Patent Publication Number: US-2017361422-A1

Title: Polishing method for turbine components

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to manufacturing methods, and more particularly to apparatus and methods for polishing workpieces. 
     A gas turbine engine includes a compressor used to pressurize intake air which then flows to a downstream combustor and one or more turbines. A compressor includes one or more rotors each rotor comprising a plurality of airfoil-shaped compressor blades. 
     Compressor performance may be enhanced by polishing the airfoil and flow surfaces to a low surface finish. Polishing processes for producing low surface finishes are well understood and industrialized. 
     One problem with prior art polishing processes is that they have a tendency to remove material and therefore change the aerodynamic contours of components such as compressor blades. This can lead to reduced aerodynamic efficiency, 
     BRIEF DESCRIPTION OF THE INVENTION 
     This problem is addressed by a process in which a workpiece is rigidly mounted into a vibratory finishing machine. A polishing media is introduced and the machine is operated with a polishing protocol to achieve a desired improvement in surface finish without negatively affecting the geometry of the workpiece. 
     According to one aspect of the technology described herein, a method is provided of polishing a metallic workpiece. The method includes: mounting the workpiece in a hopper; loading the hopper with a polishing media comprising, by weight percent, more than 98% metallic chips, less than 2% liquid, and less than 0.05% abrasive; and oscillating the hopper for a run time, thereby generating a flow of the polishing media over the workpiece, until a predetermined surface finish is achieved on the workpiece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a perspective view of a turbomachinery rotor; 
         FIG. 2  is a schematic, partially-sectioned side elevation view of the turbomachinery rotor disk of  FIG. 1  in a polishing machine; 
         FIG. 3  is a schematic plan view of a metallic chip; and 
         FIG. 4  is a side elevation view of the metallic chip of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates schematically a turbomachinery rotor  10  comprising a disk  12  with a central bore  14  and a rim  16  defining a flow/path surface. An array of airfoils  18  extend radially outward from the rim  16 . Each airfoil  18  has a leading edge  20 , a trailing edge  22 , and a pair of opposed convex and concave side walls  24  and  26  respectively, extending between a root  28  and a tip  30 . Each airfoil  18  has a chord dimension “C” measured from the leading edge  20  to the trailing edge  22 . In the illustrated example, the airfoils  18  are physically integral to the disk  12 . For example, the airfoils  18  may be constructed separately from the disk  12  and then bonded to the disk  12  using a solid state bonding process, or the airfoils  18  and the disk  12  may be machined from a solid billet of material. This type of structure may be referred to by various names such as an “integrally bladed rotor” or “blisk”. This type of turbomachinery rotor may be used in different areas of a gas turbine engine, such as a compressor rotor or a turbine wheel. Furthermore, it will be understood that the rotor  10  is merely an example of many different types of workpieces that may be polished using the method described herein. The rotor  10  may be constructed from various metal alloys, for example a titanium or nickel-based alloy. 
     As noted above, compressor performance may be enhanced by polishing the airfoils  18  and adjacent flowpath surfaces to a low surface roughness, herein referred to as a “low surface finish”, for example the surface finish may be about 16 Ra or less. 
       FIG. 2  illustrates an exemplary polishing machine  32  suitable for carrying out the method of the present invention. The polishing machine  32  includes a hopper  34  mounted to a base  36  by an elastic connection, such as the illustrated springs  38 . Means are provided for driving the hopper  34  with an oscillatory motion. In the illustrated example, a pair of electric motors  40  are mounted to the hopper  34 . Each motor  40  drives (e.g. by rotation) one or more eccentric weights (not shown). Thus operation of the motors  40  causes the hopper  34  to oscillate or shake in motion having both lateral and vertical components. The motors  40  may he controlled in a known manner so that the oscillation at a desired amplitude and frequency. 
       FIG. 2  also illustrates an exemplary fixture  42  which may be used to mount a turbomachinery rotor  10  to the hopper  34 . The fixture  42  incorporates a base  44 . lower and upper covers  46  and  48  respectively, a central post assembly  50 , a hollow central column  51 , a lower clamping element  52 , and an upper clamping element  53 . In use, base  44  with central post assembly  50  can be mounted to the floor  54  of the hopper  34  and left in place as a semi-permanent installation. The fixture  42  may be assembled outside the hopper  34  by placing the lower cover  46  over the central column  51 , then placing the rotor  10  over the lower cover  46 , then placing the upper cover  48  over the rotor  10 . The lower clamping element  52  is then placed over the upper cover  48  and engaged with the central column  51  to clamp the central column  51 , upper and lower covers  46 ,  48 , and rotor  10  securely together as a rigid subassembly. This subassembly may then be placed over the central post assembly  50  and clamped in place with the upper clamping element  53 . As an alternative procedure, the hollow central column  51 , upper clamping element  53 , and lower cover  46  could be left mounted to the base  44  in the hopper  34 . The rotor  10 , upper cover  48 , and lower clamping element could be assembled to the complete fixture  42  inside of the hopper  34 . The fixture  42  functions to secure the rotor  10  in the horizontal orientation and to mask off the disk  12 , while leaving the airfoils  18  and surrounding portions of the rim  16  exposed. 
     Once the rotor  10  is secured in the hopper  34 , hopper  34  is loaded with a polishing media  56 . The polishing media  56  includes an abrasive, metallic chips, and a liquid. 
     The abrasive takes the form of particles, for example aluminum oxide particles. Abrasive particles are commonly characterized by a metric known as “grit size”, with larger grit numbers corresponding to smaller particle diameters and smaller grit numbers corresponding to larger particle diameters. Commonly, a smaller grit designation is referred to as a “coarse” abrasive, while a larger grit designation is referred to as a “fine” abrasive. 
