Spot welding (or resistance spot welding, RSW) is the most applied resistance welding process for joining sheet metals. The spot welding process starts with pressing two pointed copper alloy electrodes at the location to be welded. While two electrodes hold metal sheets, the welding current flows through the spot and generates heat. The heat will melt the metal and form a weld nugget at the interface of metal sheets. The weld nugget size is largely dependent on the electrode tip contact area.
It has been used for many years in many industries.It is the predominant joining process for assembling car bodies and large components in the automotive industry. At the same time, the aerospace industry uses this method for interior components. Also widely used for manufacturing of furniture and domestic equipment etc.
The focus on high quality and developments quickly to the end-user, means that spot welding issues need to be solved quickly and in a cost-efficient way.
Materials for spot welding
The material’s resistivity influences heat generation. The thermal conductivity and the heat capacity influence the heat transfer. Generally, high resistance and low thermal conductivity metals are the best for spot welding.
– Steels and AHSS
Low-carbon steels, stainless steels and the new generations of advanced high strength steels (AHSS) are generally good materials for spot welding due to their low thermal conductivity and high resistance.
Spot welding zinc-coated steel is possible but it needs different welding parameters and more frequent electrode tip dressing. During the welding process, the zinc coating will be alloyed with the copper electrodes. The copper electrode degrades more rapidly due to this surface contamination which can result in poor weld quality. Slightly high weld force is used to squeeze out the melted zinc layer and a higher welding current may be necessary to compensate the weld force.
– Aluminum Alloys
Aluminum alloys have lower resistance and higher thermal conductivity which means it conducts electrical energy and heat very easily. The low melting point of aluminum alloys can cause the sheet materials sticking to electrodes. Therefore electrode cleaning or tip dressing is critical in spot welding of aluminum alloys. To weld aluminum, a very high current and short weld time are required. It also means cooling the electrodes is very important.
– Copper Alloys
Copper alloys have low resistivity and high thermal conductivity, so it is difficult to generate heat even with a high welding current, and heat will quickly transfer away. They are rather difficult to weld with resistance spot welding. Instead, copper and copper alloy are excellent materials for electrodes.
In the electrical industries, welding or brazing copper alloy is very common for electrical circuit breakers and switches. Projection welding with contact designs is one way to concentrate the heat for resistance welding. Also, surface coating or braze material such as brass, silver alloy or nickel with less conductivity can increase resistance at weld interface thus facilitate welding of copper alloys. For welding copper alloys, tungsten electrodes are often used to generate more heat in electrodes and then transfer to the joint interface.
Spot welding problems
The commonly encountered problems include the following:
- electrode misalignment
- shunt current loss between welded spots
- cold weld (or false weld) especially in spot welding of multiple sheets
- spot welding of dissimilar metals such as between aluminum and steel sheets
- electrode indentation on sheet surface at the welded point
- surface coatings
- surface oxides and contact resistance
- dynamic resistance
The solutions for problems
The most cost-efficient way to solve these problems is to work with simulations in combinations with a real live test. You need to focus on:
- optimization of the spot welding process
- interactive CAE design with real welding process conditions
- welding simulation enhanced artificial intelligence
You can optimize the process in a short time with software, whereas if you need to guess based on your experience it may take a lot of time and material before you have reached your goal.