Civil aircraft pursues long life, high weight loss, safety and low cost. For the fuselage structure, it mainly depends on new materials, new structures and new technologies, and the three are complementary. The design of new structures promotes the development of new materials and new technologies, and the development of new technologies and new materials promotes the realization of new structures, especially with the pursuit of performance and benefits of civil aircraft, the structural materials of civil aircraft bodies have changed greatly In order to achieve aircraft weight reduction and low-cost operation, the traditional aluminum alloy as the primary structural material of civil aircraft is gradually replaced by composite materials and titanium alloys.
As a kind of aircraft structural material, titanium alloy has the characteristics of high specific strength, high temperature resistance, corrosion resistance and good compatibility with composite materials. It is beneficial to reduce the weight of the aircraft, improve the safety of the aircraft and reduce the overall cost. As the representative of the latest generation of civil aircraft, the amount of titanium alloy reached 15% and 10% respectively, fully reflecting the advantages of titanium alloy and manufacturing technology in foreign aircraft applications. The following focuses on two new manufacturing technologies that use titanium alloy powder and superplastic plates as semi-finished products, and proposes in-depth research work from the perspective of engineering applications, with a view to providing new technical channels for the future application of titanium alloys for civil aircraft.
Laser forming and repair technology (LRF / LR)
Laser forming technology is an advanced solid forming technology developed in the mid-1990s. This technology is based on the “discrete + stacking” forming idea, and comprehensively utilizes high and new technologies such as laser, computer, metallurgy and new materials. The realization of high-performance complex structure, compact metal parts without mold, fast and direct near net forming, has broad application prospects in the aviation field [1-6].
The basic principle is: build a three-dimensional model of the part, and “slice” the model at a certain thickness, that is, discrete the 3D data information of the part into a series of 2D contour information; then use laser cladding to layer the powder layer by layer according to the 2D contour information Stack to obtain 3D entities; finally perform subsequent processing (a small amount of machining, heat treatment, etc.). Compared with traditional manufacturing technology, this technology has the following characteristics:
(1) High flexibility: No special tooling fixtures and large-scale raw materials are needed, and flexible processing is realized through software program drive;
(2) Precision: The product complexity (shape and size) is limited, and it is suitable for forming large / cavity (heart) / thin-wall structural parts, which can achieve near net forming of products;
(3) High quality: the rapid melting and solidification of high-energy laser obtain a dense structure, and the mechanical properties reach or exceed the level of forgings;
(4) Low cost: the material utilization rate is high (much higher than the forging utilization rate), the subsequent machining volume is small, the entire manufacturing cycle from raw materials to parts is short (relative to forging can be shortened by more than 2/3), and the overall cost is lower than traditional Manufacturing Technology;
(5) Matching: According to the actual use needs, it can realize the manufacture of different alloys or “variable components” parts to meet the service conditions and performance requirements of different parts.
From the analysis of the above characteristics, this technology can avoid the harsh requirements of traditional manufacturing technology on equipment and large-scale raw materials for large-scale integral components. For complex cavity structures, this technology solves the processing difficulties that are difficult or impossible to achieve with conventional forming processes . At the same time, it solves the problems of low material utilization, large deformation resistance, long processing cycle and high manufacturing cost in traditional manufacturing technology.
The application of laser forming and repair technology in China is not very mature. To realize the engineering application of this technology on civil aircraft, the following research work needs to be carried out as soon as possible in accordance with the requirements and structural characteristics of civil aircraft to ensure the safe and reliable application of civil aircraft.
(1) Deepening research on the internal mechanism of laser forming and repair, including basic research on forming and heat treatment process and organization, performance control, internal stress distribution law and elimination, and suppression of deformation cracking;
(2) Research on laser forming and repair manufacturing technology, developing complete sets of equipment for engineering applications, improving forming stability, improving real-time detection methods, and achieving the best matching of accuracy (size and shape) and rate;
(3) Research on laser forming and repairing quality evaluation technology, establishing a complete set of technical document system, including manufacturing standards and test standards, etc .;
(4) Laser forming and repairing full-scale structure static and fatigue assessment verification, based on airworthiness standards for compliance verification, to ensure the safe and reliable use of civil aircraft.
