Welding suitability of coated steel materials
In practice, mainly coated steel materials are used and processed. Galvanised versions predominate among the metallic coatings. With these types of coating, the electrical conductivity and thus the weldability are given. The thickness and uniformity of the coating influence the welding suitability. The thinner and more even the coating, the better. The coating metals usually have better electrical conductivity, are softer and have a lower melting point than the base material.
Depending on the thickness of the coating, the contact areas between the electrode and the sheet metal increase when the electrode force is applied, the current density decreases as a result and the welding current must be increased due to the lower contact resistances. In addition, during resistance welding, the coating creates foreign layers on the electrode working surface, which in turn lead to increased heating of the electrode working surface due to the increased welding current and thus to increased wear and a decrease in the electrode level.
This effect increases with the thickness of the coating. Special pre-treatment of the coated surfaces is not normally necessary. In principle, coated steel sheets can be processed with the usual resistance welding equipment. As listed above, however, it must be possible to guarantee the higher welding current and a higher electrode force. Sufficient electrode cooling and good setting and resetting behaviour of the electrodes have a positive effect on the standing quantity of the electrodes. The adjustment range of the welding parameters is generally narrower than for uncoated materials. To compensate for the influences listed above when resistance welding coated materials, controls with milling and stepper functionalities are recommended.
In addition, systems with constant current or better adaptive control become advantageous. Especially with adaptive control systems, the electrode levels can be positively influenced and the influences of coating variations can be compensated.
As always, you can find more information in the DVS leaflets and from your Harms & Wende partner.