Welding of a Two-Phase Ni 3 Al Alloy
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WELDING OF A TWO-PHASE Ni 3 AI ALLOY HUAXIN LI AND T. K. CHAKI State University of New York, Department of Mechanical and Aerospace Engineering, Buffalo, NY 14260 ABSTRACT Autogeneous gas tungsten arc welding was performed on plates
(about 3 mm thick) cut from cast ingot of a Ni 3Al alloy (Nir3.8 oAli5 .s 2 CTs.33 MO1 .6 8Zro.52 Bo.o5 ), known as IC-396M. The alloy was susceptible to cracking in the heat-affected zone (HAZ). The cracks in HAZ were liquation cracks and occurred along dendritic boundaries and around eutectic cells. The fusion zone (FZ) contained many fine eutectic cells (about 5-10 •tm in diameter) and there were microcracks (1-5 tim in size) in the cells. Two types of eutectic structures were observed in the base metal, HAZ and FZ of IC-396M. One had web-like structure and the other had lamellar structure, the later having high enrichment with Zr. The role of Zr enrichment in dendritic boundaries and eutectic cells in causing liquation cracks will be discussed. INTRODUCTION Polycrystalline nickel aluminide (Ni 3 AI), alloyed with a small amount of boron (a few hundred p.p.m.) has large room temperature ductilitity [1]. Besides boron, zirconium is added [2] to Ni 3 AI in small proportion (less than 1 at. %) to provide solid solution strengthening. However, ductile Ni 3 Al compound doped with B only or with B and Zr looses its ductility at intermediate temperatures of 400-800°C [3,4], when they are subjected to tensile stresses in air. The embrittlement, known as •dynamic embrittlement" (it occurs within the short time span of the test), can be overcome by addition of moderate amounts of chromium [2,5]. A series of advanced Ni 3 Al alloys containing B, Zr, Cr and Mo have been developed [2,5] at Oak Ridge National Laboratory, TN. Weldability is a key issue in the development of a commercial alloy, because welding is the primary means of joining parts. David and coworkers [6,7] performed autogeneous gas tungsten arc (GTA) and electron beam (EB) welding on several Ni 3 AI alloys, some of them containing iron as an alloying element. The specimens were made by arc melting followed by repeated rolling and annealing. Their study showed that ductile Ni 3 Al is highly susceptible to hot cracking in the HAZ with cracks occuring along Mat. Res. Soc. Symp. Proc. Vol. 288. @1993 Materials Research Society
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the grain boundaries. The alloys can be successfully welded only by EB welding operated within a narrow range of welding speeds and beam focus conditions. In an earlier paper Li and Chaki [8] have reported observation of a large number of liquation cracks in the HAZ of a single-phase Ni 3Al alloy containing 0.34% Zr and 0.10% B by atom. In this paper, we report the nature of weld cracking in cast ingots of a two-phase (7 and -') Ni 3 A1 alloy containing B, Zr, Cr and Mo. EXPERIMENTAL PROCEDURE Ni 3 Al alloy used in this study contained (at.%) 15.62% Al, 8.33% Cr, 1.68% Mo, 0.52% Zr, 0.05% B and balance Ni. The alloy is called IC-396M. Cast ingots (250 mm x 120 mm x 18 mm) were obtained from Armco Inc., O
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