to high-strength aluminum alloy strengthened by heat treatment. LD 10CS alloy has been successfully applied to many large launch vehicles and solid missiles. Because of its excellent ultra-low temperature performance, it has also been applied in three-stage liquid hydrogen and liquid oxygen propellant tanks.
It should be pointed out that LD 10 alloy has poor weldability, a greater tendency to form hot cracks during welding, and is sensitive to various factors in the welding process, and the fracture toughness of welded joints is low, especially when there are welding defects in the weld, the specimens often explode at low pressure in the hydraulic strength test.
In 1970s, in the initial stage of developing LD 10 alloy rocket propellant tank, the welding technology encountered great difficulties. The "double-sided three-layer welding" process invented in the "three-in-one" research (front backing, covering, back root cleaning and sealing welding) makes the performance of welded joints meet the design requirements. In the welding production practice of LD 10, it is concluded that the plasticity of welded joints can meet the application requirements if the elongation of welded joints is not less than 3%. In the following years, "elongation not less than 3%" has been regarded as an important acceptance index. ?
For decades, the welding technology is mainly argon arc welding (TIG), including manual and automatic argon arc welding. In terms of welding process, in order to reduce welding residual stress and deformation of welding structure, welding heat input is usually reduced as much as possible in the selection of welding process. Especially for aluminum alloy strengthened by heat treatment, due to the influence of welding thermal process, there is a softening zone in the welding heat affected zone, which has good plasticity and low strength. The strength coefficient of welded joint is 0.5 ~ 0.7. ?
Why does LD 10CS tank adopt double-sided three-layer welding process? Theoretical analysis and practical results show that if this welding method is not adopted, the plasticity of LD 10CS aluminum alloy welded joint will be poor, and cracks will easily appear at the toe on the back of the weld. When double-sided three-layer welding, root cleaning and back sealing welding can eliminate this crack. At the same time, due to the large heat input, the heat affected zone is annealed or overaged to varying degrees, which reduces the hardness and improves the plasticity. The fracture position of welded tensile specimen is the welding softening zone. In this way, in the structure, the plasticity and deformation of the softening zone make up for the lack of plasticity of the fusion zone under complex stress state. However, after the weld of storage tank is repaired, sometimes low-pressure blasting still occurs.
Due to the special requirements of double-sided welding, automatic welding and new welding technologies (such as vacuum electron beam welding, variable polarity plasma welding, etc.) are applied. ) limited. This is because the welding heat input of argon arc welding is greater than that of high energy beam vacuum electron beam welding, and considering the structural adaptability of welded joints, it is difficult to apply new welding technology with concentrated welding heat input, which restricts the application of new welding technology. ?
In welding production, the common defect of aluminum alloy weld is weld porosity. Hydrogen is the main cause of porosity in the welding process of aluminum and its alloys. The hydrogen content in the base metal, the moisture absorbed by the oxide film on the surface of the welding wire and the base metal, and the moisture in the arc column atmosphere are all important sources of hydrogen in the weld porosity. Aerospace welding workers have made unremitting efforts to ensure the smooth delivery and launch of aerospace welding products. However, due to various factors and conditions, there are still some out-of-tolerance pores in production. ?
As for welding materials, special welding plates are used abroad, and the hydrogen content of the parent metal is less than 2× 10-7? . However, there is no requirement for hydrogen content in domestic aluminum alloy sheet manufacturing technical conditions.
2.2 overview of welding between aluminum alloy 22 19 and aluminum-lithium alloy
22 19 high-strength aluminum alloy is characterized by good weldability, good mechanical properties and stress corrosion resistance in the range of -253℃ to +200℃, low sensitivity to welding hot cracks, and good plasticity and low-temperature toughness of welded joints. It has been used as the main structural material of propellant tank in the United States, and Saturn V I tank in the United States adopts 22 19 aluminum alloy. In the former Soviet Union, 120 1 (equivalent to 22 19) aluminum alloy was widely used in space shuttle energy and blizzard. ?
Similar to 22 19 aluminum alloy, the S 147 aluminum alloy developed in China has a low tendency to produce welding cracks, but it is very sensitive to porosity, especially in the fusion zone and dense porosity, which are the main defects affecting the performance of welded joints.
