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What is the progress of research on aerospace sheet metal manufacturing technology?

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Sheet metal parts constitute the framework and aerodynamic shape of aircraft fuselage, and are an important part of aircraft components. The number of parts is large (accounting for 50% of the total number of parts of the whole aircraft), the variety is large, the shape is complex, the quality requirement is high, and the processing difficulty is great. Sheet metal parts manufacturing technology is one of the basic manufacturing technologies that enable the aircraft to obtain structural efficiency and excellent performance at the same time. Still, also one of the pillars of aerospace manufacturing engineering technology, and its advanced degree is an important measure of a country’s aerospace manufacturing capacity and level of significance. In recent years, the continuous introduction of new-generation aircraft has promoted the development of aviation manufacturing technology in various countries, and sheet metal manufacturing technology has also made great progress.

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Super Plastic Forming / Diffusion Bonding

 

Superplastic Forming/Diffusion Bonding (SPF/DB for short) is a method of manufacturing monolithic structures with hollow interlayers by using certain materials that possess both superplasticity and diffusion bonding in a specific temperature range and completing superplastic forming and diffusion bonding in a single thermal cycle. The forming method is advanced by nearly no margin of the overall structure of the manufacturing technology.

It is an advanced near-surplus-free monolithic structural component manufacturing technology. Its in-depth development and wide application have had a significant impact on the design and manufacture of modern aerospace structures, and it is regarded as the high-efficiency cost-effective manufacturing technology for large and complex structural components in the 21st century in the U.S.A. The main technological advantages of SPF/DB are reflected in the following:

(1) high part economy, which can be replaced by a part of the original need for many parts assembled by mechanical connection or welding of large components, greatly reducing the number of parts and fixtures, shortening the manufacturing cycle, reducing manufacturing costs;

(2) good structural integrity, materials in the diffusion connection after the interface completely disappears, so that the entire structure as a whole, greatly improving the structure of fatigue and corrosion resistance;

(3) can form the general method is difficult to achieve the complex shape of the parts, the material in the superplastic forming process can withstand a lot of deformation without rupture, such as titanium plate can be formed out of the bending radius is small to the thickness of the material parts, which is the use of conventional cold-forming methods can not be done or need to be many times forming can be achieved;

(4) The diversity of material choices enhances design flexibility, making it possible to combine the advantages of different materials and use hollow sandwich structures to optimise load-carrying efficiency and reduce structural weight, thereby meeting the needs of diverse applications.

(5) The flow stress of the material in the process of superplastic forming is very small, so it is possible to use small tonnage equipment to form large-sized structural parts, and the processed structural parts have no rebound, no residual stress, and high forming accuracy.

From the early 30s of the 20th century, SPF technology was introduced, with the increasing demand for lightweight, long life, high strength and high stiffness in the field of aerospace manufacturing, SPF/DB technology has gone through three stages of basic process research in a small laboratory, engineering application research on typical structures, and production application and in-depth development of complex structures.

At present, SPF technology is widely used in engineering in the world aerospace field, for example, there are more than 70 pieces of SPF/DB structural parts in F-15; there are more than 20 pieces of titanium alloy SPF/DB structural parts in F-18; and SPF/DB combined structure is also used in F-22, such as the rear fuselage titanium alloy superplastic forming/diffusion connection heat shield, etc. After using SPF/DB structural parts, it can make the aircraft weight reduction of 10%~30%. With the use of SPF/DB structural components, the weight of the aircraft can be reduced by 10-30%, and the cost can be reduced by 25-40%.

Component specifications from small size to the development of large components, such as the U.S. F-22 aircraft rear fuselage using 8 high-strength titanium alloy SPF/DB heat shield, size 915mm × 635mm × (1mm ~ 4mm); B-2 aircraft on the titanium alloy SPF/DB parts size of 1200mm × 3600mm × 6.3mm. The structural form of the plate from the plate and plate diffusion connection to the plate and solid diffusion connection development, such as Beijing, China, and the development of the plate and solid diffusion connection. The structure form from plate and plate diffusion connection to plate and solid diffusion connection development, such as the Beijing Institute of Aeronautical Manufacturing Engineering developed missile rudder, airfoil, the first in China to successfully realise the hollow-solid hybrid structure of the SPF / DB (Fig 1), in the stiffness, strength to meet the design requirements of the case, to achieve a weight reduction of 50%.

