Weldability of electrogalvanized versus galvanized interstitial free steel sheets by resistance seam welding

In this research, joining of interstitial free (IF) steel with two kinds of galvanized and electrogalvanized coatings was investigated using resistance seam welding in an automotive fuel tank. In this welding process, heat was generated by resistance against the flow of current; then the joint was made by simultaneous application of pressure. Therefore, the most important factors in this process were welding current, welding time, welding speed and electrode force. This research, was based on the effects of two parameters, welding current and speed on microstructure and mechanical properties (shear-tensile strength and hardness) of the weld joint. The results, showed the decrease of nugget size and increase of joining zone thickness in each galvanized and electrogalvanized sheet by raising welding current at low speeds. With the increase of welding speed in the constant current, nugget size decreased and thickness of joining zone increased. In each galvanized and electrogalvanized sheet, hardness grew from the base metal towards the center of the weld. Therefore, the maximum hardness always was in the center of the weld. In a comparative mode and constant current of 23,600 A, the electrogalvanized sheet needed more welding speed for obtaining approximately equal strength to the galvanized sheet. Also, at equal speed, the electrogalvanized sheet needed lower welding current in order to gain approximately equal strength to the galvanized sheet. With increasing the welding current, grain size increased and as a result, the brittle fracture increased.