Resistance spot welding method

A method of resistance spot welding to join materials to be welded by squeezing the materials between a pair of electrodes and passing current while applying pressure, the materials being a plurality of overlapping metal sheets, the method comprising:

• performing test welding to form an appropriate nugget comprising:
• passing welding current with constant current control between electrodes in two or more welding stages,
• observing an electrical property between the electrodes,
• calculating a time variation of an instantaneous amount of heat generated per unit volume and a cumulative amount of heat generated per unit volume from the electrical property between the electrodes, and
• storing as a target value for each stage the time variation of the instantaneous amount of heat generated per unit volume and the cumulative amount of heat generated per unit volume that are calculated from the electrical property between the electrodes;
• subsequently, performing actual welding in two or more welding stages corresponding to the two or more stages in the test welding using, as a standard, a time variation curve of the instantaneous amount of heat generated per unit volume obtained by the test welding;
• and
• performing adaptive control welding as a time variation amount of an instantaneous amount of heat generated per unit volume deviates during any stage of the two or more stages in the actual welding from the time variation curve by a difference, the adaptive control welding controlling a current passage amount in order to compensate for the difference during a remaining welding time in the stage so as to match a cumulative amount of heat generated in the actual welding to the cumulative amount of heat generated that is determined in advance in the test welding.