Patent Publication Number: US-8980067-B2

Title: Method and apparatus for etching the surfaces of integrally bladed rotors

Description:
This application claims priority to German Patent Application DE102009021561.1 filed May 15, 2009, the entirety of which is incorporated by reference herein. 
     This invention relates to a method and an apparatus for etching the surfaces of integrally bladed rotors (blisks) of aircraft gas turbines. 
     The surfaces of the rotors of turbines or compressors of aircraft gas turbines are, as is generally known, subjected to an etching process after finish machining by mechanical or chemical removal processes to enable a structural examination to be subsequently made and structural defects, if any, to be better imaged and revealed and, finally, the in-service failure safety of critically classified rotary components to be improved. Surface etching of rotors made of nickel-base material can, among others, be accomplished by immersion in an etchant. The etching effect of this process is, however, not strong enough to enable a satisfactory structural examination to be made and positively and reliably detect all structural defects. It has also been proposed that the surface of unbladed rotor disks made of nickel-base material be etched electrolytically. Here, the electrochemical removal of surface material is effected by the transfer of electric charge between the metallic material of the workpiece and a liquid electrolyte. In this process, the rotor disk is immersed into the electrolyte and connected as anode (positive pole of a direct-current source). The flow of current via the electrolyte and a cathode, in connection with the electrolyte, causes material to be removed from the metal surface by metal ions going into solution. The apparatus provided for the performance of the etching process includes a container which is filled with electrolyte and has cathodes arranged on the sidewalls and, disposed above the container and suspended from a contact bar forming the positive pole, a holding device for the rotor disk thus acting as anode. On unbladed disks, electrolytic etching using this apparatus, as compared to pure immersion etching, enables a stronger etching effect to be obtained and structural examination to be improved. For the structural examination or the safe detection of structural defects of bladed rotors (blisks) in nickel-base material, electrolytic etching using the known apparatus is only restrictedly useful because the etching effect in the blade area, due to the complex blade geometry and the current losses occurring from the contacts at the rotor disk to the blade tips, is inconsistent and also lower in relation to the disk surface. Therefore, the removal of surface material from blisks for the purpose of subsequent structural examination is performed by use of the more consistent, but less intense, immersion etching process by which, however, reliable structural examination is not ensured. 
     A broad aspect of this invention is to provide a method and an apparatus for etching the surfaces of integrally bladed rotors (rotor disks) in nickel-base material such that a reliable detection of structural defects in ensured in all surface areas. 
     The present invention provides that nickel-base, integrally bladed rotors for aircraft gas turbines are electrolytically etched for structural examination in that, from all parts of the rotor connected as anode, a uniform current flow via the electrolyte to the main and auxiliary cathodes is produced in the blade areas remote from the electrical contact points at the rotor disk by providing additional auxiliary cathodes disposed aside the main cathodes, thereby obtaining a sufficiently strong and consistent etching effect for blisks made of nickel-base material and ensuring reliable structural examination also in the bladed area. 
     According to the present invention, an apparatus for obtaining uniform current flow from the entire surface of the rotor connected as anode via the electrolyte includes an electrolyte-filled container with main cathodes arranged therein and, suspended from a contact bar, a holding device with contacting and retaining pins for retaining and electrically contacting the rotor at the rotor disk. In the area of the blades integrally formed onto the rotor disk, auxiliary cathodes are arranged which are adaptable in form, size and arrangement to the blade geometry and the current flow to be obtained via the electrolyte to control the etching effect in the blade area. Besides uniform etch attack, the auxiliary cathodes related to the blades also provide for protection against burning of the blade tips. 
     In a further development of the present invention, the auxiliary cathodes for influencing the current flow and the etching effect are arranged at one or both blade edges and/or at the blade tips and/or each between the blades at a distance corresponding to the respective requirements. 
     In a further development of the present invention, the auxiliary cathodes are provided as an annular disk spaced from the blade side edges or as a tubular section spaced from the blade tips or also as a ring with a certain, for example circular, cross-sectional shape. The auxiliary cathodes may also include a plurality of individual cathode sections arranged beside, above, beneath or between the blades. 
     In a further development of the present invention, the holding device has a retaining bracket at whose bottom end first and second retaining arms are provided. Arranged at the mutually facing free ends of the retaining arms are the contacting and retaining pins engaging on both sides with the thickened inner area of the rotor disk and connecting to the positive pole of a direct-current source. Thus, the wetting of the rotor is only interrupted at the points of engagement between the contacting and retaining pins and the rotor disk. 
     In development of the present invention, the auxiliary cathodes are attached to the retaining bracket in an electrically insulated manner and connected to an electric conductor routed in an insulated manner on the retaining bracket and connecting to the negative pole of a direct-current source. 
     The first retaining arms are attached directly to the retaining bracket, and the second retaining arms are threadably attachable to the retaining bracket by a threaded connection to clamp the rotor disk between the contacting and retaining pins. 
    
