3.3.1. Weld ring
The ring feature, which appeared in the C-scan images and
thought to be associated with the spherical electrodes and
associated indentations, required further clarification. To establish
the nature of the ring, selected welds were milled on both
weld surfaces to completely remove all indentation. These samples
were then re-examined and it was found that a ring
corresponding to the former area of indentation was still evident
in the C-scan images. Further analysis suggested this ring was
likely to be caused by a feature of the spot welding process.
Fig. 12 shows the relationship between C-scan image and features
of the spot welding process; an 8.8 mm diameter ring, and the
weld nugget shown in the figure is correlated with the changes in
welding force, resistance and current. The use of spherical
electrodes results in dynamic changes in distribution of applied
force and consequently resistance as the weld current is applied.