Gas turbines, in particular aircraft engines, must meet exceedingly stringent requirements in terms of reliability, weight, performance, economy and service life. In recent decades, aircraft engines have been developed, particularly for use in the civil sector, which have fully satisfied the above requirements and have attained a high level of technical perfection. The selection of material, the search for new types of suitable material, as well as the quest for novel manufacturing processes have played a decisive role in aircraft engine development. Since gas turbines are subject to high stresses and, therefore, defective regions may form on the gas turbine during operation, it is also crucial that highly developed repair processes be devised, to enable the defective regions to be repaired reliably, safely, quickly and cost-effectively.
The most important materials employed today for aircraft engines or other types of gas turbines are titanium alloys, nickel alloys (also called superalloys) and high-strength steels. The high-strength steels are used for shaft parts, gear parts, for the compressor housing and the turbine housing. Titanium alloys are typical materials used for compressor parts, in particular for compressor blades. Nickel alloys are suited for the heat-exposed parts of the aircraft engine, thus, for example, for the turbine blades. The latest generation of gas turbine components is manfactured from directionally solidified or monocrystalline materials, besides being weight-optimized, the components also being structurally designed to have ever thinner wall thicknesses.
The tendencies described above in the development of gas turbine components, namely the search for increasingly improved materials, and the increasingly weight-optimized structural components, place very high demands on the manufacturing processes, as well as on the repair processes, which also include welding processes.
However, highly heat-resistant superalloys, which, namely, may be present as directionally solidified materials and as monocrystalline materials, exhibit a high susceptibility to cracking and to distortion during welding processes. Accordingly, structural components made of the above materials are only workable or repairable to a less than satisfactory extent using conventional welding methods.
The German Patent No. DE 43 27 189 C2 describes a repair welding method for the blades of gas turbines. The method it discusses provides for a butt welding of a previously prepared repair surface, either plasma arc welding (PAW), laser-beam welding or electron beam welding being used as the butt welding method. In this case, a CO2 laser is used as a laser source.
The German Patent No. DE 196 30 703 C2 describes a method and a device for the repair welding of structural components manufactured from a nickel-based alloy. In the repair welding method according to German Patent No. DE 196 30 703 C2, the structural component to be welded is inductively heated, either tungsten-inert-gas welding (TIG) or plasma arc welding being used as the welding method.