Mikkola, PPM Consulting & Design LLC, Bedford, N.H., who spent a decade working in the investment casting industry before starting a consulting firm. “The typical [metal- caster] doesn’t have the knowledge of the solidification and grain growth. It is limited to a small niche that has a strong technical staff, and that has limited the process’ growth.” The segment of metalcasters mak-
ing single crystal parts has reason to protect its intellectual property. The high-performance parts the manu- facturers produce go to customers that understand the value of paying for improved properties (commercial airlines, the U.S. Air Force, power generator manufacturers, the U.S. Navy and cruise lines), and to preserve those relationships, single crystal casters are heavily invested in limiting the growth of competition.
Secrets Don’t Make Friends What is known is that blade produc-
ers are focused on the “hot section” of the turbine, the end from which energy is expelled after the combustion process has taken place. The 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- 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. “The fracture mechanisms at high temperatures are generally controlled by grain boundaries,” said Bob Mc- Cormick, casting process engineer for Power Systems Manufacturing, Jupiter, Fla. “The more grain boundaries you have at high temperature, [the more cracking] you have. The grain bound- ary is the weak link for failure. [Be- cause single crystal castings] eliminate 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 in particular don’t like to share their pro- prietary methods of eliminating these grain boundaries and inducing grain growth, a process known as “seeding.”
Shown are turbine blades produced with (from left to right) equiaxed, directionally solidified and single crystal grain structures. Single crystal castings are produced with no grain boundaries, which makes them capable of withstanding the extremely high temperatures that are found in the hot section of some turbine engines.
May/June 2011 Metal Casting Design anD PurChasing 37
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