When spot welding sheet material through an adhesive (or sealant), it is important that the adhesive is displaced consistently at the interface under the influence of the electrode force.
The adhesive paste should have a sufficiently low viscosity to allow this squeeze-out, but it should not slump or run out of the completed joint before the adhesive is cured. Single part, heat curing adhesives are normally used as they have a consistent viscosity. However, the temperature of the adhesive can have an effect. Two part, cold curing adhesives may begin to cure between mixing and final welding, especially if there is a delay in assembly of the components. This could cause weldability problems.
The result of the adhesive imparting a higher electrical resistance at the interface is to create more heat in the weld, with a higher risk of weld splash and variability of weld quality.
In some cases, the adhesive in the flange can react some of the electrode force applied and thus effectively reduce the contact force between the metal surfaces. Consequently, a higher force may be preferred to compensate for this and improve local squeeze-out of the adhesive. Also, the establishment of a steady contact resistance may take longer than with bare metals, so the squeeze time may need to be increased.
In order to allow for an initially high resistance, upslope of current has been suggested as a means of establishing good contact prior to the high welding current pulse.
Production problems that can occur include variability in the volume of adhesive applied (which may affect the squeeze-out characteristics) and contamination of the electrode tips. Adhesive on the tips is likely to cure and present a high resistance contact surface, which could lead to a 'blow-out', causing damage to the tips and the component.
Weldbonding (knowledge summary)