Phase-Field Simulation of Fusion Interface Events during Solidification of Dissimilar Welds: Effect of Composition Inhom
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UCTION
EVER since Metzger et al.Õs substantial initial work on welding of dissimilar metal combinations,[1] a considerable amount of effort has gone into determining the weldability of specific dissimilar combinations (for example, the overviews by Sun and co-workers[2,3] summarize the progress made until the middle of the last decade). During recent years, computer simulations and experiments have helped us gain a better understanding of the fluid flow effects and development of the macroscopic weld pool geometry in dissimilar welds.[4–7] Studies have also been undertaken to elucidate the underlying mechanisms of microstructure formation in these welds in terms of fundamentals of solidification theory.[8–11] In this latter set of experiments, regions close to the fusion interfaces revealed microstructures that were remarkably different from the ones observed during welding of similar metals, and in this article, we seek to address some of the issues related to this aspect of dissimilar welding.
SUBHRADEEP CHATTERJEE, Research Associate, T.A. ABINANDANAN, Associate Professor, and KAMANIO CHATTOPADHYAY, Professor, are with the Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Materials Behavior: Far from Equilibrium’’ as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India. Article published online October 30, 2007 1638—VOLUME 39A, JULY 2008
Nelson et al.,[8] while studying welding of different grades of steel in contact with a monel (Ni-rich Ni-Cu alloy) filler alloy melt, reported that when the primary solidifying phase had a crystal structure different from the base metal, epitaxial growth of the substrate (commonly found in welds between similar metals) was replaced by a chill zonelike microstructure. This microstructure consisted of numerous small grains, which nucleated heterogeneously on the substrate grains at the fusion interface. In contrast, they observed epitaxial growth when the composition change was not accompanied by a corresponding change in the crystal structure, implying a dominant role of the crystal structure of the primary phase over that of a difference only in composition. However, the results of Phanikumar et al.[9] for laser welding of Cu and Ni, which form an isomorphous system with fcc structure, indicate that the composition gradient can affect interface microstructure even when the solidifying phase has the same crystal structure. They noted that while the Ni interface did exhibit epitaxy, the Cu interface was irregular and the fusion line was clearly separated from the alloyed weldment by an unmixed zone. In a later experiment on the immiscible Fe/Cu system, Phanikumar et al. also observed[10] the formation of Fe-rich bands at the Cu interface, which they attributed to a phase separation reaction in the liquid followed by peritectic solidification. For a closely rel
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