1．Arc welding
 
Arc welding is currently the most widely used welding method. It includes: hand arc welding, submerged arc welding, tungsten gas shielded arc welding, plasma arc welding, gas metal arc welding and so on. Most arc welding is based on the arc of combustion between the electrode and the workpiece. When forming the joint, the filler metal may or may not be used. When the electrode used is a wire that is melted during the welding process, it is called a molten arc arc welding, such as hand arc welding, submerged arc welding, gas shielded arc welding, tubular wire arc welding, etc.; the electrode used is not melted during the welding process. When a carbon rod or a tungsten rod is used, it is called non-melting arc welding, such as tungsten argon arc welding, plasma arc welding, and the like。
 
（1）Hand arc welding

Hand arc welding is one of the earliest and most widely used welding methods in various arc welding methods. It is an electrode and a filler metal with an externally coated electrode. The arc is burned between the end of the electrode and the surface of the workpiece being welded. On the one hand, the coating can generate gas to protect the arc under the action of arc heat, and on the other hand, it can produce slag covering the surface of the molten pool to prevent the interaction of the molten metal with the surrounding gas. The more important role of slag is to produce a physicochemical reaction with molten metal or to add alloying elements to improve weld metal properties. The hand arc welding equipment is simple, light and flexible. It can be applied to the welding of short seams in maintenance and assembly, especially for welding in hard-to-reach areas. Hand arc welding with the corresponding electrode can be applied to most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and their alloys
 
（2）Submerged arc welding

Submerged arc welding is a wire that is continuously fed as an electrode and a filler metal. During welding, a layer of granular flux is coated on the upper surface of the weld zone, and the arc is burned under the flux layer to melt the end of the wire and the local base material to form a weld bead. Under the action of arc heat, the upper part of the flux melts the slag and undergoes a metallurgical reaction with the liquid metal. The slag floats on the surface of the molten metal pool, which can protect the weld metal, prevent air pollution, and produce physical and chemical reactions with the molten metal to improve the weld metal and performance. On the other hand, the weld metal can be made. Slowly licking. Submerged arc welding can use a larger welding current. Compared with hand arc welding, its biggest advantage is the good weld quality and high welding speed. Therefore, it is particularly suitable for welding straight seams of large workpieces. And most use mechanized welding. Submerged arc welding has been widely used in the welding of carbon steel, low alloy structural steel and stainless steel. Because slag can reduce the joint cooling rate, some high-strength structural steels, high-carbon steels, etc. can also be submerged arc welded
 
（3）Tungsten gas shielded arc welding

This is a non-melting gas shielded arc welding that utilizes an arc between the tungsten electrode and the workpiece to melt the metal to form a weld. During the welding process, the tungsten electrode does not melt and acts only as an electrode. At the same time, argon or helium is fed from the nozzle of the torch for protection. Additional metals can be added as needed. Internationally known as TIG welding. Tungsten gas shielded arc welding is an excellent method for joining sheet metal and bottoming because it provides excellent control of heat input. This method can be used for almost all metal connections, especially for welding aluminum, magnesium, metals that form refractory oxides, and reactive metals such as titanium and zirconium. This welding method has a high weld quality, but its welding speed is slower than other arc welding.
 
（4）Plasma arc welding

Plasma arc welding is also an infusible pole arc welding. It uses a compression arc between the electrode and the workpiece (called a forward transfer arc) to achieve welding. The electrode used is usually a tungsten electrode. The plasma gas that produces the plasma arc can be argon, nitrogen, helium or a mixture of both. It is also protected by an inert gas through the nozzle. Filler metal may be added during welding or without filler metal. In plasma arc welding, the arc penetration ability is strong due to the straightening of the arc and the high energy density. The small hole effect produced by plasma arc welding can be used for the non-grooving of most metals in a certain thickness range, and the penetration and weld uniformity can be ensured. Therefore, plasma arc welding has high productivity and good weld quality. However, plasma arc welding equipment (including nozzles) is relatively complicated and requires high control of welding process parameters. Most metals that can be welded by tungsten gas shielded arc welding can be plasma arc welded. In contrast, for the welding of extremely thin metals of 1 mm or less, plasma arc welding can be easily performed.
 
