Friction stir welding of advanced high strength (bainitic) steels for automotive applications

Abstract

Friction stir welding has been emerging to be the most prominent and sought after metal joining technology as it leaves the signatures of minimal phase transformation during welding and precise weld joint. On the other hand, Bainitic steel has been the candidate steel for automotive applications as it fuses and balances the characteristics mechanical strength and deformability. It can also be tuned playing with various strengthening mechanisms. Most importantly, in automotive sector, the ductility of cut-edges (stretch-formability), fatigue, and welding (about 300 spot welds in a car) are quite essential. In this connection, bainite can help in optimizing these key contents. Advanced bainitic steels (microalloyed bainitic steels, and carbon-free bainitic steels) are also being developed to improve the formability performance, sensitivity to damaging systems, and toughness compared to traditional bainitic steels. The present paper emphasizes the feasibility of friction stir welding on bainitic steel through depth of penetration at different stir speeds. The optimization of stir speed was done through profiling temperature changes (thermography) that occur during welding. The microstructural analysis was done to analyze the resulting phases during welding. To support the microstructural analysis, microhardness evaluation was also undertaken at all weld conditions.