Superplastic forming and diffusion connection technology (SPF / DB)
Superplastic forming and diffusion connection (SPF / DB) is a near-marginless advanced forming technology that has been the focus of development and application in the aviation field for many years. By forming the whole component in one heating and pressurization process, no intermediate treatment is required and can be effectively Reducing structural weight and improving material utilization can provide greater freedom for design and have broad application prospects [11-16].
The basic principle is: the superplasticity of metals and alloys and the interfaceless integration feature of diffusion welding are used. When the superplastic temperature of the material and the diffusion welding temperature are similar, the inflation or die forging method is used to complete the superheating in one heating and pressurization process. Plastic forming and diffusion connection are two processes, so as to manufacture large-scale integral components with high precision and complexity. The technology has the following characteristics:
(1) Low forming pressure / large deformation without damage;
(2) Accurate outline dimensions, no residual stress and rebound effect;
(3) Save equipment and shorten manufacturing cycle;
(4) Improve structural performance, improve structural integrity, and extend the life of the body;
(5) Reduce manufacturing costs and reduce structural weight.
From the analysis of the above characteristics, SPF / DB simplifies the part manufacturing process and assembly process, reduces the number of parts (standard parts) and tooling, eliminates a large number of connection holes, avoids connection cracks and fatigue problems, and is conducive to improving structural durability and reliability It is especially suitable for processing parts with complex shapes, such as aircraft wings, fuselage frames, engine blades, etc. For titanium alloys, SPF / DB solves the shortcomings of titanium alloy cold forming and machining difficulties, and promotes the use of titanium alloy integral components. Compared with conventional metal structures, sandwich structures have sufficient fatigue strength, good plasticity and fracture toughness.
SPF / DB has been widely used abroad in military and civilian aircraft, showing huge technical and economic benefits, but it is still in the early stage of application in China, and this technical advantage has not been fully utilized. According to the requirements of civil aircraft use, main structural characteristics, etc., in order to realize the engineering mature application of this technology, the following research work needs to be carried out as soon as possible:
(1) SPF / DB structure design technology. At present, SPF / DB technology is mostly used for laminate structure, which has insufficient strength. Therefore, we should vigorously develop new SPF / DB components that combine volume forming and diffusion connection.
(2) SPF / DB manufacturing control technology, including forming process organization evolution and deformation mechanism, process process control and processing process automation, structural integrity and stress and deformation control, to achieve organization and performance matching.
(3) SPF / DB quality assessment and inspection technology, establish design performance database, develop low-cost inspection technology, improve inspection accuracy, formulate quality control procedures and inspection standards.
(4) SPF / DB structural static and fatigue assessment verification, conformity verification based on airworthiness standards, to ensure the safe and reliable use of civil aircraft.
With the development of the social economy and the national pursuit of quality of life, domestic demand for aircraft with lower fuel consumption, longer life and better performance will become more urgent. The characteristics of light weight, high strength, corrosion resistance and high temperature resistance of titanium alloy provide more freedom for the design and manufacture of civil aircraft, which is conducive to the realization of the performance of civil aircraft and improve the market competitiveness. It is a rare opportunity for researchers in manufacturing technology research.
For a long time, due to the physical and processing characteristics of titanium alloy, the traditional manufacturing technology for processing titanium alloy parts has the disadvantages of long cycle time, low material utilization rate, and high cost. Based on aircraft development cycle and economic considerations, aircraft designers often have to The abandonment of preference has largely hindered its expanded application on aircraft. With the advent of LRF and SPF / DB technologies, it has solved the problems of low utilization rate of titanium alloy materials and difficult processing, and also shortened the manufacturing cycle and lowered the manufacturing cost. At present, although the two new technologies are still in the stage of research and preliminary application in China, it is believed that as long as the scientific and technical personnel engaged in titanium alloy materials, manufacturing and aircraft design work together, they also assume the responsibility of promoting the engineering application of LRF and SPF / DB technologies. It will surely provide new technological approaches for the future development of civil aircraft.