Prospects of Use of Laser Cladding Technology for Restoration of Compressor Blades of Gas Turbine Engines
Requirements to the operational characteristics of machine parts and units are constantly growing. It stimulates the accelerated development of high-efficiency technologies for design of new functional materials. Respectively, the demand in equipment intended for allotment of higher anticorrosive, tribological and strength characteristics to operating surfaces of the products, which are operated under adverse climatic and mechanical conditions, is growing. Laser technologies become very prospective technologies in surface engineering area. At the expense of creation of high density of heat power in the area of impact on the surface, they allow performing its treatment without need in volume heating of part and ensure reaching ultrahigh speeds of heating and cooling of laser impact area. As a result, the capabilities of generation of surface layers of mechanical products with unique operational characteristics arise.
Laser cladding as technology of repair operations is in potential demand by the companies, which are involved in the restoration of the parts used in engine building, nuclear power engineering, petrochemical production, mineral resource, metal-working and other industries where used machines and mechanisms are subjected to the impact of aggressive media and surface wear.
At the present time, the gas turbine engines (GTE) are broadly applied in aviation and ground equipment, and the range of their application is constantly expanding. More than 1000 pieces of only ground and marine GTEs are produced in the world, and 70% of them account for the power engineering sector. In this area the broadest capacity range of GTE is applied, from 16 kW to 300 MW. They are used in the capacity of drivers of electric generators in power plants in simple, cogeneration and combined cycles producing electric energy and heat, which are supplied to consumers in the form of steam and hot water. The annual sales volume of gas turbine equipment is approximately 20–22 billion US dollars. The need of maintenance of competitive capacity of domestic GTE at the world market is obvious. Not only technical and operational characteristics of goods influence on the selection of GTE but also its price and operation cost.
Reliability of gas turbine engines mainly depends on the operation reliability of compressor and turbine blades because they are the most loaded parts. The blades are exposed to the static, dynamic and cyclic loads. Besides, the turbine blades experience the cyclic thermal stresses, they operate under the conditions of aggressive gas media at high temperature and they are subjected to gas corrosion . GTE blades have complex spatial geometry and they are produced from difficult-to-deform materials: heat-resistant, titanium and aluminum alloys. Therefore, higher requirements by the structure of metal, its chemical composition, mechanical properties, geometry and elimination of defects are made in relation to them. The special attention is paid to such defects as forging folds, perforations, burnings, thermal damages .
According to operation rules, the operating turbine blades of GTE have the service life set by developer (usually 25000 hours). After the depletion of service life, the operating blades are subject to repair. In general case, the repair consists in execution of the following operations: dismantling of blades, cleaning, flaw detection, restoration of microstructure of base material, restoration of geometry and shape, restoration of surfaces and quality control.
The most simple and common methods of repair of GTE operating blades is argon-arc welding. But this method has a number of adverse factors: due to direct fusion of the base metal by arc the significant zone of thermal influence with large-grained structure is formed, which requires the further heat treatment; the excess materials with the size of several millimeters are formed, which require the further mechanical treatment.
In order to reduce the heat-affected area, the method of argon-arc welding by homogeneous membranous fusion of base metal at the depth of 0.2–0.3 mm, which decreases the risk of crack formation in heat-affected area (HAA), is suggested . In this method, the base metal is fused at the expense of heat of fused facing material. Suggested method does not exclude the generation of deformations. But volume deformations are replaced with linear ones. In order to remove the stresses, restored blades require the further annealing at the temperature 950 °C within 3 hours. Also, the further mechanical treatment is needed. Existing arc methods of restoration of operating blades of industrial GTE are characterized by low efficiency – the yield factor is 15–25%, material utilization rate is 10–20%.
LASER TECHNOLOGIES OF REPAIR CLADDING OF GTE OPERATING BLADES
Minimum allowance for the further mechanical treatment (about 200 µm), narrow heat-affected area (up to 100 µm), presence of fine-grained structure of built-up layer, minimum (local) energy deposition, increase of repair area of GTE blade surface, absence of heat treatment, higher mechanical characteristics of built-up layer refer to the characteristics of laser technology as opposed to argon-arc welding [3, 4]. Also, flexibility of the process, which allows using the metal powder and wire in the capacity of filler, must be mentioned here [5–7].