With the development of aerospace technology, higher requirements are put forward for the strength and thinning of aluminum alloys, and aluminum-lithium alloys have developed rapidly in recent decades. Because every time 1% Li Can is added, the weight of aluminum alloy decreases by 3%, the elastic modulus increases by 6%, and the specific elastic modulus increases by 9%. Compared with 2024 and 7075 alloys commonly used in aircraft products, the density of this alloy decreases by 7% ~ 1 1%, and the elastic modulus increases by 12% ~ 65475. Compared with the widely used hard aluminum (duralumin) д 16 (2024) alloy, the 120 alloy in the former Soviet Union has the advantages of reduced density 12%, increased elastic modulus by 6% ~ 8%, good corrosion resistance, low fatigue crack growth rate, and similar strength, yield strength and elongation.
The former Soviet Institute of Aeronautical Materials (виамнридлgндер) and others invented al in the 1960s. Mg? The welding of Li system 1420 alloy was studied. The research on welding of this alloy achieved results in 1970s. They think AM can be used for argon arc welding of this alloy. г6、AM？ With г6T and 1557 welding wires, the strength coefficient of welded joint reaches above 0.7. Pre-welding and post-welding heat treatment have great influence on the strength of welded joints. The strength of welded joint in quenching state is 78.5 MPa lower than that in quenching and artificial aging state, and the strength coefficient of welded joint can reach 0.9 ~ 1.0 by post-welding quenching and artificial aging. 1980 1420 alloy is used to manufacture the welded fuselage, fuel tank and cockpit of MIG -29 supersonic fighter, which greatly reduces the weight of the aircraft by 24%. Up to now, 1420 alloy has been successfully used for more than 30 years and is widely used in military and civil aircraft and rockets [3].
In 1980s, Russia developed 1460(Al? Cu？ Li) alloy, which was strengthened by adding Sc element, changed the grain and subgrain structure, increased the tensile strength by 30 ~ 50 MPa, and obviously improved the weldability. The welding process of 1460 alloy is basically the same as that of 1420 alloy. 120 1(Al? Cu？ Mn) alloy welding wire, scandium (Sc) element can also be added to the welding wire. After the comparative test of each component, it is recommended to use CB- 1207 or CB- 12 17 welding wire. The composition of welding wire is AL? Adding copper, scandium, zirconium, titanium, etc. On the basis of Cu, the specific components need to be further understood. The application of this welding wire can significantly reduce the sensitivity of weld hot cracks. The strength of argon arc welded joint is more than 250 MPa, and the strength coefficient of welded joint is more than 0.5. The strength and hardness of welded joint are improved after post-weld heat treatment. 〔4~8〕? This kind of welding wire can ensure that the joint is crack-free and the grain is fine. By reasonable selection of welding process and preparation before welding, the welded joint without air holes can be obtained.
2 195(Al? Cu？ Lee? Mg) high strength Al-Li alloy, replacing the original 22 19 alloy used for 25 ~ 40 years. The newly designed SLWT (ultra-light tank) is 5% lighter than the original tank, that is, 3 405 kg, of which LH2 tank is 1 907 kg, LO2 tank is 736 kg, cabin section is 34 1 kg, and others are 422 kg. Every time the weight of 1 kg is reduced, the payload can be increased by 1 kg, thus increasing the payload by 3 405 kg. The United States produced 120 SLWT and completed all space flight plans [9 ~ 10].
2 195-T8 alloy tank is welded by 4043 welding wire and variable polarity plasma arc welding (VPPA). VPPA has higher arc temperature, higher arc voltage and more concentrated heat. The key of VPPA welding 2 195-T8 Al-Li alloy is the back protection of the weld. Al-Li alloy contains active lithium element, which is easy to be oxidized if the back surface is not well protected during welding. 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 of152 mm. When welding, the "protective box" walks with the welding gun, so that the oxygen in the weld zone is less than 0.5%. In addition, a stainless steel tube with a diameter of 5 1 mm and a length of 229 mm was developed, which was installed on the back of the workpiece, moved with the welding gun during welding, and effectively protected the weld seam on the back. If these two protection devices are used at the same time, the effect is better.
3 Several promising technologies
3. 1 variable polarity plasma arc welding technology (VPPA)
1978, the Marshall space center of NASA decided to use the variable polarity plasma arc welding technology to partially replace the tungsten argon arc welding process to weld the outer tank of the space shuttle. The outer tank of the space shuttle is made of 22 19 aluminum alloy and welded with 6400 m weld. After 100% X-ray inspection, no internal defects were found, and the weld quality was obviously improved compared with TIG multi-layer welding. ?
