1. Manual arc welding
Manual arc welding is the oldest welding method developed and still the most widely used among all arc welding techniques.
It employs an externally coated welding rod as electrode and filler metal, with the arc burning between the end of the welding rod and the surface of the part to be welded.
Under the thermal effects of an electric arc, the coating can generate gas to protect the arc on the one hand, and on the other hand, it can produce slag to cover the surface of the weld pool, preventing the molten metal from interacting with the surrounding gas. .
The most significant role of slag is to undergo physical-chemical reactions with the molten metal or to introduce alloying elements, thus improving the properties of the weld seam.
Arc welding equipment is simple, portable and flexible in operation. It can be used to weld short seams in repairs and assemblies, especially for welding in hard-to-reach areas.
With the appropriate welding rod, arc welding can be applied to most industrial carbon steels, stainless steel, cast iron, copper, aluminum, nickel and their alloys.
2. Tungsten Inert Gas (TIG) Welding
This is a type of gas shielded arc welding with a non-consumable electrode, where an arc between the tungsten electrode and the workpiece causes the metal to melt and form a weld seam.
During the welding process, the tungsten electrode does not melt and only serves as an electrode.
Simultaneously, argon or helium gas is fed into the welding torch nozzle for protection. Additional metal can be added as needed, a process known internationally as TIG welding.
Tungsten inert gas (TIG) welding is an excellent method for joining thin sheet metal and for root pass welding due to its superior control over heat input.
This method can be applied to almost all metal connections, especially useful for welding metals such as aluminum and magnesium, which form refractory oxides, as well as reactive metals such as titanium and zirconium.
Although this welding method offers high quality welds, its speed is slower compared to other arc welding techniques.
3. Gas Metal Arc Welding (GMAW)
This welding method utilizes the heat of the burning arc between the continuously fed welding wire and the workpiece. The arc is protected by gas sprayed from the torch nozzle.
Gas metal arc welding typically uses shielding gases such as argon, helium, CO2, or a mixture of these gases.
When argon or helium is used as a shielding gas, it is called metal inert gas (MIG) welding, a term commonly used internationally.
When a mixture of inert gas and oxidizing gas (O2, CO2) is used as shielding gas, or when CO2 gas or a mixture of CO2 + O2 is used, it is universally called Metal Active Gas (MAG) welding.
The main advantages of MAG welding include the ability to weld conveniently in a variety of positions along with high welding speed and deposition rate.
MAG welding is compatible with most major metals, including carbon steel and alloy steels. In contrast, gas metal arc welding (GMAW) with inert gas shielding is suitable for stainless steel, aluminum, magnesium, copper, titanium, zirconium, and nickel alloys. This welding method can also be used for spot welding.
4. Plasma arc welding
Plasma arc welding is a type of arc welding with a non-consumable electrode. It uses a compressed arc between the electrode and the workpiece (known as transferred arc) to perform welding.
The electrode normally used is made of tungsten. The plasma gas that generates the plasma arc can be argon, nitrogen, helium or a mixture of the two.
Additionally, an inert gas is used for shielding through the nozzle. During welding, filler metal can be added, although it is not always necessary.
During plasma arc welding, due to its straight arc and high energy density, arc penetration is strong. The keyhole effect produced during plasma arc welding allows for butt welding of most metals within a certain thickness range without the need for a groove, ensuring consistent fusion and even weld seams.
Therefore, plasma arc welding has a high productivity rate and excellent weld quality. However, plasma arc welding equipment, including the nozzle, is relatively complex and requires high control over welding process parameters.
Most metals that can be welded with tungsten inert gas (TIG) welding can also be welded with plasma arc welding.
Compared to this, plasma arc welding can be performed more effectively for extremely thin metals less than 1 mm.
5. Flux cored wire arc welding
Flux-cored wire arc welding also uses the burning arc between the continuously fed welding wire and the workpiece as the heat source for welding, which can be considered a type of gas metal arc welding. The welding wire used is tubular, filled with various flux components.
During welding, shielding gas, mainly CO2, is added externally. The flux, when heated, decomposes or melts, thus providing slag to protect the weld pool, alloy diffusion, and arc stabilization.
Flux core arc welding, in addition to the benefits of gas metal arc welding mentioned above, is metallurgically superior due to the internal flux function. This method can be applied to weld various joints of most ferrous metals.
Flux-cored arc welding has been widely adopted in several advanced industrial countries. The term “flux cored wire” is what we currently call “tubular welding wire”.
6. Resistance welding
This category of welding methods uses resistance heat as the energy source, including molten slag resistance heat-fed electric slag welding and solid resistance heat-fed resistance welding. Electric welding with slag, which has unique characteristics, will be discussed later.
