The Assessment of Second Pulse Effects on the Microstructure and Fracture Behavior of the Resistance Spot Welding in Advanced Ultrahigh-Strength Steel TRIP۱۱۰۰

In this study, the effects of the second pulse resistance spot welding on the microstructure and mechanical properties of TRIP۱۱۰۰ steel were evaluated. The thermal process after welding was designed to improve metallurgical properties with pulse currents of ۶kA, ۹kA and ۱۲kA after initial welding with ۱۰kA current. The effect of the second pulse on mechanical and microstructural properties was investigated. The fracture of the welds was for pulsed samples of ۶kA and ۹kA PO with CTS test. Due to existence of the microstructure including the equaxial dendritic and finer in FZ in the pulsed current ۹kA, the maximum fracture energy and maximum force were observed. A significant decrease in the FZ hardness in ۶kA current was observed in the nanohardness results, which was attributed to existence martensitic and ferrite temper. The highest ratio of CTS / TSS was obtained for ۶kA and ۹kA, respectively, and force displacement rate was maximum in ۹kA. The fracture surfaces included dendrites and dimples. The results of partial fracture revealed separation in the coherent boundaries of the coarse grain of the annealed region.

Keywords:

resistance spot welding , second pulse current , fracture surface , shear tensile , dendritic microstructure