The improvement of weld quality in medium frequency direct current resistance spot welding.
Zinc coated steels are widely used in the automotive industry, because of the
improved protection against corrosion. Their use has consequences for the
resistance welding process, which is the most widely used method of joining body
panels. The zinc coating alloys with the copper electrode, resulting in increased
electrode wear, and a reduction in electrode life. The welding current must be
increased, because of the reduced contact resistance and thus heavier cables and
power sources are required.
A novel form of power source, the Medium Frequency Direct Current inverter, offers
advantages over the traditional AC transformer. The higher operating frequency
results in a lighter transformer, and a smaller welding current may be used, because
the DC welding current generates heat at a constant rate, and is thus more effective
than an AC power source. A potential advantage of this technology is that the
increased frequency allows improved resolution in monitoring and control.
Novel signal conditioning circuitry was developed, allowing significant improvement
in the time resolution of the voltage and resistance signals. A series of welding trials
was conducted, while monitoring the welding process. The correlation between weld
quality and various process variables was assessed, and a control algorithm to
compensate for electrode wear was proposed. This algorithm, based on a constant
voltage principle, was implemented on a bespoke welding timer. A significant
improvement in electrode life was obtained using this technique.
The control algorithm was shown effective experimentally, but practical limitations
do not permit testing under all possible conditions. A numerical model of the spot
welding process, using Finite Difference technique, was developed. Following
successful validation, the model was used to predict the performance of the control
algorithm under various conditions of electrode wear. The results indicate that a
constant voltage algorithm can compensate for an increase in electrode tip
diameter, but that a change in contact resistance may result in unsatisfactory welds.