Abstract
Dual phase (DP) steels have high strength, while maintaining outstanding elongation capacities. This is possible using a well-controlled thermomechanical process that produces a perfect phase combination in the DP microstructures. However, automotive makers are required to weld the DP steels, which generates a soft zone in the microstructure. In this work, 1.6 mm-thick DP980 steel sheets were welded by gas metal arc welding process to analyze the response of the welded soft zone to cyclic loading conditions. Conducted macrographic and metallography analyses revealed good quality in the appearance of the welded joints, with a complete fusion of the DP980 joint and without the presence of discontinuities. Low cycle fatigue tests of the DP welded joints were conducted under a constant amplitude strain control mode. The welded joints experienced a fatigue life reduction with respect to the DP980 steel of ~16% at strain amplitudes of 0.2, 0.3, and 0.4%. For strain amplitudes larger than 0.6%, the fatigue life of the welded joint was reduced by 39%. Weld thermal cycles combined with metallography analysis indicated that a tempered process of the martensite during the welding was responsible for the soft-zone formation and the poor fatigue response.
Highlights
Advanced High-Strength Steels (AHSS) is a group of steels widely used by the automotive industry because of their superior mechanical properties obtained by well-controlled thermomechanical processes and the resulting microstructures
Dual phase sheet steel of 1.6 mm-thick with a nominal tensile strength of 980 MPa were properly welded by the Gas Metal Arc Welding (GMAW) process using a heat input of 302 J·mm−1
The dual martensite-ferrite phases were extensively modified by the GMAW process, presenting a martensite content increment of 54% in the HT-Heat Affected Zone (HAZ) and 60% in the LTHAZ
Summary
Advanced High-Strength Steels (AHSS) is a group of steels widely used by the automotive industry because of their superior mechanical properties obtained by well-controlled thermomechanical processes and the resulting microstructures. The adequate control in the martensite formation, morphology, and distribution can improve the mechanical strength, while the ductility is still sufficient for the sheet metal forming process [1,3–5] Due to their high formability, the DP steels (and all the AHSS group) are suitable to fabricate lighter components with smaller thickness, reducing the final weight of the automotive body structure [4,6]. This weight reduction results in enhanced fuel efficiency with less exhaust emissions and allows a larger flexibility and the customization in the vehicle design process [1]
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