Abstract:
Method and apparatus ( 20 ) for forming a smooth metal surface ( 42 ) on a metal substrate ( 22 ). A melt pool ( 32 ) solidifying under a layer of molten electrolytic slag ( 34 ) on the metal substrate is subjected to a DC current ( 12 ) between a cathode ( 28 ) in contact with the molten slag and the substrate, thereby causing anodic leveling of the surface. The cathode may be buried in a layer of flux material ( 26 ) which is melted by a laser beam ( 30 ) traversing the substrate. A filler material ( 24 ) may be melted coincidently in an additive process. The flux material includes electrolytic, optically transmissive and viscosity reducing constituents.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to apparatus and methods for laser fabrication and repair of metal components, and particularly relates to electrochemical smoothing of a solidified melt pool through electrolytic liquid slag thereon. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is often desired to produce a smooth surface on a metal article to control geometry or to improve performance or appearance. Metal deposition processes utilizing a flux material, such as submerged arc welding or flux core arc welding, sometimes produce a pock marked surface due to the accumulation of gas such as carbon monoxide at the interface between the molten metal and slag resulting from melting of flux and reaction with carbon. The present inventors have developed processes for depositing superalloy materials using a laser heat source to melt powdered superalloy material and flux. See, for example, United States patent application publication number US 2013/0136868 A1. It is expected that some applications of such flux assisted laser deposition processes may be susceptible to pock marking or may otherwise require post-deposition processing to achieve a desired surface finish. 
         [0003]    Electropolishing is an electrochemical process that deburrs and smoothes a surface of a metal article, and it is one post-deposition process that may be used to smooth the surface of a laser-deposited material. The surface is immersed in an electrolyte and is connected to positive direct current, making it an anode. Current flows from the surface to a cathode through the electrolyte via metal ions removed from the surface. Burrs and other projections become areas of high current density and are preferentially eroded, resulting in a process called anodic leveling. This is effective on many surface shapes including complex, high resolution surfaces that are not amenable to mechanical smoothing. Electropolishing and other surface smoothing processes add time and expense to any material deposition processes, and thus, further improvements are desired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention is explained in the following description in view of the drawings that show: 
           [0005]      FIG. 1  is a schematic front sectional view of an apparatus according to aspects of the invention. 
           [0006]      FIG. 2  is an enlarged schematic front sectional view of an apparatus according to further aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0007]    The present inventors have devised a technique for electropolishing newly solidified metal formed during laser material deposition repair or fabrication by using molten flux/slag on the deposit as the electrolyte. The combined flux-assisted laser deposition/electropolishing process may produce a smoother surface at a lower cost on a shorter schedule than prior art sequential deposition/smoothing processes. 
         [0008]      FIG. 1  is a schematic sectional view of an apparatus  20  operating on a substrate  22  according to aspects of the invention. A layer  24  of a filler material may be placed on a surface  23  of the substrate. A flux layer  26  is placed on the filler layer  24  or directly onto the substrate for a non-additive repair. A refractory cathode  28  is placed in contact with the flux. The cathode is an electrical conductor with a higher melting point than the laser processing temperature that reaches the cathode—for example, higher than the melting point of the filler material  24 . Exemplary cathode materials include niobium, molybdenum, tantalum, tungsten, and rhenium. The cathode may be formed as a plurality of wires in the flux. Spaces between the wires allow laser heat penetration to the filler material  24  or substrate. For example, the wires may be parallel or may form a screen with interstitial spaces. The cathode  28  and substrate (anode)  22  are connected to a DC power source  12  as illustrated. A laser beam  30  is directed onto the flux  26 , creating a melt pool  32  of filler material and/or substrate metal covered by melted flux material which forms a molten slag  34 . The laser beam  30  progresses in relative direction  36  over the flux  26 , leaving the melt pool and molten slag to solidify into a solidified deposit  38  and solidified slag  40 . The melt pool may have a higher solidification temperature than the molten slag, so the melt pool  32  solidifies first, leaving a zone E where the solidified deposit  38  is covered by molten slag  34 . Alternatively, the melt pool  32  may solidify first regardless of its solidification temperature relative to that of the molten slag due to heat transfer into the substrate  22 . Under the influence of the DC power source  12 , the region E of molten slag  34  above solidified deposit  38  enables a period of electropolishing (anodic leveling)  10  of the solidified surface  42  of the deposited filler material (or substrate material for non-additive embodiments) until the slag solidifies. 
         [0009]    The present inventors have disclosed flux compositions that are useful for the laser deposition of superalloy material. See United States patent application publication US 2015/0027993 A1, incorporated by reference herein. The flux  26  of the present invention contains electrolytic constituents that are liquid at the laser processing temperatures of the filler material. For example, the flux may form liquid slag in a temperature range above 1300° C. at an atmospheric pressure of 1013 millibars. An embodiment of flux may include one or more of the following: 
         [0010]    a) 40-80 wt % CaF 2    
         [0011]    b) 5-40 wt % Al 2 O 3    
         [0012]    c) 1-15 wt % SiO 2    
         [0013]    d) &gt;0-20 wt % MnO 
         [0014]    e) &gt;0-15 wt % CaO 
         [0015]    f) &gt;0-7 wt % MgO 
         [0016]    g) &gt;0-7 wt % TiO 2    
         [0017]    h) &gt;0-10 wt % Fe 2 O 3  and/or Fe 3 O 4    
         [0018]    In another embodiment, a filler layer  24  is not provided. The melt pool  32  is formed by melting the surface  23  of the substrate  22  for crack repair and surface restoration. Alloy constituents that have been depleted near the surface of the substrate, such as aluminum, may be restored by constituent additions in the flux  26  as pure elements, metal compounds, or alloys and in various forms including powder and foil. 
         [0019]      FIG. 2  schematically illustrates the laser beam  30  being turned on A and off B as it passes respectively between or over the wires of the refractory cathode  28 . The spaces between the wires of the cathode allow the laser beam to penetrate through the flux to the filler metal  24  or the substrate  22  without direct impingement onto the wires of the cathode  28 , which by way of applied electrical current  12  accomplish electropolishing  10 . 
         [0020]    It is advantageous to make the flux optically transparent or translucent to laser light, as described by the present inventors in United States patent application publication US 2014/0220374 A1, which is also incorporated by reference herein. This can be done by constituting the flux of optically transmissive constituents in a range of 5-60 wt % or 20-40% wt %, as examples. Optically transmissive constituents include metal oxides, metal salts, metal silicates, and various fluorides. Examples include alumina (Al 2 O 3 ); silica (SiO 2 ); zirconium oxide (ZrO 2 ); sodium silicate (Na 2 SiO 3 ); potassium silicate (K 2 SiO 3 ); zinc selenide (ZnSe); magnesium, calcium, and barium fluorides (MgF 2 , CaF 2 , BaF 2 ); and other compounds capable of optically transmitting laser energy, for example as generated from Nd:YAG and Yb fiber lasers. Some optically transmissive constituents are also electrolytic constituents. The following list provides exemplary ranges of constituents for a flux that is both optically transmissive and electrolytic: 
         [0021]    a) 40-80 wt % CaF 2    
         [0022]    b) 5-40 wt % Al 2 O 3    
         [0023]    c) 1-15 wt % SiO 2    
         [0024]    It is also advantageous that the molten slag have low viscosity to facilitate leveling of the surface  23  of the deposit by surface tension and/or by facilitating the release of gasses from the interface of the molten metal and flux. Viscosity may be reduced by including in the flux one or more viscosity reducing constituents totaling a greater proportion than any viscosity increasing constituents such as Al 2 O3 3 , TiO 2 F, and SiO 2 . Viscosity increasing constituents (VIC herein) form a network of covalent bonds, while viscosity reducing constituents (VRC herein) interfere with such network formation. Such properties of materials can be found in available handbooks and online resources such as provided by the ASM International professional society. 
         [0025]    Examples of viscosity reducing constituents include one or more of CaO, MnO, Fe 2 O 3 , CaF 2 , Na 3 AlF 6 , MgO, Na 2 O (maximum 5 wt %), and K 2 O (maximum 5 wt %). Some exemplary ranges of low viscosity, optically transmissive, electrolytic fluxes are shown in the following table. In general, the flux may contain one or more electrolytic constituents; one or more optically transmissive constituents (OTC), including any electrolytic constituents that are also optically transmissive; and one or more viscosity reducing constituents (VRC) totaling a greater weight % than any viscosity increasing constituents (VIC). 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Electro- 
                   
