Diffusion Brazing NiAl with Self-Generated Filler Metal
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DIFFUSION BRAZING NiAl WITH SELF-GENERATED FILLER METAL
Thomas J. Moore* and Joseph M. Kalinowski** * NASA - Lewis Research Center, MS 49-3, Cleveland, OH 44135 ** Sverdrup Technology, 2001 Aerospace Pkwy, MS 49-3, Brook Park, OH 44142 ABSTRACT Conventional fusion welding procedures are not likely to be useful for joining NiAl because of its poor low temperature ductility. Thus, the potential use of this material could be limited. This study was designed to demonstrate the feasibility of using a diffusion brazing process to fabricate NiAl components. A unique feature of this process is the development of self-generated filler metal through elevated temperature vacuum exposure. Butt joints were made in extruded bar and joint quality was evaluated using metallographic techniques. Sound joints, which were produced with complete grain growth across the braze interface, were indistinguishable from the base metal. Thus, the feasibility of diffusion brazing NiAl was established. INTRODUCTION NiA! is of interest for advanced turbine engine applications because of its physical properties and oxidation resistance [1]; however, in order to fully utilize NiA! in structural applications, suitable welding procedures must be developed. NiA! is very difficult to join by fusion welding processes because of its poor ductility below the ductile-to-brittle transition temperature of - 775 K [2]. Preliminary studies at NASA-Lewis have shown that cracking is a major problem in the electron beam welding of NiAl. For example, a preheat on the order of 1000 K may be required to avoid cracking in 2 mm thick sheet. Since this order of preheat would be difficult for industrial applications, diffusion brazing was thought to be a more viable process. The objective was to produce sound braze joints in NiAl without the addition of any foreign material. Diffusion brazing NiAl was accomplished in a unique two-step process. In the first step, a glaze of self-generated filler metal was formed on the faying surfaces simply by heating the NiAl in vacuum. In the second step, the braze was made by bringing the faying surfaces together, holding them in place under slight pressure, and reheating to melt the self-generated filler metal and produce coalescence. Some brazed joints were hot isostatically pressed (HIP'ed) to further assure joint soundness. Joint quality was evaluated by metallography. MATERIALS AND PROCEDURE The NiAl material was in the form of 10 mm diameter bar resulting from the extrusion of cast billets. The composition in atomic percent was as follows: Ni - 49.6 Al 0.20 C - 0.08 0, with N being
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