Aluminum alloy welding adopts variable polarity plasma welding technology, and the thickness of aluminum alloy can reach 25.4 mm in one pass. Its technological feature is that there is a through hole in the center of the welding pool during the welding process, and vertical welding process is usually adopted in actual production, which is not only beneficial to the formation of the front edge of the weld, but also to the escape of hydrogen in the welding pool and the reduction of porosity defects. Therefore, it is called "zero defect welding". ?
During the Eighth Five-Year Plan period, based on the introduction of a foreign company's variable polarity plasma welding system, the welding process tests of LF6, LD 10 aluminum alloy plates (thickness 3 mm, 6 mm, 10 mm) were carried out [1]. ?
During the Ninth Five-Year Plan period, the research on variable polarity plasma welding technology was jointly carried out with Harbin Institute of Technology, and the prototype of variable polarity plasma welding equipment was developed. The welding process tests of LF6 and LD 10 aluminum alloy plates (thickness 3 mm, 5 mm, 12 mm) were carried out, and tank simulators with longitudinal and circumferential seams were welded, which solved the problems of arc drilling and arc filling, and the end-to-end connection of welds.
With the application of 22 19 aluminum alloy and 2 195 aluminum-lithium alloy, variable polarity plasma welding technology has a broad application prospect in the welding production of medium and large storage tanks in the future.
3.2 Local Vacuum Electron Beam Welding Technology
Because the vacuum electron beam welding process is to weld the workpiece in a vacuum environment, high quality weld can be obtained. At the same time, the high energy density of electron beam makes the weld narrow, the depth-width ratio is large, and the welding stress and deformation are small, which has been widely used in various industrial fields, especially in national defense industry.
However, for some large components, such as the tank shell of launch vehicle, if vacuum electron beam welding technology is used, a large vacuum chamber with a volume of hundreds of cubic meters is needed, and this kind of electron beam welding equipment is very expensive. In order to solve this problem, foreign countries began to design and apply local vacuum electron beam welding equipment. Instead of putting the whole workpiece into a vacuum chamber, a vacuum environment is established at the local weld to complete the welding.
The former Soviet Union applied local vacuum electron beam welding technology to the welding of different types and sizes of rocket fuel tank shells. In the welding of longitudinal seam, butt girth seam and flange girth seam of shell, local vacuum electron beam welding technology is adopted. Used for girth welding of φ2.5m diameter shell in the early 1990s. The longitudinal seam of energy rocket tank is welded by local vacuum electron beam, with a wall thickness of 42 mm, and the local seal is sealed by magnetic fluid seal, rubber ring seal and other technologies. ?
During the Ninth Five-Year Plan period, 1 domestic partial vacuum electron beam flange girth welding machine (patent number: ZL 002631776.6) [12] was developed in cooperation with the Institute of Electrical Engineering, Chinese Academy of Sciences. The electron gun and the upper vacuum chamber adopt dynamic sealing structure, and the workpiece and the upper and lower vacuum chambers adopt static sealing structure. The electron gun can move in polar coordinates during welding. The radial movement of the electron gun is driven by a stepping motor, and the displacement is detected by a grating ruler. The circular rotation is driven by AC servo motor, and the optical code disk detector is angularly displaced. The secondary electronic welding seam centering system is used to realize welding seam trajectory teaching. Using two-stage microcomputer control and PLC to control welding parameters, flexible welding can be realized, that is, welding flange girth with diameter of100 ~ 300 mm. The vacuum degree of local vacuum chamber reaches 5× 10-3Pa, which is higher than that of foreign similar products. ?
In the future thick-walled structures of 22 19 aluminum alloy and 2 195 aluminum-lithium alloy spacecraft, especially in the production of flange girth welding with high requirements on welding residual stress and deformation, it is of great significance to apply local vacuum electron beam welding technology to improve welding quality.
3.3 Gas pulse TIG and MIG welding technology
In aerospace industry, TIG and MIG processes are widely used in aluminum alloy welding, and argon and helium are used as shielding gases, among which argon is widely used.
