An understanding of these features provided confidence to
assess the remaining welds produced at different weld currents.
Fig. 9 shows the diameters of a series of scanned welds against
weld currents. The calibration factor used for all measurements
was derived from the milled area, which has a pre-defined
15.0 mm diameter. The weld diameter quoted is an average of
the minimum and maximum co-ordinate values measured by the
square box as shown in the figure. As expected, an increase in the
size of weld nugget and indentation ring with respect to increasing
weld current can be clearly seen. The results also suggest a
tendency for weld nuggets to grow preferentially from one side of
the indentation ring. These tendencies are attributed to several
factors. As the current increased the amount of heat input
increased, leading to the weld nugget expanding and filling the
area of indentation. In addition, the variation of surface characteristics
of the sheet and the electrode; for example, sheet surface
lubricant distribution, uniformity of pre-treatment and electrode
Fig. 3. Typical A-scan and C-scan results. alignment, affects the position of highest resistance, and thereforealter weld initiation point. The fact that the welds appear to have
initiated from indentation ring may also be attributed to the local
change in welding force, and is explained in detail in Section 3.3.1.