Super complete welding knowledge (IV)

1. Resistance welding

This is a kind of welding method using resistance heat as energy source, including electroslag welding using slag resistance heat as energy source and resistance welding using solid resistance heat as energy source.

Resistance welding is generally a welding method that makes the workpiece under a certain electrode pressure and uses the resistance heat generated when the current passes through the workpiece to melt the contact surface between the two workpieces to realize the connection. Higher currents are usually used. To prevent arcing on the contact surfaces and to forge the weld metal, always apply pressure during welding. When conducting this type of resistance welding, the surface of the weldee is of primary importance for obtaining stable welding quality. Therefore, the contact surfaces between electrodes and workpieces and between workpieces must be cleaned before welding.

advantage:

① When the nugget is formed, it is always surrounded by a plastic ring, the molten metal is isolated from the air, and the metallurgical process is simple.

② The heating time is short, the heat is concentrated, so the heat affected zone is small, and the deformation and stress are small. It is usually unnecessary to arrange correction and heat treatment procedures after welding.

③ It does not need filler metals such as welding wires and electrodes, and welding materials such as oxygen, acetylene and argon, so the welding cost is low.

④ Simple operation, easy to realize mechanization and automation, and improved working conditions.

⑤ With high productivity and no noise and harmful gas, it can be programmed into the assembly line together with other manufacturing processes in mass production. However, flash butt welding needs to be isolated due to spark splashing.

Disadvantages:

① At present, there is a lack of reliable nondestructive testing methods, and the welding quality can only be checked by destructive tests of process samples and workpieces, as well as by various monitoring technologies.

② The lap joint of spot welding and seam welding not only increases the weight of the component, but also causes low tensile strength and fatigue strength of the joint due to the included angle around the nugget between the two plates.

③ The high power of the equipment and the high degree of mechanization and automation make the equipment more expensive and difficult to maintain, and the commonly used high-power single-phase AC welding machine is not conducive to the normal operation of the power grid.

Scope of application:

It is widely used in automobile, airplane, instrument, home appliance, building steel bar, etc., and has a wide range of applicable materials, but the resistance welding weldability of easily oxidized metals is slightly poor. It is mainly used for welding thin plate components with thickness less than 3mm. All kinds of steel, aluminum, magnesium and other non-ferrous metals and their alloys, stainless steel can be welded.

2. Friction welding

Friction welding is a solid phase welding using mechanical energy as energy source. It uses the heat generated by mechanical friction between two surfaces to realize metal connection. The heat of friction welding is concentrated at the joint surface, so the heat affected zone is narrow. The pressure must be applied between the two surfaces. In most cases, the pressure is increased at the end of heating, so that the hot metal is combined by upset forging. Generally, the joint surface does not melt.

Friction welding has a high productivity. In principle, almost all metals that can be hot forged can be friction welded. Friction welding can also be used for welding of dissimilar metals. It shall be applicable to workpieces with circular cross section and a maximum diameter of 100mm.

It is a solid pressure welding method that uses the heat generated by the friction between the contact end faces of weldments to make the end faces reach the thermoplastic state, and then rapidly applies the upset force to realize welding.

Friction welding has the following advantages:

① The welding quality is stable, the size accuracy of weldments is high, and the rejection rate of joints is lower than that of resistance butt welding and flash butt welding;

② High welding productivity, 5-6 times higher than flash butt welding;

③ Suitable for welding dissimilar metals, such as carbon steel, low alloy steel, stainless steel, high-speed steel, copper stainless steel, copper aluminum, aluminum steel, steel zirconium, etc;

④ Low processing cost, power saving, no need for special cleaning of weldments;

⑤ It is easy to realize mechanization and automation, and the operation is simple. There is no spark, arc light and harmful gas in the welding workplace.

Disadvantages:

It is difficult to weld non-circular section by workpiece rotation. Disc shaped workpieces and thin-walled pipe fittings are difficult to be clamped and welded. Limited by the power of the spindle motor of the welding machine, the maximum cross section that can be welded by friction welding is 20000mm at present ²。 Friction welding machine has large one-time investment cost and is suitable for mass production.

Applications: dissimilar metals and dissimilar steel products, such as copper aluminum transition joints in the power industry, high-speed steel structural steel tools for metal cutting, etc; Structural steel products, such as utility boiler coil, valves, tractor bearing shells, etc.

3. Diffusion welding

Diffusion welding is generally a solid phase welding method using indirect heat energy as energy source. It is usually carried out under vacuum or protective atmosphere. During welding, the surfaces of the two weldments shall be contacted under high temperature and large pressure and kept warm for a certain time to reach the distance between atoms, which will be combined through simple mutual diffusion of atoms. Before welding, it is not only necessary to clean impurities such as oxides on the workpiece surface, but also to ensure the welding quality if the surface roughness is lower than a certain value.

Diffusion welding Under the protection of vacuum or protective atmosphere, at a certain temperature (lower than the melting point of the base metal) and pressure, the smooth and clean surfaces to be welded that contact each other will closely contact after micro plastic flow, and the atoms will diffuse with each other. After a long period of time, the original interface will disappear, and the welding method of complete metallurgical bonding will be achieved.

Diffusion welding has the following advantages:

① It can realize the welding between various kinds of the same materials and different materials without damaging the properties of the materials to be welded, and can be used to manufacture double-layer or multi-layer composite materials;

② Able to weld workpieces with complex structure and large thickness difference;

③ The composition and structure of the joint are uniform, reducing the stress corrosion tendency;

④ The welding deformation is small and the joint precision is high, which can be used as the final assembly and connection method of components;

⑤ It can be carried out simultaneously with other processing technologies (such as vacuum heat treatment), and multiple joints can be welded at the same time to improve productivity.

Disadvantages: diffusion welding has high requirements for surface processing and cleaning of weldments, long welding time, low productivity, high cost and large equipment investment.

Application:

Welding between dissimilar metals with large difference in melting point or metallurgical incompatibility, welding between metals and ceramics, and welding of titanium, nickel and aluminum alloy structures. It is not only applied to the cutting-edge technology fields such as atomic energy, aerospace and electronic industry, but also extended to the general machinery manufacturing industry.