Deformation and fracture of miniature tensile bars with resistance-spot-weld microstructures
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I. INTRODUCTION
DUE to the combination of high strength and ductility, dual-phase steels are being actively investigated for future automotive applications.[1] The term “dual-phase steel” refers to the predominance of two phases in the ferrous microstructure, viz., the relatively soft body-centered-cubic ferrite, and the relatively hard body-centered-tetragonal martensite. The beneficial ferrite martensite mixture in dual-phase steels is typically produced after annealing in the so-called intercritical temperature range, where ferrite and austenite are stabilized. This annealing is immediately followed by rapid cooling (or quenching)[2] to transform the austenite into martensite by displacement. To create adequate compromises on strength and ductility, dual-phase steels are fabricated with fine ferrite grains decorated with various amounts of coarse, segmented-looking martensite islands. Compared to precipitation-strengthened or solid-solution-strengthened lowalloy[3] steels, dual-phase steels possess a slightly lower initial yield strength (YS), a continuous flow behavior due to sufficient active slip systems in the ferrite phase, and a more uniform and higher total elongation.[1] These last two properties explain the good formability of the dual-phase steels which, when combined with high strength, have made them appealing to the automotive industry. Like other automotive materials, dual-phase steels must be resistance spot weldable, meaning that welds fabricated in these new automotive materials must fulfill a range of requirements.
WEI TONG, Associate Professor, HONG TAO and NIAN ZHANG, Graduate Students, and XIQUAN JIANG, Postdoctoral Fellow, are with the Department of Mechanical Engineering, Becton Engineering Center, Yale University, New Haven, CT 06520-8284. Contact e-mail: [email protected] MANUEL P. MARYA, formerly Researcher, Department of Metallurgical and Materials Engineering, Colorado School of Mines, is Senior Materials Engineer, NanoCoolers Inc., Austin, TX 78735. LOUIS G. HECTOR, Jr. and XIAOHONG Q. GAYDEN, Staff Research Scientists, are with the Materials and Processes Lab, General Motors R&D Center, Warren, MI 48090-9055. Manuscript submitted October 15, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
Of all joining processes, resistance spot welding is the most established in the automotive industry, where it has been used for decades to join steel sheets.[4,5] In this robust process, high electrical currents are forced between two axisymmetric copper-based electrodes such that sufficient resistance heating is generated at the contacting interfaces of overlapping sheets, to melt and solidify mixed volumes of the various sheets within a fraction of a second. Due to the rapid heating and subsequent cooling, weld microstructures are considerably different from pre-existing microstructures.[4–7] In dual-phase steels, strengthening occurs in most of the weld joint as the overall fraction of martensite is dramatically increased. In steels, the extent of this microstructural strengthening, as we
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