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Slot Welds and Plug Welds
Plug welds and slot welds are specialized types of welds that are made to join two pieces of overlapping metal together, without the need to lay down a fillet weld along the edge of the metal. These welds may be used for a variety of reasons. One is aesthetic – a properly laid slot or plug weld may be almost invisible once you finish the metal piece with paint or other coatings, whereas a weld bead is always visible to some extent.
Another possible reason is that a weld bead along the edge of the metal might cause problems with other fittings in some circumstances. Slot welds and plug welds are used frequently in automotive welding applications, where a large weld bead running along the edge of two panels would often be either hideously ugly, or would interfere with fitting the car’s parts together successfully because they would create a raised edge.
Making plug welds
Plug welds are fairly straightforward to make, considering that the main task in producing them is to drill a hole in the upper piece of metal so that the weld can be made through both pieces of metal. The overlapping piece is pierced with a series of holes for plug welds, while the overlapped piece is typically left intact. Plug welds are only slightly stronger than simply bolting or riveting the pieces of metal together, and cannot be used for areas subject to heavy loads, but they still have many effective applications.
The holes for the plug welds must be wide enough so that welding in their confines can be successful. If the holes are narrow and deep, like a well, then they will fill up with filler metal, but the heat of the arc will be unable to penetrate to the overlapped piece of metal and create a weld. The hole must be sufficiently broad so that the arc will melt not only the filler metal but into the overlapped metal as well. This will ensure that the weld provides a good, solid bond between the two pieces.
Plug welds are typically made in rows, like rivets. You can even drill through the horizontal plate in a tee joint and plug weld it to the edge of the vertical plate in order to hold this plate in place from yet another direction.
Making slot welds
Slot welds are long, narrow slots in the overlapping metal which work much the same as plug welds. In effect, they are also a form of lap welding, just as plug welds are. A slot weld must be wide enough to provide heat flow sufficient to involve the overlapped metal plate in the welding pool also, for a good, solid bond. Slot welds are usually aligned in the direction in which forces will act on the weld, though this varies occasionally depending on the specific application you are using them for.