Friction stir welding technology and its developme

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Friction stir welding technology and its development in aircraft manufacturing

the development of a variety of new aircraft at home and abroad in the 21st century, such as domestic new generation fighters, carrier based aircraft, large transport aircraft and large passenger aircraft, as well as foreign fifth generation fighters, long-range bombers, new passenger aircraft and unmanned aerial vehicles, has promoted the rapid development of aircraft manufacturing technology. Among them, friction stir welding technology is a new highlight of advanced aircraft manufacturing technology, and has been applied in the manufacturing of a variety of new aircraft

among many advanced aircraft manufacturing technologies, friction stir welding has the characteristics of good compatibility of welding materials and high joint performance for the connection of aircraft metal materials (aluminum alloy, titanium alloy); For the thin-walled complex metal structure of aircraft, many friction stir welding methods such as straight seam welding, spot welding, bonding compound welding, function curve welding and space trajectory welding can be used to realize the integrated manufacturing and assembly of new aircraft, and even the repair and restoration of structural parts of aircraft in service. So far, friction stir welding technology has been used to manufacture aircraft wings, fuselages, wing boxes, floors, sealed compartments, stiffeners and other parts, which plays a significant role in improving aircraft performance and production efficiency and reducing production costs

in this paper, friction stir welding technology suitable for aircraft structure manufacturing will be discussed and introduced in detail, and its engineering application and development will be described

friction stir welding technology features and advantages

friction stir welding is a new solid-phase welding method of machining. In the process of friction stir welding, the stirring head is rotated and inserted into the interface of the workpiece to be welded. Due to the close contact and continuous friction between the shaft shoulder and the stirring needle of the stirring head and the material of the workpiece to be welded, the friction heat generated raises the temperature of the metal material to be welded near the stirring head and thermoplasticizes it. When the stirring head rotates forward along the welding interface, the thermoplasticized metal is transferred backward by the stirring head and the dynamic upsetting pressure is applied at the same time, Under the combined action of heat and force, the metallurgical diffusion connection of materials is realized, and a dense welded joint is formed behind the mixing head

the heat sources of friction stir welding are mainly mechanical friction heat and plasticized material deformation work. The welding process is a temperature adaptive dynamic equilibrium process, and the welding temperature is always lower than the melting point of the material to be welded. Although it has been reported that the welding process will produce the instantaneous melting of low melting point alloys, on the whole, friction stir welding is still a solid-state welding process. At the same time, due to the low welding temperature, friction stir welding has good adaptability to materials and structures, high joint performance and small deformation. It also has the characteristics of energy saving, environmental protection and amenity

Table 1 shows the comparison of mechanical properties between friction stir welded joints and base metal of typical aluminum alloy materials. It can be seen from the table that the friction stir welded joint has excellent performance, and sometimes the joint performance is close to or higher than the base metal. For example, when the friction stir welded joint of 5A06 aluminum alloy and 7A52 aluminum alloy is tested for tensile strength, the fracture position occurs on the base metal, indicating that the FSW technology welding 5A06 and 7A52 aluminum alloy can achieve the goal of equal strength with the base metal; In terms of plasticity index (elongation), the elongation of 5A06 aluminum alloy f SW joint is almost the same as that of base metal. Of course, 2A14, 2219 and 6063 materials shown in the table belong to heat-treatment aging precipitation strengthened aluminum alloy. The welding process is subject to the effect of thermal cycle, resulting in the desolvation of strengthening phase, and the joint strength index is lower than the base metal, but the research shows that the joint strength can be restored to the same strength as the base metal through post heat treatment

for aircraft structure, using friction stir welding to realize aircraft manufacturing has obvious technical and economic advantages

first of all, friction stir welding can provide new methods and ideas for aircraft design. For example, aircraft designers can give priority to commercial high-strength aluminum alloy materials to replace expensive composite materials, and use friction stir welding to realize the welding of different kinds of aluminum alloys or even aluminum lithium alloys, so as to reduce material costs and costs

secondly, different kinds of friction stir welding technologies such as seam welding, spot welding and bonding compound welding can be used to realize the "rivetless" and "nondestructive" manufacturing of aircraft panel structures, maintain the consistency of aircraft material properties and structural integrity, and also have the effect of sealing and weight reduction

third, ribbed panel structures are widely used in aircraft. The traditional manufacturing methods are riveting and bonding. Now, the mass aluminum alloy extruded profiles can be welded by friction stir welding to realize the efficient and low-cost manufacturing of aircraft ribbed panel structure

fourth, the standardization and digital manufacturing of aircraft structural parts can be achieved by using large-scale precision CNC friction stir welding equipment. Using friction stir spot welding robot can realize the "assembly line" manufacturing and assembly of aircraft parts, and improve the automation level of aircraft manufacturing

friction stir welding technology in aircraft manufacturing

friction stir welding is very suitable for the connection of straight and long welds, with the characteristics of stable performance and high efficiency. However, in order to meet the needs of different materials and structures, after more than 10 years of research and development, a variety of friction stir welding technologies with different characteristics have been developed, such as friction stir welding circumferential seam welding technology, spot welding technology and spatial curve technology. According to the structure and characteristics of aircraft, the friction stir welding technology suitable for aircraft manufacturing will be introduced below

