What Is Superplastic Forming?

Superplastic forming is a process method in which a superplastic metal is used as a mold cavity blank, and in a superplastic state, a process punch is pressed into the blank to realize the mold cavity forming process. ^[1] Refers to the method of forging and forming using the superplasticity of certain metals under certain conditions. The plasticity of a metal is usually expressed as an elongation, and its value is generally less than 40%. However, under certain conditions, the metal is superplastic, which is characterized in that the elongation can be increased by dozens to hundreds of times, up to more than 2000%.

Metal superplasticity was first discovered in 1920. Rosenhain et al. Found that Zn-4Cu-7Al alloy can bend nearly 180 ° without cracking when bending at low speed, which is very different from ordinary crystal materials. In 1964, WA Backofen, IR Turner, and DH Avery published an epoch-making article "Zn-Al alloy superplasticity". The last paragraph of the article mentioned that "Nothing is better than applying polymer and glass forming technology to metals. Forming and even more amazing things, "has since opened the prelude to metal superplastic forming. In 1968, the British Liland Motor Company produced a car upper cover and door inner panel of industrial Zn-22% Al eutectoid alloy using a superplastic forming process method, setting a practical precedent for superplastic forming technology.

Many companies in the United States have the ability to produce superplastic formed, superplastic formed / diffused connecting members. Superplastic forming, superplastic forming / diffusion connection parts are widely used in a variety of advanced models of aircraft and engines. For example, F-15 has more than 70 superplastic forming / diffusion connection structural parts; F-18 has titanium There are more than 20 alloy superplastic forming / diffusion connection structural members; in F-22, a large number of superplastic forming / diffusion connection combined structures are also used, such as the rear fuselage titanium alloy superplastic forming / diffusion connection heat insulation plate. At the same time, the application and development speed and scale of European superplastic forming technology are also very fast. Many companies in the UK have strong production capacity of titanium alloy superplastic forming / diffusive connection structural parts. Rolls-Royce has developed a titanium alloy wide chord non-shoulder hollow fan blade using superplastic forming / diffusion connection technology, which is a world leader. Russia has the Ufa Superplasticity Research Institute, the world's largest superplasticity research institution, and French superplastic forming companies have the capacity for mass production. German MBB company uses a superplastic forming process to produce the propellant box on the satellite.

Since the 1970s, China has been conducting basic research on superplastic forming / diffusion connection, and has successively carried out research work on the development of typical structural parts, mold selection experiments, performance testing, quality control and inspection. Among them, the units that carry out research and application of superplastic forming technology in China mainly include Beijing Institute of Aeronautical Manufacturing Engineering, Harbin Institute of Technology, Western Institute of Technology, China Southern Airlines and so on. After nearly 40 years of hard work, the application of superplastic forming and diffusion connection has reached a certain scale. In terms of military and civilian use, more and more superplastic forming / diffusion connection members are used, the size of members is also getting larger, and the structure of the members has also changed from single-layer and two-layer to complex three-layer, four-layer and five-layer structures. Wait. Especially in the aerospace field, it has been widely used and has obtained good technical and economic benefits.

Since the 1960s, scholars from various countries have conducted a lot of research in superplastic materials science, mechanical mechanism, forming science and other aspects and have initially formed a relatively complete theoretical system. Superplasticity is both a science and a process technology. It can be used on small tonnage equipment to achieve precision molding of parts with complex shapes and other plastic processing processes that are difficult or impossible. ^[2]

The superplastic forming process can be divided into pneumatic forming, hydraulic forming, dieless forming, dieless drawing according to the forming medium; according to the original blank form, it can be divided into volume forming, sheet forming, tube forming, cup protrusion forming and so on. Among them, in the field of aerospace, the most widely used superplastic forming method is sheet pressure forming, also known as blow molding. Blow molding is a sheet forming technology that uses low energy and low pressure to obtain a large amount of deformation. By designing and manufacturing a special mold, a closed pressure space is formed between the mold and the sheet. After the sheet is heated to the superplastic temperature, under the action of gas, the blank is superplastically deformed and gradually approaches the mold profile until the same mold Completely fit into a predetermined shape. Materials with superplasticity include titanium alloys, aluminum alloys, magnesium alloys, high temperature alloys, zinc aluminum alloys, aluminum lithium alloys, and the like.

The most widely used superplastic forming technology is the superplastic forming / diffusive joining combined process technology combined with the diffusion joining technology. The metal material has both the characteristics of superplasticity and diffusion joining in a temperature range. An integral component with a space sandwich structure. It can be divided into single-layer, two-layer, three-layer and four-layer structures according to the number of initial blanks of forming members. Hollow sandwich structure parts formed by superplastic forming / diffusion connection process have the advantages of good formability, large design freedom, high forming accuracy, no springback, no residual stress, high rigidity, short cycle, and reduced number of parts. Because this structure reduces the number of parts and connectors, eliminates a large number of connection holes, and avoids possible cracks in the connection holes during the connection process, greatly improving the durability and damage tolerance of the structure. In addition, this structure can achieve the best stiffness-to-weight ratio. Superplastic forming / diffusion connection structure is used to replace conventional metal structural parts, which can generally reduce the structural weight by 10% to 50%, and the manufacturing cost can be reduced by 25% to 40%. The economic benefits brought by it are considerable and widely used in aerospace field. Titanium alloy has excellent superplasticity and diffusion connection in the same temperature window. It is the most widely used superplastic forming / diffusion connection process material. The overall frame, beam, wall plate, mouth cover, The doors, wings, and rear fuselage heat shields are made of superplastic forming / diffusion connection multi-layer hollow structure, and the weight reduction benefits are outstanding. In addition, the use of superplastic forming / diffusion connection technology to form a wide-chord hollow fan blade in a large ducted turbofan engine represents the highest level of development and application of superplastic forming / diffusion connection technology. ^[3]