79 1. INTRODUCTION The continuous development of advanced high-strength steels (AHHSs) for combining high strength and high ductility with suitable impact resistance for automobile applications requires the study of weldability [1]. Automobile manu- facturers should improve the fuel efficiency and safety standards of their vehicles [2]. To address the above issues, it’s very important to design and select a material with high strength, forma- bility and also good weldability [3]. Thus, trans- formation induced plasticity (TRIP) steels have received widespread attention for automotive application due to their excellent combination of strength and ductility. The microstructure of these steels consists of ferrite, bainite, martensite and more than 5% of retained austenite. The retained austenite is the most important phase in TRIP Abstract: In this study, the effects of the second pulse resistance spot welding on the microstructure and mechan- ical properties of transformation induced plasticity 1100 steel were evaluated. The thermal process after weld- ing was designed to improve metallurgical properties with pulse currents of 6 kA, 9 kA, and 12 kA after initial welding with 10 kA current. The effect of the second pulse on mechanical and microstructural properties was investigated. The fracture of the welds was for pulsed samples of 6 kA and 9 kA pull out with mechanical test. Due to the existence of the microstructure including the equiaxial dendritic and 'ner in fusin zone in the pulsed current of 9 kA, the maximum fracture energy, and maximum force were observed. A signi'cant decrease in the FZ hardness in 6 kA current was observed in the nano-hardness results, which was attributed to the existence of martensitic and ferrite temper. The highest ratio of CTS/TSS was obtained for 6 kA and 9 kA, respectively, and force-displacement evaluation was maximum in 9 kA. 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. steel because its strain-induced transformation to martensite during deformation causes work hard- ening of steel, which delays the onset of necking and eventually leads to a high ductility [4]. The main method used for sheet metal joining in the automobile industry is the resistance spot welding (RSW). This is because of its reliability, fastness, and suitability for automation [5]. The alloy content and high strength of AHSSs limit their weldability and often fracture modes with low-bonding energy are seen. In general, in the RSW, there is a critical weld nugget size, where the fracture mode changes from the low-energy interface fracture (IF) to the ductile peripheral fracture or pull out (PO) mode [6]. In recent years, studies have been done on the pulsed welding and heat treatment after welding on TRIP steels. For example, Balltazar Hernan- dez et al. [7], performed resistance spot welding Iranian Journal of Materials Science & Engineering Vol. 16, No. 2, June 2019 RESEARCH PAPER The Assessment of Second Pulse Effects on the Microstructure and Fracture Behavior of the Resistance Spot Welding in Advanced Ultrahigh-Strength Steel TRIP1100 I. Hajiannia 1,* , M. Shamanian 1 , M. Atapour 1 , R. Ashiri 2 and E. Ghassemali 3 *i.hajiannia@ma.iut.ac.ir Received: June 2018 Revised: September 2018 Accepted: October 2018 1 Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran. 2 Department of Materials Science and Engineering, Dezful Branch, Islamic Azad University, Dezful,Iran. 3 School of Engineering, Jo¨nko¨ping University, Jo¨nko¨ping, Sweden. DOI: 10.22068/ijmse.16.2.79 Downloaded from ijmse.iust.ac.ir at 9:18 IRDT on Saturday July 4th 2020 [ DOI: 10.22068/ijmse.16.2.79 ]