1 straight butt friction stir welding technology

ordinary friction stir welding generally refers to the friction stir welding that can realize the straight butt weld. This longitudinal seam welding technology is simple and practical, the mixing tool (mixing head) is simple, the control parameters are few, and the functional requirements of the equipment are low. As long as the flatness of the welding backing plate is within 0.2m m, the butt joint gap is less than 1/10 of the plate thickness, and the thickness difference of the workpiece is within 0.5mm, the welding can be realized by simple straight clamps. Generally, it is suitable for the welding of flat butt joint structures of flat plates and profiles, such as the longitudinal seam butt joint of the flat structure of aircraft fuselage skin and the floor structure of large aircraft profiles in aircraft manufacturing. Figure 1 shows the aircraft ribbed wall panel structural products made by straight butt friction stir welding

there are many kinds of friction stir welding equipment for straight butt longitudinal joints, including small bench friction stir welding equipment for research and teaching, cantilever friction stir welding equipment for thin-wall cylinder structure, and vertical friction stir welding equipment for aerospace fuel storage tank. The mechanical fatigue spring testing machine of Jinan experimental machine factory is a welding equipment that uses electromechanical and reducer connecting cams to drive connecting rods to do reciprocating motion, And large gantry friction stir welding equipment for high-speed train body panels. Figure 2 shows the friction stir welding equipment for wide profile siding that can meet the manufacturing of large aircraft floor structure products in China at present. The equipment can adapt to the friction stir welding manufacturing of various cross-section structural profile products. The welding thickness can reach 15mm, the effective welding length can reach 18000m m, and the size of 18m can be achieved. The problem of being controlled by people is still prominent × Friction stir welding of 6m wide profile wallboard

Figure 1 flat butt friction stir welding manufacturing aircraft ribbed wall panels

2 plane curve butt friction stir welding technology

plane curve friction stir welding technology is generally required for aircraft hatch cover structural products and hollow radiator structural products. The key of this technology is to keep the tip of the mixing tool focused along the curve weld within the thickness range of the workpiece, and enable the mixing tool to realize tangent adjustment along the welding curve track within the range of 0 ~ 5 ° welding inclination during the welding process. The plane curve friction stir welding joint is mostly designed for lap joint structure. The gap of the workpiece is required to be close, and the height difference is not more than 0.5mm. The adjustment and follow-up of the inclination angle in the welding process are continuous and stable, otherwise it is easy to produce tunnel type and local accumulation type cavity defects. At present, China friction stir welding center has used the planar curve friction stir welding technology to realize the friction stir welding manufacturing of many aircraft hatch cover products and hollow radiator products of electronic and radar systems, as shown in Figure 3

plane curve friction stir welding equipment is relatively complex. Ordinary CNC milling machine can realize plane machining as long as there are two coordinate axes linked, but friction stir welding must have three coordinate axes interpolated and linked in the same plane to realize the tracking of plane curve track, and at the same time, the rotation of spindle and the adjustment of height direction can realize welding. Figure 4 shows the five axis three linkage table type planar curve friction stir welding equipment developed by China friction stir welding center and the planar curve products welded

3 lapping function friction stir welding technology

in the manufacturing of large aircraft fuselage, the lapping structure between "I", "Z" and "L" stiffeners and skin panels is very common, which is usually connected with rivets. The manufacturing process requires multiple processes such as hole making, hole enlarging, gluing and riveting, and each riveting point is an independent operation process, and the consistency of riveting points is poor. In the bending test, the riveted structural parts of aircraft skin and ribs fail in the form of overall failure of riveted joints, which is easy to cause the chain failure reaction of aircraft structure, and the consequences are very serious

friction stir welding has been used to lap longitudinal seams instead of riveting in the manufacture of fuselage skin structures of new large aircraft, as shown in Figure 5. This manufacturing method of "welding instead of riveting" has the advantages of reducing weight and improving structural integrity. Although this structure has advantages over the traditional aircraft riveting manufacturing process, due to the "wedge" notch on both sides of the lap weld, it is a potential crack source, which may affect the fatigue performance and service life of aircraft structural parts. Friction stir welding based on lap straight seam has obvious process imperfections, especially the consistency of the "wedge" notch direction on both sides of the weld, which may cause unidirectional failure or performance degradation of structural members. Therefore, for aircraft thin-walled lap structures, China friction stir welding center has developed the function curve lap friction stir welding technology

function curve lap friction stir welding technology is to use plane curve friction stir welding equipment to do geometric function trajectory friction stir welding movement along the direction of lap workpiece, such as sine function curve or cosine function curve. The test shows that the strength of friction stir welding with function curve locus is more than 25% higher than that of straight weld. In China, China friction stir welding center is committed to the application of this technology in aircraft thin-walled ribbed structures. In foreign countries, NASA has applied this technology to the new generation of "cosmic God" rockets Φ Friction stir welding manufacturing of 5.5m fuel storage tank; Airbus in Europe has begun to use the function curve friction stir welding technology in the manufacture of wall panel structural parts of a series of large passenger aircraft

4 friction stir spot welding technology for aircraft assembly

for friction stir welding of aircraft panel structure, a new "friction stir spot welding" (FSSW) technology is emerging, which is a new technology suitable for the manufacturing and assembly of aircraft complex structures, and will have a great impact on the manufacturing and assembly system of aircraft aluminum alloy structures in the future

in traditional welding knowledge, spot welding mainly refers to resistance pressure spot welding. Its principle is to use the resistance heat energy formed by large current passing through the welding interface to locally melt the welding interface and cool it under the action of pressure

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