What is resistance welding?

Resistance welding is a thermoelectric process in which heat is generated at the point where the parts to be joined are joined. An electric current is passed through the parts under pressure for a precisely controlled time. It is called resistance welding because the resistance of the workpieces and electrodes is used in combination or in opposition to generate heat at their joint. Resistance spot welding is one of the pressure welding processes.

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What is resistance welding?

Resistance welding makes use of the fact that materials have a higher electrical resistance at the pressure points when they are heated under pressure. This allows them to be welded there with an applied current.

The materials and their melting temperature are known. Theoretically, the current that must be introduced into them to enable sufficient heating with the existing resistance can then be calculated. In reality, however, practical experience is used, as the calculations quickly become complicated due to different conditions that influence the amount of energy required.

These conditions include the fact that the welding current is passed through machine elements made of different materials, such as copper and brass alloys. This changes the resistances, the mass, the distribution of the mass and the cross-sections of the machine elements through which the current flows. The material itself has its own resistance values and also its own specific heat capacities, which must also be taken into account.

This includes the fact that when using high-frequency alternating current, impedances occur in the machine elements, which also have an influence on the continuous current. Furthermore, there are additional resistance values at each connection point that is present in the current flow. If single-phase alternating current is used, the inductive components in the secondary circuit must also be taken into account.

This makes it very easy to see that it would be a very complex undertaking to try to analyze the different boundary conditions precisely. In practice, it is therefore better to rely on experience, empirical data and information from laboratory tests. Rules are also provided in tables for this purpose.

What are the advantages of resistance welding?

Various advantages can be identified for resistance welding. These look like this in practice:

• No additives: Usually, no filler materials need to be added during resistance welding. This applies to both the filler material, which eliminates a cost factor, and the shielding gas, which also reduces costs, but also the impact on the immediate working environment and the environment.
• Automation capability: It is relatively easy to use automated processes for resistance welding. They have become widespread in the industry over the past decades and can be used quickly and, above all, cost-effectively.
• Very short welding times: The welding process itself takes place in just a few milliseconds. This means that the release of thermal energy is concentrated accordingly, which limits distortion in the workpieces.
• Good controllability: Resistance welding is very easy to control. This allows it to be carried out very precisely as a process and the success is very easy to reproduce. Especially in the area of medium frequency welding, it is easy to set and adjust the welding power precisely. The force, voltage and penetration depth can be measured during the work. This allows the welding parameters to be continuously monitored. This makes it easier to control the process and allows an assessment to be made after the work has been carried out.
• High energy efficiency: This point may be surprising, especially when you know that currents of up to 100,000 amperes are used. However, what makes the process so energy-efficient are the short welding times and the targeted introduction of the current. This ensures a high level of efficiency with very low losses.
• No change to the workpieces: No additives are added. This means that the elements in the workpieces do not change, even at the seams. Furthermore, there is no increase in weight.

How is resistance welding used in practice?

In theory, resistance welding is very easy to carry out. The workpieces are pressed together with an appropriate force. The current is applied and the connection is made. In practice, however, there are many different influences that can impair the welding result. Therefore, these must be taken into account from the outset and the process must be controlled accordingly in order to exclude these influences.

Start with the mains voltage. It is essential to keep this stable. If there are voltage fluctuations that exceed a value of ± 6%, the weld seam will be unsafe.

How does the resistance welding process work?

The process of resistance welding is highly dynamic. This is also due to the fact that the electrodes move towards each other during the work. In order to achieve a constant result, the contact forces must act constantly on the contact surfaces and these must not move.

The pressing forces on the surface must also be sufficiently high. This is the only way to avoid the shearing effects that would otherwise occur when the current is introduced.

If the sheets to be welded are unclean or already slightly scaled, the contact pressure must be increased. The current itself must be moderately high and the welding time must be extended.

Once the current has been applied, it is important to allow the spot welds to cool down properly. To do this, the pressure must be maintained. This is the only way to achieve sufficient seam quality and avoid the formation of impurities.

The electrodes should be applied vertically to the welding material. The effective surface must be flat or spherical, depending on the welding task. If necessary, the electrodes should be replaced, not re-filed.

If the sheets have a thickness of more than 3 mm, it is better to select the multi-pulse welding method. This reduces the penetration depth while at the same time increasing the welding reliability.

It is possible to join metal sheets with very different thicknesses. Even ratios of 1:20 are no problem. In this case, the point diameter must be determined according to the thinner sheet. However, it is possible to increase this by up to 20% in order to achieve an appropriate result.

What does the material rule look like?

It is also essential to consider the material to be welded when selecting the welding parameters. The heating that can be achieved depends on the resistance in the material. There are two simple rules of thumb for this. According to the first rule, the better the material conducts the current, the more current is required. According to the second rule, the welding current can be weaker if the melting point of the material is lower.

The higher the requirements for the quality of the welding result, such as uniformity, safety or beauty, the more important metrological monitoring is. The focus must not only be on the electrode. The current must also be monitored, as it has the greatest influence on the result.