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  A repair and overhaul methodology for aeroengine components
  Oguzhan Yilmaz a, , Nabil Gindy b, Jian Gao c
  A Department of Mechanical Engineering, The University of Gaziantep, Gaziantep 27310, Turkey
  B School of Mechanical, Materials and Manufacturing Engineering, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK
  C School of Mechanical and Electronic Engineering, Guangdong University of Technology, 729 East Dongfeng Road, Guangzhou City 510090, PR China

  Article info
  Article history:
  Received 24 January 2008
  Received in revised form
  21 April 2009
  Accepted 1 July 2009
  Keywords:
  Blade repair and overhaul
  Aeroengine parts
  Reverse engineering
  Free-form surface reconstruction
  Abstract
  Repair and maintenance of aeroengine components have been one of the main tasks to be overwhelmed by aerospace industries due to continual increase in raw material and manufacturing costs. In this paper, an advanced methodology for the repair of complex geometry and expensive components via reverse engineering, free-form surface modeling and machining is presented. The methodology has been successfully implemented on a critical aeroengine component, i.e. thin-curved compressor blade. The approach involves the integration of 3D non-contact digitization, adaptive free-form surface reconstruction and multi-axis milling operation. Each operation is individually automated and inter-connected each other in order to reduce the total repair time. The workshop results have shown that the proposed repair methodology can be considered a reliable and time-cost effective as compared with the current repair approaches.
  & 2009 Elsevier Ltd. All rights reserved.

  1. Introduction
  Aeroengine components, such as blades, vanes, combustors and shafts, are faced to harsh environments and long-hours operation. Due to high temperature and pressure, as well as foreign object impacts, engine components may have various defects, for instance distortion, wear, impact dents and cracks. The consequences of such an event can be fatal and the financial costs would be high. To extend the service life of these parts, repair and overhaul is an inevitable choice as the replacement is far more expensive.

  Blades are the most critical parts of an aeroengine and assembled along the engine stages in different sizes and roles. A turbine blade is described an aerodynamic body having special geometry characterized mainly by an airfoil cross-section; blade design relies heavily on aerodynamic/thermodynamic theory to achieve a design with maximum efficiency [1]. A small change in blade geometry can lead to a large change in engine performance. Therefore, control of the blade shape is critical to the design and even in repairing process [2]. In a blade repair process, it is crucial to maintain the original blade shape and to achieve maximum efficiency for the restored blade, such as chord and thickness dimensions must lie in acceptable ranges defined by engine manufacturers. For aeroengine components, blade tip repair occupies the major repair volume of all parts due to oxidization and crack during operation [3]. Thereby, the blades are often removed from service long before the blade has reached its service life. So that, a considerable effort is expended to dimensionally restore the blade tip. A blade repair process mainly involves four stages: pre-inspection, welding, machining and post-inspection. Currently, most of the repair processes for the repair of engine components are carried out manually. Although some steps of repair process has updated, such as worn areas of blade is built-up by laser cladding, excess material on the blade is still removed by conventional grinding–hand polishing and the inspection is visually carried out. There are a limited number of studies have been performed on specific aspects of blade repair. Brinksmeir [4] proposed an integrated process chain for the aeroengine components (stationary vanes and rotating blades in the first compressor stage of a turbine). In the proposed system, a 3D optoelectronic sensor device, simulation and cutting path planning systems were used to repair a component using a master model. Similarly, Bremer [5,6] presented a reverse engineering (RE)-based repair system for aeroengine parts. A touch-trigger probe inspection technique and milling operation with modifying the master cutting path model are the basis of the developed system. 3D optoelectronic sensor is one of the preliminary versions of current 3D optical devices. Although the touch-trigger probes are easy to use, the process is slow and has been found an inappropriate for the complex geometries compared to the laser and optical scanners. In fact, the repair of each component is a unique set of problems since the component after worn or deposited no longer represents its original shape [3]. On the other hand, using a master model in a repair process may be a solution for simple geometries, such as straight blades but more complex geometries certainly need more attention in inspection, reconstructing and machining
  processes. For instance, a thin-curved blade profile cannot be created by probing and the reconstruction of nominal blade model will be difficult. Alternatively, repair of aeroengine components can also made by robotic systems. These systems [7,8]are mainly used for machining operations, i.e. grinding and polishing, to remove fairly less excess material from blade surface, such as coatings. Despite these approaches give more flexibility in terms of removing excess material from blade surface, particularly minor damages are repaired due to less accuracy and lack of reliability of these systems.

