The goal of a resistance brazing schedule is to bring a thin sheet of braze alloy to its melting point and form a fillet around the periphery of the part being joined. The squeeze cycle is basically the same as in spot welding – put the parts under the desired holding force. The off cycle is the same – cool and resolidify the braze material. What varies is the weld or heating portion of the cycle. Spot welds generally take a few cycles, or milliseconds. The time is generally less than a second. A resistance braze frequently will take several seconds up to many. It depends upon the size of the part and how conductive the material is.
Many small parts can be run automatically similar to a spot weld. Other larger parts can be automated also. In some cases infrared cameras watch the fillet area for braze material temperature. These can communicate with the weld control and maintain this temperature as desired. If resistance brazing two modest size copper parts braze times of 20 – 30 seconds are not unusual. In a job shop situation for short runs the heat input sometimes is controlled manually to not overheat the electrodes and machine systems. It may be manually pulsed. For longer runs automated systems can be developed.
The other difference in resistance brazing is that some brazes also employ a flux. This also has to melt and dissipate and be activate at the time braze material melts. This adds to the complexity of visual observation.
Of coursed the goal is to end up with the desired braze joint thickness of 0.0015-0.003” for best strength. This is only accomplished by control of the welding force at a low level. A large force can push most of the braze alloy out of the joint. Holders are available for this force control if your machine cannot be dialed down.
Low Force Adjustable Holder
Reference: CMW Products Inc. Catalog
Tuffaloy Products Catalog
RWMA, Resistance Welding Manual. Section 1, Chapter 6