Projection Welding

Questions and Answers

Resistance brazing is not spot welding so the electrode face geometry does not have to concentrate the heat into a small area. Frequently the electrode face is machined to conform to the surface that it is in contact with. Frequently the electrode will contact the entire work piece surface. In some cases the electrode faces is milled out so the work piece can be nested into the face of the electrode in order to insure proper part placement and alignment. A small vacuum is sometimes pulled through the center of the electrode to hold the part in place before the electrode closes. This is very common for small part brazing.

Resistance brazing is a variation of the resistance welding process. Joules Law H= I2 RT provides the heat just as in spot welding. There is a squeeze weld and hold cycle. There are electrodes. To differentiate the process a thin piece of braze allow is placed between the parts being joined. The goal is to heat the braze material until it melts and bonds to both mating substrates. Then the current is turned off and the braze material solidifies. The result is a brazed joint bonding the parts together. Low force holders are frequently used.

Follow up describes the ability of the welding equipment to maintain full force while the projection weld collapses. This is very critical because any variation will change the contact resistance and thus the heat being generated in that projection. Generally this would result in more heat and the projection might overheat and start to expel molten material. If the material expels there may not be anything left to form a nugget. This weld nugget collapse occurs very quickly and some large press welders have excess weight to move causing inertia or a faulty gun cylinder may stick and not be able to follow up properly.

Low weld strength can be caused by anything which retards weld nugget growth or formation. It can also be caused by the condition of the materials being welded.

Expulsion of molten particles from between the two sheets of metal being welded can be caused by several items. All are related to causing excessive heat between the two work pieces or not containing the weld nugget during welding.

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