Experimental study and finite element analyses of electrode wear mechanisms during the resistance spot welding of galvannealed steel

The wear mechanisms of electrodes used on resistance spot welding of galvannealed steels were studied. The study focused on the inter-relationship among the steel properties, welding parameters and electrode wear.; Six different galvannealed steels were studied using a standard constant current welding test. With the same kind of Cu-Zr electrode, the tests were performed with the electrode force fixed at 600 lbs and the welding time fixed at 12 cycles for all the steels studied. The welding current is set at just below the expulsion limit for each of the steels. The microstructure and mechanical properties of these steels were examined by SEM and microhardness tests. The face profiles for electrodes subjected to various numbers of welds were examined using carbon imprint tests and low magnification optical microscopy. The alloys formed on the electrode face were studied by the EDS and WDS quantitative analyses and linescans. Changes in the microhardness of the electrode material near the electrode face during the electrode wear process were also studied. Combined with the experimental examination, a sequentially coupled finite element analysis procedure was used to analyze the detailed distribution and evolution of the electrical current, temperature and stress throughout the process of making a weld. These analyses have greatly enhanced the understanding of the experimental observations.; The results of this study indicate that the welding current is the dominant factor influencing electrode life. When the electrode force and the welding time are fixed, the welding current is determined by the steel properties. Thicker steel sheets and higher steel sheet surface hardnesses will result in smaller welding current. When the electrode force and welding time are fixed, steels requiring higher welding currents will yield shorter electrode lives. With increasing welding current, the top and bottom electrodes in this study showed increasingly different wear behaviors. Electrodes used on steels requiring higher welding current developed an edge pit near the top electrode periphery, that gradually evolved into a large pit at the bottom electrode face center. Toward the end of the electrode life, these electrodes result in a "three spots" feature nuggets that are shifted toward the top electrode. This kind of electrode failure is characterized by "nugget shifting". In contrast, under smaller welding current, the top and bottom electrode wear approximately the same. As the electrode face diameter increases gradually, not all the face area is in good contact with the steel sheet during welding. The localized contact areas for the top and bottom electrodes are symmetric and become fragmented gradually toward the end of the electrode life. The fragmentation of the electrode face areas which can be in good contact with the steel sheet results in the shunting of the welding current. When the electrical current density is reduced to a certain level by the shunting effect, the nugget can not be formed and the electrodes are declared to be failed. This kind of electrode failure is characterized by nugget shrinking. The welding current also has a big influence on the other electrode wear mechanism as the electrode mushrooming, face alloying, etc.