The role of hydrostatic stress on the plastic deformation of asperities is examined in the context of the adhesion theory of friction. A lower bound analysis gives the frictional shear strength to be proportional to the normal component of the deviatoric stress, rather than the total stress as is currently held in the Coulomb friction model. When applied to specific regimes, the new model is shown to agree qualitatively with various existing models for friction, including the Coulomb and Prandtl rules.

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The gas turbine industry is continuously driven to achieve higher thermodynamic effi ciency, higher electrical output, and higher reliability through turbine design improvements. The specifi c component of interest in this article is the turbine wheel, which is the rotating hub on which turbine blades are mounted. The wheel is mechanically loaded by both axial and centrifugal forces and thermally loaded by heat that is conducted from the turbine blades. Currently, the turbine wheel is forged from an ingot that is triple-melted, but nucleated casting is under development as a long-term option. This article describes the investigation into nucleated casting technology for future turbine wheel production.

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The clean metal nucleated casting program is a cooperative research program between GE Energy and Allvac, sponsored by the National Institute of Standards and Technology under the Advanced Technology Program. The goal of the program is to develop a spray-casting technology for the production of extremely large, segregation-free, superalloy ingots for use in turbine wheels in land-based gas turbines. The raw material for the process is a superalloy vacuum induction melted (VIM) electrode; in the CMNC process it is melted in a bottompouring Electro-Slag Remelting (ESR) furnace that forms a stream of liquid superalloy for subsequent atomization and collection in a withdrawal mold. Gas atomization of the stream to form a spray occurs in a chamber where spray distance and gas-to-metal flow rates can be adjusted to cool the metal to a desired level before collection. The approach taken in the R&D program is to address key technical risks associated with the system through construction of a one-ton research plant. These risks include the design and operability of the ESR furnace, pouring system, and collection system. Computational models of all key components of the process are developed and validated against experiments performed on the research plant. The validated models will be used to extrapolate to commercial-sized ingots.

Ceramic Inclusions -A ProblemA pilot plant has been constructed to demonstrate the concept of using a combination of electroslag refining (ESR) and an induction-heated, segmented, water-cooled copper guide tube (CIG) to melt, refine, and deliver a stream of liquid metal to a spray forming process. The basic ESR system consists of a consumable electrode of the alloy to be melted, a liquid slag, and a watercooled copper crucible. The liquid slag is heated by passing an ac-electric current from the electrode through the slag to the crucible. The liquid slag is maintained at a temperature high enough to melt the end of the electrode. As the electrode melts, a refining action takes place-oxide inclusions are exposed to the slag and are dissolved. Droplets of molten metal fall through the slag and are collected in a liquid metal pool contained in the crucible below. By the addition of the induction-heated, segmented, water-cooled copper guide tube (CIG) to the bottom of the crucible, a liquid metal stream can be extracted from the liquid metal pool. This stream makes an ideal liquid metal source for atomization and spray forming. The pilot plant has been operated at a melt rate of 15 to 25 kg/min with the Ni-base superalloys Alloy 718, Rene' 9.5 and Rene' 88. Process optimization and cleanliness evaluation studies are in progress. Ceramic inclusionscan reduce low-cycle-fatigue (LCF) life in both the powder metallurgy approach and the spray forming approach to the preparation of superalloys [5,6]. Mechanistically, a large ceramic inclusion will crack early in life and act as a crack starter for the surrounding superalloy. Direct observation of this event is shown in Figure I. The aluminum oxide particle cracked very early in life and the crack rapidly extended into the surrounding superalloy.