Study of electrode wear mechanism in consecutive spot welding -spot weldability of Zn-Ni electrogalvanized and organic-silicate composite steel sheets (report I)-

摘要

Electrode life is of significant concern in spot weldability of coated steel sheets. The factors which dominate electrode life are still indistinct, although there has been an extensive number of investigations. One of the reason for this may bethat electrode wearing process is not clarified. The purpose of this study is to define the electrode wear process and its mechanism focused on a distinction of alloying layers at electrode face during consecutive welding.Electrode life when consecutive spot-welding Zn-Ni electrogalvanized steel sheet is achieved over 10000 welds, whereas it is only 4000 welds for organic-silicate composite steel sheet. Electrode used on organic-silicate composite steel sheet exhibitedmore rapid wear than electrode used on Zn-Ni electrogalvanized steel sheet.In the case when welding Zn-Ni electrogalvanized steel sheet, the primary factor in electrode wear is a series of Cu (electrode material) /Zn (coating element) interaction, γCu-Zn formation on electrode face and its peeling off. Zn-Fe alloy is depositedand formed into a projection at the center of electrode face. This projection have a potential for keeping high current density when electrode wear is proceeding. This phenomenon can be thought of as one of the main factors responsible for goodconsecutive spot weldability.In the case when welding organic-silicate composite steel sheet, electrode/steel interface experiences high temperature exceeding the melting point of Cu-Zn during welding. Therefore, the principal factor of electrode wear would be a series of Cu/Zninteraction, alloying and melting. The locomotion of partial current-passed area is observed during consecutive welding. It would correspond to the movement of micro projection caused by partial current flow and melting on electrode face.