What is electric resistance welding? Find out the
application, process, and cost. Learn about spot welding and seam welding,
advantages and disadvantages of electric resistance welding.
Electric resistance welding (ERW) produces coalescence of
faying surfaces. Heat to form the weld comes from electrical resistance of
material, rather than time and force. Factors influencing weld temperatures
include work piece proportions, coating, electrode (material, geometry, pressing
force), weld current, and weld time. Small pools are formed at the point of
most resistance, the connecting surfaces. As a high current passes through
metal, electrical resistance results.
Advantages
ERW results in efficient energy use, limited work piece
inconsistencies, high productivity, easy automation, little air pollution, and
no filler requirements. A properly welded seam is often stronger than the
original material. Durability is a result of a joint forged from heat and
pressure.
Cost
Equipment cost can be high. Cost per weld can be as little
as four cents. Depending on application and manufacture rate, a return on your
investment may still be seen in short order. Spot welding is also more cost
effective than riveting.
Spot Welding
Resistance welding joins overlapping metal sheets, studs, projections,
electrical wiring hangers, some heat exchanger fins, or some tubing. Power
sources and welding equipment should be sized to material and material
thickness. Two copper electrodes clamp the sheets together and a current passes
through. Heat is generated at the point of highest electrical resistance, where
the surfaces touch. The rising temperature causes increased resistance and a
molten metal pool between electrodes. When the current stops the metal solidifies
under pressure. The process is accelerated by water cooled copper electrodes
that remove surface heat.
Seam Welding
ERW seam welding produces a weld at the faying surface. Two similar
metals may be seamed at a butt joint or an overlap joint. Seam welding is often
an automated process. Butt welding is similar, but differs in that butt welding
welds the entire joint at once. Seam welding starts at one end and forms a weld
progressively.
Seam welding relies on two electrodes. Usually made from copper,
these electrodes provide the pressure or resistance as well as the current. The
disc shaped electrodes rotate as the material passes between them. There is constant
contact with the material for long continuous welds. In some cases the
electrodes help move the material.
The seam is created when the electrical resistance of the
work piece is higher than the rest of the circuit. The joint reaches melting
point by the current. Semi-molten surfaces are pressed together. The welding
pressure creates a fusion bond.
Power Sources
Power sources include a transformer that supplies energy. The
weld joint may be infused with low voltage or high current AC power.
Limitations
Thickness is limited by welding power source output. If excessive
heat is applied or heat is applied too quickly expulsion may occur. If the
force between base materials is too low, the coating is too thick or too
conductive, the high pressure will escape the containment force of the tips. Thinner
metal will have less strength than welds without expulsion. ERW solid state
bonds may result in joints with poor peel strength. Reflow braze bonds joining
two dissimilar materials may result in bonds with poor peel and shear strength.
The reflow braze process also requires a longer heating time.