Evaluation of Formability and Mechanical Behavior of Laser-Welded Tailored Blanks Made of Interstitial-Free and Dual-Pha
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A laser-welded tailored blank (LWTB) can be made up of two or more steel sheets of different thicknesses and strengths welded together and subsequently formed to the required shape. In a LWTB, thinner material can be placed in less critical regions with the benefit of weight reduction, while thicker and stronger material can be positioned in more critical areas. Thus, using LWTB, weight reduction of vehicles can be achieved without compromising the crashworthiness. The weld in a LWTB is most critical. Overall performance of the blank largely depends on this weld. Therefore, prior to successful implementation, the weld properties must be extensively studied, especially when one part of the LWTB is made up of advanced highstrength steel such as dual-phase (DP) steel. Static testing of laser welds has been carried out to establish a data bank for design purposes. However, it is unwise to use static test data in vehicle structural design where cyclic service loads must be considered. Fatigue properties of laser welds are reported in the literature. Ono et al.[1] have also studied the fatigue properties of LWTB and observed that, in a differential thickness joint, fractures occurred on the thinner sheet side, and they have concluded that differential thickness joints have lower fatigue strength than equal thickness joints. This is obvious since, because of the higher strength and hardness of the weld, fatigue deformation will concentrate on the lower thickness sheet side. To assess the SUJIT CHATTERJEE, RAJIB SAHA, and M. SHOME, Researchers, and R.K. RAY, Visiting Scientist, are with R&D, Tata Steel, Jamshedpur-831007, India. Contact e-mail: sujitchatterjee123@ gmail.com Manuscript submitted September 20, 2008. Article published online March 6, 2009 1142—VOLUME 40A, MAY 2009
fatigue properties of the weld, it should be ensured that the fatigue failure takes place only through the weld. The impact property of the weld is also needed to be evaluated as it is important for assessing crashworthiness. Bayraktar et al.[2] have studied the impact properties of different laser welds using soft steels and reported the ductile brittle transition temperatures. However, the impact property of LWTB using DP and other high-strength steels has not been reported yet. Due to the inherent nonhomogeneity, flow of material in LWTB during forming is considerably different from the flow of homogeneous blanks. Abundant research works related to LWTB forming are available in published form.[3–8] Most of this work has been carried out with low-strength formable grade steels. Lisok and Piela[3] studied the drawability of LWTB and observed that, during drawing, the weld line moves toward the material with greater strength or thickness. Sunders et al.[4] studied the influence of weld location on formability of LWTB using extra deep drawing (EDD) and high strength low alloy (HSLA) steel sheets. They found that the change in the deformation pattern of LWTB depends largely on the differential strengths of the steel sheets joined. Chan et al.[5,6] studied
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