     In processing a workpiece one possible practice is to begin a polishing operation using a coarser grit, then to proceed through progressively finer and finer grits until a desired surface finish is achieved. As will be explained further below, the polishing method described herein can be carried out using multiple process steps wherein each step uses an abrasive of a different grit size. 
     As an alternative to using multiple grit sizes, the abrasive may include abrasive particles held together into larger groups or “clumps” by a binding agent. This combination permits the effective grit size of the abrasive to begin at a coarser value, and as the clumps break down into smaller clumps and/or individual particles, the effective grit size becomes finer. This property permits the abrasive to start out coarse, and become more fine during the polishing process. A nonlimiting example of a suitable binding agent is TRI-AL 860 available from S. P. M. Mould Polishing System srl of Conigliaro, Italy. 
     A relatively small mass of abrasive is provided in comparison to the size and volume of the workpiece. In order to provide a medium to distribute the abrasive evenly and to provide backing for the abrasive, a medium of dense, soft chips is provided. For example, soft metals such as zinc or copper may be used. 
       FIGS. 3 and 4  illustrate an example of a metallic chip  58 . The illustrated metallic chip  58  consists essentially of copper and has two spaced-apart side edges  60  connected by two end edges  62 . 
     In the illustrated example, the metallic chip  58  has a parallelogram shape in plan view. Stated another way, each end edge  62  intersects the opposed side edges  60  at an angle that is off-perpendicular by an amount θ. In the illustrated example the angle θ is about 30°, but this may vary, for example about 20° to about 40°. It has been observed that the parallelogram shape with non-perpendicular angles is effective to permit free flow of the metallic chips  58  during a polishing process, and to prevent “bridging” or interlocking of the metallic chips  58  with each other that would inhibit free flow. 
     The dimensions of the metallic chips are sized relative the object to be polished. In other words, larger chips would be used for larger workpieces and smaller chips would be used for smaller workpieces. The example metallic chip  58  has an overall length “L” on the order of 7 mm (0.28 in). The metallic chips  58  are generally thin enough to bend slightly under their own weight. For example their thickness “T” may be on the order of 1 mm (0.040 in). 
     A suitable liquid such as water is provided as an agent to separate and lubricate the metallic chips  58  so as to permit the metallic chips  58  to flow readily. 
     A surfactant may be added to the liquid to reduce its surface tension. The specific product used is not critical and any commercially available soap may be used. For example, commercially available detergent is suitable to serve this purpose. Depending on the specific application, special surfactants may be used to meet applicable environmental regulations. 
     Preferably, the polishing media has the following approximate composition by weight: metallic chips more than 98%, liquid less than 2%, abrasive less than 0.05%. These values may be varied to suit a specific application. An example of one suitable specific composition for the polishing media is as follows, by approximate weight percent: copper chips 98.8, water 1.16, surfactant 0.03, abrasive 0.01. 
     Once the rotor  10  is mounted and the media  56  is loaded, the process can begin by starting the motors  40  and operating them at a selected speed to achieve a selected frequency of oscillating motion. 
     The polishing process continues for a run time until the desired surface finish is achieved. For an initial run of a specific component, the run time may be determined by trial and error. Subsequent run times may be then be predetermined based on testing results of the initial run (e.g., measurements from a profilometer, coordinate measuring machine, etc.). Preferably, the run time is about 2.5 hours or less. Testing has shown that periodically reversing the direction of the motors  40  at a predetermined time interval is helpful in producing a consistent and acceptable end result. A nonlimiting example of a suitable time interval for reversing the direction is about 15 minutes. 
     If an abrasive with a binder is used as described above, the total process time may occur in a single uninterrupted session. Alternatively, if varying grits of abrasive are used, the total process time may be divided into shorter sessions adding up to the predetermined total time. For example, if the total desired process time is one hour, this may comprise 20 minutes of processing each for coarse, medium, and fine grits of abrasive. 
     The desired surface finish will vary with the specific application. As used herein, the surface roughness is characterized by the arithmetic average roughness value (Ra), expressed in microinches. For example the surface roughness may be less than 16 microinches Ra, preferably less than 8.5 microinches Ra. Using an exemplary rotor  10  comprising a titanium alloy, the process described herein can achieve an average surface roughness of 8.5 Ra with a run time of approximately one hour. This result is achieved while limiting reduction in the chord dimension C, (“chord loss”) to no more than 0.03 mm (0.001 in), while causing no negative impact to the airfoil leading edge shape or rounding of the airfoils tips. As another example, using an exemplary rotor  10  comprising a nickel alloy, the process described herein can achieve an average surface roughness of 6 Ra with a run time of approximately 2.5 hours. This result is also achieved while limiting chord loss to no more than 0.03 mm (0.001 in), and causing no negative impact to the airfoil leading edge shape or rounding of the airfoils tips. 
     When the polishing process is complete, the hopper  34  is emptied of media  56  and the rotor  10  is removed from the fixture  42 . The rotor  10  may be cleaned of excess media  56 , for example by a water rinse and drying. 
     The method described herein has several advantages over the prior art. In particular, testing has shown that the polishing method described herein is effective to obtain a desired surface finish while minimizing loss of material. In particular, the method prevents unacceptable loss in the chord dimension C which has a strong effect on aerodynamic efficiency of the airfoils  18 . It is believed that this result is due at least in part to the metallic chips  58  having a size which is large enough to “flow around” thin workpiece features such as the leading edge  20  and trailing edge  22  of the airfoil  18 , without significantly damaging or abrading those features. 
     The foregoing has described an apparatus and method for polishing. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may he combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying potential points of novelty, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.