Results of laser cladding with the use of repetitively-pulsed solid-state Nd:YAG laser (HTS-Mobile 300 – Experimental Design Bureau "Bulat") are given in the paper . However, the main disadvantage of developed technology consists in the absence of automation of technological process: filler wire is manually supplied by operator into the area of laser radiation during the cladding process. There is technological complex for pulse laser cladding with the system of automatic supply of wire, automatic focusing of laser beam and periscopic nozzle in the production range of EDB "Bulat". Such automation allows enhancing the quality and efficiency of repairing process. Technologies of pulse laser cladding allow building up the alloys based on nickel, cobalt and titanium . Examples of domestic and foreign complexes based on repetitively-pulsed solid-state lasers are shown in Fig. 1.
But even these complexes have disadvantages: they do not have cassette accessories, which allow easy transition from single-piece cladding of each blade to automated cladding of the set of one-type blades.
Results of powder building-up of chords and edges of compressor blades from titanium alloy using the continuous-wave fiber laser with maximum capacity of 1 kW are given in the paper . Building-up was performed using the device LENS 850R produced by OPTOMEC (USA) (Fig. 2).
Results given in the paper proved the principal ability to use this category of equipment for the restoration repair of operating blades. At the same time, the significant allowance for mechanical treatment, increase of microhardness of built-up metal in comparison with the base by 1.5–2.5 times; broad heat-affected area (about 0.9 mm), unfused powder particles on the surface of built-up layer, presence of internal defects obtained by the authors in the course of the experiments prove the complexity and multiple factors of the technological process of laser cladding and necessity of its optimization with the use of theoretical and experimental methods.
The way of development of repair technologies through design of the complexes, which would combine control, mechanical treatment (preliminary and further) and laser cladding systems, seems to be prospective.
Such approach was implemented in Reclaim project executed by the consortium of English companies: Renishaw, Electrox, TWI, Precision Engineering Technologies, Cummins Turbo Technologies, Airfoils Technology International and De Montfort University. The software was developed by Delcam company. The British State Administration of Technological Strategy invested more than half a million pounds into this project.
Results of restoration of engine turbine blades using the automated technological complex RECLAIM are described in the paper . The technology of adaptive laser cladding and equipment, which is similar to RECLAIM complex by its technical characteristics, are offered by the employees of Fraunhofer Institute and Beam Machines company [12, 13].
The experience of restoration building-up of entrance edges of steam generator blades implemented at the rotor shaft is of interest  (Fig. 3).
Diode laser with the capacity of 3 kW was used in the capacity of laser radiation source. The main positive results of this paper should include the capability of restoration of blades of steam generator bladed disk at the rotor shaft, which provides the significant economy of monetary resources in comparison with the technologies requiring the removal of defective blade from the structure of bladed disk , which can reach 2.3–4.5 million US dollars according to authors. The disadvantages of paper results include the occurrence of porosity in the area of welding of blade and built-up layer and impossibility to apply this technology for the restoration of large areas due to the occurrence of significant deformations and crack formation (Fig. 4).
The methods of fabrication and repair of aircraft engine parts by the direct laser growth (DLG) and selective laser melting are well known . The advantages of such technologies include the capability to make parts with irregular shapes from expensive materials with unique properties and minimum allowance for the further mechanical treatment; there are no deformations in the process of fabrication/repair; heat-affected area is minimal. The disadvantages include absence of technology testing in respect to the restoration of worn-out parts of turbine blades and use of import equipment.
Restoration of the blades of heat-resistant alloys based on titanium and nickel, which are installed at bladed disk in GTE compressor PD-14, using the device TruLaserCell 7020 produced by Trumpf is described in the paper  (Fig. 5). Results of metallographic studies showed the existence of clear boundary between the base and built-up material, absence of defects and fine-needled structure of built-up layer indicating high cooling speeds. In respect to the advantages of used methods the authors also mentioned its adaptivity, which is relevant for blade surfaces with various degrees of wear.
