Frequently Asked Questions
Resistance spot welding involves melting a weld nugget of the interface of the sheets to be joined. Relatively high electrode forces are used to contain the melted material and to minimise heating between the electrode and the sheet surface. An indentation is normally produced on each sheet surface during welding, up to 10 or 20% of the sheet thickness, when the softened material is squeezed by the electrodes.
In some cases, a good surface appearance is required on one side of the joint (face side). While it is virtually impossible to eliminate the mark completely when making welds to the normal weld diameter requirements, there are ways of minimising the marks.
- On the face side, a flat electrode or backing bar can be used. This must have a smooth surface and be well aligned to both the sheet itself and to the opposing electrode tip. Good cooling is an advantage.
- A self-aligning flat electrode can also be used to allow some self-alignment to the sheets being joined. However, they have a limited movement capability and poorer cooling capability compared to a solid electrode. It is still important that the opposing electrode is well aligned.
- Series welding enables welds to be made against a flat backing bar. For thin sheet, this bar is normally copper alloy and provides the current link between welding points. If the face side sheet is sufficiently thick, then an insulating backing can be used and the welding current flows through the sheet itself to the return electrode.
- Projection welding can be employed and the projections are punched in the sheet opposing the face sheet. A number of welds can be made simultaneously. A large flat electrode on the face side can minimise marking, as with spot welding, provided that the mechanical wear of the electrode (in the form of a shallow indentation) is controlled.
Coated steels present a particular problem for making minimum marking welds as the alloying of the coating with the electrode causes rapid electrode wear and the surface of the sheet shows the marking of the worn surface in the face electrode.
In summary the main factors in minimising marking are:
- Good alignment of the electrode contact faces and workpiece.
- Smooth surface of the face side electrode.
- Good cooling.
It is important to check that weld formation is not affected when using electrodes with a dissimilar contact area. The position of the nugget tends to form nearer the electrode with the smaller contact area. This can be a particular problem when the face side sheet is thinnest.
A useful compromise for controlling marking in special cases could be to use an electrode with a large face radius on the face side to achieve a smooth shallow indentation with good visual appearance.