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Jupiter, Fla. “Te more grain boundar- ies you have at high temperature, [the more cracking] you have. Te grain boundary is the weak link for failure. [Because single crystal castings] elimi- nate the boundary, you up the ante for where they can survive.” According to Mikkola, trade secrets abound in the single crystal blade pro- duction world because companies don’t like to share their proprietary methods of eliminating these grain boundaries and inducing grain growth, a process known as “seeding.” “Seeding is a technique where you


have a crystal in the mold that has the orientation that you want in the cast- ing,” Mikkola said. “It actually ‘seeds’ the growth of the crystal in the exact same direction. Believe it or not, there is a lot of trial and error to figure out how to seed. Tere are trade secrets that are beyond just the science.”


Step-By-Step Through the Process


Quality single crystal castings start in the melting process, where tight controls are necessary to prepare the melt to produce the desired microstruc- ture. Tis area of the operation is one of the most concealed in the industry, with multiple manufacturers declining to divulge their secrets. Next, the molds are created in much the same way as typical investment casting molds—wax injection into tooling followed by (typi- cally) manual tree assembly. Before pouring the molds, they


Turbine blade makers like Howmet, Precision Castparts Corp. and Chromalloy (shown) are highly secretive about their processes.


section” of the turbine, the end from which energy is expelled after the combustion process has taken place. Te parts used in this section of the engine are exposed to the harshest high-temperature conditions (on the order of 1,800F) and therefore suffer the greatest amount of wear. To put up with that wear, single


crystal components are grown in a way that enhances their strength and thermal properties in one direction, as needed in the application. For turbine components in both aero and land based power generation turbines, pro-


34 | MODERN CASTING August 2011


ducers of single crystal nickel superal- loys must control the grain orientation relative to key component features, according to Kath Clay, Hexmat Mate- rials Consultancy Ltd., Matlock, U.K., and Peter Quested, National Physical Laboratory, London, who have been involved in the development of a stan- dardized system of measuring the grain structure of single crystal components. “Te fracture mechanisms at high


temperatures are generally con- trolled by grain boundaries,” said Bob McCormick, casting process engineer for Power Systems Manufacturing,


must be seeded. To do so, microstruc- tural crystals are placed in the mold in the direction of grain growth desired for the entire casting. “Te seed actually pushes the


growth of the crystal in the exact same direction of the seed already placed in the mold,” Mikkola said. With a properly seeded investment


casting mold in hand, single crystal castings are produced by controlling the solidification of the molten alloy in the mold using a heated coil or some other thermal device. Te coil is removed from the mold slowly in the direction grain growth is desired. Equiaxed and direc- tionally solidified parts also are produced using such controlled solidification tech- niques, but the process is much faster. According to Tom Trotter, vice president


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