This causes the metal to melt and on subsequent application of pressure, the liquid regions undergo a process called coalescence. When cooled, this coalesced liquid metal mixture undergoes solidification and the weld is complete, thus giving us one continuous piece of metal.
The heat energy required for the process may be obtained from a number of sources. Some options that can be used are gas flames, electric arcs and ultrasound. Most often carried out in an industrial environment in open air, there may arise certain situations where the welding is required to be carried out marine or even in space. The sources of energy will have to be selected accordingly, since certain sources may not work well in certain environments. For instance, an open oxyacetylene flame in a vacuum or even underwater, is obviously impossible.
The quality of a weld, its strength and durability are largely dependant on the base metals used in the welding process. Some of the major base metals which can be joined by using the process of welding are
The suitability of alloys such as steel to welding depend on the contents, which may be a diverse collection. Steel, or more accurately, plain carbon steel is chosen as a reference material for this. To judge alloys made up of many distinct materials, we make use of a factor called the equivalent carbon content. This is used to compare the relative weldabilities of different alloys by comparing their properties to plain carbon steel. Considerable effects are seen on the weldability of a metal alloy which contains elements like carbon, chromium and vanadium, while copper and nickel have only negligible effects. As the equivalent carbon content rises, the weldability of the alloy decreases (Lincoln Electric, 1994). But this can't be helped, because plain carbon and low-alloy steels have unacceptably low strength levels, especially from an industrial perspective. High strength, low-alloy steels which contain a very small percentage of carbon and include additive elements like manganese, phosphorus, sulphur and small amounts of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, silicon, or zirconium(Schoolscience.co.uk, 2007) were developed especially for welding applications during the 1970s.
The high chromium content of stainless steel makes it less preferable for welding. Those varieties which may have been deemed weldable are susceptible to distortion due to their high coefficient of thermal expansion, and hence are prone to cracking and reduced corrosion resistance.
The chemical composition of aluminum alloys, as with any alloy, decides the weldability. Hot cracking of the alloy on welding is prevented by preheating. This reduces the temperature gradient across the welding area. However, this can reduce the mechanical properties of the base material. Another alternative is to alter the design of the joint, with a more compatible filler alloy to decrease hot cracking. Aluminum alloys should also be cleaned prior to welding, with the goal of removing all oxides, oils, and loose particles from the surface to be welded. This is especially important because of an aluminum weld's susceptibility to porosity due to hydrogen and dross due to oxygen(Lincoln Electric, 1994)
Stresses caused in a rigid structure as a result of internal strains are referred to by the term Residual Stresses. These strains are usually of a permanent nature and may have its origins at any stage in the component life cycle. Welding is one of the most significant causes of residual stresses and may cause large tensile stresses whose maximum value is approximately equal to the yield