Extract taken from "Fundamentals of Modern Manufacturing" Groover,Michael P., pages 713-714
Although welding is considered a relatively new process as practiced today, its origins can be traced back to ancient times. Around 1000 B.C., the Egyptians and others in the eastern Mediterranean area learned to accomplish forge welding. It was the natural extension of hot forging, which they used to make weapons, tools, and other implements. Forge welded articles of bronze have been recovered by archaeologists from the pyramids of Egypt. From these early beginnings up to the Middle Ages, the blacksmithing trade developed the art of welding by hammering to a high level of maturity. Welded objects of iron and other metals dating from these times have been found in India and Europe.
It was not until the 1800's that the technological foundations of modern welding were established. It was during this period that two important discoveries were made, both attributed to the English scientist Sir Humphrey Davy, firstly the electric arc, and secondly acetylene gas.
Davy observed that an electric arc could be struck between two carbon electrodes around 1801. However, not until the mid- 1800's, when the electric generator was invented, did electrical power become available in amounts sufficient to sustain arc welding. It was a Russian, Nikolai Benardos , working in a French laboratory who was granted a series of patents for the carbon arc welding process (one in England in 1885 and another in the United States in 1887). By the turn of the century, carbon arc welding had become a popular commercial process for joining metals.
Benardos's inventions seem to have been limited to carbon arc welding. In 1892, an American named Charles Coffin was awarded an U.S. patent for developing an arc welding process utilising a metal electrode. The unique feature was that the electrode added filler metal to the weld joint (the carbon arc process does not deposit filler). The idea of coating the metal electrode (to shield the welding process from the atmosphere) was developed later, with enhancements to the metal arc welding process being made in England and Sweden starting around 1900.
Between 1885 and 1900 several forms of resistance welding were developed by E.Thompson. These included spot welding and seam welding, two joining methods widely used today in sheet metalworking.
Although Davy made the discovery of acetylene gas early in the 19 th. century, oxyfuel gas welding required the subsequent development of torches for combining acetylene and oxygen around 1900. During the 1890's, hydrogen and natural gas were mixed with oxygen for welding, but the oxyacetylene flame achieved significantly higher temperatures.
These three welding processes — arc welding, resistance welding, and oxyfuel gas welding — constitute by far the majority of welding operations performed today.
Welding is a materials joining process in which two (or more) parts are coalesced at their contacting surfaces by the suitable application of heat and/or pressure. The assemblage of parts that are joined by welding is called a weldment .
Many welding processes are accomplished by heat alone, with no pressure applied, others by a combination of heat and pressure, and still others by pressure alone with no external heat supplied. In some welding processes a filler material is added to facilitate coalescence. Welding is most commonly associated with metal parts, but the process is also used for joining plastics.
Welding is a relatively new process, its commercial and technological importance derives from the following:
Although Welding has the advantages indicated, it also has certain limitations and drawbacks (or potential drawbacks):
Welding involves localised coalescence or joining together of two metallic parts at their faying surfaces. The faying surfaces are the part surfaces in contact or close proximity that are to be joined. Welding is usually performed on parts made of the same metal, but some welding operations can be used to join dissimilar metals.
Over 50 different types of welding operations have been catalogued by the American Welding Society. They use various types or combinations of energy to provide the required power. Welding processes can be divided into two major groups: (1) fusion welding and (2) solid-state welding.
Fusion welding processes use heat to melt the base metals. In many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and strength to the welded joint. A fusion welding operation in which no filler metal is added is referred to as an autogenous weld. The fusion category comprises the most widely used welding processes and includes the following general groups (initials in parentheses are designations of the American Welding Society):
Arc welding (AW). Arc welding refers to a group of welding processes in which heating of the metals is accomplished by an electric arc. Some arc welding operations also apply pressure during the process, and most utilise a filler metal.
Resistance welding (RW). Resistance welding achieves coalescence using heat from electrical resistance to the flow of a current passing between the faying surfaces of two parts held together under pressure.
Oxyfuel gas welding (OFW). These joining processes use an oxyfuel gas, such as a mixture of oxygen and acetylene, to produce a hot flame for melting the base metal and filler metal, if one is used.
Other fusion welding processes. In addition to the preceding types, there are other welding processes that produce fusion of the metals joined. Examples include electron beam welding and laser beam welding .
Certain arc and oxyfuel processes are also used for cutting metals.
Solid-state welding refers to joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process is below the melting point of the metals being welded. No filler metal is utilised in solid-state processes. Some representative welding processes in this group include the following:
Diffusion welding (DFW). In diffusion welding, two surfaces are held together under pressure at an elevated temperature and the parts coalesce by solid-state fusion.
Friction welding (FRW). In this process, coalescence is achieved by the heat between two surfaces.
Ultrasonic welding (USW). Ultrasonic welding is performed by applying a moderate pressure between the two parts and using an oscillating motion at ultrasonic frequencies in a direction parallel to the contacting surfaces. The combination of normal and vibratory forces results in shear stresses that remove surface films and achieve atomic bonding of the surfaces.
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