In projection welding the design or shape of the part is used to make discreet individual point or contacts to concentrate the current during the welding process. In some applications a full ring projection or part edge is used to form a complete seal. The TO-5 transistor can is an example of a hermetic sealed device. The full edge of the bottom comes in contact with the lid and a full hermetic ring weld is formed. The actual current flow is controlled by the part design. The electrode is there to deliver force and current into the two work pieces and provide follow up force as the weld is made. Frequently Class 2 & 3 electrodes are used. In nut welding applications Elkonite facing electrodes are commonly used on the sheet steel side. The use of dissimilar electrode materials in projection welding is common. Normally it is done to improve mechanical wear or to resist the heat being generated.
Projection welding traditionally has been performed using AC controls and power supplies. The welders frequently have been traditional press welders and recently fixed station weld guns. both provide good alignment and can be designed for low inertia fast follow up systems. Rocker arm machines are not a good choice because they apply force in an arc and do not hold good alignment. In all cases fast follow up cylinders or servos are necessary for good quality.
With the advent of MFDC (Mid Frequency) controls and their accuracy these units are a logical player in projection welding. To make them suitable in this application, fast rise time technology has been developed because standard MFDC has a slow current rise. Another system that has shown some application for nut and stud welding is CD (capacitive discharge welders). This technology has been around for years. In the recent years CD has evolved into higher energy machines capable of nut and stud welding.
Welding Journal, July 2017
Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Manual Section 3
Tuffaloy Product Inc. Catalog
CMW Inc. Resistance Welding Products Catalog
A resistance braze offers a few advantages over traditional spot welding:
First it allows the joining of materials that otherwise cannot be resistance welded. Generally they are very conductive materials and hard to get hot enough to form a weld nugget. Coppers and brasses would be an example of these.
Secondly a resistance braze can be performed at a lower temperature than a spot weld. We do not have to bring the work piece up to near its melting point. We only need to bring the braze shim to its melting point which is much lower. This means the work piece can preserve some or all of its mechanical properties. Post heat treatment may not be necessary.
The third advantage is that we can control the area of heat application to a small area of the part. We are only heating to a modest temperature and we can protect other areas if desired with cooling. The rest of the part stays strong for the end application.
Reference: CMW Products Inc. Catalog
Tuffaloy Products Catalog
RWMA, Resistance Welding Manual. Section 1, Chapter 6
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.
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.