1. Shielded Metal Arc Welding (SMAW)
Shielded Metal Arc Welding (SMAW) is the earliest developed and still the most widely used welding method among various arc welding methods. It uses a coated welding rod as the electrode and filler metal, with the arc burning between the end of the welding rod and the surface of the workpiece.
The coating on the welding rod can generate gas to protect the arc and produce slag to cover the surface of the molten pool, preventing the interaction of molten metal with the surrounding gas. The more important role of the slag is to react physically and chemically with the molten metal or add alloy elements to improve the properties of the weld.
SMAW equipment is simple, lightweight, and flexible to operate. It can be used for welding short seams in repair and assembly, especially in hard-to-reach areas. SMAW with corresponding welding rods can be used for most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel, and their alloys.
2. Gas Tungsten Arc Welding (GTAW)
Gas Tungsten Arc Welding (GTAW), also known as Tungsten Inert Gas (TIG) welding, is a non-consumable electrode gas shielded arc welding method that uses the arc between the tungsten electrode and the workpiece to melt the metal and form a weld. The tungsten electrode does not melt but only acts as an electrode, while the nozzle of the welding torch supplies argon or helium gas as a shield. Additional metal can be added as needed.
GTAW can control the heat input very well, making it an excellent method for connecting thin metal sheets and for backing welding. This method can be used for connecting almost all metals, especially for welding aluminum, magnesium, metals that can form refractory oxides, and active metals like titanium and zirconium. The weld quality of this welding method is high, but its welding speed is slower than other arc welding methods.
3. Gas Metal Arc Welding (GMAW)
Gas Metal Arc Welding (GMAW), also known as Metal Inert Gas (MIG) welding when using inert gas such as argon or helium as a shielding gas, is a welding method that uses a continuous wire as the electrode and the arc burning between the wire and the workpiece as the heat source. The welding torch nozzle sprays gas to protect the arc and welding. The commonly used shielding gases for GMAW are argon, helium, CO2 or a mixture of these gases.
When using a mixture of inert gas and oxidizing gas (O2, CO2) as the shielding gas, or using CO2 or CO2+O2 mixture as the shielding gas, it is called Gas Metal Arc Welding with active shielding gas (MAG) in international standards.
The main advantages of GMAW are that it can be conveniently used for welding in various positions, and it has a fast welding speed and high deposition rate. GMAW with active shielding gas can be used for most major metals, including carbon steel and alloy steel. GMAW with inert shielding gas is suitable for stainless steel, aluminum, magnesium, copper, titanium, zirconium, nickel alloys, and can also be used for spot welding.
4. Plasma arc welding
Plasma arc welding is also a non-melting pole arc welding method. It uses compressed arc between the electrode and workpiece (called transferred arc) to achieve welding. The electrode used is usually tungsten. The plasma gas that generates the plasma arc can be argon, nitrogen, helium or a mixture of the two. In addition, inert gas is used for protection through the nozzle. During welding, filler metal can be added or not.
During plasma arc welding, due to its straight arc, high energy density, and strong arc penetration ability, the small hole effect generated during welding enables butt welding without groove opening for most metals within a certain thickness range, while ensuring uniform melting and weld seam quality.
Therefore, plasma arc welding has high productivity and good weld quality. However, plasma arc welding equipment (including nozzles) is relatively complex and requires high control over welding process parameters.
Tungsten gas shielded arc welding can be used for welding most metals and can be adopted for plasma arc welding. Compared with it, plasma arc welding is easier to carry out for welding metals thinner than 1mm.
5. Tube wire arc welding
Tube wire arc welding also uses a continuous burning arc between the continuously fed welding wire and workpiece as a heat source for welding, which can be considered as a type of melting pole gas shielded welding. The welding wire used is tubular and contains various components of flux.
During welding, protective gas is added, mainly CO2. The flux is decomposed or melted by heat, playing a role in slag protection of the molten pool, infiltration of the alloy, and stable arc.
In addition to the advantages of melting electrode gas protection arc welding mentioned above, pipe-shaped filler wire arc welding has additional metallurgical benefits due to the action of the welding flux inside the pipe. Pipe-shaped filler wire arc welding can be applied to weld various joints of most ferrous metals. It has been widely used in some advanced industrial countries. “Pipe-shaped filler wire” refers to what is now known as “flux-cored wire”.
