Polarity in resistance spot welding has to do with the direction of the current flow in the welding process. With direct current (DC) the welding circuit can either be straight, or reverse polarity. When the machine is set for straight polarity, the current flows from the electrode to the weld surface and creates considerable heat in the metal. When the machine is in reverse polarity, the current is backwards and is flowing from the metal to the electrode causing a greater concentration of heat at the electrode. In an opposed weld, there are three material interfaces: two electrode-to-part interfaces and one part-to-part interface. Since conductive electrodes are generally placed against resistive materials, and vice-versa, it is not uncommon to have dissimilar junctions in welding (see blog Spot Welding? Remember the old adage 'opposites attract.') To take advantage of polarity, the negative electrode should be placed on the most resistive part and the positive electrode against the most conductive part. This will increase the heat at the interface of the two materials and reduce the heat at the electrode–to-part interfaces. Reversing the polarity will cause the material interface to be colder and may cause electrode sticking.
For more information read our paper 'The Effect of Polarity on the Resistance Welding Process'
And for more information about the resistance welding process read our Fundamentals of Resistance Welding.