（5）MIG gas arc welding

This welding method uses an arc that is continuously burned between the welding wire and the workpiece as a heat source, and the gas ejected from the torch nozzle protects the arc for welding. The protective gas used for the gas arc shielded arc welding is argon gas, helium gas, CO 2 gas or a mixture of these gases. When argon or helium is used as shielding gas, it is called molten inert gas shielding arc welding (referred to as MIG welding in the world); when inert gas and oxidizing gas (O2, CO2) are used as shielding gas, or When the CO2 gas or the CO2 + O2 mixture gas is a shielding gas, or when the CO2 gas or the CO2 + O2 mixture gas is used as a shielding gas, it is collectively referred to as a molten-polar active gas-protected arc welding (referred to as MAG welding in the international market). The main advantage of the gas-shielded arc-shielded arc welding is that it can be easily welded at various positions, and also has the advantages of high welding speed and high deposition rate. Molten active gas shielded arc welding can be applied to most major metals, including carbon steel and alloy steel. MIG welding is suitable for stainless steel, aluminum, magnesium, copper, titanium, zirconium and nickel alloys. Arc spot welding can also be performed using this welding method.
 
（6）Tubular wire arc welding

Tubular wire arc welding is also performed by using an arc that burns between a continuously fed wire and a workpiece as a heat source, and can be considered as a type of gas metal arc welding. The wire used is a tubular wire containing various components of flux. When welding, a protective gas is added, mainly CO. The flux is decomposed or melted by heat, and plays a role in protecting the pool, infiltrating the alloy and stabilizing the arc. In addition to the advantages of the above-described gas-shielded arc-shielded arc welding, the tubular wire arc welding has advantages in metallurgy due to the action of the flux in the pipe. Tubular wire arc welding can be applied to the welding of most ferrous metal joints. Tubular wire arc welding has been widely used in some advanced industrial countries.
 
2.Resistance welding
 
This is a type of welding method in which resistance heat is used as an energy source, including electroslag welding using slag resistance heat as an energy source and resistance welding using solid resistance heat as an energy source. Since electroslag welding has more unique characteristics, it will be introduced later. Here we mainly introduce several types of solid resistance heat energy resistance welding, mainly welding, seam welding, projection welding and butt welding. Resistance welding is generally a welding method in which the workpiece is subjected to a certain electrode pressure and the contact surface between the two workpieces is melted by the resistance heat generated when the current is passed through the workpiece. Larger currents are usually used. In order to prevent arcing on the contact surface and to forge the weld metal, pressure is always applied during the welding process. When performing this type of resistance welding, the surface of the workpiece to be welded is of the utmost importance for obtaining a stable welding quality. Therefore, the electrode and the workpiece and the contact surface between the workpiece and the workpiece must be cleaned before welding. Spot welding, seam welding and projection welding are characterized by large welding current (single phase) (thousands to tens of thousands of amps), short energization time (several cycles to several seconds), expensive equipment, high complexity, and high productivity, so it is suitable for large Mass production. Mainly used for welding thin plate assemblies with thickness less than 3mm. Various types of steel, aluminum, magnesium and other non-ferrous metals and their alloys, stainless steel, etc. can be welded.
　　
3．High energy beam welding
 
This type of welding method includes: electron beam welding and laser welding.

（1）Electron beam welding.

Electron beam welding is a method of welding by the heat generated by a concentrated high-speed electron beam bombarding the surface of a workpiece. In electron beam welding, an electron beam is generated by an electron gun and accelerated. 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 vacuuming time) is long, and the workpiece size is limited by the size of the vacuum chamber. Compared with arc welding, the main characteristics of electron beam welding are that the weld has a deep penetration, a small melt width, and a high purity of the weld metal. It can be used for precision welding of very thin materials or for welding of very thick (up to 300mm thick) components. All metals and alloys that can be melt welded by other welding methods can be electron beam welded. Mainly used for welding of high quality products. It can also solve the welding of dissimilar metals, oxidizable metals and refractory metals. But not suitable for high volume products.

（2）Laser welding

Laser welding is a welding of a laser beam that is focused by a high-power coherent monochromatic photon stream. This welding method usually has continuous power laser welding and pulse power laser welding. The advantage of laser welding is that it does not need to be carried out in a vacuum. The disadvantage is that the penetration force is not as strong as that of electron beam welding. Accurate energy control during laser welding enables welding of precision micro devices. It can be applied to many metals, especially to weld some difficult-to-weld metals and dissimilar metals.
 
