Tailoring Electrode Life Expectancy in Pre-pulse Resistance Spot Welding of Coated High-Strength Interstitial Free Steel

The present work addresses the unique problem of predicting electrode life in resistance spot welding (RSW) of galvannealed (GA) coated high-strength interstitial free (HIF) steels at variable sheet thickness. The weld quality is assessed as a function of dynamic resistance (DR) for 0.7, 1.0- and 1.2-mm thickness HIF steels with 90 gsm GA coating. The pre-pulse and main welding pulse is optimized first to generate the weld growth curve. The high DR of thicker sheets promotes faster weld nugget growth and early expulsion. The expulsion current increases from 8 kA to 9 kA with a reduction in thickness from 1.2 to 0.7 mm. The finite element (FE) simulation indicates that the highest peak temperature is achieved at the sheet-sheet interface for the thicker substrate. The smaller conduction length of the thinner sheet accustoms to elevated electrode temperature for a considerable time and propagates the electrode wear. 0.7 mm sheet exhibits electrode life of only about 300 spots compared to more than 1500 for 1 and 1.2 mm thick sheets. The similar quantity of thin coating promotes better alloying for the thinner sheets than for thicker sheets. The electrode failure occurs early for 0.7 mm sheet due to the reduced current density and tip conductivity caused by significant deposition on the electrode surface. This study reveals that controlling electrode-sheet interface temperature is critical for 0.7 mm sheets to improve weld quality and enhance electrode life.