As far as TIG welding is concerned, there are two processes: AC argon arc welding and DC direct helium arc welding. Compared with argon (Ar), the minimum ionization energy of helium (He) is higher, and the arc voltage is higher under the same other conditions and parameters. Therefore, helium arc welding has high arc temperature, large welding heat input and high energy density. Compared with argon arc welding, helium arc welding has deeper penetration and fewer welding defects, especially welding blowholes.
According to the data, DC direct helium arc welding has no effect of cathode atomization to remove oxide film in AC argon arc welding, and the damage degree of oxide film depends on the length of arc, so DC direct helium arc welding adopts short arc welding to remove oxide film. This makes it more difficult to fill wire in welding, and DC direct helium arc welding has not been widely used because of the limitation of equipment and other factors.
In order to take advantage of the high heat of helium arc and avoid the disadvantages caused by pure helium, gas pulse Ar+He TIG and MIG welding technology are used to weld aluminum alloy abroad, which can greatly reduce welding porosity. ?
Drawing lessons from foreign experience, the research of gas pulse TIG welding technology has been started in recent years. The preliminary test shows that the gas pulse (Ar+He)TIG welding process has obvious effect on suppressing welding porosity of aluminum alloy S 147. The 7 mm flat plate can be penetrated at one time without groove, and the surface gloss is the same as that of argon arc welding, so as to avoid the darkening of the weld surface of DC direct helium arc welding. The welding process and operability are the same as those of argon arc welding, and the arc length is not particularly limited. This is of great application value to the application of S 147 aluminum alloy and 2 195 aluminum-lithium alloy which are sensitive to porosity in future vehicles. ?
3.4 Friction stir welding technology
Aluminum alloy is widely used in aerospace industrial aircraft structures. Because of the poor weldability of some materials, riveted structures have to be adopted. Friction stir welding invented by TWI in 199 1 provides a new idea for joining such materials [13]. Because this method belongs to solid phase welding, it is especially suitable for melting non-ferrous metals with poor weldability. Compared with fusion welding method, it will not produce welding defects related to melting, such as hot cracks and blowholes. However, due to the limitation of this method, its application is limited to the workpiece with simple structure.
The principle of friction stir welding is to use the heat generated by friction to quickly heat the metal around the special-shaped finger of the high-speed rotating stirring head to form a very thin thermoplastic metal layer. With the movement of the stirring head, the weld of friction stir welding is formed. At present, the aluminum alloys successfully studied by friction stir welding are: 2000 series (Al? Cu), 5000 series (Al? Mg), 6000 series (Al? Mg? Si), 7000 series (Al? Zn), 8000 series (Al? Li). 1998, the space defense laboratory of American Pop company applied this technology to the welding of some rocket parts. At present, ESAB company is manufacturing a commercial friction stir welding machine, which is planned to be installed in TWI in 2002 to weld workpieces with a size of 8 m× 5 m. it is estimated that the thickness of the workpieces that can be welded is1.5 ~18 mm. some universities and scientific research institutes in China have also started research work in this field. it is reasonable to believe that the most promising application of friction stir welding technology in China will be aviation.
3.5 Welding repair technology
Welding repair of aluminum alloy structural parts is an inevitable problem for spacecraft in production and use. In welding production, due to accidental factors such as material, structure, equipment, technology and environmental conditions, there will be welding defects exceeding the standard after welding, which need to be repaired. Although the traditional manual TIG welding method is simple and easy to operate, due to the large local welding heat input, it may cause grain growth and local toughness reduction, and at the same time cause large residual stress at the repair welding site, which often becomes the crack source of "low pressure blasting". On the other hand, the reusable launch vehicle in the future may have some defects, such as cracks, which need to be repaired by welding. At this time, the launch vehicle is covered with heat insulation material, which has very strict requirements on temperature rise, so the welding process of centralized heat transfer and small heat transfer must be adopted.
Friction plug welding technology [14] was invented by Cambridge welding research institute in England in 1995. Loma Company and Marshall Flight Center of NASA studied the repair welding technology, and it was used to repair the external fuel tank in 2000. This is a new welding repair technology. Drill a wedge-shaped hole in the weld defect, and insert a wedge-shaped rotary plug with the shape similar to the hole in the hole. When rotating at high speed, the complete wedge plug rubs against the hole surface to generate heat and realize welding. Welding parameters include plug diameter, rotation speed, applied pressure and plug displacement. It is different from fusion welding repair, which requires repeated grinding and filling to remove defects. The strength of welding repair is 20% higher than that of ordinary TIG fusion welding repair, which improves the mechanical properties of the repaired parts and is not easy to produce welding defects. The repair process can also greatly reduce the repair time and cost.