6. Resistance Welding
Resistance welding is a type of welding method that uses resistance heating as energy, including slag welding with resistance heating and resistance welding with solid-state heating. Because slag welding has unique characteristics, it will be introduced later. Here, several types of resistance welding with solid-state heating will be mainly introduced, including spot welding, seam welding, projection welding, and butt welding.
Generally, resistance welding involves applying a certain electrode pressure to the workpiece and using the resistance heating generated when the current passes through the workpiece to melt the contact surface between the two workpieces to achieve connection. Large currents are usually used.
In order to prevent arcing on the contact surface and to forge the weld metal, pressure must be applied throughout the welding process. When performing this type of resistance welding, the surface of the workpiece to be welded is of paramount importance for obtaining stable welding quality. Therefore, the contact surfaces between the electrode and the workpiece, as well as between the workpieces, must be cleaned before welding.
The reason why spot welding, seam welding, and projection welding are widely used is that they have large welding currents (single-phase) ranging from several thousand to tens of thousands of amperes, short power-on time (from several cycles to several seconds), expensive and complex equipment, and high productivity. Therefore, they are suitable for mass production of thin sheet components less than 3mm in thickness. Various types of steel, non-ferrous metals and alloys such as aluminum, magnesium, stainless steel, etc., can be welded.
7. Electron Beam Welding
Electron beam welding is a method of welding using the thermal energy generated when a high-speed electron beam strikes the surface of a workpiece.
During electron beam welding, an electron gun generates and accelerates the electron beam. The commonly used electron beam welding methods are 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 preparation time for welding (mainly vacuum pumping time) is longer, and the size of the workpiece is limited by the size of the vacuum chamber.
Compared with arc welding, the main features of electron beam welding are large weld penetration depth, small weld width, and high weld metal purity. It can be used for precision welding of very thin materials, as well as for welding of very thick components (up to 300 mm).
All metals and alloys that can be welded by other welding methods that involve melting can be welded by electron beam welding. It is mainly used for welding products that require high quality. It can also solve the welding of dissimilar metals, easily oxidized metals, and refractory metals. However, it is not suitable for mass-produced products.
8. Laser Welding
Laser welding is a welding method that uses a high-power, coherent monochromatic photon beam focused as a 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 require a vacuum environment, but the disadvantage is that its penetration capability is not as strong as electron beam welding. Laser welding allows for precise energy control, making it possible to perform precision welding on micro-devices. It can be applied to many metals, especially for solving the welding of difficult-to-weld metals and dissimilar metals.
9. Brazing
The energy source for brazing can be chemical reaction heat or indirect thermal energy. It uses a metal with a lower melting point than the base material as a filler material, and heats it to melt it. Through capillary action, the filler material is drawn into the gap between the joint contact surfaces, wets the surface of the base material, and forms a brazed joint by mutual diffusion between the liquid phase and solid phase. Therefore, brazing is a welding method that involves both solid and liquid phases.
Brazing is performed at a low heating temperature, so that the base material does not melt, and no pressure needs to be applied. However, before brazing, certain measures must be taken to remove oil stains, dust, oxide films, and other contaminants from the surface of the workpiece to ensure good wettability and joint quality.
When the liquid phase line of the brazing material is higher than 450℃ and lower than the melting point of the base metal, it is called hard brazing; when it is lower than 450℃, it is called soft brazing. According to different heat sources or heating methods, brazing can be classified as flame brazing, induction brazing, furnace brazing, dip brazing, resistance brazing, and so on.
Since the heating temperature during brazing is relatively low, the impact on the properties of the workpiece material is small, and the stress deformation of the welded joint is also small. However, the strength of the brazed joint is generally lower, and its heat resistance is poor.
Brazing can be used to weld various metal materials such as carbon steel, stainless steel, high-temperature alloys, aluminum, copper, etc. It can also connect dissimilar metals and metals with non-metals. It is suitable for welding joints that are not heavily loaded or work at room temperature, and is particularly suitable for precision, micro, and complex multi-brazed seam welding.
10. Submerged Arc Welding
Submerged arc welding is a welding method that uses the resistance heat of the slag as the energy source. The welding process is performed in a vertical welding position within an assembly gap formed by the end faces of two workpieces and two water-cooled copper sliders on both sides. During welding, the end of the workpiece is melted by the resistance heat generated by the current passing through the slag. According to the shape of the electrode used during welding, submerged arc welding can be divided into wire electrode, plate electrode, and nozzle electrode.
