Gas Turbine Blade Metallurgy and Fabrication
Turbines, whether as part of a power plant or a jet engine, experience high temperatures, jolting vibrations, and large stresses as part of normal operation. The turbine disc and blade have to be specially designed to withstand his harsh operational environment without failing. These stresses are even larger in gas turbine engines, such as those that power commercial and military aircraft. Specialized materials and manufacturing processes have been developed to ensure quality of the turbine blades and disc and minimize the probability of operational failure.
The turbine itself is only one part of a gas turbine engine, and is situated aft of the combustion chamber. The turbine consists of a central disc and a series of blades that are fixed to the central disc. Each of the blades is angled to “catch” the hot gas that is exiting the combustion chamber and imparts a torque on the central shaft. The largest turbine blades, those used in the GE 9H engine, are be 18 inches long and weigh over 30 pounds each. Some engine designs use multiple turbine discs in series along the central shaft.
Turbine Blade Metallurgy
Early turbine blades were fabricated from polycrystalline alloys, meaning that each blade was made up of a complex grain structure. Advances in materials engineering allowed manufacturers to fabricate polycrystalline blades where the grain was aligned into a single direction, increasing the strength and working life of each blade. More recent developments include the fabrication of monocrystalline blades, meaning that each blade consists of a single grain or crystal of the metal alloy. Because there are no grain boundaries in a monocrystalline part, the overall strength of the part is greatly increased.
Each engine manufacturer uses a proprietary alloy for its turbine blades. These high-strength alloys, called “superalloys”, are primarily nickel-based. Even with these high-tech alloys, gas turbine engine temeratures often exceed the melting point of these materials, and the turbine blades require complex cooling mechanisms to maintain component temperatures beneath the melting point of the alloy.
Turbine Blade Manufacturing
Turbine blades are fabricated using investment casting methods inside of vacuum chambers. The turbine blades are finely machined to a precise shape and laser machining is used to add tiny cooling holes into the blade.The surfaces of the blades are coated with a ceramic thermal barrier coating to increase their life.
Single-crystal turbine blades were pioneered by Pratt and Whitney Aircraft (now part of United Technologies Corporation). As described in Mechanical Engineering Magazine, fabrication requires “carefully controlled mold temperature distributions to ensure transient heat transfer in one dimension only, to a water-cooled chill plate.” Crystals begin to form at the surface of the chill plate and grow up into the mold chamber. The crystals have to pass through a narrow helical channel called the “pigtail”, which only allows a few crystals to pass. As the metal solidifies from bottom to top, “crystal elimination takes place so that only one crystal emerges from the pigtail into the blade root, to start the single crystal structure of the airfoil itself.” Through continued design and development, Pratt and Whitney achieves over 95% yield from its monocrystalline turbine blade fabrication process.
The advances in turbine blade design and fabrication has boosted gas turbine engine efficiencies to as high as 60%, resulting in reduced operational costs.
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