Electron-beam welding at high temperatures has been found to be a suitable process for joining structural components made by casting certain superalloys. This process can be used in the fabrication of superalloy parts that must withstand high operating temperatures. Examples of such parts include exhaust ducts of advanced aerospace engines and end caps on turbine buckets.
The superalloys in question are γ´- strengthened nickel-base alloys that contain either >3 weight percent Al or >6 percent Ti. Strain age cracks form in such alloys upon cooling after welding or on subsequent reheating to aging temperatures. The cracks result from a combination of residual stresses produced during welding and aging cycles. Heretofore, the formation of the strain age cracks has made it impossible to utilize these superalloys to make welded structures.
The development of the present process brought electron-beam welding together with vacuum heat treatment to provide a new industrial capability. Electron-beam welding has long been used to produce structural weldments in a wide variety of alloys, but, heretofore, has not been successful for welding nickel-base superalloy structures. Nickel-base superalloys are frequently heat-treated in vacuum furnaces to impart the very properties for which they were selected.
In the present process, a heat-treating furnace is placed in the vacuum chamber of an electron-beam welding machine. A superalloy structure to be welded is placed in the furnace. Prior to and during welding, the furnace is used to heat the entire superalloy structure to a temperature at or near the solution temperature of the alloy. Maintaining the entire structure at this temperature reduces or eliminates the thermal stresses produced by the differential thermal expansion during welding. Further, maintaining this temperature for a while after welding affords some relief of solidification stresses, thereby helping to prevent subsequent strain age cracking. The structure can then be cooled rapidly and later aged or can be aged during the cooling cycle after welding has been completed. An additional advantage is that the reduction in thermal stress prevents the formation of liquation cracks in the heat-affected zones of the welds.
This work was done by Thomas J. Kelly 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 category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16686.