In order to restore the surface of GTE compressor blades, a number of authors suggest using the technology of cathode-ray building-up [18, 19]. Despite the advantages, which are similar to the advantages of laser cladding, this method has restrictions by overall dimensions of restored workpieces depending on the size of vacuum chamber, for the generation of vacuum in which the time is also needed.
Results of laser restoration of the blades of gas transmission stations using nickel-based powders Inconel 625 and Inconel 738 as fillers are specified in the paper . Solid-state laser with the wavelength of 1.07 µm was used in the capacity of laser radiation source. In both variants, high-quality beads were obtained; in case with the material Inconel 738 the following heating of building-up area was additionally used. Also, authors mention the advantages of fiber laser in comparison with СО2 laser. The positive result consisted in the presence of control system of weld bead geometry for the purpose of provision of minimum allowance for the further mechanical treatment; also the evaluations of technological capacity of "treatment" of surface cracks of the blades of nickel alloys during the laser cladding were mentioned in the papers [21–23].
Analysis of the papers in the area of repair of GTE operating blades showed the industry interest in the implementation of laser technologies. The series of works were performed with respect to the testing of laser cladding technologies for the blades with various application ranges using the devices of different types. Aviation and power machine building refer to strategically significant industries. Therefore, it is required to enhance the level of technological import independence of production processes with respect to new products, repair and maintenance of used products. Major part of technological complexes of laser treatment, which are currently used at the enterprises of aviation engine building and repair, is imported.
DEVELOPMENT OF DOMESTIC TECHNOLOGICAL COMPLEX FOR RESTORATION OF GTE OPERATING BLADES USING LASER CLADDING METHOD
In order to establish the restoration of GTE operating blades using the method of laser cladding with the support of Government of Russia (Ministry of Education and Science of the Russian Federation) within the framework of Government Decree No. 218, the employees of the Institute of Laser and Welding Technologies of Peter the Great St. Petersburg Polytechnic University (ILET SPbPU, Saint Petersburg) are developing the robotized technological complex of laser cladding on the basis of order of CJSC "Plakart". The general concept of complex is given in Fig. 6. The complex has the following components: fiber laser with the capacity of 700 W (1), control system (2), water cooling system (3), industrial robotic manipulator (4), laser cladding head (5) and powder feeder (6).
The special attention in the complex development will be paid to the nozzle part of technological head, which allows providing the coefficient of utilization of built-up powder of not less than 0.5 at the width of built-up beads of 0.8–1.5 (2) mm. Also, the cassette accessories will be developed for the installation of set of one-type blades, and it will allow increasing significantly the efficiency of building up process and reducing the time expenditures connected with installation of individual blades.
Technical parameters of developed complex were determined on the basis of the results of preliminary technological experiments in building up of nickel – and cobalt-based heat-resistant alloys (in the form of powders) on GTE operating blades made of the materials ZhS32-VI and ChS70-VI (Fig. 7).
Metallographic studies of thin sections and X-ray analysis of restored ridges of operating blades showed that there are no pores, cracks, scabs and unfused powder particles in all samples. In case of building up of the powder Stellite 6 on the blade made of ZhS32-VI the welding line is clear (see Fig. 7a). In case of building up of Inconel 625 on the blade made of ChS70-VI and EP648 on the blade made of ZhS32-VI the welding line is wavy with insignificant mix of materials (see Fig. 7b, c). All layers were built-up with minimum allowance for the further mechanical treatment (Fig. 8).
The results of performed studies showed that the technology of laser cladding is capable to replace used technologies of restoration of gas turbine engine blades having reduced the cost of repairing cycle and increased the resource of their inter-repair operation. Also, the fabrication of adaptive automated complex will make it possible to implement the technology of laser cladding providing the independence of domestic manufacturer on import equipment.