Technology ℱ Integrated Composite Structure Manufacturing Technology

 

In order to obtain the best structural performance and economic benefits, advanced aircraft continue to pursue the goals of high weight reduction, low cost and long life in structural design. To this end, the integration of the new generation of aircraft structures has been greatly improved: more than a dozen parts or even dozens of parts are integrated into an integral structure with a size ranging from more than ten meters to tens of meters, minimizing the weight paid by the connection, the stress concentration caused by the connection, and the man-hours and tooling required for the manufacture and assembly of many small and medium-sized parts, so as to reduce the manufacturing and maintenance costs. Therefore, the application of integrated composite structures has become the development direction of the new generation of aircraft structures.

 

1 Curing and Forming Technology of Composite Wall Panels for Large Aircraft

 

The “T”-shaped reinforced wall panels of the composite wing of Airbus A350XWB (see Figure 3) and the “I”-shaped reinforced wall panels of the composite wing of Boeing 787 are used. Manufacturing of composite wing panels for large aircraft: In terms of curing forming method, considering economy and quality reliability, ribs and skins are generally bonded and co-cured, and there are also forming methods that use co-curing and secondary bonding; automatic tape laying technology is used for skin laying; precise positioning technology is used for long stringer assembly; in the design of curing mold structure, the uniformity of temperature field and the influence of thermal expansion are mainly considered, and the mold material is generally Invar steel.

 

 

(1) Forming and Curing Process of Multi-wall Box Segment with I-shaped Wall

 

The forming of multi-wall box segment with I-shaped wall can adopt the overall co-curing process, adhesive co-curing process or secondary adhesive process. In addition to considering the feasibility of the layer, the convenience of tooling loading and unloading, and the uniform pressure during the curing process, the division of the layer unit and the core mold design should also meet the coordination of the top shape of the structural ribs after forming and the accuracy of the size. The upper and lower mold clamping and core mold position control in product production are important guarantees for the curing or bonding quality and shape accuracy, see Figure 4.

 

 

(2) Forming and Curing Process of Multi-wall Box Segments with π Ribs

 

After the skin panels with π ribs and composite wall (honeycomb or foam sandwich panels, etc.) are co-cured and formed, paste glue is applied to the π structure of the two panels, assembled with the wall in sequence, and then cured (Figure 5). The mold design and assembly tooling design of this scheme are the key to ensuring the precise correspondence of the π rib positions of the upper and lower panels. The fluidity and curing temperature of the adhesive used have specific requirements.

 

 

 

2 Automated Manufacturing Technology of Main Load-bearing Integral Components

 

The automated manufacturing technologies adopted abroad mainly include automatic tape laying, automatic tow laying and other technologies. They are the only means for manufacturing large-scale composite wall panels and composite fuselage cylinders, and have significantly improved the production efficiency of composite materials and parts. Internal quality reduces costs, making it possible to optimize composite material performance and low cost. Figure 6 represents the application and development of automatic tape laying and automatic tow laying technology.

 

 

 

Automatic Tape Laying Technology

 

Automatic tape laying technology can realize the continuous and automatic completion of prepreg tape cutting, positioning, stacking, rolling and other processes. It is widely used in the laying of large wall panel components with small and medium curvatures. Compared with manual laying, it has stable quality and reduces manufacturing costs by 30%~50%.

 

Automatic tape laying technology has matured in Europe and the United States. The mechanical system of tape laying machines, CAD/CAM software, and laying process technology are widely used in the manufacture of aviation composite structural parts. The wing skins of B1 and B2 bombers, NavyA6 bombers, F-22 fighters and Boeing 787, 777 aircraft, A380 and A400M aircraft, as well as the horizontal stabilizer skins of C-17 transport aircraft, A330, A340 and the central wing boxes and wing beams of some aircraft are all manufactured using automatic tape laying technology, see Figure 7.

 

 

Automatic Tow Laying Technology

 

Automatic tow laying technology overcomes the bottleneck problems of bridging and slipping that are difficult to solve in winding processes, as well as the bottleneck problems that are difficult to solve in automatic laying when laying wing surfaces with steps and concave surfaces with small curvature radii. It can realize continuous tow laying of three-dimensional trajectories of complex-shaped integral structural parts, and can adaptively adjust the tow width according to the laying width of the running trajectory to realize the laying of complex internal curved surface structures such as convex surfaces, concave surfaces, steps, thickening, and corners.

 

There are two structures of foreign automatic tow laying equipment, vertical structure and horizontal structure. The horizontal structure is more suitable for the laying of closed cylindrical composite components, and the vertical structure is more suitable for the laying of large fuselage-like wall panel composite components.

 

The representative applications of automatic tow laying technology in the aviation field include: the rear fuselage of the V-22 aircraft, the S-shaped composite air intake of the F-22 and F35, the fuselage section of the Boeing 787, and the rear fuselage of the A380, as shown in Figure 8.

 

 

3. Manufacturing Technology of Secondary Load-bearing Integral Structural Parts

 

The liquid forming method of composite material structure is mainly used for the forming of complex and secondary load-bearing integral structures except the main load-bearing wing surface and the main load-bearing cylinder of the fuselage. Liquid forming technology mainly includes preform preparation and RTM, RFI and VARI resin transfer technology.

 

Preform Preparation and Stitching Technology

 

Preform preparation technology refers to the technology of using fabric stitching to make the skeleton of the part structure. Preform preparation is the key link of liquid transfer molding technology. The stitched preform uses a sewing machine to stitch the laid fabric to enhance the longitudinal connection of the fabric, meet the impact resistance requirements of the composite material, and use the sewing method to connect the laminated blocks of each part to form a structural preform.

 

Selection of stitching equipment: For flat parts and structural parts with small curvature, a gantry stitching device is used; for structural parts with complex shapes, a manipulator stitching device is used.

 

RTM Forming Technology

 

RTM forming method is resin transfer molding forming method: laying a fiber-reinforced preform in the mold cavity, vacuuming to remove the gas in the preform and the mold cavity, and applying pressure to inject resin into the closed mold cavity until the fiber-reinforced preform in the entire cavity is completely infiltrated, and finally solidified. RTM technology has been widely used in secondary load-bearing integral structures, such as the manufacturing of vertical tail stage integral structures (Figure 9).

 

 

VARI Forming Technology

 

VARI (vacuum assisted resin infiltration forming) technology is another forming technology developed on the basis of RTM forming process. It aims to degas the preform under vacuum environment and realize the flow of resin and penetration of fibers. The resin content is controlled by overflowing excess resin, and the bubbles wrapped in the resin are taken away, and finally cured and formed at room temperature or heating conditions. The void content of VARI formed composite structural parts is generally lower than the porosity of laminate structural parts.

Since the flow of resin in VARI forming process is only by vacuum traction, which is far less than the injection pressure of RTM, the resin viscosity is required to be lower and the flow path is shorter during forming. It is suitable for manufacturing extra-large composite components formed at room temperature and medium temperature. Figure 10 is a typical application of VARI forming technology.

 

 

RFI Forming Technology

 

RFI forming process is resin film infiltration forming, which is a kind of composite liquid forming process. Its main principle is to lay resin film between the preform and the mold according to the structural requirements. During the curing process, the resin film is heated and melted. Under the action of vacuum and pressure, the resin liquid penetrates into the corresponding parts of the preform and completes the curing and forming. RFI technology has been used in the wing trailing edge and rear pressure bulkhead of A380, most bulkheads of Boeing 787 fuselage and other aircraft structures.