Abstract:
A method for removing a metal layer comprising the steps of providing a part having a slot, providing a porous metallic cathode comprising a recess bounded by a wall having an outer surface corresponding to the slot, inserting the porous metallic cathode into the slot, introducing an electrolyte into the recess of the porous metallic cathode, and removing a portion of an inner surface of the slot by flowing an electric current between the part and the porous metallic cathode.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation-in-part of Ser. No. 10/867,229, filed Jun. 14, 2004, entitled APPARATUS AND METHOD FOR WHITE LAYER AND RECAST REMOVAL, the disclosure of which is incorporated by reference herein as if set forth at length.  
       TECHNICAL FIELD  
       [0002]     The invention relates to an apparatus, and method for using such an apparatus, for removing small amounts of surface metal from a part. More particularly, the invention relates to a method for removing white layer and/or recast debris from metal parts.  
       BACKGROUND OF THE INVENTION  
       [0003]     Machining slots, particularly blade retention slots, using SAM (Super Abrasive Machining) or wire EDM (Electrical Discharge Machining) often times results in the creation of unwanted material upon the machined surface. In particular, SAM tends to produce undesirable, thin (approximately 0.0001 inch) localized areas consisting of white layer and bent grains. Similarly, wire EDM tends to produce an undesirable thin (approximately 0.0001 inch) uniform layer of recast material along the surface cut.  
         [0004]     As white layer and recast material is generally unwanted and may have an unacceptable deleterious effect on the operation of parts such as blade retention slots, it is desirable to precisely and uniformly remove a thin (up to approximately 0.0005 inch) layer so as to remove all of the white layer and/or recast material. Once such white layer and/or recast material is removed, the disk slots may optionally then be conventionally shot peened to provide desirable compressive stresses. Unfortunately, SAM or EDM re-machining would produce the same metallurgical damage as described above.  
         [0005]     What is therefore needed is a method for removing small amounts of material from the working surfaces of blade retention slots, so as to precisely and uniformly remove undesirable layers of white layer or recast material. Such method must be able to precisely and uniformly remove a thin layer of approximately 0.0005 inches from the inner surface of a slot.  
       SUMMARY OF THE INVENTION  
       [0006]     Accordingly, it is an object of the present invention to provide an apparatus, and method for using such an apparatus, for removing small amounts of surface metal from a part. More particularly, the invention relates to a method for removing white layer and/or recast debris from metal parts.  
         [0007]     In accordance with the present invention, a method for removing a metal layer comprises the steps of providing a part having a surface from which material is to be removed, providing a porous metallic cathode comprising a recess bounded by a wall having an outer surface corresponding to the part surface, inserting the porous metallic cathode onto the part surface, introducing an electrolyte into the recess of the porous metallic cathode, and removing a portion of the part surface by flowing an electric current between the part and the porous metallic cathode.  
         [0008]     In further accordance with the present invention, a cathode comprises a wall structured to form a porous electrical cathode having a recess, a first retaining plate attached to a first end of the porous electrical cathode, a second retaining plate attached to a second end of the porous electrical cathode, and a third retaining plate attached between the first end and the second end of the porous electrical cathode, and an electrolyte conduit inserted through the first retaining plate into the recess.  
         [0009]     In further accordance with the present invention, a method for removing metal layers comprises the steps of providing a part having a plurality of slots, providing a porous metallic cathode comprising a recess bounded by a wall having an outer surface corresponding to the slot, inserting the porous metallic cathode into one of the plurality of slots, introducing an electrolyte into the recess of the porous metallic cathode, removing a portion of an inner surface of the one of the plurality of slots by flowing an electric current between the part and the porous metallic cathode while introducing the electrolyte, removing the porous metallic cathode from the one of the plurality of slots, moving the part and the cathode relative to one another such that another one of the plurality of slots is aligned with the porous metallic cathode, and repeating the introducing step.  
         [0010]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is an illustration of the metal anode and porous metallic cathode of the present invention.  
