Flash welding

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Flash welding is a resistance welding method that doesn’t use filler metals. Two metal pieces are kept apart by a small gap chosen based on thickness and the desired weld. An electric current runs through the pieces, and the gap creates resistance and heat, forming a high-temperature arc that melts the ends. When they’re hot enough, the pieces are pressed together and fused into a solid weld.

Two main timing factors matter: flash time and upset time. Flash time is how long the arc lasts. Upset time is how long the pieces are pressed together while they weld. The flash must heat the metal enough but not melt away too much. The upset time controls bonding and the expulsion of impurities. If upset time is too short, impurities may remain; if it’s too long, you can soften or distort the parts. In many setups, the process is controlled by how far the pieces flash apart (for example, a set distance like 5 mm) rather than by time, and upset is controlled by how far the pieces are pressed.

There is usually a hold time after upsetting to let the joint cool and finish bonding.

Flash welding is used to join railroad rails, creating long welded rails that are smoother and need less maintenance because there are no gaps. It can also weld dissimilar metals, including nonferrous metals, with inserts that preserve the properties of both parts—for example when connecting a switch or crossing to carbon steel rails.

Beyond railways, flash welding is used in the metal building industry to extend the length of angle iron, and in the aluminum industry to join aluminum, steel, and copper in busbars. Steel provides strength, copper conductivity, and aluminum offers a cost–conductivity advantage.