This section mainly introduces various types of resistance welding that use solid resistance heat as a power source, including spot welding, seam welding, projection welding and butt welding.
Resistance welding is a method that melts the contact surfaces between two workpieces using the resistive heat generated when current passes through the workpieces under a certain electrode pressure. This process generally employs a large current.
To avoid arcing at the contact surface and to weld the seam metal, pressure must be applied consistently during welding. In this type of resistance welding, cleaning the surface of the part is essential to achieve stable weld quality.
Therefore, it is essential to clean the contact surfaces between the electrode and the workpiece and between the workpieces before welding.
Spot welding, seam welding, and projection welding are characterized by high welding current (single-phase, from a few thousand to tens of thousands of amps), short energization time (from a few cycles to a few seconds), expensive and complex equipment, and high productivity, making them suitable for mass production.
These methods are mainly used to weld assemblies of thin sheets less than 3 mm thick. They can weld all types of steels, non-ferrous metals such as aluminum and magnesium, their alloys and stainless steel.
7. Electron beam welding
Electron beam welding is a method that utilizes the thermal energy produced when a high-speed, concentrated beam of electrons strikes the surface of a workpiece.
During electron beam welding, an electron gun generates and accelerates the electron beam.
Common types of electron beam welding include: high vacuum electron beam welding, low vacuum electron beam welding, and non-vacuum electron beam welding.
The first two methods are carried out inside a vacuum chamber. The preparation time for welding (especially the time for vacuum pumping) is quite extensive and the size of the part is limited by the size of the vacuum chamber.
Compared to arc welding, electron beam welding is distinguished by its deep weld penetration, narrow fusion width and high metal purity. It is versatile, capable of precision welding on thin materials as well as handling very thick components, up to 300mm.
All metals and alloys that can be fusion welded using other methods are suitable for electron beam welding. It is mainly used for welding high-quality products.
Furthermore, it can solve the welding problems associated with dissimilar metals, easily oxidized metals and difficult-to-melt metals. However, it is not suitable for mass-produced items.
8. Laser welding
Laser welding uses a monochromatic, coherent, high-power stream of photons, focused on a laser beam, as a heat source for the welding process. This welding approach typically involves continuous power laser welding and pulsed power laser welding.
The advantage of laser welding is that it does not need to be conducted in a vacuum, but the disadvantage is that its penetrating power is not as strong as electron beam welding.
Laser welding allows precise control of energy, thus enabling precision welding of microdevices. It can be applied to many metals, especially solving the welding of some different and difficult to weld metals.
9. Brazing
The energy for brazing can come from the heat of the chemical reaction or indirect thermal energy. It employs a metal with a lower melting point than the material being brazed as filler.
This metal melts upon heating and capillary action draws the filler into the gap in the joint's mating surface, wetting the surface of the metal being brazed.
This process results in a joint welded by mutual diffusion between the liquid and solid phases. Therefore, brazing is a welding method that involves solid and liquid phases.
Brazing operates at a relatively low heating temperature, leaving the base metal unmelted without requiring any applied pressure.
However, it is necessary to take certain measures to clean the surface of the workpiece from oil, dust and oxidation layers before brazing. This is a crucial step to ensure good wetting of the part and the quality of the joint.
Brazing is classified as hard brazing when the liquidus line of the brazing alloy is above 450°C but below the melting point of the base metal. When it drops below 450℃, it is called soft brazing.
Depending on the heat source or heating method, brazing can be categorized into flame brazing, induction brazing, furnace brazing, immersion brazing, resistance brazing, and more.
Given the relatively low heating temperature during brazing, there is minimal impact on the material properties of the part, with reduced stress deformation. However, the strength of welded joints tends to be lower, with low heat resistance.
Brazing can be used to join carbon steel, stainless steel, high-temperature alloys, aluminum, copper and other metallic materials. It also allows the connection of dissimilar metals as well as metals and non-metals.
It is particularly suitable for joints that bear low loads or operate at room temperature, and especially applicable for complex, miniature and precision multi-welded seam parts.
10. Electroslag welding
Electroslag welding is a method that uses the resistive heat of molten slag as an energy source. The welding process is conducted in an assembly gap formed by the end faces of two parts and two water-cooled copper sliders in a vertical welding position.
During welding, the resistive heat generated by the electric current passing through the molten slag is used to melt the ends of the parts.
Depending on the shape of the electrode used during welding, electroslag welding can be categorized into electroslag welding with wire electrode, electroslag welding with plate electrode and electroslag welding with consumable nozzle.