         [0012]      FIG. 2  is a diagram of the apparatus of the present invention showing the retaining plates  
     
    
       [0013]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0014]     It is therefore a teaching of the present invention to provide an apparatus, and a method for using such an apparatus, to precisely and uniformly remove a thin layer of unwanted material from a surface to be treated, which is exemplified in the present disclosure as the inner surface of a slot, preferably a blade retention slot. This is accomplished by utilizing the part into which there is machined the blade retention slot as an anode. A metallic cathode comprises a porous, corrosion resistant, metallic material such that the outer surface of the metallic cathode is similar in shape to, but smaller than, the inner surface of the slot formed into the metal anode. An electrolyte is then injected into an interior cavity or recess of the porous metallic cathode and permitted to diffuse through the cathode and into the space between the metallic cathode and the metal anode. An electrical current is then produced to flow between the metal anode and the metal cathode at a rate and for a time sufficient to remove a precisely controlled, generally uniform layer from the inner surface of the slot.  
         [0015]     With reference to  FIG. 1 , there is illustrated in detail the apparatus of the present invention. Metal anode  13  is illustrated having a gap  17  machined into it from which unwanted material is to be removed. Metal anode  13  may be constructed of any metal. In a preferred embodiment, metal anode  13  is formed of nickel-based alloys, nickel-based superalloys, and titanium alloys. While shown with reference to a blade retention slot, gap  17  is not so limited. Rather, gap  17  may be any recess fabricated into metal anode  13 . Gap  17  is formed having an inner surface  11  upon which is located unwanted white layer and/or recast material (not shown) as described above. Typical thicknesses of such unwanted white layer and recast material are of up to approximately 0.0001 inches in thickness.  
         [0016]     Porous metallic cathode  5  forms a recess bounded by a wall  19  of a generally uniform wall thickness  3 . As constructed, porous metallic cathode  5  possesses an outer surface  7 . The shape of outer surface  7  is of a shape similar to that formed by the inner surface  11  of metal anode  13 . While the shapes of the inner surface  11  of metal anode  13  and the outer surface  7  of porous metallic cathode  5  are similar, the outer surface  7  of porous metallic cathode  5  is smaller so as to enable porous metallic cathode  5  to fit within the concave recess bounded by the inner surface  11  of metal anode  13 . Preferably, the outer surface  7  of porous metallic cathode  5  is between 0.005 and 0.025 inches smaller than the inner surface  11  of metal anode  13 . This results in a gap  17  formed between the outer surface  7  of porous metallic cathode  5  and the inner surface  11  of metal anode  13  extending for between approximately 0.005 and 0.025 inches. In a preferred embodiment, gap  17  extends for approximately 0.015 inches between inner surface  11  and outer surface  7 .  
         [0017]     As noted above, wall  19  is of a substantially uniform wall thickness  3 . In operation, an electrolyte is introduced into the concave recess formed by wall  19  and permitted to diffuse through the porous metallic cathode  5  and into gap  17 . It is therefore desirable that the electrolyte diffuses at a substantially even rate across the entire outer surface  7  of porous metallic cathode  5 . This is achieved by fashioning porous metallic cathode  5  of a wall  19  of substantially uniform wall thickness  3 .  
         [0018]     In order to permit an electrolyte introduced into an interior cavity of porous metallic cathode  5  to permeate the wall  19  and fill up gap  17 , thereby performing a conduit for electric current between porous metallic cathode  5  and metal anode  13 , porous metallic cathode  5  must be formed of a material providing pores through which the electrolyte may travel. Porous metallic cathode  5  is therefore formed of a porous, and preferably corrosion resistant metal. More preferably, such a metal is formed of porous stainless steel. Most preferably, the metal used to form porous metallic cathode  5  is approximately 100 micron porous stainless steel. A preferred method of forming porous metallic cathode  5  is to wire EDM a portion of porous stainless steel so as to produce a porous metallic cathode  5  of a desired geometry wherein the outer surface  7  of the porous metallic cathode  5  corresponds to the inner surface  11  of the metal anode  13  as described above.  
