Spot Welding

Questions and Answers

Glidcop is a tradename for a dispersion strengthened copper material classified by the RWMA. The most common form is RWMA Class 20. It is frequently used to resistance weld galvanized steel. There are other uses but this is a common usage and will be assumed the subject matter for this inquiry.

RWMA Class 20 is used in a manner similar to RWMA Class 1 and Class 2. It is most often used at or near expulsion levels of the weld lobe. It’s claims to fame are resistance to heat and anti-sticking. At various intervals during it use it is changed and dressed and reused. The other more common method in automation, the robot moves the electrodes periodically to a dressing station while the next part is loaded.

Weld Lobe

The question asked is: what is the life expectancy of RWMA Class 20? The answer depends upon what is being welded, how close is the weld schedule is to the high end of the weld lobe, how much expulsion is being experienced, how much material is being dress away?

To start this off “RO” stands for Reverse Osmosis. This is a filtration process that uses a semi permeable membrane to filter out larger particles in the water. It removes most minerals. “DM” stands for Demineralized Water. It is also called Deionized water. It also removes the minerals from the water. As the name implies the ions are removed. The pro for both is they produce pure water.

AWS has a standard for water used in resistance welding equipment:

AWS Standard J1.2M/J1.2:2016 Guide to the Installation and Maintenance of Resistance Welding Machines

Yes, upslope might help in the reduction of expulsion or spatter during resistance welding.


It was described that the lower power applied during upslope gives the components a chance to anneal and relax and come into full contact before full power is applied. This reduces the possibility for material expulsion. An example is poor alignment or badly formed projections can sometimes be helped with upslope.

The answer is yes. MFDC can be used to heat 10 mm bars.

In this application the equipment could be running nearly continuously. In the design of the equipment care must be taken to make sure the equipment is cooled adequately to insure it does not overheat.  Additionally the MFDC equipment must be sized properly.

Properly cooled and sized MFDC will handle this job nicely and perform well.

This is a classical application of Joules Law:

Joules Law

The heat is being generated by the resistance to the flow of current through the material.


Reference: RWMA, Resistance Welding Manual 4th Edition

Voids and cracks are not desirable in any resistance welding nuggets. In some material systems there is a tendency for cracks to form – such as Aluminum. In other materials oxides and surface impurities can end up in the nugget and voids or cracks form. Some cracks or voids form due to the volume expansion and contraction from liquid to solid state.

Depending upon the material and product and the industry there are specifications. Aircraft has specifications. Automotive has specifications. Individual companies have specifications.

Per AWS D8.1 Specification for Automotive Weld Quality – Resistance Spot Welding of Steel

Automotive specifies that cracks or voids lengths combined in the specified weld nugget area cannot exceed 25% of the nugget width. Therefor 1/3rd the nugget diameter is not acceptable in the automotive industry.

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