A continuous weld made between or upon overlapping members, in which coalescence may start and occur on the faring surfaces, or may have proceeded from the outer surface of one membel: The continuous weld may consist of a single weld bead or a series of overlapping spot welds. See Figure S-2. See also ARC SEAM WELD and RESISTANCE SEAM WELDING.
Seam welds are made with resistance welding equipment in high-production manufacturing. Seam welds are typically used to produce continuous gas- or liquid-tight joints in sheet metal assemblies, such as automotive gasoline tanks. This process is also used to weld longitudinal seams in structural tubular sections that do not require leak-tight seams. A resistance seam weld is made on overlapping workpieces and is a continuous weld formed by overlapping weld nuggets, by a continuous weld nugget, or by forging the joint as it is heated to the welding temperature by its resistance to the welding current.
In most applications, two wheel electrodes, or one translating wheel and a stationary mandrel, are used to provide the current and pressure for resistance seam welding. Seam welds can also be produced using spot welding electrodes; this requires the purposeful overlapping of the spot welds in order to obtain a leak-tight seam weld. Two variations of this process are lap seam welding, using two wheel electrodes (or one wheel and a mandrel) and mash seam welding, which makes a lap joint primarily by high-temperature plastic forming and diffusion, as opposed to melting and solidification.
In mash seam welding, overlap is maintained by clamping or tack welding the pieces. The electrode wire seam welding process uses an intermediate wire electrode between each wheel electrode and the workpiece. This process is used almost exclusively for welding tin mill products to fabricate cans.
Butt joint seam welding is done with the edges of the sheets forming a butt joint. A thin, narrow strip of metal fed between the workpieces and the wheel electrode is welded to one or both sides of the joint. The metal strip bridges the gap between the workpieces, distributes the welding current to both sheet edges, adds electrical resistance, and contains the molten weld nugget as the nugget forms. The strip serves as a filler metal, and produces a flush or slightly reinforced weld joint.
Two seam welds can be made in series, using two weld heads. The two heads may be mounted side by side or in tandem. Two seams can be welded with the same welding current, and power demand will be only slightly greater than for a single weld.
A tandem wheel arrangement can reduce welding time by 50%, since both halves of a joint can be welded simultaneously. Thus, for a joint 182 cm (72 in.) long, two welding heads can be placed 91 cm (36 in.) apart, with the welding current path through the work from one wheel electrode to the other. A third continuous electrode is used on the other side of the joint. The full length of the joint can be welded with only 91 cm (36 in.) of travel. See RESISTANCEWELDING (RW) and TUBEMANUFACTURE.