Application of electron beam welding technology in aerospace

Time:2017-09-18 12:00:00

Various types of rockets, satellites, spaceships, planetary vehicles, space stations, and solar power stations used in aerospace technology have their common features, structural components, and various instruments used. They not only require extremely reliable component quality. Can withstand a variety of harsh environments, such as strong vibration, high temperature and low temperature alternating impact caused by changes in sunlight, weightlessness, cosmic ray radiation, operation in ultra-high vacuum environment; and requires small size, light weight and good air tightness. Therefore, the structural design, material selection and processing technology of aerospace components are extremely demanding. It has been proved that in order to meet the above characteristics, electron beam welding technology is one of the indispensable powerful tools. This is because electron beam welding technology has the following characteristics:

(1) Electron beam welding has a high energy density (106 W/cm2). For any material, including high melting point tungsten, molybdenum and other materials, the weld seam can be melted quickly. Generally, no welding rod is used, and the material of the part itself is welded.

(2) Electron beam welding is carried out in a vacuum to prevent oxidation of materials and other harmful gases. The pressure difference between the molten pool and the vacuum atmosphere is beneficial to the discharge of gas contained in the molten metal of the weld, reducing weld porosity, increasing airtightness, and improving weld strength.

(3) Electron beam welding not only has high energy density, but also can obtain a large weld width-to-width ratio due to its special welding mechanism. The weld seam is deep and narrow, so the deformation of the welded parts is small. For thicker parts, the 100kW electron beam welding machine can weld 200mm steel once, and it needs to be filled multiple times by ordinary welding method, which will cause large deformation of parts.

(4) When welding two parts made of materials with large physical properties (such as heat conduction or heat capacity), the two materials can be instantly melted and solidified at the same time, such as copper and steel, and the welding of extremely thin parts and thick parts Can achieve.

(5) The electron beam can be gathered very finely and the deflection is convenient, so that very fine parts can be welded. It is possible to weld hard-to-reach solder joints, so it is very suitable for special structures and particularly fine parts for electron beam welding.

(6) The energy density is high, the welding speed is fast, and the heat-affected zone range is small, which does not adversely affect adjacent semiconductor devices or other heat-sensitive devices.

Below we give a few examples of the application of electron beam welding in space navigation.

The space station to be built and the solar power station being planned, these need to be transported into space by space shuttle or carrier rocket, and assembled. Riveting and bolting will greatly increase the structural weight. Electron beam welding is the most Ideally, no welding rods are required and no additional structural weight is added. Another advantage of using electron beam welding in space is that it does not require a vacuum system, which greatly simplifies the welding equipment.

Figure 1 shows the cosmic space electron beam welder developed in the United States in the 1960s for the repair of the Apollo lunar module. This is because the lunar module is on the surface of the lunar surface, in case of inadvertent impact. The leg was broken and could not take off and return to the earth. After detailed analysis, demonstration and scientific experiments confirmed that electron beam welding is the most ideal and realized. The future space station and solar power station will fly in space for many years, and the area is large, which is inevitably damaged by the impact of meteorites. It also needs to be repaired by electron beam welding machine.

Figure 1 Cosmic space electron beam welder

All parts of the space used for the navigation of the universe are welded except for a small part that needs to be welded in space (as mentioned above). Most of the parts can be finished on the ground. Due to the excellent welding performance of electron beam welding and the special requirements of cosmic instruments, the variety and number of parts using electron beam welding are numerous. Only some typical welding examples are selected below.

Modern flight and interplanetary stations need to provide a living atmosphere for human beings. The spacecraft and the interstellar station are filled with gas with the composition and pressure equivalent to the earth's atmosphere. Because of the inevitable leakage, gas needs to be supplied from the earth. It is obvious that the larger the air leakage of the aircraft, the larger the amount of replenishment, which imposes a great burden on the ship. The cosmic space is ultra-high vacuum, which requires higher airtightness of the aircraft. If you have used the Gemini constellation (the two people in the spacecraft travel between the Earth and the Moon), the atmosphere will remain less than two weeks. If the occupants increase, or the voyage becomes longer, especially when you are away from the Earth, the air cannot be replenished. Next, it will bring serious problems. Therefore, very strict requirements are imposed on the airtightness of the spacecraft. There are several factors in the formation of gas leakage, but the welding quality of the aircraft is one of the factors. It is confirmed by experiments that the electron beam welding can obtain the minimum leakage.

