Electric resistance welding
[1] Electric resistance welding is widely used, for example, in manufacture of steel pipe and in assembly of bodies for automobiles.
[2] The electric current can be supplied to electrodes that also apply clamping pressure, or may be induced by an external magnetic field.
In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials.
The thickness is limited by the output of the welding power source and thus the equipment range due to the current required for each application.
When the current is passed through the electrodes to the sheets, heat is generated due to the higher electrical resistance where the surfaces contact each other.
When the current is stopped the copper tips cool the spot weld, causing the metal to solidify under pressure.
Because both tests are destructive in nature (resulting in the loss of salable material), non-destructive methods such as ultrasound evaluation are in various states of early adoption by many OEMs.
The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials.
When high strength in shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting.
Typically the process requires a longer (2 to 100 ms) heating time at low weld energy.
A transformer supplies energy to the weld joint in the form of low voltage, high current AC power.
The joint of the work piece has high electrical resistance relative to the rest of the circuit and is heated to its melting point by the current.
Most seam welders use water cooling through the electrode, transformer and controller assemblies due to the heat generated.
[3] Electric resistance welded (ERW) pipe is manufactured by cold-forming a sheet of steel into a cylindrical shape.
Initially this manufacturing process used low frequency AC current to heat the edges.
Over time, the welds of low frequency ERW pipe were found to be susceptible to selective seam corrosion, hook cracks, and inadequate bonding of the seams, so low frequency ERW is no longer used to manufacture pipe.
