Super complete welding knowledge (III)

1. Gas welding

It is a fusion welding method that uses the heat generated when combustible gas burns in oxygen to melt the base metal welding joint to realize connection. Gas welding is a welding method using gas flame as heat source.

The oxygen acetylene flame using acetylene gas as fuel is most widely used. Because the equipment is simple and convenient to operate, but the heating speed and productivity of gas welding are low, the heat affected zone is large, and it is easy to cause large deformation. Gas welding can be used to weld many ferrous metals, nonferrous metals and alloys.

The combustible gas mainly includes acetylene, liquefied petroleum gas, etc. Taking acetylene as an example, the flame temperature can reach 3200 ℃ when it burns in oxygen. There are three types of oxyacetylene flame:

·Neutral flame: the volume mixing ratio of oxygen and acetylene is 1~1.2, acetylene is fully burned, and it is suitable for welding carbon steel and non-ferrous alloys.

Carbon flame: the volume mixing ratio of oxygen and acetylene is less than 1, and acetylene is surplus, which is suitable for welding high carbon steel, cast iron and high-speed steel.

Oxidation flame: the volume mixing ratio of oxygen and acetylene is greater than 1.2, and the oxygen is excessive, which is suitable for brazing of brass and bronze.

Gas welding has low flame temperature, slow heating speed, wide heating area, wide welding heat affected zone, large welding deformation, poor protection of molten metal during welding, and difficult to ensure welding quality, so its application is few. However, gas welding has the characteristics of no power supply, simple equipment, low cost, convenient movement and strong universality, so it has practical value in non power supply occasions and field work. At present, it is mainly used for welding thin steel plates (0.5~3mm thick), copper and copper alloys and repair welding of cast iron.

2. Gas pressure welding

Like gas welding, gas pressure welding also uses gas flame as heat source. During welding, heat the ends of the two butt joint workpieces to a certain temperature, and then apply sufficient pressure to obtain a firm joint. It is a solid phase welding. Gas pressure welding does not add filler metal, and is commonly used for rail welding and reinforcement welding.

3. Electroslag welding

Electroslag welding is a welding method that uses resistance heat of slag as energy source. The welding process is carried out in the vertical welding position, within the assembly gap formed by the end faces of the two workpieces and the water-cooled copper sliders on both sides. During welding, the resistance heat generated by the current passing through the slag is used to melt the end of the workpiece. According to the electrode shape used in welding, electroslag welding is divided into wire electrode electroslag welding, plate electrode electroslag welding and nozzle electroslag welding.

Features of electroslag welding:

In the welding process of electroslag welding, except for an arc process at the beginning, the rest are stable electroslag processes, which are essentially different from submerged arc welding.

The advantages of electroslag welding are:

The thickness of weldable workpiece is large (from 30mm to more than 1000mm), and the productivity is high. It is mainly used for welding butt joints and T-joints on the section. Electroslag welding can be used for welding various steel structures, and also for assembly welding of castings. Due to slow heating and cooling, wide heat affected zone, coarse and tough microstructure, electroslag welding joints generally need to be normalized after welding.

Limitations of electroslag welding:

① As the welding pool is large, heating and cooling are slow, and it is easy to overheat in the weld and heat affected zone to form coarse structures, electroslag welding usually uses normalizing treatment after welding to eliminate coarse grains in the joint.

② Electroslag welding is always carried out in a vertical manner, instead of flat welding. Electroslag welding is not suitable for workpieces with thickness less than 30mm, and the weld should not be too long.

Classification and application of electroslag welding

Electroslag welding mainly includes wire electrode electroslag welding, plate electrode electroslag welding, nozzle electroslag welding and tube electrode electroslag welding.

Wire electrode electroslag welding is the most commonly used electroslag welding method. It uses welding wire as the electrode. According to the different thickness of the weldment, one or more welding wires can be used; The thickness of weldment that can be welded by single wire welding is 40~60mm. When the thickness of weldment is greater than 60mm, the welding wire shall swing laterally; Three wire swing can weld 450mm thick weldments.

Wire electrode electroslag welding is mainly used to weld weldments with a thickness of 40~450mm and weldments with long welds, as well as girth welds of large weldments.

Application: mainly used in the heavy machinery manufacturing industry to manufacture forged welded structural parts and cast welded components, such as the base of heavy machine tools, high-pressure boilers, etc. The thickness of weldments is generally 40~450mm, and the materials are carbon steel, low alloy steel, stainless steel, etc.

4. Electron beam welding

Electron beam welding is a method of welding with the heat energy generated when concentrated high-speed electron beam bombards the workpiece surface. During electron beam welding, an electron gun generates an electron beam and accelerates it.

Commonly used electron beam welding includes: high vacuum electron beam welding, low vacuum electron beam welding and non vacuum electron beam welding.

