Seam welds are a series of spot welds in a straight line. They can be overlapping or separated. Overlapped welds are used in fluid tight application and spaced welds in normal structural weld situations. A washing machine drum would be a water tight application. If the welds are separated the process is sometimes referred to as roll spot welding.
Seam welding will generally use the same alloys as spot welding. The higher conductivity lower strength workpieces will be welded using Class 2 and the stronger work pieces will use Class 3. As in the case of spot RWMA Class 2 is the most used product for seam welding. Most seam welding wheels are ideally cross forged to break up the original cast billet grain structure. There are two schools of thought on this forging. One is to forge blanks after being sliced from a cast billet. The other is to forge the cast billet then slice the blank. Both processes break up cast structure but to different degrees. The individual forged blank process will inherently cost more since you forge each blank individually. If you have a difficult heavy wheel wear application you might want to consider paying the extra dollars.
Some wheels are made by cutting ends off rod and others by cutting out of plate. Neither of these products would be expected to hold up in heavy duty applications.
Seam welding is very similar to spot welding. There is still an electrode but it is two opposing wheels rather than two pointed electrodes. Force and current is applied through the wheel just as in spot welding. The control timing and wheel speed determine the spacing of the spots. They can be made to overlap and be fluid tight or separated. If separated this is called a roll spot weld. Fluid tight applications could be dishwashers, washing machines, tin cans and metal drums.
Repeat or pulsing refers to repeating the weld and hold portion of a weld schedule. Normally there is a very short off period of one or two cycles with continued application of force between the two weld sequences. This gives you the ability to dissipate the heat energy a little further into the part away from the center of the weld nugget.
Aluminum has a tendency to form voids or cracks in the weld nugget as it solidifies. These voids are generally not desirable. In many cases this condition can be improved with good force follow up or application of a secondary forging force near the end of the weld cycle.
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