Fatigue Performance of Friction-Stir-Welded Al-Mg-Sc Alloy
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INTRODUCTION
AL-MG-SC alloys belong to a new class of high-performance aluminum alloys developed for critical structure applications. Minor additions of Sc to conventional 5xxx series aluminum alloys lead to the formation of uniformly distributed nano-scale coherent Al3Sc precipitations that are extremely stable under annealing and intense plastic strain.[1,2] These particles are very effective for the pinning of dislocations as well as grain boundaries, and thus impart significant strengthening and promote microstructure stabilization. High specific strength and stiffness of these alloys are also promising for cryogenic applications.[2] Conventional fusion welding of the Al-Mg-Sc alloys leads to dissolution of the unique precipitates and respective degradation of material strength.[1,3] In this regard, friction-stir welding (FSW), an innovative ‘‘solid-state’’ joining technology, appears to be particularly attractive for these materials. Due to the relatively low heat input associated with the FSW process, this technique is believed to preserve the fine coherent particles, and thus provide high service properties of joints. Given the potential advantage of the FSW, several research efforts have been made recently to evaluate the feasibility of this technology for the joining of Al-Mg-Sc alloys, and so far very encouraging results have been obtained.[1,3–6] Fine coherent particles have indeed been found to be mainly preserved in the weld zone.[1,3–8] Additionally, the microstructure that develops in the stir zone has been shown to be exceptionally fine-grained, DARIA ZHEMCHUZHNIKOVA, Researcher, and RUSTAM KAIBYSHEV, Professor, are with the Laboratory of Mechanical Properties of Nanostructural Materials and Superalloys, Belgorod State University, Pobeda 85, Belgorod 308015, Russia. Contact e-mail: [email protected] SERGEY MIRONOV, Researcher, is with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-6-02 Aramaki-aza-Aoba, Sendai 980-8579, Japan. Manuscript submitted October 15, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
with a mean grain size often being less than 1 lm.[9–16] As a result, the joint efficiency of the friction-stir weldments typically achieves ~80 to 100 pct, thus being significantly higher than that in the welds produced by conventional tungsten inert gas (TIG) welding.[1,3,5,6] It is important to emphasize that the examination into the mechanical properties of the Al-Mg-Sc friction-stir welds has so far been limited to microhardness measurements, tensile tests,[1,2,5,6] or dynamic impact tests.[17] However, considering the potentially wide range of engineering applications of these joints, their fatigue performance also requires examination. To the best of the authors’ knowledge, the fatigue behavior of friction-stir-welded Al-Mg-Sc alloy is not studied well. The sole work in this area is the recent study by Besel et al. in which a relatively low fatigue strength has been reported.[18] It should be emphasized however that significant material softeni
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