Study on the Formation and Characterization of the Intermetallics in Friction Stir Welding of Aluminum Alloy to Coated S
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RECENTLY, the automotive industry has been concentrating on multimaterial fabrication, which can make vehicles substantially lighter. Adoption of this fabrication in practice is closely linked with the development of efficient methods of joining high-strengthlow-alloy steel sheets with competitive structural grades of aluminum alloys.[1,2] From a practical point of view, both sound joints between dissimilar materials that enable multimaterial design methodologies and low-cost fabrication processes have to be established,[1–3] but the availability of a sound joining technique for dissimilar materials is indispensible. Joining of dissimilar metals by conventional fusion welding techniques is difficult due to large differences in thermophysical properties such as melting point, thermal conductivity, thermal expansion leading to high distortion, and residual stresses, and also in metallurgical characteristics resulting in the formation of brittle intermetallic phases that generally form by solid-state reaction.[4] These intermetallic compounds H. DAS, Senior Research Fellow, and T.K. PAL, Professor, are with the Metallurgical and Material Engineering Department, Welding Technology Centre, Jadavpur University, Kolkata 700032, India. Contact e-mail: [email protected] R.N. GHOSH, Professor, is with the Metallurgical and Materials Engineering Department, IIT Kharagpur, Kharagpur, 721302 West Bengal, India. Manuscript submitted October 24, 2013. Article published online July 11, 2014 5098—VOLUME 45A, OCTOBER 2014
(IMCs) generally result in mechanical degradation of the joint.[5,6] This can be avoided possibly by using friction stir welding (FSW), which is a solid-state joining process and does not involve any melting.[5,6] The use of FSW in the joining of aluminum alloys to steel has recently begun. The feasibility of FSW of aluminum to steel and the importance of an Al/steel interface on weld strength has been considered by numerous authors.[7–9] The initial investigations on FSW of aluminum alloys to steel reported the presence of IMCs such as Fe4Al13, Fe2Al5, and FeAl4.[5,7–9] However, the effect of such intermetallic phases on the mechanical properties of the weld has not been evaluated.[5] Tanaka et al.[10] established that the joint strength increased exponentially with a decrease in IMC thickness. A similar opinion was also expressed by Kimapong and Watanabe.[11] The thickness of the intermetallic layer at the weld interface induced by process parameters such as rotational speed, axial force, and their influence on the degradation of the mechanical properties of the weld joint was reported subsequently by a few researchers.[10–14] Also, improvement of the joint strength in the IMC layer seems to be necessary, but an IMC layer that is too thick may initiate crack and propagate easily through brittle IMC.[11] Bozzi et al.[15] reported an optimum IMC layer thickness of 8 lm developed under a rotational speed of 3000 rpm and tool penetration depth of 2.9 mm in friction stir spot welding of aluminum to steel. While making
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