There are two types of welding: namely Gas welding and Arc welding.
Under the category Gas welding, there are two methods that can be classified as subcategories. First among these is oxyacetylene welding, which is the joining of two metals using the flame of an oxyacetylene torch , filler metal can be used for thicker steel, though this method is usually used for thin metals not exceeding 3mm in thickness. If a filler rod is used it is the same material and composition as the base metal.
Brazing is another subcategory of gas welding, using the oxyacetylene flame, a brazing rod and brazing flux, similar as well as dissimilar metals can be joined. Materials that are held close together with a very small gap in between create a suction that allows brazing to penetrate deeply into the workpiece. A large variety of metals can be joined by brazing, such as brass and steel, or brass and bronze. On site the most commonly used brazing rod is the silver solder rod, which is covered in flux, making it faster and easier to use.
There are many subcategories of arc welding, firstly- i will mention those that are used on site.
Shielded metal arc welding is a process whereby an electric current is passed through a welding electrode to create an electric arc. The shielded electrode consists of a metal rod and flux coating that, when the arc is struck, create an oxygen free environment that protects the welding bead from contamination . This method of welding is very versatile, as a wide range of metals can be joined- depending on the electrode used. For example, aluminium can be welded using aluminium welding electrodes.
Metal inert gas welding replaces the need for electrodes by creating an oxygen free atmosphere with gas , this is ususally a mixture of carbon dioxide and other elements. On site the gas used is called argoshield, it is supplied by afrox. Deposit metal is supplied to the workpiece by a spool of welding wire. The most commonly used welding wire is standard copper coated mild steel wire, however, there are types of welding wire that allow for welding without the need of a protective gas shield, the drawback to this is that it creates a flux coating on the workpiece, which must then be chipped , when using this welding wire, the process effectively becomes submerged arc welding, as the inner core of the wire contains flux with alloying properties
Tungsten Inert Gas welding utilizes a non consumable tungsten electrode and shielding gas, an arc is struck with the tungsten electrode which is then used to fuze the metal of the workpiece . TIG welding is used for very precise welding on thin plate, it can be used along with a filler rod on workpieces of up to 3mm of thickness. An advantage of TIG welding is that it is not limited to ferrous metals, and if filler rod is unavailable for a workpiece, strips of material from the workpiece can be used instead
Plasma arc welding uses the same principles as GTAW (TIG) , the difference being that in PAW the tungsten electrode is confined within the body of the torch, allowing the plasma arc to be seperated from the shielding gas. This plasma arc is then forced through a fine bore nozzle at high velocity , allowing for greater penetration. Plasma arc processes are much the same as Oxyacetelyne processes, only it is not restricted to ferrous metals and can be used to weld or cut any metal.
High pressure welding has two methods, 'wet' welding, which is done underwater and requires a specialized underwater electrode, and 'dry' or hyperbaric welding, which entails sealing the workpiece off and filling the chamber with gas- usually a mixture of helium and oxygen or argon- forcing out the water and creating a pressurized chamber for the workpiece to be welded.