Seam welding uses a wheel-shaped electrode (Fig. 9.4) to make either a series of overlapping spot welds to form a continuously welded and leak tight seam or a number of spot welds spaced apart - roll-spot welding. The requirements on electrodes and surface finish are the same as for spot welding. The shunt effect of the closely spaced nuggets and the short weld times mean that higher currents are necessary than for spot welds. Typical welding parameters are given in Table 9.4.
Higher welding forces will be needed for harder alloys and lower values for softer alloys. Welding parameters for three phase frequency converter
9.4 Typical resistance seam welder showing the copper wheel electrodes. Courtesy of British Federal.
Table 9.4 Seam welding conditions. Single phase AC units. Hardened 5XXX series alloy
units are similar to those in Table 9.4 except that welding current needs to be increased by between 0.5 and 2.5 times, the higher values for the thicker materials.
Pick-up on the electrode wheel can be a problem and may require the wheel to be cleaned after only one revolution. Mechanised cleaning systems that remove the contamination in-process by wire brushing or abrasive means have been successful in maintaining continuous production.