Fig. 1

Fig. 1 SPF/DB building blocks for hollow-solid hybrid structures

Shot Peening Technology

 

Shot peening technology is the use of high-speed projectile flow impact on the surface of the metal plate, so that the sprayed surface material produces plastic deformation to the four sides of the extension, the surface area increases, while the material deformation by the consistent limitations, so that the plate to produce a bending deformation of a forming method, is one of the main forming methods of wall plate parts. The main features of shot peening forming technology include:

(1) low cost of parts manufacturing: no need for forming moulds, simple process equipment, short process preparation cycle.

(2) forming parts with strong fatigue resistance: parts on and off the performance of the presence of residual compressive stress.

(3) forming a wide range: the size of the part is not subject to the specifications of the blast chamber; can be formed into a variable thickness of the skin wall plate, but also can be formed with a ribbed overall wall plate; can be formed into a single curvature of the simple shape of the part, but also can be formed into a complex double-curvature shape of the part.

Since the early 1950s, when shot peen forming technology was first successful in the production of wall panel parts for the Constellation aircraft, the technology has been widely used in the aerospace field for all types of aircraft manufacturing. This includes many military aircraft such as the EM120, A10, F15, etc., as well as civil aircraft such as the Airbus A310 to A340 series and the Boeing 707 to 777 series. In addition, it is used in the manufacture of launch vehicles such as Ariane 4 and 5, ATLAS II, and others.

The design of shot peened parts has also evolved progressively from simple shapes in the early days to more complex curved structures, with thicknesses varying from uniform to non-uniform, and with larger part sizes.

Currently, shot peening has developed into a mature and specialised means of wall plate processing at Boeing, Airbus and Metal Improvement. The maximum length of the whole wall plate can be formed to more than 35m, and can be formed with bending and torsion of the saddle-shaped complex surfaces, the structure of the form also includes the mouth cover, tabs, reinforced edges, mitre, reinforcement, short cross bars and other combinations of complex structures. As shown in Figure 2 for the U.S. Metal Improvement Corporation using pre-stressed shot peening technology to manufacture the A380 supercritical wing under the wall plate, it is so far the largest length and thickness of the largest component obtained by shot peening technology.

Fig. 2

Fig. 2 Airbus A380 aircraft and its shot peening of the outer wing under the wing surface of the whole wall plate

At the same time, the German KSA company will be applied to digital technology in the shot peening process, the development of automatic shot peening technology, reducing the manual calibration, wall plate shot peening shape accuracy of 0.3mm ~ 0.5mm, a pass rate of 100%, shot peening processing a part as soon as 2h, greatly improving the quality and efficiency of parts manufacturing. This technology has been successfully applied to the shot peening of the whole wall plate of the A380 laser-welded fuselage.

Beijing Institute of Aeronautical Manufacturing Engineering to catch up with the world’s cutting-edge technology, 2006 successfully developed the ARJ21 aircraft supercritical wing overall lower centre wall plate, the part for the complex saddle-shaped with torsion profile, variable thickness with local reinforcement and the structure of the mouth and cover area, so that China has become the world’s few masters of large-scale supercritical wing wall plate shot blasting technology of the overall wall plate of the country.

2009, the use of independently designed and developed shot blasting path design software, combined with the CNC machine, the shot blasting path design software, and the shot blasting path design software. In 2009, using the independently designed and developed shot peening path design software, combined with CNC shot peening technology, the digital design of the shot peening process was realised, making it possible to accurately shoot peen complex parts. In the same period, the world’s first complex saddle-shaped high-strength overall wall plate (Fig 3), so that the shot peening technology into the world’s leading level.

Fig.3

Fig.3 Shot peening of a reinforced monolithic wall plate process validation part

2012, the use of pre-stressed shot peening technology to successfully test the production of aluminium-lithium alloy welded reinforced wall plate simulation (Fig 4), proved the feasibility of welded reinforced overall wall plate shot peening, for the future lightweight, high-efficiency welded reinforced overall wall plate application laid an important technical foundation. Technical basis for the future application of lightweight, efficient welded ribbed monolithic wall plates.

Fig. 4

Fig. 4 Simulation of a welded reinforced monolithic wall plate with shot peen forming

Spinning technology

 

Metal spinning forming technology refers to the blank with the core mould rotation or spinning tool around the blank and the core mould rotation, spinning tool and core mould relative feed and make the blank pressure and produce continuous point-by-point deformation art, mainly used for processing thin-walled rotary body workpiece. Spinning technology as a typical continuous local plastic forming technology, is to achieve the thin-walled rotary parts less cutting-free processing of advanced manufacturing technology, widely used in machinery, electronics, chemical industry, automotive and aerospace and other fields of the defence industry.

