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When a gap exists in a resistance spot welded (RSW) joint, the lack of intimate contact between the members at the faying interface can have a significant influence on sheet steel spot weldability, especially for advanced high strength steels. Several test joints simulating gap conditions observed in typical auto body structures have been designed for study of the effect of gap magnitude on the resistance spot weldability of such AHSS as DP980, DP780 and DP600. Tests show that an existing gap reduces the confinement of the molten zone during the welding process, increasing the probability of expulsion and therefore decreasing the current range. Increasing the electrode force helps reduce the gap influence and broadens the current range. Employing a longer weld time also has a beneficial effect when used in combination with an appropriate electrode force.

Resistance spot welding (RWS) guns driven by servomotors instead of pneumatic cylinders are called servo guns. They bring many new features to RWS process. In this study, the influences of servo guns on RSW process are systematically investigated based on comparative experiments. In addition, the costs of servo guns are also analyzed. The long-term applications of servo guns will be cost effective due to their technical features and savings on pneumatic systems although the acquisition cost of servo guns is high. Therefore, servo gun is an excellent alternative RSW machine for sheet metal assembly.