Resistance Spot Welding
Spot welding is the best-known resistance welding method. It is used for joining thin sheet materials (up to 3 + 3 mm) by overlap joints, and is widely used, e.g. in the automotive industry. An ordinary private car can have up to 5 000 spot-welded joints. The high current, in combination with a rapid heating time, means that the thermal energy input is efficiently used: very little is conducted away to the surrounding metal. Spot welding therefore has several advantages over other methods of welding sheet metal, such as:
- Little deformation of the workpiece, as the thermal energy is more or less restricted to the immediate vicinity of the weld.
- Very high rate of production for mechanised processes.
- Easy to automate, with high consistency, which is therefore suitable for mass production.
- Low energy requirement and little pollution.
- Fast: resistance welding of 1 + 1 mm sheet, for example, takes 0.20 s.
- No filler materials required.
- Little special training required.
- Less environmental impact than when welding with an arc.
Two electrodes clamp the two sheets of metal together with a considerable force, while passing a high current through the metal. Thermal energy is produced as the current passes the electrical contact resistance between the two sheets.
(Weman K., Welding processes handbook, p. 80)
Resistance Seam Welding
Seam welding is used in the same way as spot welding, and operates on essentially the same principle. The difference is that two wheel-shaped electrodes are used, rolling along (and usually feeding) the workpiece.
The two wheels should be of the same size, in order to prevent the part from being deflected towards one of them. The actual contact profile can be designed in a number of ways, in order to suit the shape of the part to be welded. The current may flow continuously while welding is being carried out, or intermittently to produce a series of spots, so closely positioned as to produce a single, continuous weld. An unavoidable problem of seam welding is that some of the current 'leaks' through the completed weld.
As the electrode rollers rotate, they do not need to be lifted between each spot, as with spot welding. If the weld does not have to be continuous, seam welding can be used therefore to position spots some regular distance from each other, which can be carried out quicker than ordinary spot welding.
(Weman K., Welding processes handbook, p. 83)
Resistance Projection Welding
As with seam welding and spot welding, projection welding is used to join two overlapping sheets of relatively thin metal. The process involves pressing a number of 'dimples' in one of the plates, welding the two plates together at the same time.
The method can also be used for welding metal sheet to the ends of bars, rods or pipes, or for welding nuts to sheets. Wire grids (i.e. the crossing points of the wires) are also particularly suitable for projection welding.
An advantage of the process, relative to spot welding, is that there is less wear and tear on the electrodes due to the greater contact area.
(Weman K., Welding processes handbook, p. 83)
Dip Brazing
In dip brazing, either a molten salt bath or a molten metal bath accomplishes heating. In both methods, assembled parts are immersed in the baths contained in a heating pot. Solidification occurs when the parts are removed from the bath.
Dip brazing achieves fast heating cycles and can be used to braze many joints on a single part or on multiple parts simultaneously.
(Groover M. P., Fundamentals of modern manufacturing: Materials, processes, and systems ,p. 751)
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