As a basic principle, the workpiece surfaces should be dry and bare and not have any coatings unsuitable for welding. Dust and fingerprints have just as negative effects on the welding result as oil and deep drawing agents. Paint and anodization may even prevent the lighting of the drawn arc. After the welding process, an insufficiently prepared surface can be recognized by a dark circle around the welded stud. As a rule of thumb, the workpiece must be cleaned the more thoroughly the faster the welding process is conducted.
The greatest challenge when welding aluminum is the natural oxide layer that forms on the light metal under the influence of oxygen. Oxides in the welding zone may have the following effects:
- Decreased hardness/increased brittleness;
- Higher risk of incomplete fusion;
- Increase of the arc blow effect.
The latter occurs when the electrical arc is deflected from its center axis due to changes in the surrounding electromagnetic field. Arc blow can cause asymmetrical melting, or an unfavorable burn through, as well as welding mistakes, such as increased porosity, an uneven weld collar or undercut bases, which have a negative effect on quality.
To remove the oxide layer, it is possible to use mechanical methods (e.g., chip removal) as well as chemical treatments. Manual grinding and unsuitable abrasives, on the contrary, can have negative effects. This may result in scratches on the surface of the oxide layer, which make it harder to remove impurities. Insofar as other methods do not qualify, it is recommended to leave the aluminum oxide on the surface of the workpiece.
During drawn-arc stud welding, a white residue appears around the welded stud. This should be removed prior to welding in the same area to avoid scattering results.
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