For example, the nose cone of the space shuttle, the screw isolation catcher, the outer receiver of the aero-engine, the exhaust cone, the receiver shell, the missile cowl, the guidance module and so on. Its main advantages include a high material utilization rate, high product precision, good product organization performance, good process flexibility, easy-to-realize mechanization and automation, long mould life, small production cost, etc. With the rapid development of aerospace and other high-tech industries, the thin-walled rotary shell parts require more lightweight, integral, complex shapes, and more demanding performance requirements, spinning technology has also experienced a new technological leap. Mainly embodied in:

(1)high-strength aluminium alloy, titanium alloy, and high-temperature alloys, such as high strength the amount of difficulty to deform the material increased significantly, and hot spinning technology came into being. Such as Beijing Aviation Manufacturing Engineering Research Institute used high-temperature alloys to develop an aircraft engine magazine shell (Fig 5), instead of the original forging process, through the study of temperature on the high-temperature alloy spinning deformation behaviour of the mechanism to obtain the temperature and the mandrel speed, spinning wheel feed speed and other process parameters to coordinate the relationship, to solve the hot spinning temperature control, forming quality control and other key technologies to reduce the amount of subsequent machining, improve the mechanical properties of the parts. The mechanical properties of the parts are improved.

Fig. 5

Fig. 5 Spinning forming an engine magazine shell

(2) The demand for large-size, high-precision complex thin-walled parts is increasing, and multi-pass composite spinning is becoming more and more perfect. For example, Northwestern Polytechnical University uses a combination of numerical simulation and experimental methods, based on a self-developed simulation platform, to carry out the correlation study of uneven plastic deformation behaviour and forming defects, blanks, process and mould parameters affect the law of the study, to achieve the composite spinning of large and complex thin-walled shells with transversal internal reinforcement accurate forming.

Beijing Aviation Manufacturing Engineering Research Institute used GH3536 high-temperature alloy to successfully develop an aircraft engine external magazine, the part is a large thin-walled complex rotary shell containing a straight section, conical section, arc and flange edge, through the study of multi-channel composite spinning process under the role of composite physical field of plastic deformation behaviour of the material, to solve the thermal state, multi-channel composite spinning temperature and defects in the control and coordination of the relationship between the precision of the formation of key technologies, to achieve the difficult to deform the material composite spinning precision forming.

By studying the plastic deformation behaviour of the material under the action of the compound physical field in the process of multi-pass composite spinning, we have solved the problems of hot state, multi-pass composite spinning temperature and defect control, and the coordination of forming accuracy.

Hot forming technology

 

Hot forming technology is a sheet metal part manufacturing technology where the part blank is stamped and formed at high temperatures. The main use of metal materials is heating to soften the nature of the material to reduce the deformation resistance of the material, improve the forming limit, reduce the elastic rebound, and improve the precision of forming. It is mainly applied to titanium alloy, magnesium alloy, molybdenum alloy and other materials that are difficult to deform at room temperature. The main process methods include bending, deep drawing, expansion, flanging, turning holes, local forming and so on.

From the 1950s, with the large number of applications of titanium alloy in aerospace and military, hot forming technology has developed rapidly, and quickly entered the engineering application stage. At present, hot forming technology is widely used in military aircraft, civilian aircraft, aircraft engines, missiles and other manufacturing areas, such as aircraft fuselage skin, tail fairing, spacer frame, engine nacelles, hot end parts, adapter section, missile shells, wings and other parts (Fig 6).

Fig. 6

Fig. 6 Typical thermoformed parts

At present, with the long life of civil aircraft, lightweight, military aircraft with high stealth, missile flight speed to improve the demand is increasingly strengthened, to meet the performance requirements, hot forming technology also maintains constant innovation, mainly reflected in:

(1)large size, no spare parts assembly demand, enhance the shape of hot forming precision control: large size parts lead to large parts rebound, parts and mould thermal expansion is not easy to correct, and no spare parts assembly on the parts of the profile and the external dimensions of the requirements of a more stringent.

For example, the TC4 titanium alloy aft fuselage upper wall plate of a certain type of aircraft developed by Beijing Aviation Manufacturing Engineering Research is a complex shape with torsion and double curvature, which is difficult to form. The project team through the study of high-temperature deformation behaviour of TC4 titanium alloy materials, the establishment of a thermal material constitutive model, combined with numerical simulation technology to successfully predict the rebound of the parts; and through the material forming limit test research, the establishment of thermal forming limit curve, according to which the mould is modified, to achieve the large-scale moulding of the fuselage. Accordingly, the mould is corrected, and the large and complex titanium alloy parts are formed accurately in the hot state.

