Microstructural and mechanical characterization of welded joints on innovative high-strength steels

The car-bodies are more and more frequently constructed with innovative high-strength steels, both to reduce the vehicles weight and to improve the passenger safety. The car-body parts are cold formed from steel sheets (manufactured by continuous casting, hot and cold rolling, and continuous heat treatment), and assembled by resistance spot welding or less frequently, by laser welding The welded joints obtained with the latter methods on two high-strength steels, with similar ultimate tensile strength, are examined here. An innovative, 18% Mn, austenitic TWIP steel, which exhibit a very good combination of strength and ductility, is compared with a widely used DP steel, which consists of ferrite and martensite obtained by intercritical heat treatment and fast cooling The size and shape, defects, and microstructure of each welded joint are evidenced by metallographic examinations. Moreover, the tensile properties and the stress-life fatigue behavior of both as-received and welded specimens are compared. The fatigue strength exhibit a sharp reduction after the resistance spot welding, whereas laser welding has a much smaller effect.