                 Viscosity 
               
               
                 Embodi- 
                 lytic 
                 Optically 
                 Reducing 
               
               
                 ment 
                 weight % 
                 Transmissive (OTC) 
                 (VRC) 
               
               
                   
               
             
             
               
                 A 
                 40-80% CaF 2   
                 Included in the electrolytic, 
                 VRC &gt; VIC 
               
               
                   
                   
                 but also may include other 
               
               
                   
                   
                 OTCs e.g. MgF 2  and BaF 2 . 
               
               
                 B 
                 5-40% Al 2 O 3   
                 Included in 
                 VRC &gt; VIC 
               
               
                   
                   
                 the electrolytic 
               
               
                 C 
                 1-15% SiO 2   
                 Included in 
                 VRC &gt; VIC 
               
               
                   
                   
                 the electrolytic 
               
               
                   
               
             
          
         
       
     
         [0026]    For example, in one embodiment the flux may comprise 1-15 weight % of SiO 2  as an optically transmissive and electrolytic component; and at least one further electrolytic component selected from the group of CaO and MgO; and a viscosity reducing proportion of one or more components including CaF 2  having a total weight % greater than a total weight % of any and all viscosity increasing components in the flux 
         [0027]    Upon cooling of the apparatus  20  following laser processing, the slag is removed to reveal the smooth surface  42 . The cathode  28  is encased in the solidified slag  40  and it may facilitate slag removal from the substrate  22 . The cathode  28  may be reused by mechanically breaking the brittle slag off of the cathode  28 . 
         [0028]    The invention overcomes the following obstacles: 
         [0029]    a) Electrolytes used for prior art electropolishing vaporize at the laser processing temperatures of molten metal. Exemplary conventional electrolytes include mixtures of sulfuric acid and phosphoric acid, perchlorates with acetic anhydride, and methanolic solutions of sulfuric acid. 
         [0030]    b) Conventional cathode materials such as lead, copper, and stainless steel, would melt at the laser processing temperatures of high-temperature superalloys. 
         [0031]    c) A cathode in the flux or molten slag could block the laser beam used to melt the filler or substrate and may be damaged by the beam. 
         [0032]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.