  This paper presents a repair and overhaul methodology developed for aeroengine components. The purpose of the developed methodology is to restore the components nominal geometry satisfying with tolerances defined by engine manufacturer after being worn or damaged. Automated and adaptive solutions have been achieved in order to reduce the total repair time and for part-to-part variation. The methodology is an integrated approach of 3D optical measurement, free-form surface reconstruction and machining operations. The three stages are inter-connected and inter-faced each other through compatible file transfer. A 3D optical scanner has been used for inspection processes in order to acquire fast and accurate measurement compared to touch-trigger and laser probes. RE is the key process in blade repair and uses acquired data obtained from measuring devices [9,10] . Hence, in this study, a RE-based adaptive free-form surface reconstruction approach has been adopted for part-to-part variation in the repair process. In a previous work of the authors [11] , a RE-based approach was developed and presented for recreating the defect area geometry where the original geometry has been damaged. In this work, the proposed adaptive RE reconstruction method is only for re-modeling of deposited surfaces of components. Based on the reconstructed geometry, a machining strategy on which mainly focuses thin edge curved blades, has been developed. Thin-curved blades have been chosen to implement the proposed repair approach since these components are the most difficult to repair ones due to their delicate and complex shape.

  2. General structure
  The proposed repair system consists of a chain of different processes. The general structure of the proposed repair system is shown in Fig. 1. The digitization of the part surface is necessary for the inspection of damaged or deposited area. In this work, the GOM ATOS II-400 non-contact optical measurement system has been used to acquire 3D data from the part surface. The process uses projection technology and guarantees high data quality with a minimum of noise. Inspection of the part surface is utilized for identification and positioning of damaged/deposited area by comparing nominal CAD model (Reference) and digitized data (Data) of the part surface. The nominal CAD model can either be the initial design model of the part or the reconstructed surface model.

  At the end of the inspection process, a decision is made whether the part can be repaired, scrapped; or the repair is not required. This decision is totally subject to the results of the comparisons and the inspection process. Also, manufacturer recommendations on dimension and tolerance limits are taken into consideration during the decision-making process. If the part is decided to be repaired, the comparison results, such as position, maximum amount of wear and deposition, and the reconstructed surface CAD model will be used to initiate removal or deposition process. In this work, although the additive process has not been included, the same inspection procedure can be implemented to investigate the worn areas and the necessary data is sent as an input to the additive operation, i.e. laser cladding. The proposed repair process has only concentrated on the parts already metal deposited. To remove the excess material from the surface, a machining strategy has been developed. The machining strategy firstly deals with the error-free tool path generation necessary for milling operation. Following the tool path generation, machine codes are produced and CNC machining is performed. Postinspection is the last step in order to complete the whole repair process. Final digitization and comparisons are made in this step and repaired part can be eventually used in service after fulfilling acceptable dimension and tolerances. Information integration has been targeted to propose a reliable and accurate repair/overhaul process. In the following sections, each stage of the whole system is introduced by implementing on the repair of an aeroengine component: high-pressure (HP) single-crystal thin-curved compressor blade.

  Fig. 1. An overview of the proposed system.
  3. Implementation: thin-curved blade repair
  Repair technology is becoming an appropriate and justified strategy used for aeroengine components because of their high value and long lead time. For instance, manufacturing cost of a
  turbine blade is estimated E`780 while corresponding repair cost of the same component is between E`160 and E`260. If repair of a component costs less than manufacturing of a new one and extends overall life time to an acceptable limit, which is nearly half of its expected service life, repair can be regarded as a true alternative [12] . Turbine and compressor blades are the most important components used in a gas/jet engine and also are working in excessive conditions. Therefore, different types of damage are encountered on the blade tips, airfoils and edges caused from internal or external effects, as shown in Fig. 2. In this work, HP compressor blades have been chosen to implement the proposed repair methodology. Fig. 3 shows the blade and its location in the engine. The blade airfoil surface has a bidirectional twist and a thin cross-section ( o1.1 mm) as well as small edge radii (o0.3 mm).

