The single-parameter, in-process monitor and automatic control systems for the resistance spot welding process have been studied by many investigators. Some of these have already been commercialized and used by sheet metal fabricators. These control systems operate primarily on one of the three process parameters: maximum voltage or voltage drop, dynamic resistance, or thermal expansion between electrodes during nugget formation.Control systems based on voltage or dynamic resistance have been successfully implemented for industrial applications. A great amount of experience on these two control methods has been accumulated through trial-and-error approaches. The expansion-based control system is not commonly utilized due to lack of experience and understanding of the process.Since the expansion displacement between electrodes during welding responds directly to the weld nugget formation, this control parameter provides a better means to produce more precise spot welds. However, the control algorithm of this method is more complex than that for the other two methods. Fruitful development of a such system can not be obtained by trial-and-error approaches.This paper presents a systematic approach to develop the expansion-based control algrithm for resistance spot welding. The finite element method was used to simulate the welding process and to determine the physical response of the joint material to the various welding conditions. Direct correlations between nugget formation and expansion displacement between electrodes were obtained.By systematic computer simulations, the weldability characteristic curves for resistance spot welding were developed. These weldability curves show inadeguate welding conditions which would cause nugget expulsion and current shunting. A welding duration curve, which shows appropriate time for squeeze, weld, and hold cycles, was developed and used as a basis for in-process resistance spot welding control.