Characteristics of Resistance Spot Welding for 1 GPa Grade Twin Induced Plasticity Steel

Abstract

For the last few decades, formability and weldability of steels have been intensively investigated to increase the productivity and the safety in automotive companies. Recently, steel which has both high ductility and high tensile strength is required to reduce vehicle weight. Twin induced plasticity (TWIP) steel has high tensile strength as well as improved ductility and is applicable to automobiles. Since, TWIP steel is an austenitic alloy which contains high content of Mn, it would have different resistance spot welding characteristics comparing with dual phase (DP) steel. In this work, TWIP steel, the newly developed steel, was introduced. To identify the resistance spot welding characteristics of 1 GPa grade TWIP steel, the experiments for both DP and TWIP steels were conducted. Resistance and power signals were measured to analyze the welding characteristics. The suitable welding ranges were also obtained with the lobe diagrams for these steels. In order to analyze the weld, shear tension and hardness tests, and a microstructure analysis were conducted. Both steels show a different microstructure in base metal, heat affected zone, and nugget. Because of the different microstructures between the two steels, the two steels showed different hardness distribution and tensile shear strength in the weld. Also, the differences in dynamic resistance signal, welding power signal, and welding heat input are shown between the two steels. These distinctions in microstructure and welding process characteristics caused the difference in suitable welding range. As well, this study provides guidelines to the application of DP and TWIP steels in resistance spot welding for vehicle manufacturing process.