Advanced Welding Processes: Technologies and Process Control

Monitoring and control of welding processes 199
Deviation monitors
Dynamic resistance. The quality of resistance spot welds may vary due to
the surface conditions of the plate being welded, electrode wear, electrode
force fluctuations, current shunting and transient changes in the welding
parameters. Various methods of monitoring spot weld quality in order to
detect the effect of these variations have been proposed but the dynamic
resistance technique is probably the most suitable for industrial application.
[219, 220] In this system, the instantaneous welding current and voltage are
measured and the resistance of the metal between the electrodes is calculated.
If computer-based techniques are used to collect the data, the resultant dynamic
resistance curve may be plotted on the monitor screen immediately after the
weld has been completed. The normal dynamic resistance curve for a
satisfactory weld in plain carbon steel is shown in Fig. 10.7. The curve may
be divided into three zones:
∑ Zone A: the area corresponding to the reduction of contact resistance;
∑ Zone B: the area corresponding to resistive heating and increasing resistivity
of the material;
∑ Zone C: the area corresponding to a reduction in thickness and weld
nugget growth with consequent steady reduction in resistance.
Changes in shape of the curve indicate deviations from the normal weld
nugget formation and, at the limit, a defective weld. A computer-based dynamic
resistance instrument [221] may be used to monitor the performance of the
process in the following manner.
The welding parameters are set and a test specimen is welded with the
monitor set in its ‘test’ mode. The welded specimen is checked visually and
by destructive testing and, if a satisfactory weld has been achieved, the
dynamic resistance curve which has been obtained is stored as a master for
the application (if the weld is unsatisfactory the parameters are adjusted and
a new test is conducted). With the equipment switched to its ‘monitoring’
mode, the dynamic resistance curves of subsequent welds are compared with
Resistance (mW)
A B C
Number of cycles
10.7 Typical dynamic resistance curve for a normal spot weld.