The location of the weld nugget is determined by the locating the resistance center of the part being welded. The “Relative Resistivity Diagram” explains this graphically.
Every component and contact point in the weld has a resistance and contributes to the weld nugget location.
By definition - If the upper and lower halves of the resistances are equal, and RI is the largest Resistance then the weld will occur in the center. If there is any imbalance then the nugget will move up or down toward the hottest spot. This is called heat balance.
The most common cause of displaced weld nuggets are:
Poor Heat Balance – This is caused by any factor which affects the relative resistance diagram above. This could be electrode face size and material, work piece thickness and material, number of thicknesses, arrangement of multi thickness work pieces and forces applied.
Electrode Misalignment – Misalignment will change the working face size touching the part and cause the desired current to flow through a smaller or larger face area which changes the heat pattern. The smaller surface area of course would get hotter.
Poor Fit Up – If the part does not fit up and the welder is being used as a forming tool then part of the desired forging force has been spent on forming and is not available for the welding operation. This leads to higher contact resistances Rc and nuggets tend to form near that part surface.
Other factors which can play a part but are less prevalent are both low and high conductivity electrode materials. Either variation from the desired level will cause the heat to flow toward or away from that side of the part.