Welding is a joining process that produces coalescence of materials (typically metals or thermoplastics) by heating them to welding temperature, with or without the application of pressure or by the application of pressure alone, and with or without the use of filler materials.
Most commonly, workpieces are welded by melting both of them and adding more molten metal or plastic to form a pool that cools to form a strong joint. The energy to form the joint between metal workpieces most often comes from a flame (e.g. oxy-acetylene) or an electric arc, but welding by laser beam, electron beam, ultrasound and friction processes is well established. Energy for fusion welding of thermoplastics typically comes from direct contact with a heated tool or a hot gas.
Welding differs from soldering and brazing in that enough heat is applied to melt the materials to be joined. Soldering uses solder, a lower-melting-point material.
History
The history of joining metals goes back several millenia, but before the end of the 19th century, the only process available was forge welding, where blacksmiths pounded heated metal repeatedly until bonding occurred. The Greeks of the first millenium B.C. knew how to heat treat steel, and it is other ancient peoples also knew basic welding principles. Forge welding developed extensively during the first half of the 2nd millenium AD, and in 1540, Vannoccio Biringuccio published De la pirotechnia, which includes descriptions of the forging operation. Craftsmen of the Renaissance era were skilled in the process, and the industry continued to grow during the following centuries. However, with the discovery of the electric arc by Sir Humphrey Davy in 1801, and subsequent developments during that century, arc welding became the most commonly used method of metallurgically joining metals.
In 1865, an Englishman named Wilde was granted a patent for his process of melting pieces of iron together. The electric arc did not make inroads into practical usage until 1881, with the introduction of carbon-arc street lamps. During that decade, many developments were made in the arc welding process, including the use of a metal electrode (instead of carbon) and of an insulated handle that permitted manual operation (patented by Russian scientist Nikolas de Benardos in 1887). Additionally, two other welding process were developed during the last two decades of the 19th century, namely resistance welding and oxyacetylene welding. Oxyacetylene welding at first was more popular because of its portability and relatively low cost, but as the 20th century progressed, it fell out of favor for industrial applications. It was largely replaced with arc welding, as metal coverings for the electrode that stabilize the arc and shield the base material from impurities were developed, commonly known as flux.
World War I caused a major surge in the use of welding processes, with the various military powers attempting to determine which of the several new welding processes would be best. The British primarily used arc-welding, even constructing a ship, the Fulagar , with an entirely welded hull. The Americans were more hesitant, but began to recognize the benefits of arc welding when the process allowed them to repair their ships quickly after a German attack in the New York Harbor at the beginning of the war. Arc welding was first applied to aircraft during the war as well, as some German airplane fuselages were constructed using the process.
During the 1920s, welding applications began slowly increasing. The application of coverings for the metal electrodes became much cheaper in 1927 when an extrusion process was developed, and this fed major expansion in the role of arc welding during the 1930s and during World War II. Major advancements in the use of automatic welding, AC current and flux types were made during those years, and inert gases began to be used to allow the welding of reactive metals like aluminum and magnesium. This led to the creation of two commonly used processes, gas tungsten arc welding (then known as tungsten inert gas welding) and plasma arc welding.
The limitations of gas tungsten arc welding included the inability to weld thick sections, and this led to the development of a consummable electrode and ultimately gas metal arc welding, announced in 1948. During this time, several important developments were made, such as the use of iron powder in electrode coverings, the use of argon-helium inert gas mixtures, and ultimately, the much cheaper use of carbon dioxide as an often satisfactory replacement for argon and helium. In 1958, the flux-cored arc welding process debuted, in which the self-shielded wire electrode could be used with automatic equipment, resulting in greatly increased welding speeds.
Further developments in welding have continued, but new processes (such as laser beam welding and electron beam welding) generally are designed for specialized applications. Research also has shifted toward assuring that individual welds for particular applications meet specifications.
Welding processes
Arc welding processes
Arc welding processes use a welding power supply to create an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-cosumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, and filler material is sometimes used as well.
Shielded metal arc welding
- See main article at shielded metal arc welding
Shielded metal arc welding (SMAW), also known as manual metal arc welding (MMA) or stick welding, uses electric current to strike an arc between the consumable electrode rod and the base material. The electrode is made of steel and is covered with a flux that protects the weld area from oxidation and contamination by producing CO2 gas during the welding process. The electrode core itself acts as filler material, making a separate filler unnecessary. The process is very versatile, requiring little operator training and inexpensive equipment. However, weld times are rather slow, since the consummable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though speciality electrodes have made possible the welding of cast iron, nickel, aluminum, cooper, and other metals. It is one of the most common welding techniques, and is used extensively in construction.
Gas metal arc welding
- See main article at gas metal arc welding
Gas metal arc welding (GMAW), also known as metal inert gas welding (MIG), is a manual or automatic welding process that uses an automatic wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. Since the electrode is continuous, welding speeds are greater for GMAW than for SMAW. However, because of the additional equipment, the process is less portable and versatile, but still useful for industrial applications. The process can be applied to a wide variety of metals, both ferrous and non-ferrous.
Flux-cored arc welding
- See main article at flux-cored arc welding
Similar to GMAW, flux-cored arc welding (FCAW) uses the same equipment but use wire that consists of a steel electrode surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits higher welding speed and greater metal penetration.
Gas tungsten arc welding
- See main article at gas tungsten arc welding
Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding, is a manual welding process that uses a non-consumable electrode made of tungsten, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. It can be used on nearly all weldable materials, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in aircraft and naval applications.