The advantages of electroslag welding include its ability to weld large thickness parts (ranging from 30 mm to over 1000 mm) and its high production rate. It is predominantly used for welding butt joints and T-joints.
Electroslag welding can be used in welding various steel structures, and also in welding the assembly of castings.
Due to the slow heating and cooling process, the electroslag welding joint has a wide heat-affected zone with coarse microstructures, resulting in higher toughness. Therefore, it generally requires post-welding heat treatment.
11. High frequency welding
High-frequency welding employs solid-state heat resistance as the energy source.
During the welding process, high-frequency current generates resistance heat within the part, heating the surface of the welding area to a molten or nearly plastic state.
Subsequently, a forging force is applied (or not), resulting in the fusion of the metals. Therefore, it is a kind of solid state resistance welding method.
High-frequency welding can be categorized into high-frequency contact welding and high-frequency induction welding based on how the high-frequency current generates heat in the workpiece.
In high-frequency contact welding, high-frequency current is transferred to the workpiece through mechanical contact. In high-frequency induction welding, high-frequency current induces an electrical current within the workpiece through the coupling effect of an external induction coil.
High frequency welding is a highly specialized welding method that requires dedicated equipment depending on the product.
It offers a high production rate, with welding speeds of up to 30m/min. It is mainly used for welding longitudinal or spiral seams in pipe manufacturing.
12. Gas welding
Gas welding is a type of welding method that uses a gas flame as a heat source. The most commonly used is the oxy-acetylene flame, with acetylene as the fuel.
Although the equipment is simple and easy to use, gas welding has a slower heating rate and lower productivity. It also produces a larger heat affected zone and is likely to result in significant deformation.
Gas welding can be used to join many ferrous metals, non-ferrous metals and their alloys. It is typically used for repairing and welding single-piece thin sheets.
13. Pressure Gas Welding
Pressure gas welding, like gas welding, uses a gas flame as a heat source. During the process, the ends of the two pieces to be joined are heated to a certain temperature and then enough pressure is applied to obtain a robust joint.
This method is a type of solid phase welding. During pressure gas welding, no filler metal is added. It is commonly used for rail welding and rebar welding.
14. Explosive Welding
Explosive welding is another solid-state welding method that uses heat from a chemical reaction as an energy source.
However, it takes advantage of the energy generated from an explosive detonation to facilitate the joining of metals. Under the influence of a blast wave, two pieces of metal can be accelerated and impacted to form a metallic bond in less than a second.
Of all welding methods, explosive welding offers the widest range for joining dissimilar metals. It can fuse two metallurgically incompatible metals at various transition joints.
Explosive welding is commonly used for surface coating of large flat plates and is an efficient method for manufacturing composite plates.
15. Friction welding
Friction welding is a solid-state welding process powered by mechanical energy. It uses the heat generated by mechanical friction between two surfaces to achieve metal connection.
The heat in friction welding is concentrated at the joint, so the heat-affected zone is narrow.
Pressure must be applied between the two surfaces, and in most cases the pressure is increased at the end of the heating phase, causing the heated metal to undergo twisted forging and bond. Normally, the joint surface does not melt.
Friction welding offers high productivity and, in theory, virtually all metals that can be hot forged can be friction welded. This technique can also be used to weld dissimilar metals.
It is applicable to parts with a maximum circular cross-section diameter of 100 mm.
16. Ultrasonic Welding
Ultrasonic welding is a solid-state welding method that relies on mechanical energy as an energy source.
During the process, the part under relatively low static pressure is subjected to high-frequency vibrations produced by the acoustic post. This induces intense friction on the surface of the joint, heating it to welding temperature and forming a bond.
Ultrasonic welding can be used to join most metallic materials, facilitating the welding of metals, dissimilar metals and the joining between metals and non-metals.
This method is suitable for the repetitive production of thin metal wires, sheets or sheets less than 2-3 mm thick.
17. Diffusion Welding
Diffusion welding typically uses indirect heat as the energy source for solid phase welding. It is usually carried out under vacuum or in a protective atmosphere.
During the welding process, the surfaces of the two parts to be welded are brought into contact under high temperatures and substantial pressure, and are held there for a certain period of time to achieve interatomic distances. Subsequent atomic diffusion results in binding.
Before welding, not only does the surface of the workpiece need to be cleaned of oxides and other impurities, but the surface roughness must also be below a certain value to ensure the quality of the weld.
Diffusion welding has practically no harmful effects on the properties of the materials to be joined.
It can be used to weld a wide range of homogeneous and heterogeneous metals, as well as some non-metallic materials such as ceramics.
Furthermore, diffusion welding is capable of joining complex structures and components with significant differences in thickness.
