Gas turbine engines, such as those used to power modern commercial aircraft or in industrial applications, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor.
In operation of a gas turbine engine, fuel is combusted in the combustor in compressed air from the compressor thereby generating and high-temperature combustion exhaust gases, which pass through the turbine. In the turbine, energy is extracted from the combustion exhaust gases to turn the turbine to drive the compressor and also to produce thrust. The turbine includes a plurality of turbine stages, wherein each stage includes of a stator section formed by a row of stationary vanes followed by a rotor section formed by a row of rotating blades. In each turbine stage, the upstream row of stationary vanes directs the combustion exhaust gases against the downstream row of blades. Thus, the blades of the turbine are exposed to the high temperature exhaust gases.
Each turbine blade typically has an airfoil-shaped hollow body having a concave surface and a convex surface extending between a leading edge of the blade body to a trailing edge of the blade body. The blade body extends generally radially outwardly from a blade root, whereat the blade root is attached to the turbine rotor disk by a dovetail joint, to a blade tip at the distal end of the blade body. In operation, in order to reduce the passage of combustion exhaust gases outside the blade tips as the blades rotate, thereby reducing turbine efficiency, a tight clearance is established by having the blade tips pass in extremely close proximity to the turbine casing or to actually contact the rub surface of a blade outer air seal. In either case, over time the blade tips of the rotating turbine blades are subject to wear from contact with either the engine casing or the rub surface of the blade outer air seal.
As a result of the associated physical wear and also oxidation due to expose to the high-temperature of the combustion exhaust gases, the blade tips erode over time in service, the turbine blades actually become shorter. As a consequence, the tip clearance at cruise becomes larger and turbine efficiency is degraded. Therefore, it is customary to take gas turbine engines out of operation for overhaul as necessary to service various parts of the engine. As part of the servicing of the engine, it is conventional practice to inspect the turbine blade tips and remove blades having excessively eroded blade tips. Because turbine blades are usually made of expensive superalloys in order to withstand the high temperatures to which the blades are exposed, and because turbine are often cooled through internal cooling air passages, the presence of which make the blades very expansive to manufacture, it is customary to restore the blades, rather then simply scrapping the removed blades. The removed blades are restored by rebuilding the eroded blade tip sufficiently to return the blade body to its original design length using various techniques for depositing repair alloys unto the eroded tip of the removed blade, thereby salvaging the blade. However, conventional restoration methods require the damaged blades to be removed from the engine, restored, and then replaced in the engine, which necessarily requires the engine to remain out of service for an extended period.