The advantages of submerged arc welding are: it can weld thick workpieces (from 30mm to greater than 1000mm) and has high productivity. It is mainly used for welding cross-sectional joints and T-joints.
Submerged arc welding can be used for welding various steel structures and for component welding of castings. Due to slow heating and cooling, the welded joint of submerged arc welding has a wide heat-affected zone, coarse microstructure, and low toughness, so generally, post-weld heat treatment is necessary.
11. High Frequency Welding
High frequency welding is a welding method that uses the resistance heat of solids as an energy source. During welding, high-frequency current is used to generate resistance heat inside the workpiece, heating the surface layer of the weld area to a melting or near-plastic state. Then, (or not) top forging force is applied to achieve the bonding of metal. Therefore, it is a solid-phase resistance welding method.
High-frequency welding can be divided into contact high-frequency welding and induction high-frequency welding according to the way in which high-frequency current generates heat in the workpiece. In contact high-frequency welding, high-frequency current is transmitted to the workpiece through mechanical contact. In induction high-frequency welding, high-frequency current is induced inside the workpiece through the coupling of an external induction coil.
High-frequency welding is a highly specialized welding method that requires dedicated equipment based on product needs. It has high productivity, and welding speed can reach 30m/min. It is mainly used for welding longitudinal or spiral seams when manufacturing pipes.
12. Gas Welding
Gas welding is a welding method that uses gas flame as a heat source. The most commonly used gas flame is the oxygen-acetylene flame. Due to its simple equipment and easy operation, gas welding has low heating speed and productivity, a large heat-affected zone, and is prone to deformation.
Gas welding can be used for welding many black metals, non-ferrous metals, and alloys. It is generally suitable for repair and single-piece thin plate welding.
13. Pressure Welding
Like gas welding, pressure welding also uses gas flame as a heat source. During welding, the end of two mating workpieces is heated to a certain temperature, and then sufficient pressure is applied to obtain a strong joint. It is a solid-phase welding method. No filler metal is added during pressure welding, and it is commonly used in rail welding and rebar welding.
14. Explosion Welding
Explosion welding is another solid-phase welding method that uses chemical reaction heat as an energy source. However, it uses the energy generated by explosives to achieve metal connection. Under the action of explosion waves, two metals can be accelerated and impacted to form a metal bond in less than one second.
Among various welding methods, explosion welding has the widest range of combinations of dissimilar metals. Explosive welding can weld incompatible metals into various transition joints. It is mostly used for large flat coating surfaces and is an efficient method for manufacturing composite panels.
15. Friction Welding
Friction welding is a solid-phase welding method that uses mechanical energy as an energy source. It uses the heat generated by mechanical friction between two surfaces to achieve metal connection. The heat of friction welding is concentrated at the joint surface, so the heat-affected zone is narrow. Pressure must be applied between the two surfaces, and in most cases, the pressure is increased when the heating is terminated to combine the hot metal by top forging, and the joint surface generally does not melt.
Friction welding has high productivity, and in principle, almost all metals that can be thermally forged can be friction welded. Friction welding can also be used for welding dissimilar metals. It is suitable for workpieces with a maximum diameter of 100mm with a circular cross-section.
16. Ultrasonic Welding
Ultrasonic welding is also a solid-phase welding method that uses mechanical energy as an energy source. During ultrasonic welding, the welding workpiece is subjected to high-frequency vibration produced by a sonotrode under low static pressure, which causes strong fracture friction on the mating surface and heats it up to the welding temperature to form a bond.
Ultrasonic welding can be used for welding between most metal materials and can achieve welding between metals, dissimilar metals, and metals and non-metals. It is suitable for repetitive production of metal wires, foils, or sheet metal joints below 2-3mm.
17. Diffusion Welding
Diffusion welding is generally a solid-phase welding method that uses indirect thermal energy as an energy source. It is usually performed under vacuum or in a protective atmosphere. During welding, the surfaces of the two workpieces are brought into contact and held at high temperature and pressure for a certain period of time to achieve the atomic distance required for atomic diffusion bonding. Prior to welding, not only do the surface impurities such as oxides need to be cleaned, but the surface roughness also needs to be below a certain value to ensure welding quality.
Diffusion welding has almost no harmful effects on the properties of the welded materials. It can weld many same-species and different-species metals as well as some non-metallic materials such as ceramics. Diffusion welding can weld complex structures and workpieces with large differences in thickness.