(2) Diversification of materials, expanding the field of application of hot forming: along with the parts of the service environment temperature increases, the use of new materials is essential. Such as a certain type of missile flight speed increased to 3 times the speed of sound, and the service temperature of its parts as high as 400 ℃, at this time the ordinary TC4 material can not meet the needs of use. Beijing Aviation Manufacturing Engineering Research is committed to the study of new materials such as TNW700 hot forming technology and will write a new chapter for China’s titanium alloy technology.

(2)Automation and Intelligent Level Enhancement: With the progress of science and technology, automation and intelligence have become an important trend in the development of hot-forming technology. Through the introduction of advanced CNC machine tools and robotics, it is possible to achieve precise control of the hot forming process and reduce the errors brought about by human operation.

At the same time, combined with the Internet of Things technology, real-time monitoring of the material temperature, pressure and forming process parameters to ensure the stability of the production process and the quality of the finished product. For example, the Beijing Aviation Manufacturing Engineering Research Institute has realised the automation of the whole process from material preheating, and forming to cooling by establishing an intelligent hot-forming production line, which has significantly improved production efficiency and part consistency.

(3)Integration of green manufacturing concept: In the face of increasingly severe environmental problems, hot forming technology is also constantly transforming into green manufacturing. By optimising process parameters and using renewable materials to reduce energy consumption and waste generation, hot forming technology is moving towards sustainable development.

For example, in the production of a certain type of civil aircraft, the project team reduced the heat loss of the material by improving the forming process and used a more environmentally friendly cooling medium to reduce the impact on the environment. The application of this green manufacturing concept not only improves production efficiency but also contributes to the realisation of sustainable development in the aerospace sector.

Ultrasonic processing technology

 

Ultrasonic machining technology is an advanced technology that utilises high-frequency ultrasonic vibration for material machining. The technology transmits ultrasonic energy to the cutting tool, causing the tool to produce tiny vibrations when it comes into contact with the material, which reduces the cutting force and improves the cutting efficiency, as well as improves the machining accuracy and surface quality. Ultrasonic machining is particularly suitable for handling hard and brittle materials such as ceramics, glass and certain metal alloys, and is widely used in aerospace, microelectronics and medical devices. Mainly embodied in:

(1) Precision machining capability: ultrasonic machining can achieve high-precision microfabrication, suitable for the manufacture of complex shapes and precision parts. Its vibration effect can effectively reduce the cutting force and reduce material damage, thus improving the quality of the finished product. For example, in the aerospace field, ultrasonic machining is commonly used in the manufacture of high-precision aircraft engine components and turbine blades.

(2) Material diversity: the technology applies to a variety of materials, including metals, plastics and composite materials. With the continuous emergence of new materials, ultrasonic processing technology is also constantly expanding its field of application. For example, for some special alloys or new composite materials, ultrasonic processing can effectively reduce the processing difficulty of the material and enhance its performance.

(3) automation and intelligence: with the development of science and technology, ultrasonic processing equipment gradually realizes automation and intelligence. For example, combined with CNC technology, ultrasonic processing machine tools can achieve efficient fully automated processing, reduce human error, and improve production efficiency. At the same time, the Internet of Things technology monitors the parameters of the processing process, to ensure the stability of the processing quality.

(4) Green manufacturing: ultrasonic machining technology can effectively reduce material waste and energy consumption due to its low cutting force and high efficiency, in line with the concept of green manufacturing. In some applications, the project team has optimised ultrasonic machining parameters to reduce the impact on the environment and promote sustainable development in the aerospace sector. This incorporation of green concepts not only improves processing efficiency but also contributes to reducing the environmental burden of the manufacturing process.

Table 1 Links and differences between advanced aerospace sheet metal fabrication technologies

table 1

Conclusion

 

With the increasing demand for high efficiency, long life, low cost and high stealth of new military and civil aircraft, aviation manufacturing technology has ushered in a comprehensive upgrade. Aerospace sheet metal forming technology has made unprecedented progress in this context, which is mainly reflected in several aspects:

Firstly, the mature application of digital technology has significantly promoted the development of precise sheet metal forming technology, improving manufacturing accuracy and efficiency;

Secondly, the diversification of materials and the complexity of parts have given rise to emerging sheet metal forming technology, which meets the needs of complex design;

Finally, the solid basic research technology reserves have shown powerful energy at critical moments, providing strong support for technological innovation.

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