The Apollo manned lunar module in the United States uses an adjustable liquid rocket engine. The quality of the engine is related to the life safety of the astronauts. The welding is required to be flawless. The electron beam welding can meet the requirements. The engine has 64. The parts are electron beam welded, in which the flow control consists of a valve and its manifold, and 90% is electron beam welded. Its materials are 321 ~ 347 stainless steel, 6061 aluminum alloy, 17 ~ 4, 17 ~ 7, PH stainless steel and 6A1 ~ 4V titanium, as well as some actuators, test standards and command control boxes, etc., can be input very small due to electron beam welding The energy and deformation are small and the electronic components and sensor components sealed in the welded parts are not damaged. The short-time small thruster for aerospace is shown in Fig. 2: the explosive 2 is sealed in the outer casing 1, and the spout 3 is sealed with a 0.05 mm304 stainless steel film 4. When the explosive is ignited, the high-pressure gas breaks through the film to generate thrust. Requires 134 parts to be welded once. Due to the structural requirements, the weld is only 1mm away from the explosive. The surface temperature of the explosive should not exceed 150 °C during welding and packaging. Otherwise, the explosive will be ignited. Only by electron beam welding and strict control of its parameters can the requirements be met. .

Figure 2 Spacecraft Booster

Many of the propellers of spacecraft have successfully solved their difficult problems by electron beam welding because of their special structure and materials. If there is an interplanetary aircraft, the propeller uses an electric rocket, and its emitter vaporizes and dissipates mercury or sputum. Its ions are pulled out from the surface and accelerated to a certain speed under the action of the accelerating pole potential to form the required thrust. The larger the surface area of the emitter, the larger the free amount and the higher the efficiency, and the porous tungsten is the best choice. The porous tungsten also needs to be firmly integrated with the support tungsten block by electron beam welding, and the tungsten support must be welded to the crucible, but the tantalum and tungsten are directly welded, the alloy becomes brittle, and the titanium is used as the intermediate medium. Electron beam brazing to obtain crack-free soldering.

Titanium alloy has a high strength to mass ratio and is an important structural material for aerospace. With argon arc welding, the ductility is poor, very brittle, and with electron beam welding, the quality of the joint welding is much better, such as welding 5A1, 2.5Sn; 4A1, 3Mo, 1V; 6A1, 4V; 13.5V, 11Cr, 3A1, etc. The strength of these titanium alloy welds by electron beam welding can reach the equivalent strength of the substrate, and the impact strength is even higher than that of the substrate. Niobium alloy has a higher strength to mass ratio, and the frame member of the Apollo spacecraft door is made of niobium alloy and is welded by electron beam. The missile housing shown in Figure 3 uses a non-vacuum electron beam welding schematic:

Figure 3 Schematic diagram of the non-vacuum electron beam welding missile shell

Tool steel, hot-worked die steel (5%Cr, 0.4%C) is suitable for certain missile and spacecraft skeleton materials. It is competitive because its price is much lower than that of titanium alloy. Its ultimate strength is 300,000 psi. Electron beam heat treatment can improve its properties. The refractory metals such as tungsten, molybdenum, niobium and tantalum have a wide application in aerospace, but have a strong tendency to inhale and brittle. It is difficult to guarantee the quality of conventional arc welding, and electron beam welding can be avoided in vacuum. Inhale, reduce brittleness, increase their ductility and impact strength. Welded and molybdenum materials have a weld strength comparable to that of the substrate.

Some parts of different parts will be operated under different atmospheres (such as high or low temperature, corrosion or not; dust state, etc.) and different working conditions (such as difference in load size and properties, heat transfer, hardness and wear resistance requirements, etc.) With a material to meet the different requirements of different components, sometimes have to give up reasonable structure such as changing the structure, increasing the size, weight, processing steps. The superior characteristics of the electron beam are a good solution to the welding problem between most dissimilar metals to meet the ideal structure of the part. In the aerospace processing technology, some non-metallic materials such as ceramics, graphite, glass, or welding problems between these materials and high-melting metals are encountered, and electron beam welding can be successfully solved. The fusion welding technology between these non-metallic materials and non-high melting point metals has not been solved, but electron beam brazing can solve this problem. It can wet the surface of the parts by electron beam evaporation under high vacuum conditions. , so that the brazing material is easy to spread and achieve high quality brazing. Electron beam welding can solve the welding problem of most dissimilar materials, so that the parts produced are small in size, light in weight and good in performance.