         [0019]     With reference to  FIG. 2 , there is illustrated the porous metallic cathode  5  of the present invention shown from the side. Attached to the porous metallic cathode  5  are a plurality of retaining plates  21 ,  23 ,  25 . Through one such retaining plate  25  is inserted an electrolyte conduit  15  through which electrolyte  27  may be introduced into the interior recess of porous metallic cathode  5 . In a preferred embodiment, electrolyte conduit  15  has a cross section, preferably non-circular, facilitating the gripping of electrolyte conduit  15  to avoid unwanted rotation during operation. Retaining plates  23 ,  25  are of a shape similar to that formed by outer surface  7  of porous metallic cathode  5  and are attached to both the front and rear ends of porous metallic cathode  5 . As such, retaining plates  23 ,  25  serve to insure that electrolyte  27  introduced into an interior recess of porous metallic cathode  5  via electrolytic conduit  15  does not immediately flow out of the front or rear ends of porous metallic cathode  5 . Similarly, retaining plate  21  serves to prevent electrolyte  27  introduced into an interior recess of porous metallic cathode  5  via electrolyte conduit  15  from exiting through the bottom of porous metallic cathode  5 . As illustrated, electrolyte conduit  15  is attached to retaining plate  25  such that electrolyte  27  introduced into electrolyte conduit  15  may travel into the interior recess of porous metallic cathode  5 . In this manner, electrolyte  27  may be introduced into an interior recess of porous metallic cathode  5  via electrolyte conduit  15  at a rate and pressure so as to produce a precisely controllable rate of diffusion of the electrolyte  27  through the wall  19  of porous metallic cathode  5  and into gap  17 .  
         [0020]     In operation, porous metallic cathode  5  is positioned within gap  17 . An electrolyte  27  is then introduced into porous metallic cathode  5  via electrolyte conduit  15 . Electrolyte  27  may be either an acid-based or saline-based electrolyte. Electrolyte  27  is introduced via electrolyte conduit  15  at a rate sufficient to entirely fill gap  17  and allow for discharge electrolyte/debris  12  to exit the gap  17 . A typical flow rate for electrolyte  27  is between approximately 0.5 and 3 GPMs/inch 2 . In a preferred embodiment, the flow rate is 1 GPM/inch 2 .  
         [0021]     Once electrolyte  27  is introduced via electrolyte conduit  15 , diffuses through the wall  19  of porous metallic cathode  5 , and fills up gap  17 , an electric current is induced across porous metallic cathode  5  and metal anode  13 . The electric current is formed from providing a low voltage differential across porous metallic cathode  5  and metal anode  13 . Typical values for this voltage in the case of a part fabricated from a nickel based alloy, range from approximately 5 to 20 volts. In a preferred embodiment, the voltage is approximately 10.5 volts DC. A current sufficient to remove a layer of material of a desired thickness may be applied over a surface area of the inner surface for an amount of time sufficient to achieve the desired removal of material. The amount of current utilized may be dependent upon the surface area and thickness of material being removed from the inner surface of the metal anode. As a result, the current value, and corresponding current density (A/in 2 ), varies as the surface area and amount of material being removed varies. For example, a current of about 45 A may be applied across a surface area of about 8.65 in 2  for a current density of about 5.2 A/in 2  at a voltage of about 10.5 V for about 108 seconds to remove about 0.001 in. of material from the inner surface  11  of the metal anode  13 .  
         [0022]     The material removed from the inner surface  11  of metal anode  13  is discharged in the form of a metal hydroxide sludge partially forming discharge electrolyte/debris  12 . This debris may be discarded or may be filtered out of discharge electrolyte/debris  12  so as to leave behind relatively pure electrolyte  27  which may be reintroduced via electrolyte conduit  15  and reused.  
         [0023]     In another embodiment, the present invention may be employed to efficiently remove white layer and recast material in a plurality of slots. With reference to  FIG. 1 , metal anode  13  typically comprises a plurality of fir tree shaped slots  17  fabricated, and radially disposed, about a disk or hub each gap  17  separated from its neighbors by a uniform distance. In such an instance, porous metallic cathode  5  is inserted into a gap  17  and an electrolyte is introduced and electric current provided as described above to remove metal from the surface of gap  17 . Porous metallic cathode  5  is then removed from gap  17 , the disk or hub forming said metal anode and cathode  5  are moved relative to one another, e.g., the disk is rotated or otherwise moved, so as to bring another gap  17  in alignment with porous metallic cathode  5 , and the process is repeated.  
         [0024]     By varying the voltage across the porous metallic cathode  5  and metal anode  13 , the rate of introduction of electrolyte  27 , and the duration of time over which the voltage is applied, it is possible to remove a uniform and precisely controlled amount of material from the inner surface  11  of the metal anode  13 .  
         [0025]     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.