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resistance welding[ri′zis·təns ‚weld·iŋ]
a method of welding metals in which the pieces are heated by an electric current passing through them at the point of contact and are also pressed together (upset). A distinction is made between butt welding and flash welding, depending on the method of heating. In resistance butt welding the joint is formed as a result of melting, tight squeezing together of the pieces to be welded, and crystallization of the metal in the form of a molten core. In flash welding the pieces simply touch, but because of the high current density at the points of contact, the metal is heated rapidly and is transformed into liquid dams, which spread out, forming a thin surface layer of molten metal. All metal still in a plastic state is removed during upsetting, and a welding joint is formed over the entire contact area.
Resistance welding is done on resistance welding machines. The welding transformer of the machine lowers the line voltage to 1–15 volts. Electrodes made of copper alloys are used to supply a current of 1–200 kiloamperes and to press the pieces together. The power of such machines is 0.5-500 kilovolt-amperes. A compressive force of 0.01-100 kilonewtons (1-10,000 kilograms-force) is provided by a pneumatic-hydraulic drive or by a lever-and-spring mechanism. The current is turned on for periods from 0.01 to 10 sec by electronically controlled contactors. Resistance welding machines currently being manufactured include stationary, movable, and suspended types, as well as general-purpose and special machines.
Types of welded joints produced by resistance welding include spot welds, projection welds (a type of spot weld), seam welds (roll welds), and butt welds. The most common type is the spot weld, which makes possible the production of up to 600 welds per minute. It is used for joining extremely thin parts of electronic devices (as thin as 0.02 micron), as well as for welding steel structural assemblies from sheets up to 20 mm thick for the construction of motor vehicles, aircraft, and ships and in manufacturing agricultural machinery. Projection welding is used to join parts along previously formed projections. Spot welds or a continuous seam weld along an annular projection may be produced with the same instrument in projection welding. In seam welding, a continuous tight seam is formed by partially overlapping spot welds. The electrodes are electrically driven rollers that not only press and move the pieces but also supply the current. Seam joints are used in manufacturing gasoline tanks, pipes, and pressure vessels. Butt welds are used in joining wires, rods, and strips, and well as articles consisting of unlike metals, and to join rails, gas pipes, motor-vehicle wheels, and various parts having a complex shape and made of high-strength steel or aluminum alloys. The use of butt welding assures continuity of many processes, such as rolling of metals. Welded cross sections range from 10 to 100,000 sq mm.
Resistance welding is a highly efficient metal joining process. It lends itself readily to automation and is efficient in mass and large-series production.
REFERENCEKhrenov, K. K. Svarka, rezka i paika metallov, 4th ed. Moscow, 1973.
K. K. KHRENOV
A process in which the heat for producing the weld is generated by the resistance to the flow of current through the parts to be joined. The application of external force is required; however, no fluxes, filler metals, or external heat sources are necessary. Most metals and their alloys can be successfully joined by resistance welding processes. Several methods are classified as resistance welding processes: spot, roll-spot, seam, projection, upset, flash, and percussion.
In resistance spot welding, coalescence at the faying surfaces is produced in one spot by the heat obtained from the resistance to electric current through the work parts held together under pressure by electrodes. The size and shape of the individually formed welds are limited primarily by the size and contour of the electrodes. See Spot welding
In roll resistance spot welding, separated resistance spot welds are made with one or more rotating circular electrodes. The rotation of the electrodes may or may not be stopped during the making of a weld.
In resistance seam welding, coalescence at the faying surfaces is produced by the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes. The resulting weld is a series of overlapping resistance spot welds made progressively along a joint by rotating the electrodes.
In projection welding, coalescence is produced by the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes. The resulting welds are localized at predetermined points by projections, embossments, or intersections.
In upset welding, coalescence is produced simultaneously over the entire area of abutting surfaces or progressively along a joint, by the heat obtained from resistance to electric current through the area of contact of those surfaces. Pressure is applied before heating is started and is maintained throughout the heating period.
In flash welding, coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from resistance to electric current between the two surfaces and by the application of pressure after heating is substantially completed. Flash and upsetting are accompanied by expulsion of the metal from the joint. See Flash welding
In percussion welding, coalescence is produced simultaneously over the entire abutting surfaces by the heat obtained from an arc produced by a rapid discharge of electrical energy with pressure percussively applied during or immediately following the electrical discharge.
Most metals and alloys can be resistance-welded to themselves and to each other. The weld properties are determined by the metal and by the resultant alloys which form during the welding process. Stronger metals and alloys require higher electrode forces, and poor electrical conductors require less current. Copper, silver, and gold, which are excellent electrical conductors, are very difficult to weld because they require high current densities to compensate for their low resistance. Medium- and high-carbon steels, which are hardened and embrittled during the normal welding process, must be tempered by multiple impulses.