2 Special requirements for aerospace beam welding machine

The electron beam welding equipment used in aerospace parts is roughly divided into two categories, one is a conventional electron beam welding machine. Used to weld parts that can be assembled on the ground. This type of machine is the same as a normal electron beam welder and will not be described separately. The other type is the electron beam welding machine used in space. The United States and the former Soviet Union have been successfully developed. Because they must be transported to space for welding operations, they must adapt to the special environment of space.

(1) Light weight and small volume. This is one of the most important indicators. Because it is launched into space by rockets, space astronauts also need to carry a complete set of equipment for welding operations. Although there is no need for a vacuum system, high-voltage rectification power supplies and electron guns are still indispensable, and batteries are needed. The US developed an electron beam welding machine for moon landing. The weight of the whole machine on the earth is 150 kg, and only 26 kg on the moon.

(2) Care should be taken to protect the cosmic radiation. Because there is no atmospheric protection in the space, the radiation of the cosmic rays is very strong. It will damage some devices, especially semiconductor devices and insulating materials, and cause some components to fail. Protective measures must be taken.

(3) It is necessary to consider the influence of the weightlessness state on the function of various components.

(4) The space is ultra-high vacuum (<10-12Pa), and its components must be adapted to work in this atmosphere.

(5) The materials and components used in the equipment must withstand the effects of space temperature alternation (from -150 to +250).

For the cosmic navigation electron beam welder, the United States and the former Soviet Union have developed similar types of welders. The mainframe has a new type of space battery, which provides energy for the welder. The inverter supplies the DC power supplied by the battery to high-frequency electricity, and then is boosted and rectified by a transformer to be converted into a high-voltage electron pump of 20~30kV DC. The latter's high-pressure grade has also been thoroughly studied. The accelerating voltage is high (80kV). Although its focusing performance is good, the equipment is bulky, heavy in weight, poor in maneuverability, complicated in X-ray protection, and inconvenient to use in space. The voltage is below 15kV. It is not necessary to adopt X-ray protection. The disadvantage is that the depth of fusion is closely related to the working distance. The lower the voltage, the more obvious the impact. In the maintenance of space, mainly by hand welding, it will inevitably shake, and the resulting working distance change will bring melting. Significant changes in depth and melting width affect the quality of the weld, so the acceleration voltage of the electron gun for aerospace is not expected to be lower than 20kV. The United States developed the electron beam welding machine electron gun with a power of 1.5 kW and a high voltage of 20 kV. The size of the gun (excluding the handle) is 250 mm long, 75 mm in diameter, and the handle weight is 4.35 kg (ground).

In the space of the welding, whether the weight change will fundamentally change the weld forming mechanism, and whether the welding can be carried out smoothly, the United States and the Soviet Union used the aircraft to cause weightlessness in the early stage (when there was no spacecraft). The test can last for 25 seconds. The welding results have the same weld shape and melting capacity as under normal gravity conditions. Only some small pores appear in the welding of some individual materials such as AMT6, which is easier to use under ultra-high vacuum conditions. The gas is precipitated in the molten metal zone, and the aluminum oxide AMT6 evaporable material is more likely to cause pores. Although the welding in space has not encountered as many difficulties as originally estimated, it is necessary to strictly control the welding operation and parameter specifications.

In order to fully utilize the advanced technology of electron beam welding in aerospace technology, on the one hand, it is necessary to develop an advanced electron beam welding device suitable for use in space, and on the other hand, to deeply study the welding process in a special atmosphere of aerospace.

Space science and technology, including manned spacecraft, has been included in China's scientific research plan. The electron beam welding technology for aerospace is indispensable. The scientific and technological workers with more than 30 years of scientific and technological accumulation in this field will be able to cooperate with the overall plan. Complete the mission that history has given them.