When two materials need to be joined together in a strong, rupture-resistant way, resistance welding gets the job done through heat and pressure. Electrical current creates heat, while electrodes or other equipment apply pressure. In this way, two pieces of material (usually metal but sometimes plastics as well) are bonded together. The five different methods of resistance welding are best suited for specific types of materials, projects and joints.
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Resistance Spot Welding
Spot welding bonds together two or more metal sheets, which are held in an overlapping position between a pair of welding electrodes, one fixed and one mobile. When heavy current is directed through these electrodes, the top electrode also simultaneously adds downward pressure. The result is a weld that takes place at the spot between the two electrodes. To create further welds, the sheets are moved and repositioned.
Spot welding is commonly employed in the manufacture of automobiles, aircraft, steel household furniture and steel containers.
In spot welding, the welder can position welds strategically in small spots along the workpieces. This allows for more control and a fairly uniform line of welds along a specific edge axis on the piece that's being welded.
Spot welding generally comes at a lower cost, both in equipment and labor. Spot welding doesn't generally require the advanced level of skilled training needed by more precise or complex welding work. Additionally, spot welding is generally faster than other types of welding operations.
However, more skilled labor could be required for the ongoing maintenance of spot-welded pieces. Additionally, spot welding won't work if the pieces to be welded are of any significant thickness.
Resistance Projection Welding
Used primarily in the electrical, automotive and construction fields, projection welding connects metal sheets or component parts using electrodes. These electrodes are applied directly to the pieces to be joined together. Then, opposing forces are run through the electrodes.
The advantages to projection welding include flexibility, as the welder can weld more than one spot at a time. Additionally, the welder can position welded spots more closely to each other than is possible with spot welding. Finally, welds will look somewhat neater and less obtrusive than with spot welding.
However, projection welding can't be used on metals. Additionally, projection welding can wind up costing more, due to the higher investment in the required equipment.
Resistance Butt Welding
In butt welding, the adjoining of two surfaces takes place simultaneously over the entirety of the affected workpiece surfaces instead of in small spots.
The heat that's required to produce that connection - or what's termed coalescence in welding jargon - is created by the electrical resistance that's produced by resistance to the current that's being passed through and between those two surfaces as they face each other.
In a basic butt weld, the two pieces to be welded are first brought together under pressure. Current is then applied, heating the contact area enough to allow the applied pressure to forge the parts together. In other words, a butt weld is a single-stage operation of both current and pressure.
The welder continues to apply both pressure and current until the affected area becomes pliable and softened. The continual, even application of pressure eventually creates the welded joint, which is typically quite smooth and even.
Butt welding is most often utilized in wires and rods with small diameter measurements.
Flash Butt Welding
Flash butt welding is like butt welding, described above, except that here the welder joins the two pieces by applying light pressure then passing a heavy current through the joint. The flashing actually burns away surface irregularities.
Flash butt welding is what most people think of when they hear the word "welding." This process produces an arc at the joint that's being welded. After the weld generates enough heat, the parts are physically connected together with pressure and the current being applied at the same time.
Flash welding is often used for welding together chain links, shaft axles and rail ends. It's also used in large building construction, some types of bodywork in automobile repair and in other situations where the welder needs to join very large pieces of metal together in a secure, stress-resistant way.
Flash welding is a faster type of resistance welding since the welder doesn't need to prepare the surface to be welded as with other methods. Additionally, it generally uses less energy and costs less to finish a job. However, given the arcing it produces, it does carry a much higher risk of fire. Because of the flashing process, some metal will be lost from the surface. Finally, it can be difficult to maintain the alignment of the pieces being welded together.
Resistance Seam Welding
Seam welding is just what it sounds like: a method used to weld shut the seam that exists between two metal pieces.
Seam welding uses wheel-shaped electrodes to produce longer, continual welds for a stronger joint. While some joints need to be welded by a skilled individual welder, seam welding lends itself well to an automated machine process. The result is fast and accurate welding with incredibly strong joints. Of course, machine-welded joints are always visually inspected and reinforced by spot welders, when necessary.
Seam welding can also produce multiple parallel seams with less overlap compared to spot or projection welding methods. Additionally, seam welding produces joints that won't allow the escape of gas or liquid contents.
There are a few disadvantages: It generally costs more to achieve a seam welded joint than it would through other methods. Additionally, seam welding is only appropriate for straight axis seams. Finally, it can be difficult to create a joint for pieces thicker than 3 millimeters.