Method and system for welding workpieces

An apparatus for welding two or more workpieces that reduces temperature and residual stresses at an associated workpiece-to-electrode interface, the apparatus comprising:

• a motion-controlled welder having a welding electrode and a power source productive of weld current to the electrode;
• a processor in signal communication with the motion-controlled welder for controlling stroke and force of the electrode, and for controlling weld current to the electrode;
• a temperature sensor disposed to sense a temperature of the workpieces, and being in signal communication with the processor for communicating temperature information thereto;
• wherein the processor is responsive to executable instructions when executed thereon for:
• applying, during a first period of time, a first electrode force to the workpieces to be welded, and applying a weld current that causes heating of an associated workpiece-to-workpiece faying surface, the first electrode force being applied at a first electrode stroke;
• determining a temperature of the faying surface indicative of a weld nugget formation thereat, and in response thereto reducing the electrode force to a second level during a second period of time while maintaining a constant electrode stroke;
• during a third period of time, further reducing the electrode force to a third level while simultaneously reducing the electrode stroke to a second level;
• and
• stopping the welding after the third period of time, resulting in a weld joint having reduced residual stresses and reduced likelihood of stress crack formation.