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Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including open air, under water and in outer space. Welding is a potentially hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation.

Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join iron and steel by heating and hammering. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and electric resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding, electron beam welding,electromagnetic pulse welding and friction stir welding in the latter half of the century. Today, the science continues to advance. Robot welding is commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality.

Energy for welding

Every kind of welding needs to use energy. This energy is usually heat, but sometimes force is used to make a weld. When heat is used, it can be from electricity or from fire.

Power supplies for arc welding

A lot of electricity is used in arc welding. Some kinds of welding use alternating current like the electricity that buildings use. Other kinds use direct current like the electricity in a car or most things with a battery. Almost all kinds of welding use a lower voltage than the electricity that comes from a power plant. Arc welding requires using a special power supply that makes the electricity from the power plant usable for welding. A power supply lowers the voltage and controls the amount of current. The power supply usually has controls on it that allow these things to be changed. For kinds of arc welding that use alternating current, sometimes the power supply can do special things to make the electricity alternate differently. Some power supplies do not plug into a power plug, but instead generate their own electricity. These kind of power supplies have an engine that turns a generator head to make the electricity. The engine might run on gasoline, diesel fuel, or propane.

Energy for other kinds of welding

OFW uses a flame from burning fuel gas and oxygen to heat up the metal. This fuel gas is almost always acetylene. Acetylene is a flammable gas that burns very hot, hotter than any other gas. That is why it is used most of the time. Other gases like propane, natural gas, or other industrial gases can be used too. Some kinds of welding do not use heat to make the weld. These kinds of welding can get hot, but they do not make the metal melt. Forge welding is an example of this. Friction stir welding is a special kind of welding that does not use heat. It uses a very powerful motor and a special spinning bit to mix the metals together at the edge. This seems odd because metals are a solid. this is why it takes a lot of force to do and is very hard. The energy for this kind of welding is mechanical energy from the spinning bit.