    
     
       The present invention is more fully described in light of the accompanying drawings showing a preferred embodiment. In the drawings, 
         FIG. 1  is a schematic representation of an apparatus for electrolytically etching a blisk, 
         FIG. 2  is a schematic representation of a partial sectional view of a blisk with auxiliary cathodes differently arranged and designed in the blade area, and 
         FIG. 3  is a schematic representation of a holding device connected as anode for a blisk with auxiliary cathodes attached to the latter in an insulated manner and related to the blade area of the blisk. 
     
    
    
     The etching apparatus illustrated in  FIG. 1  includes a container  2  filled with electrolyte  1  and main cathodes  3  arranged in the container  2  and connected to the negative pole of a direct-current source. A holding device  5  is suspended from a contact bar  4  arranged above the container  2  and connected to the positive pole of the direct-current source. A rotor  6  is positioned at the bottom end of and conductively connected to the holding device  5  immersed in the electrolyte  1 , sulphuric acid in the present example, and therefore acts as an anode whose surface is to be etched. Attachment and conductive connection of the holding device  5  to the rotor disk  7  of the blisk-type rotor  6  is indicated by arrowhead  8 . Auxiliary cathodes  10  connected to the negative pole of the direct-current source are arranged in the area of the blades  9  integrally formed onto the rotor disk  7 . In  FIG. 2 , the auxiliary cathodes  10  can be provided as annular disks  10   a  spaced sidewards from the blades  9 , or as a circumferentially extending tubular section  10   b  spaced radially from the blade tips. As shown in  FIG. 2 , the auxiliary cathodes can also be provided as rings  10   c  with circular or any other cross-sections, or as cathode sections  10   d  arranged between adjacent blades  9  and protruding into the space between the blades and, additionally, adapted in shape to the blade curvature. Besides their shape, the auxiliary cathodes  10  are also variable in number, disposition and distance to the blade area. Also, any combination of the disclosed cathodes can be used in conjunction with one another. As shown in  FIG. 3 , an auxiliary cathode  10  in the form of an annular disk  10   a  and an auxiliary cathode  10  in the form of a ring  10   c  with circular cross-section can be related to the blade area of the same rotor (blisk)  6 . 
     The holding device  5  exemplified in  FIG. 3  includes, suspended from the contact bar  4 , a retaining bracket  11  with first—integrally provided—retaining arms  12  and second retaining arms  14  threadedly attachable to the retaining bracket  11  via a threaded connection  13 . The rotor disk  7  is held between contact pins  15  provided at the ends of the retaining arms  12 ,  14  and, therefore, conductively connected to the retaining bracket  11  attached to the contact bar  4 . Furthermore, the rotor disk  7  is supported on the retaining bracket  11  via a spacer  16  made of insulating material. An electrical conductor  18  routed in insulation blocks  17  on the retaining bracket  11  is connected, on the one hand, to the negative pole of the direct-current source and, on the other hand, to an auxiliary cathode  10  in the form of an annular disk  10   a  circumferentially extending at a side edge of the blades  9  and to a further auxiliary cathode  10  in the form of a ring  10   c  disposed underneath the blade platforms  19  of the blades  9 . 
     After being cleaned, the rotor  6  (blisk) is etched upon attachment to the holding device, immersion in the electrolyte  1  kept under agitation by a stirrer (not shown) and application of a specific voltage. The flow of current from the workpiece acting as an anode via the electrolyte to the cathode will not take place in the blade area via the more remotely situated main cathodes  3 , but directly via the auxiliary cathodes  10  positioned closer to the blades  9  or the blade area, respectively, and designed in accordance with the blade geometry, thus enabling a material removal to be specifically set via the auxiliary cathode parameters (shape, size, distance, arrangement) which in all blade parts is uniform and appropriate for structural examination. Upon removal from the electrolyte  1  and the holding device  5 , the rotors  6  (blisks) thus etched are multiply rinsed/cleaned and subsequently dried, being then reliably structurally examinable in all areas of the rotor disk  7  and the blades  9 . 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Electrolyte 
           2  Container 
           3  Main cathodes 
           4  Contact bar 
           5  Holding device 
           6  Integrally bladed rotor (blisk) 
           7  Rotor disk 
           8  Arrowhead—conductive connection 
           9  Blade 
           10  Auxiliary cathode 
           10   a  Annular disk 
           10   b  Tubular section 
           10   c  Ring with circular cross-section 
           10   d  Cathode sections 
           11  Retaining bracket 
           12  First retaining arm 
           13  Threaded connection 
           14  Second retaining arm 
           15  Contacting and retaining pins 
           16  Spacer 
           17  Insulation blocks 
           18  Electrical conductor 
           19  Blade platform