3.2.1.5 Pressure Diffusion Welding

Pressure welding is to weld the contact surfaces of the materials to be welded together by pressure without transition metal. Pressure diffusion welding is mainly divided into resistance welding and diffusion welding.

A resistance welding

Resistance welding is to combine the metal welded parts and press them between two copper electrodes. When the welded parts are pressed against each other, a large current is applied to the welded workpiece, and a high-intensity current is used to flow through the contact surface of the workpiece joint. A welding method in which the resistance heating effect produced by the adjacent area heats it to a molten or plastic state, thereby joining the welded parts. This type of resistance welding, commonly known as spot welding, is widely used for welding parts in vacuum chambers. The characteristics are: the welding point is firm, there is no pollution, and it does not affect the vacuum system to reach the ultra-high vacuum state. Seam welding is a developed form of spot welding, which can be used to weld laminated metal parts with a thickness of less than 2mm, and the joints are very strong.

B Vacuum Diffusion Welding

aPrinciple and application characteristics of vacuum diffusion welding

Vacuum (protective gas) diffusion welding technology is to heat two smooth and clean surfaces to be welded to a precise temperature under the protection of a precise vacuum degree or a protective atmosphere (hydrogen, argon, etc.), without adding any In the case of solder or intermediate metal, under the simultaneous action of temperature and pressure, they are in close contact with each other after microscopic plastic flow occurs, and the electrons, atoms or molecules on the contact surface of the weldment are used to diffuse and transfer each other, and form ionic bonds, metal bonds or Covalent bond, after a period of heat preservation, homogenizes the composition and structure of the welding zone, so as to achieve the perfect metallurgical connection process. It can be seen that diffusion welding mainly relies on the microscopic plastic rheology of the welding surface to achieve close contact and make the atoms diffuse to each other to achieve welding. It can complete the welding work that is difficult to achieve by other welding methods, and can also realize the welding between dissimilar materials such as dissolved, high melting point metals and non-metals, so that they can obtain high-quality welded joints. The welding surface of diffusion welding must be free of oxides and grease. The characteristics of vacuum diffusion welding are: (1) The welding process is to form a joint without the participation of a liquid phase or only a transition phase and then undergo diffusion treatment. the process of. The composition and structure of the weld can be exactly the same as that of the matrix, no as-cast structure remains in the joint, and the original interface completely disappears, so the physical, chemical and mechanical properties of the original base metal can be maintained. (2) Diffusion welding can weld high-quality metal and non-metal materials without destroying the properties of the material to be welded because the matrix is not overheated or melted. It is especially suitable for welding materials that are difficult to achieve by general welding methods, or can be welded but whose properties and structures are easily damaged during the welding process, such as dispersion-strengthened superalloys and fiber-reinforced boron-aluminum composite materials. (3) It can weld different types or even very different materials, including dissimilar metals, metal-to-metal sealing and other metallurgically incompatible materials. (4) Workpieces with complex structures and large differences in thickness can be welded. (5) The heating is uniform, the weldment is not deformed, and no residual stress is generated, so that the workpiece maintains a high-precision geometric size and shape.

Specific applications of vacuum diffusion welding: welding of aluminum alloy and stainless steel; sealing of titanium alloy and 95% alumina metal sealing; oxygen-free copper, nickel-plated Kovar and Monel alloy and 95% alumina metal sealing and 99.5 % Alumina sealing. The main feature of these materials is that the material has a great affinity for oxygen, and forms a stable and dense oxide film, especially at high temperature, it has great chemical activity to the gas, and the crystal structure and properties are easy to change. In addition, it also has a strong inhalation phenomenon, especially for oxygen and nitrogen, which are the main difficulties encountered when welding titanium alloys.

b vacuum diffusion welding process

First, the surface of the workpiece is finely processed, pre-ground and polished to obtain the required surface roughness, and then the workpiece is cleaned. The cleaning process is to remove the grease, oxide film and other adsorption layers on the surface of the material, because this is an absolute obstacle to obtaining high-quality joints. A better cleaning method is to pickle the welded workpiece in HNO2-HF solution. Then put the cleaned workpiece into the oven for baking. When assembling the workpiece, the flat and smooth welding surface must be closely attached, positioned, pressed, and put into the vacuum chamber after assembly. When the vacuum degree reaches the required value, the welding equipment starts to heat up and pressurize, and then performs diffusion welding.

Temperature, pressure and time are the key parameters of vacuum diffusion welding. These parameters have an impact on the properties of materials and the dynamics of microstructure transformation. Therefore, they must be selected reasonably according to different welding materials and the requirements of the workpiece. Among them, the welding temperature is to determine the welding seam. The most influential factor in quality. Taking the diffusion welding process of titanium alloy (Ti-6%al-4%) material as an example, for a-type titanium alloy, its temperature is about 42~56℃ lower than the transformation temperature of β-type titanium alloy. The diffusion welding temperature of 6%al-4%v titanium alloy is 927~945℃ (the transformation temperature of B-type titanium alloy is 996℃). When vacuum diffusion welding, it should be noted that the superheat of the material should not be too large, otherwise the strength and plasticity of the joint will be reduced due to grain growth. The pressure should be selected according to the different roughness of the workpiece surface, generally 4.9~9.8MPa. The welding time should be determined according to the size of the diffusion coefficient of the welding material, the surface state, the mechanical properties, and the values of temperature and pressure.

The working vacuum during diffusion welding is also an important process parameter. The test results show that although titanium has a great affinity for oxygen, a smooth welding surface can be obtained under the condition of vacuum degree of 13.3Pa. In order to obtain high-quality and high-performance welded joints, it is more appropriate to maintain the vacuum degree of welding work above 10-1Pa.