In addition, some people put forward the idea of laser repair welding. The difficulty of laser welding of aluminum alloy lies in the influence of aluminum alloy on CO2 laser beam (wavelength is 10.6? Micron) has extremely high initial surface reflectivity (above 90%), and the reflectivity of YAG laser beam (wavelength: 1.06μm) is close to 80%. Moreover, aluminum alloy laser beam is easy to produce blowholes. These problems need further study.
3.6 Welding Technology and Safety Assessment Technology of Welding Structure
Due to the particularity of aerospace products, we attach great importance to product quality and reliability. With the development of welding technology, new requirements are constantly put forward for the welding quality and reliability of aerospace products. In actual production, the quality of welding process depends not only on whether the welding of the target structure can be completed, but also on whether it has a relatively stable ability to make the welding quality reach the product acceptance standard. The concept of "weldability" answers the question of whether welding can be realized; In 1990s, the concept of "welding process margin" put forward by space welders answered the question whether the welding process can meet the welding quality standard [15]. In other words, the concept of "welding process margin" is the basis of welding process evaluation. For example, the ability to ensure welding quality can be evaluated according to the evaluation method of welding process margin, which can be divided into "qualified process", "restricted process" and "forbidden process". Of course, it is necessary to carry out necessary experimental work to evaluate a particular process. First of all, we must identify the key factors that affect the welding quality, and then we can comprehensively evaluate these factors.
Due to the limitation of current technical level and production conditions, all the properties of welded joints can not be fully evaluated only by nondestructive testing of welded joints after welding. In actual production, at present, there are only a few defects such as porosity, inclusion, crack and incomplete penetration in the inspection of aluminum alloy welds, so it is difficult to achieve 100% inspection, especially fillet welds. Even for the common blowhole defects in aluminum alloy welding, the resolution of X-ray can only detect blowholes above 0.2 mm at present, but the tiny blowholes that have a great influence on the plasticity of the joint cannot be completely judged. In a word, the welding process is still the direct factor that determines the welding quality, so it is very necessary to evaluate the quality assurance ability of welding process scientifically in production.
Aiming at the reliability evaluation of welded structures, the safety evaluation technology of welded structures has been developing continuously in recent 20 years. Only the concept of "suitability for use" principle is introduced here [16]. The principle of "suitability for use" is aimed at the principle of "perfection". In the early stage of welding structure development, it is required that there should be no defects in the process of manufacture and use, that is, the structure should be perfect, otherwise it will be repaired or scrapped; Edgar Fuchs, who later served as the director of the welding research institute, proved through a large number of experiments that even if there is a certain degree of porosity in the welded joint of aluminum alloy, the influence on the strength of the joint may be small, and unnecessary repair welding will increase the local residual stress, adversely change the microstructure, and lead to a decline in service performance. Based on this research, the welding research institute put forward the concept of "suitable for use" for the first time. After the appearance and wide application of fracture mechanics, this concept has become one of the central topics in the long-term research of welded structures, and has gradually developed into a clearly defined principle. In some countries, the standard of "suitability for use" principle applicable to the design, manufacture and acceptance of welded structures has been established.
In the evaluation standard of "suitability for use", three parameters, such as load, crack defect and fracture toughness, need to be input. The safety evaluation methods can be roughly divided into fracture mechanics method and structural test method.
4 conclusion
Aluminum alloy is one of the main structural materials of aerospace products. With the development of material technology, aluminum alloy family is growing. In the United States and Russia, 22 19, 120 1, 1420 aluminum alloys have been widely used, and 2 195 aluminum alloys have also been used. In China, the application prospect of S 147 and S 2 195 in future aerospace models can not be ignored. Manned space flight and reusable spacecraft put forward higher requirements for the reliability of welded structures. With the appearance of this technology, the application of new welding technology in aerospace welding production will surely develop by leaps and bounds. Welding automation and the guarantee of high quality and high reliability will be the basic requirements for welding technology in 2 1 century. Especially the welding technology of aluminum alloy plate and thick plate will become one of the hot spots of aerospace welding workers in recent years.
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