The first two methods are carried out in a vacuum chamber. The welding preparation time (mainly the vacuum pumping time) is long, and the workpiece size is limited by the size of the vacuum chamber. Compared with arc welding, electron beam welding is characterized by large weld penetration, small weld width and high weld metal purity. It can be used not only for precision welding of very thin materials, but also for welding of very thick (up to 300 mm thick) components.

All metals and alloys that can be fusion welded by other welding methods can be welded by electron beam. It is mainly used for welding of products requiring high quality. It can also solve the welding of dissimilar metals, easily oxidized metals and refractory metals. But it is not suitable for mass production.

Electron beam welding machine: the core is the electron gun, which is a device to complete the generation, formation and convergence of electrons, mainly composed of filament, cathode, anode, focusing coil, etc. The filament is energized to heat up and heat the cathode. When the cathode reaches about 2400K, electrons are emitted. Under the action of the high-voltage electric field between the cathode and the anode, electrons are accelerated (about 1/2 light speed), shot through the anode hole, and then converged into an electron beam with a diameter of 0.8~3.2mm through the focusing coil to shoot at the weldment, and the kinetic energy is converted into heat energy on the surface of the weldment, so that the weldment connection is rapidly melted, and the weld is formed after cooling and crystallization.

Classification of electron beam welding according to different vacuum degrees of welding chambers (places where weldments are placed):

① High vacuum electron beam welding: the working room is in the same room as the electron gun, and the vacuum degree is 10-2~10-1Pa, which is suitable for precision welding of refractory, active, high-purity metals and small parts.

② Low vacuum electron beam welding: the working chamber and electron gun are divided into two vacuum chambers. The vacuum degree of the working chamber is 10-1~15Pa, which is suitable for larger structural parts and refractory metals that are not sensitive to oxygen and nitrogen.

③ Non vacuum electron beam welding: an inert gas shield or nozzle shall be added, and the distance between the weldment and the electron beam outlet shall be controlled at about 10mm to reduce the scattering caused by the collision between the electron beam and the gas molecules. Non vacuum electron beam welding is applicable to the welding of carbon steel, low alloy steel, stainless steel, refractory metal, copper, aluminum alloy, etc. The size of weldment is not limited.

Advantages of vacuum electron beam welding:

① High electron beam energy density, up to 5 × 108W/cm ²， It is about 5000~10000 times of the common arc, with concentrated heat, high thermal efficiency, small heat affected zone, narrow and deep weld, and minimal welding deformation.

② In vacuum welding, the metal does not interact with the gas phase, and the joint strength is high.

③ The focus radius of the electron beam can be adjusted in a wide range, controlled flexibly, and has strong adaptability. It can be used to weld 0.05mm thin pieces or 200-700mm thick plates.

Application:

It is especially suitable for welding some refractory metals, active or high-purity metals and metals with strong thermal sensitivity. However, the equipment is complex, the cost is high, the size of the weldment is limited by the vacuum chamber, the assembly accuracy is high, and it is easy to excite X-ray, the auxiliary welding time is long, and the productivity is low. These weaknesses limit the wide application of electron beam welding.

5. Laser welding

Laser welding is a welding process using laser beam focused by high-power coherent monochromatic photon flow as heat source. This welding method usually includes continuous power laser welding and pulsed power laser welding. The advantage of laser welding is that it does not need to be carried out in vacuum, and the disadvantage is that the penetration is not as strong as electron beam welding.

Precise energy control can be carried out during laser welding, so precise micro devices can be welded. It can be applied to many metals, especially to solve the welding of some difficult to weld metals and dissimilar metals.

Generation of laser: the light beam with identical wavelength, frequency and direction is generated after the material is excited.

Laser features:

It has the characteristics of good monochromaticity, good directivity and high energy density. After the laser is transmitted or focused by a reflector, it can obtain a diameter of less than 0.01 mm and a power density of 1013 W/cm ² It can be used as heat source for welding, cutting, drilling and surface treatment. The materials that generate laser include solid, semiconductor, liquid, gas, etc. Among them, yttrium aluminum garnet (YAG) solid laser and CO2 gas laser are mainly used for industrial processing such as welding and cutting.

The main advantages of laser welding are:

① Laser can be bent and transmitted through optical methods such as optical fiber and prism, which is suitable for welding micro parts and parts that are difficult to reach by other welding methods, and can also be welded through transparent materials.

② With high energy density, high-speed welding can be realized, and the heat affected zone and welding deformation are very small, which is especially suitable for the welding of heat sensitive materials.

③ Laser is not affected by electromagnetic field, does not produce X-ray, and does not need vacuum protection. It can be used for welding of large structures.

④ The insulating conductor can be directly welded without stripping the insulating layer in advance; It can also weld dissimilar materials with large differences in physical properties.

The main disadvantages of laser welding are:

The equipment is expensive, the energy conversion rate is low (5%~20%), and the requirements for the processing, assembly and positioning of the weldment interface are very high. At present, it is mainly used for the welding of micro devices in the electronic industry and instrument industry, as well as the welding of silicon steel sheets, galvanized steel sheets, etc.