Welding of Materials for Energy Applications
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TRODUCTION
THE importance of materials in energy, including conversion, efficiency, exploration, transportation, and storage was stressed by an overview article by Arunachalam and Fleischer[1] and a team of researchers.[2–10] These authors clearly documented the need for sustained materials science and technological research in coal, oil and gas, nuclear, wind, solar, biomass, biofuels, and renewables (hydro, geothermal, and ocean). To realize these objectives, extensive research in the last two decades has occurred in titanium- and nickel-based superalloys[11,12] and steels.[13–15] For example, by increasing the operating temperature of nickel-based superalloys, it is possible to increase the energy efficiency of gas-fired turbine engines.[16] In certain areas like energy conversion and storage devices (e.g., fuel cells and batteries), there is a need to develop material systems that work in unison to meet the functionality of the devices. For example, in advanced lithium ion batteries,[7] the range of materials includes complex cathode compounds (e.g., LiFePO4), nanostructured anodes (graphitic carbon), and current collectors (copper and aluminum). These devices are subjected to extreme environments and often fail at the material junctions. Along the same lines, new generations of hybrid materials based on organic and inorganic substances in pre-determined geometric (nm to mm) configurations[17–19] are being proposed. Hybrid materials JOHN N. DUPONT, R.D. Stout Distinguished Professor, is with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA. Contact e-mail: [email protected] SURESH BABU, Professor, is with the Department of Materials Science and Engineering, Ohio State University, Columbus, OH. STEPHEN LIU, Professor, is with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO. Manuscript submitted March 15, 2012. Article published online February 23, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
are structures with combinations of physical and mechanical properties that are not achievable by monolithic materials. For example, Rhein et al.[20] demonstrated that low-expansion lattice structures could be made using this concept. The same concept has been extended to develop actively cooled thermostructural panels.[21] In these research directions, it is typically assumed that robust processes are available for welding or joining. However, the weldability-related issues, which require extensive trial and error optimization, often stifle the accelerated insertion of these materials into applications. The scope of the challenge is indeed unique if one realizes that the total number of joining processes (including welding and brazing) may be more than 90. One of the overarching needs identified by the manufacturing industry is framed by a schematic decision tree shown in Figure 1. In many other engineering fields, the design questions are often resolved in quantitative fashion with optimization tools. For example, optimization algorithms have been
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