An arc-welding technique for repair of a directionally solidified (DS) nickel-base superalloy article (e.g., a turbine blade or vane) has been devised. The technique equally is applicable to a part as cast or a part that has been damaged in use. Unlike ordinary gas tungsten arc (GTA) welding, this technique results in a predominantly DS microstructure in the repaired region, similar to the microstructure of the DS casting. In addition, whereas ordinary GTA welding often involves the use of a filler material different from that of the workpiece, the weld filler material used in the present technique is of the same composition as that of the DS casting, so that undesired localized chemical differences are not introduced into in the repair region.
In this technique, the weld-repair area is shielded from atmospheric contamination; typically, shielding is accomplished by placing the article in a chamber that contains a protective argon atmosphere. To minimize residual stress and distortion, the article is maintained at a high temperature (approaching the liquidus of the DS material) while the weld is being made. After the welding arc is struck, the weld filler rod is moved along the repair region parallel to the direction of the original grain structure of the article. The filler material solidifies by epitaxial nucleation and growth of grains as typically observed on most weld repairs.
The unique feature of this technique is the combination of the directional deposition of the filler material and the high process temperature. This feature promotes directional grain growth parallel to the original DS grain structure. As shown in the figure, this is much different from the microstructure of a conventional GTA weld, in which there are typically two rows of converging, columnar grains, the major axes of which are perpendicular to the direction of the original DS structure.
This work was done by Russell Wayne Smashey, John Herbert Snyder, and Bruce Leanard Borne of General Electric Co. for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Manufacturing & Prototyping category.
Inquiries concerning rights for the commercial use of this invention should be addressed to
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Attn: Steve Fedor
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Refer to LEW-16865.