4．Brazing
 
The energy source for brazing can be either chemical reaction heat or indirect heat energy. It uses a metal whose melting point is lower than the melting point of the material to be soldered as a brazing material. After heating, the brazing material is melted, and the brazing material is inserted into the gap of the joint surface of the joint by capillary action to wet the surface of the metal to be welded, so that the liquid phase and the liquid phase The solid phases are mutually diffused to form a brazed joint. Therefore, brazing is a solid phase and liquid phase welding method. The brazing heating temperature is low, the base metal does not melt, and no pressure is required. However, certain measures must be taken before welding to remove oil, dust, oxide film, etc. from the surface of the workpiece to be welded. This is an important guarantee for the wettability of the workpiece and the quality of the joint. When the liquidus humidity of the brazing filler metal is higher than 450 ° C and lower than the melting point of the base metal, it is called brazing; when it is lower than 450 ° C, it is called soldering. Brazing according to different heat sources or heating methods can be divided into: flame brazing, induction brazing, furnace brazing, dip brazing, resistance brazing, and the like. Since the heating temperature is relatively low during brazing, the influence on the performance of the workpiece material is small, and the stress deformation of the weldment is also small. However, the strength of the brazed joint is generally low and the heat resistance is poor. Brazing can be used to weld carbon steel, stainless steel, superalloy, aluminum, copper and other metal materials, as well as dissimilar metals, metals and non-metals. It is suitable for welding joints with low load or normal temperature. It is especially suitable for precision, miniature and complex multi-brass weldments.
　　
5．Other welding methods
 
These welding methods belong to different degrees of specialized welding methods and their application range is narrow. It mainly includes electroslag welding, high-frequency welding with resistance heat as energy source, gas welding, gas pressure welding and explosion welding with chemical energy as welding energy, friction welding, cold pressure welding, ultrasonic welding and diffusion welding with mechanical energy as welding energy.

（1）Electroslag welding

As described above, electroslag welding is a welding method in which the resistance heat of slag is used as an energy source. The welding process is carried out in the vertical welding position in the assembly gap formed by the end faces of the two workpieces and the water-cooled copper sliders on both sides. The end of the workpiece is melted by the resistance heat generated by the slag during welding. According to the shape of the electrode used in welding, electroslag welding is divided into wire electroslag welding, plate electroslag welding and nozzle electric slag welding. The advantages of electroslag welding are: the thickness of the weldable workpiece is large (from 30mm to more than 1000mm) and the productivity is high. Mainly used for welding of cross-section butt joints and T-joints. Electroslag welding can be used for welding various steel structures and for group welding of castings. Due to the slow heating and cooling, the electroslag welding head has a wide heat-affected zone, a large microstructure and toughness, so it is generally necessary to perform normalizing treatment after welding.
 
（2）High frequency welding

The same frequency welding is based on solid resistance heat. The resistance heat generated by the high-frequency current in the workpiece during welding causes the surface of the workpiece weld zone to be heated to a molten or close plastic state, and then the forging force is applied (or not applied) to achieve metal bonding. Therefore it is a solid phase resistance welding method. High-frequency welding can be classified into contact high-frequency welding and induction high-frequency welding according to the way in which high-frequency current generates heat in a workpiece. When in contact with high frequency welding, high frequency current is transmitted to the workpiece through mechanical contact with the workpiece. In induction high-frequency welding, high-frequency current generates an induced current in the workpiece through the coupling of the external induction coil of the workpiece. High-frequency welding is a specialized welding method that requires special equipment according to the product. High productivity and welding speed up to 30m/min. Mainly used for the welding of longitudinal seams or spiral seams when manufacturing pipes.
 
（3）Gas welding

Gas welding is a welding method that uses a gas flame as a heat source. The most widely used is an oxy-acetylene flame fueled by acetylene gas. Because the equipment is simple and easy to operate, the gas welding heating speed and productivity are low, the heat affected zone is large, and it is easy to cause large deformation. Gas welding can be used for the welding of many ferrous metals, non-ferrous metals and alloys. Generally suitable for repair and single sheet welding.
　　
（4）Pressure welding

Gas pressure welding is the same as gas welding. Gas welding also uses a gas flame as a heat source. When welding, the ends of the two butted workpieces are heated to a certain temperature, and then sufficient pressure is applied to obtain a firm joint. It is a solid phase welding. Gas filling is not filled with filler metal and is often used for rail welding and steel bar welding.
 
（5）Explosive welding

Explosive welding is another solid-phase welding method that uses chemical reaction heat as an energy source. But it uses the energy generated by the explosion of explosives to achieve metal connections. Under the action of the blast wave, the two metals can be accelerated and impacted to form a metal bond in less than one second. Among various welding methods, the combination of dissimilar metals that can be welded by explosive welding is the widest. Explosive welding can be used to weld metallurgically incompatible two metals into various transition joints. Explosive welding is mostly used for flat coatings with a relatively large surface area and is an efficient method for manufacturing composite sheets.
 