Plasma arc welding
- See main article at plasma arc welding
Plasma arc welding uses a plasma gas that flows around the electrode (usually made of tungsten), while a shielding gas protects the welding region from contamination. The arc is more concentrated that the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process, and furthermore, it is much faster. It can be applied to all of the same materials as GTAW except magnesium, and automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting, an efficient steel cutting process.
Submerged arc welding
- See main article at submerged arc welding
Submerged arc welding (SAW) is a high-productivity welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself, and combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes, since the flux hides the arc and no smoke is produced. The process is commonly used in industry, especially for large products.
Other arc welding processes
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Stud arc welding
- Atomic hydrogen welding
- Bare metal arc welding
- Carbon arc welding
- Electrogas welding
Gas welding
Oxy-fuel welding
- See main article at oxy-fuel welding and cutting
Oxy-fuel welding, also known as oxy-acetylene welding, is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work. The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of more than 3000 degrees C. The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. Other methods, such as air acetylene welding , oxygen hydrogen welding , and pressure gas welding are quite similar, generally differing only in the type of gases used. A similar process, generally called oxy-fuel cutting, is used to cut metals.
Resistance welding
- See main article at resistance welding
Resistance welding involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high amounts of current (1000-100000A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.
Spot welding
- See main article at spot welding
Spot welding is a popular welding method used to join overlapping metal sheets of up to 3mm thick. Two electrodes are simulataneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry – ordinary cars can have several thousand spot welds. A specialized process, called shot welding, is used to spot weld stainless steel.
Seam welding
- See main article at seam welding
Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited.
Other resistance welding processes
- Flash welding
- Projection welding
- Upset welding
Energy beam welding
Laser beam welding
- See main article at laser beam welding
Laser beam welding is an advanced welding procedure that employs a highly focused laser beam, providing a high energy density, that causes a small area of the workpiece to melt. The high concentration of heat results in a narrow, deep weld, minimizing material distortion in the part and permitting the easy welding of thick materials. An extremely fast process, it is often used in high production operations where precision is necessary. Its primary disadvantages include very high investment costs (though these are decreasing) and a susceptability to thermal cracking.
Electron beam welding
- See main article at electron beam welding
Electron beam welding, like laser beam welding, is used to produce deep and narrow welds, but it is characterized by even higher energy content and a smaller beam. It uses a high-energy electron beam, and as a result, the procedure can only be performed in a vacuum. It is economically competitive for large production runs of complicated parts, and especially when thick materials are joined through butt joints.
Solid-state welding
Welding costs
As an industrial process, the cost of welding plays a key role in manufacturing decisions. Different welding methods have different machine costs, labor costs, material costs, and energy costs, and as a result, cost considerations impact manufacturing decision making. Machine costs range from relatively inexpensive processes, such as shielded metal arc welding and oxy-fuel welding to methods like laser beam welding and electron beam welding, which are only used on extremely high production runs. Furthermore, the use of automation and robots can greatly increase these costs. Labor costs depend on several factors, including the deposition rate (the rate at which a mass of weld metal is melted), the hourly wage, and the operation time, which includes the time that the arc is struck and the handling time. The cost of materials includes the cost of the base material, the cost of any filler wire used, and the cost of shielding gases. Finally, energy costs depend on arc time and welding power demand, but these costs normally do not amount to more than several percent of the total welding cost.
For manual welding methods, labor costs often make up the vast majority of the total cost. Filler metal costs tend to increase when welding specialized materials, and machine costs only increase when automation enters the picture. As a result, most cost-savings measures are focused on minimizing the operation time. To do this, welding procedures with high deposition rates can be selected, and weld parameters can be fine-tuned to increase welding speed. Planning the work can also lead to an improvement in work efficiency and thus cost. Finally, welding time can be greatly reduced by mechanization and automization, but while these reduce labor costs, the cost of extra equipment and additional setup time must be considered.
Safety issues
Welding, without the proper precautions, can be a dangerous and unhealthy practice. However, with the use of new technology and proper protection, the risks of injury and death associated with welding can be greatly reduced. Because many common welding procedures involve an open electric arc or flame, the risk of burns is significant. To prevent them, welders wear heavy leather gloves and protective long sleeve jackets to avoid exposure to extreme heat and flames. Additionally, the brightness of the weld area leads to a condition called "arc eye" in which ultraviolet light causes the inflammation of the cornea and can burn the retinas of the eyes. Helmets with dark face plates are worn to prevent this exposure, and in recent years, new helmet models have been produced that feature a face plate that self-darkens upon exposure to high amounts of UV light. Welders are also often exposed to gases, such as nitrogen oxides, ozone and carbon monoxide, and fumes that can prove dangerous if ventilation is inadequate. Furthermore, the use of compressed gases and flames in many welding processes pose an explosion and fire risk if proper precuations are not taken, including the prevention of excess oxygen in the air and keeping combustible materials away from the workplace.
External links
References
- Blunt, Jane and Nigel C. Balchin (2002). Health and Safety in Welding and Allied Processes. Cambridge: Woodhead. ISBN 1855735385.
- Weman, Klas (2003). Welding processes handbook. New York: CRC Press LLC. ISBN 0-8493-1773-8.
- The Procedure Handbook of Arc Welding, Lincoln Electric, 13th Edition, 1994
Last updated: 10-22-2005 06:11:51