  3.1. Non-contact measurement
  In the measurement (digitization) process, 3D surface data is acquired from the part surface. The acquired data is then processed and a polygonal mesh surface model is obtained for the part to be repaired. The digitized mesh model can then be used either for inspection of worn out/deposited areas and surface reconstruction. For that purpose, a detailed and accurate data need to be obtained from the part surface in a reasonably shorter time and can be easily manipulated in further operations. The GOM ATOS II-400 3D non-contact optical measurement system shown in Fig. 4 has been used in the proposed repair methodology. The digitizing equipment consists of four integrated units: a camera head (sensor unit), a stand, a rotary table and a control unit connected to a computer. The camera head has two high-resolution sensors on the left and right side of the head and a light projector in the middle. The camera head is mounted to the stand to facilitate a digitizing process. Three degrees of freedom (DOF) can be obtained by tilting the camera head and other three DOFs are from sliding in two vertical directions and a rotating mechanism of the stand.
  The digitizing system calculates precise 3D coordinates up to 1.3 million data points. Individual measurement images are merged via reference points (circular marks; white dots on a black background) and measured data is made available as point clouds, sections or polygon mesh. Thus, cloud data points are polygonized and can be exported in common CAD data formats, such as IGES, VDA, STL, etc. Working principle of the digitizing system is to project a fringe light pattern onto the object surface by which images are captured via two cameras. For all-round measurement of complex objects, several partial views are joined together.
  A digitizing procedure has been developed to be integrated with the proposed repair system. The procedure was used to develop an automation programme for repetitive and adaptive scanning processes. One of the most crucial steps in the digitizing process is the determination of the number of measurements around the part, since total digitizing time is affected. In the automated digitizing process for the blade tip repair, the number of measurements around the blade has been optimized to eight after a series of trials.Fig. 5 shows the stages in the procedure.
  The developed automated digitization process has been used in the repair process of the thin-curved blade. The blade in a clamped position via a special screw-pin fixture designed and produced for digitizing, build-up and machining purposes in order to maintain the blade pose and avoid unnecessary reclamp, is firstly located onto the rotary table. The blade can accurately be positioned on the fixture and the clamping of the blade can be performed via screw-pins regardless of geometric change of blades to guarantee a high-precision repair. When the automation programme is run, the whole digitizing process is utilized without any human interaction and a smooth mesh model of the blade is obtained at nearly 5 min. In Fig. 6, the blade before clamping and after clamping is shown.
  To deal with the digitized data in further inspection and surface reconstruction processes, a common reference coordinate system (RCS) has been transformed using 3-2-1 method, which is also compatible with the five-axis milling machine tool during the RCS setting prior to machining operation. Hence, the clamping and setup time has been reduced, which also effect the total repair cycle time. The smooth polygonal mesh model of the deposited blade tip surface can be seen in Fig. 7.

  Fig. 2. Defects on blade surfaces.

  Fig. 3. An HP compressor blade (left) and a jet engine (Rolls-Royce Trent 800) on right.


  Fig. 4. GOM ATOS-II 400 digitizing system.

  Fig. 5. Digitization steps.

  Fig. 6. The blade before (a) and after the clamped position (b).

  Fig. 7. Digitized blade tip surface.
  3.2. Surface reconstruction
  Reverse engineering-based surface reconstruction of the blade tip is performed only using a digitized polygonal model of the welded blade, since the original CAD model no longer represents the current blade shape. An adaptive approach has been developed in order to reconstruct the deposited blade tip surface and it can be used to restore the blade tip geometry to be used effectively in an aeroengine after restoration. The reconstruction of the blade tip surface is based on creating a non-uniform rational B-spline surfaces (NURBS) that can be dealt with by a CAM software to be used for removal process. When modeling complex shapes, the biggest problem for designers is to determine a reasonable surface patch boundary layout where the ( u , v ) surface parameterizations of each patch are in reasonable alignment with the underlying surface structure and the continuity conditions of the adjacent surface patches. In the proposed repair system, four surface patches are created using the cross-section curves obtained from the polygonal model of the deposited blade in the reconstruction process. The approach can adaptively be applied to all types of blade tips to be repaired. These four surface patches are called leading edge (LE), trailing edge (TE), suction surface and pressure surface to be compatible with the well-known description of the airfoil of a blade surface. Although any CAD package can easily be used to implement the proposed method, Rhinoceros 3D modeling package has been chosen in this work. A step-by-step procedure of the reconstruction of the blade tip is given as follows:

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