（6）Friction welding

Friction welding is a solid phase welding that uses mechanical energy as an energy source. It uses the heat generated by the mechanical friction between the two surfaces to achieve the connection of the metal. The heat of the friction welding is concentrated at the joint surface, so the heat affected zone is narrow. Pressure is applied between the two surfaces. In most cases, the pressure is increased at the end of the heating, so that the hot metal is combined by upsetting, and generally the bonding surface does not melt. Friction welding has a high productivity, and in principle almost all metals capable of hot forging can be friction welded. Friction welding can also be used for welding dissimilar metals. Applicable to workpieces with a maximum diameter of 100 mm with a circular cross section.
 
（7）Ultrasonic welding

Ultrasonic welding is also a solid phase welding method that uses mechanical energy as an energy source. When ultrasonic welding is performed, the welding workpiece is subjected to high static vibration by the high frequency vibration generated by the acoustic pole, and the joint surface is strongly cracked and heated to the welding temperature to form a joint. Ultrasonic welding can be used for welding between most metal materials, enabling metal, dissimilar metals, and welding between metals and non-metals. It can be applied to the repeated production of wire, foil or sheet metal joints of 2 to 3 mm.
 
（8）Diffusion welding Diffusion welding is generally a solid phase welding method using indirect thermal energy as an energy source.
 
It is usually carried out under vacuum or a protective atmosphere. When welding, the surfaces of the two workpieces to be welded are contacted at a high temperature and a large pressure for a certain time to reach the distance between atoms, and are combined by mutual diffusion of atoms. Before welding, it is not only necessary to clean impurities such as oxides on the surface of the workpiece, but also the surface roughness is lower than a certain value to ensure the welding quality. Diffusion welding has almost no detrimental effect on the properties of the material to be welded. It can weld many of the same and different metals as well as some non-metallic materials such as ceramics. Diffusion welding can weld complex structures and workpieces with widely varying thicknesses.
　　
Process parameters for laser welding.
 
1、Power density. Power density is one of the most critical parameters in laser processing. With a higher power density, the surface layer can be heated to the boiling point in the microsecond time range, resulting in a large amount of vaporization. Therefore, high power density is advantageous for material removal processing such as punching, cutting, and engraving. For lower power density, the surface temperature reaches the boiling point and it takes several milliseconds. Before the surface layer vaporizes, the bottom layer reaches the melting point, which is easy to form a good fusion weld. Therefore, in conduction laser welding, the power density is in the range of 104 to 106 W/cm2.
 
2、Laser pulse waveform. Laser pulse waveforms are an important issue in laser welding, especially for sheet welding. When a high-intensity laser beam is incident on the surface of the material, the metal surface will be reflected by 60 to 98% of the laser energy, and the reflectance varies with the surface temperature. The reflectance of the metal changes greatly during the action of one laser pulse.
 
3、Laser pulse width. Pulse width is one of the important parameters of pulsed laser welding. It is an important parameter that is different from material removal and material melting. It is also a key parameter that determines the cost and volume of processing equipment.
 
4、The effect of the amount of defocus on the quality of the weld. Laser welding usually requires a certain amount of disengagement because the power density at the center of the spot at the laser focus is too high and it is easy to evaporate into holes. The power density distribution is relatively uniform across the planes that exit the laser focus.
 
There are two ways to defocus: positive defocusing and negative defocusing. The focal plane is located above the workpiece for positive defocusing, and vice versa for negative defocus. According to the theory of geometric optics, when the distance between the positive and negative defocus planes and the welding plane are equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different. In the case of negative defocusing, a greater penetration can be obtained, which is related to the formation of the molten pool. Experiments show that the laser heating 50 ~ 200us material begins to melt, forming liquid phase metal and appearing vaporization, forming commercial pressure steam, and spraying at a very high speed, emitting dazzling white light. At the same time, the high concentration of vapor moves the liquid phase metal to the edge of the molten pool, forming a depression in the center of the molten pool. When negative defocusing, the internal power density of the material is higher than the surface, and it is easy to form a stronger melting and vaporization, so that the light energy is transmitted to the deeper part of the material. Therefore, in practical applications, when the penetration depth is required to be large, negative defocusing is used; when welding thin materials, positive defocusing should be used.