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Electric welding






This type of welding is a pressure process, as is forge welding. It consists of heating to their plastic temperature the surfaces of parts to be joined, then applying pressure mechanically to achieve complete union of the parts. The heating is accomplished by passage of a heavy, localized electric current. Alternating current of common commercial voltage is employed for this purpose.

In all types of resistance welding, selection of proper electrodes is important to the success of the process. Electrodes must possess a high degree of thermal and electrical conductivity, and they also must possess strength and hardness.

Small pools of molten metal are formed at the weld area as high current (1000-100, 000 A) 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.

Types of resistance welding are: spot welding; butt welding; flash welding; percussion welding; seam welding; projection welding.

Spot Welding. — Spot welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously 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 made by industrial robots.

A spot-welding machine is the simplest form of resistance-welding device; therefore it is widely adapted throughout the metal-working industry.

Butt-Welding. — By this process the parts to be welded are gripped mechanically and pressed together while heat is ge­nerated by passage of a heavy current through the area of the joint.

Butt welding is used extensively for resistance welding work. Pipes, tubing, bars, rods, light- and medium-weight structural shapes, etc., may be butt welded.

Flash Welding. — This is another type of butt welding, the difference consisting in the method of heating the base metals. The electrodes are clamped to the parts to be joined, and a heavy current is passed through them. The movable electrode brings the surface of the metal it holds into contact with the surface of the metal held in the stationary electrode. Just be­fore the two pieces meet, an arc developed in the short gap causes sufficient heat to make the surface to be joined plastic.

Percussion Welding. — This process is similar to flash welding inasmuch as it relies upon a sudden discharge of electrical energy (produced by the arc) at the joint to generate sufficient heat to make the surfaces to be joined plastic. The pressure required to accomplish the weld at the proper moment is obtained by employing a heavy spring.

Seam Welding. — This is a resistance-welding process which consists of passing two or more base metals between electrodes which are in the form of rollers. The current is transmitted through the electrodes to the metals. The mechanical pressure required for forcing them together, as well as the current is transmitted by the roller-shaped electrodes. The welded joint thus produced is similar to a seam made in cloth.

Refrigerators, metal containers, gasoline tanks, automobile mufflers, aircraft fabrications, pipes for furnaces and stoves, and other products requiring pressure-tight seams are rapidly produced by seam welding.

Projection Welding. — This process is similar to spot welding and consists of joining two pieces of metal, one or both of which have mechanically formed projections. The purpose of the projection is to localize the current.

The current and pressures required for this type of welding are heavier than those applied in spot welding. Various sheet-metal parts used in automobile and aircraft production are welded by this process.

Electric Arc Welding. — This is a fusion-welding process in which surfaces to be joined are fused by the heat of an electric arc. By bringing the work and the electrode together as conductors, an electric circuit is established; then, by separating the conductors, an electric arc is created in which the electrical energy is converted into heat.


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