Laser Spot Welding of Cold-Rolled Boron-Free Ni 3 Al Foils
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3AL has attracted interest as a promising hightemperature structural material because of its excellent high-temperature strength and oxidation/corrosion resistance.[1] Recently, we have successfully developed thin foils of boron-free Ni3Al with a 23-lm thickness by cold rolling of single crystals.[2–5] These heat-resistant foils provide a high potential for lightweight hightemperature structural applications, for example, honeycomb structures.[6] Welding is a key technology in assembling honeycomb structures. Laser spot welding has been known to be one of the most suitable techniques for this purpose. With a highly concentrated heat source, the weld size is small, and it is thus possible to suppress the generation of undesirable defects such as cracking and distortion of the work piece. We adopted this technique for assembling honeycomb structures.[6] There is, however, a serious concern about cracking during the fusion welding of Ni3Al. As is well known, boron-free polycrystalline Ni3Al is extremely susceptible to intergranular cracking both at room temperature and at high temperatures, and it fractures at very small strain in tension.[1] Because our cold-rolled foils do not contain any boron, intergranular cracking can occur in both the fusion zone (FZ) and heat-affected zone (HAZ) where thermal stress is generated during welding. TOSHIYUKI HIRANO, Group Leader, and MASAHIKO DEMURA, and YA XU, Senior Researchers, are with National Institute for Materials Science, Ibaraki 305-0047, Japan. Contact e-mail: [email protected] KYOSUKE KISHIDA, Assistant Professor, is with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. KATSUHIRO MINAMIDA, General Manager, is with Laser Technology Center, Nippon Steel Technoresearch Corporation, Chiba 293-0011, Japan. Manuscript submitted July 31, 2006. Article published online April 26, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
To date, the welding behavior of Ni3Al has been studied on bulk plates thicker than 0.7 mm using gas tungsten arc,[7,8,9] electron beam,[7,10,11] and CO2 laser[12,13,14] in pursuit of crack-free welding conditions. These previous studies reported that in the bulk Ni3Al alloys, intergranular cracking predominantly occurs in the HAZ and occasionally in the FZ, and that the proper addition of B together with other elements such as Fe, Hf, and Zr can effectively improve the resistance to HAZ and FZ cracking.[7,8,10,12] Needless to say, no studies have been performed on thin Ni3Al foils. The objective of this study is to investigate the welding microstructures of the cold-rolled boron-free Ni3Al foils using laser spot welding and to determine the proper welding conditions that produce no cracking.
II.
EXPERIMENTAL
Boron-free Ni3Al (Ni-24 at. pct Al) foils with 30- and 100-lm thicknesses were used for the welding tests. They were fabricated by the 98 and 95 pct cold reductions of single crystals, respectively, as already described.[2,3,4] No intermediate annealing was used in the rolling process, and thus
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