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Summer 2020 Contents: Issue No. 47
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Electrode Life Issues in Resistance Welding
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Resistance welding is one of the few welding processes where the welding energy, in the form of electrical current, is transferred to the parts being welded through direct physical contact via welding electrodes. Given that resistance welding typically results in welding temperatures at or close to the melting point of parts being welded, and given the intimate contact between electrodes and parts, the electrode tips are also exposed to very high temperatures during the welding process. Such thermal exposure, in addition to the high welding forces and chemical reactivity between electrodes and parts, can lead to rapid degradation of the electrode surfaces, which can then result in frequent stoppage to clean/replace the electrode tips. In this newsletter, we will discuss the factors that can damage the electrode surfaces and steps to be taken to avoid such damage.
- Welding force too low
If the welding force is set too low, the welding current faces greater resistance as it jumps across the electrode/part interface. Such high resistance at the interfaces, also known as contact resistance, can lead to overheating/arcing at the interface and cause sticking of the electrode to the part. In such situations, welding force should be increased in conjunction with other parameters to produce an acceptable weld without electrode arcing/sticking. - Welding force too high
Excessively high welding force can cause the electrode tips to flare out over time and increase the contact area. High welding forces may also produce electrode surface cracks or even cause small parts to the tip to break off, especially if they are made of tungsten or molybdenum alloys. Reducing welding force will help the situation, but only to the extent that the force is still sufficient to provide necessary coalescence to form strong weld. - Weld head follow-up is poor
One of the main functions of the weld head is to make sure that the welding force is consistently applied during the welding cycle; also known as follow-up. If follow-up is poor, the electrode/part interface resistance can increase substantially and again lead to excessive heating and arcing at the interface. Usually, spring-loaded weld heads have the best follow-up. - Welding current too high
In order to make a weld in the shortest amount of time, a user may set the corresponding current a bit too high such that even if welding force and follow-up are satisfactory, heat generation at the electrode/part interface is excessive and can lead to arcing/sticking of the electrodes. In such instances, the welding current should be reduced and welding time extended suitably. - Welding current rise time too fast
If the welding current increases too rapidly (even if the maximum current is set to reasonable value), there will once again be too much heating at the electrode/part interface and lead to arcing/sticking. A good rule is to have the rise time set at about 20% of the total welding time. - Electrode/Part reactions
In some applications, the electrode reacts with the part surface and forms a layer of reaction products that will sometimes adhere to the electrode tip surface. These reaction products are often quite resistive and lead to additional heating of the electrode/part interface leading to formation of more reaction products. The chain reaction continues to a point where the electrodes have to be replaced or cleaned. Common situation where such reactions make life difficult for the welding personnel include welding galvanized steel, welding aluminum, and welding nickel plated components; all using copper electrodes. Selection of electrode material is based on multiple factors including chemistry, conductivity, and strength; surface reactions are sometimes unavoidable and steps, such as electrode cooling discussed below, have to be taken to reduce detrimental effect of reactions. - Electrode cooling
Sometimes the only way to avoid reaction product buildup is to keep the electrode tip as cool as possible; a cold tip is less likely to react with the part surface. Cooling is provided by circulating chilled water right up to the electrode tip; the tips are designed to be easily replaceable. When the electrodes are less than 0.5” in diameter, it is difficult to have cooling water circulated up to the tip. For smaller electrodes, typically 0.25” and under, cooling is provided by the body of the electrode itself while the tip is narrowed down to required size. Such electrodes can also be composites, with tips made of molybdenum/tungsten while the body is made of copper for cooling. - Surface contaminants
It is quite common to have parts that have surface residue from prior processing step; can be oils, plating bath residues, or deposits from vapors from adhesive processes. Such contaminants can burn and form a sticky carbon residue that will have to be frequently cleaned from the electrode surface. Obvious solution is to wash the parts before welding. Manual handling of the parts and associated risk of transferring finger oils is usually not a problem for welding. However, if parts are considered at risk of such contamination, use of gloves provides sufficient protection. - Electrode surface finish
Surface roughness is an important factor in controlling electrode/part interface resistance. In order to keep the resistance low, the electrode surface should be polished to a smooth finish. In practical terms, a polish with a 600-grit sandpaper is quite sufficient. However, it is important to use proper tooling to produce a clean finish without changing the surface profile. It is quite common to have profiled electrodes, often a dome shape tip, and it is important to maintain the required profile during any re-grinding/re-surfacing operations.
Electrode cleaning, polishing, resurfacing, and replacing are all activities that are an added cost to the welding operation, both in terms of materials and time. A welding engineer would be wise to pay proper attention to choosing the correct electrode material, geometry, cooling, welding equipment, and process parameters so as to have an efficient welding process.
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