Frequently Asked Questions
When making spot welds to tube, an electrode or mandrel inside the tube would be the best solution. However, this is often impossible where the tube is of small diameter, long or shaped. In these cases, the electrode is used at the point of welding in contact with the sheet, and the current return may be an electrode or die on the reverse side of the tube. Alternatively, a second electrode against the sheet side of the tube may be used. By this means, series welding is possible, provided that the wall is sufficiently thick, relative to the sheet.
It is usually most convenient to use one electrode against the sheet and a return electrode from the back of the tube. In this case, the electrode against the sheet should be shaped to concentrate the current. A truncated cone is usually used, although a ball ended electrode, e.g. EN ISO 5182 type E or F, can be an advantage when the tube is flat. A larger flat electrode should normally be used to contact the tube. Alternatively a V shaped electrode can be used to nest the tube in the case of round tube, provided that the alignment of the tube is constant relative to the electrode. The V shape is preferable to a half cylindrical shape as a perfect fit is difficult to maintain.
As the tube wall thickness is unsupported under the weld point itself, the electrode force may need to be reduced compared to welding the equivalent sheet materials. Weld time should be similar to that used for the equivalent sheet materials, and welding current is adjusted to achieve a plug or button fracture on peel testing. The welding range or tolerance of the welding condition is likely to be reduced compared to welds in sheet material.
A fused weld nugget can be achieved if the tube is substantially thicker than the sheet, but poor fusion often occurs with thinner wall tubes. In general, a thickness ratio of tube wall to sheet thickness of at least 2:1 should allow good weldability. As this ratio is reduced, it becomes more difficult to achieve reliable weld formation. A coating (e.g. zinc) on the sheet and/or tube decreases the weldability of a given thickness combination.
When welding to round tube, the initial line contact of the tube with the sheet provides additional current concentration and assists the weld formation, allowing welds to be achieved at a thickness ratio less than two (for uncoated sheets). It is important to maintain good electrode alignment and accuracy of contact position when welding to round tube, otherwise variable weld quality and distortion of the weld area can occur.
Alternative methods of joining sheet to tube include arc spot, plug or laser welding.