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Fabricated blade, showing complexity of internals
addressing deficiencies in current shell systems; and (3) investigating novel cooling concepts for increased thermal gradients during the solidi-fication process.
The thrust of the VIM furnace efforts is definition of the factors that will improve control of mold temperatures and thermal gradients. Howmet conducted furnace surveys on the GEPS 9H blade casting pro-cess to update and validate a solidi-fication process model. Computer models were also developed to ana-lyze potential and current furnace materials. These models will pro-vide the tools to optimize the VIM furnace design. Howmet has dem-onstrated that improvements of up to 40 percent in the thermal gradi-ent are attainable by enhancing the current system.
The shell systems activities ad-dress the additional requirements imposed on the ceramic mold with increased casting size. For example, longer casting times induce shell creep, thicker shells reduce thermal

Fabricated blade showing bonding plane
Siemens Westinghouse, in con-junction with the National Institute for Science and Technology (NIST), PCC, and Howmet, has moved the fabricated component approach to prototype production. Efforts have determined segments and bonding planes, developed coreless SX cast-ing technology for the segments, developed fixtures to bond the seg-ments, verified the structural integ-rity, and designed non-destructive evaluation (NDE) methods. Both fabricated stage 1 vanes and blades are to be used on the SWPC ATS unit.
gradients, and the larger and heavier molds lead to structural and handling problems. Howmet investigated materials additives to strengthen the shell, and additives to improve ther-mal conductivity.  Under some con-ditions, additives reduced creep deflection by 25–90 percent. Simi-larly, material additives achieved improvements in thermal conduc-tivity of up to five times under some conditions.
As indicated above, maintain-ing a high thermal gradient at the solidification front is critical to pre-venting casting defects and enhanc-ing yields. Novel cooling methods have the potential for achieving revolutionary increases in thermal gradients. The research being car-ried out is defining the heat transfer mechanisms necessary to design such novel cooling methods. Work to date has shown that the maximum thermal gradient may be limited by three rather than one resistance mechanism. By identifying the principal rate limiting thermal char-acteristic, a significant increase in thermal gradient may be achieved.
T
he advances in materials and
manufacturing technology
needed to effectively transfer
aerospace technology to the
large land-based turbine systems
represented the single greatest
challenge to meeting ATS goals.
Only through mutual investments
in extensive R&D under ATS part-
nerships was the challenge suc-
cessfully met and a foundation
laid for further advancement.
　
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