In resistance brazing we are not spot welding but we still are passing modest currents so an adequate force is necessary to avoid arcing. The main goal of the force is to hold the parts together for current flow and avoid arcing so the braze alloy is not arced out of the joint. This is usually a modest force developed by trial with set up parts. Joule Heating H= I2rt will then generate the energy necessary for the braze material to melt. When molten braze metal is visible in the filet the current can be turned off and allowed to cool under pressure until solidified. Then the electrode can open and the part is ready to be ejected or removed. The temperature that the part reached at the joint is lower than it would have been if spot welded but it will be hot so caution is required.
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.
Projection welding schedules can be developed using standard spot welding schedules. For a single projection the weld time, current and force can be based upon available spot weld schedules. When welding multiple projections are being welded at one time and they are relatively far apart multiply the current times the number of projections. If the projections are close together and aligned, the total current will be reduced. Weld force is determined in the same manner. Weld time is the same no matter how many projections. Remember that in set up you always start on the cold side of power and work up to where you think you need to be to avoid damage to product and for the safety of people.
Weld schedules are available in the references below for some standard materials.
AWS and RWMA both publish spot welding schedules that can be used to develop a projection welding schedule. They also have published projection welding schedules for many materials.
Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Manual Section 3
Tuffaloy Product Inc. Catalog
CMW Inc. Resistance Welding Products Catalog
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.
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