Patent Publication Number: US-2020291780-A1

Title: Nickel-chromium-aluminum composite by electrodeposition

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application No. 61/914,307 filed on Dec. 10, 2013 and titled Active Flutter Control of Variable Pitch Blades, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF USE 
     The present disclosure relates to a composite including nickel-chromium alloy and aluminum, and alloys or compounds formed by nickel, chromium and aluminum, and more particularly to a nickel-chromium-aluminum (Ni—Cr—Al) alloy applied to gas turbine parts for wall restoration and bond coat, a method for electrodepositing the Ni—Cr—Al alloy and associated heat treatment, and coated articles. 
     BACKGROUND 
     High and low pressure turbine parts including turbine vanes or airfoils are made of nickel based superalloys. These components are protected against the high temperature environment by a thermal barrier coating (TBC). In the TBC, a bond coat disposed in between the top oxide layer and the substrate superalloy provides an aluminum reservoir, which supply aluminum diffusing outwards to form protective α-Al2O3, an adherent thermally grown oxide (TGO). Thus, the bond coat is critical for protecting gas turbine components from high temperature oxidation. Like aluminum, chromium tends to form dense oxide chromia in a high temperature environment, providing hot corrosion protection. These elements allow the parts made from nickel alloys to perform well in gas turbine engines. 
     Turbine vanes are occasionally removed from service due to the loss of wall thickness during such repair processes as coating stripping, recoating, grit blast cleaning, and chemical processing which typically remove some base metal and often reduce component wall thicknesses below the required minimum thickness. 
     Thinned turbine vanes or airfoils are either replaced with new parts or scrapped unless the lost wall thickness is restored by adding metal materials that include key elements (e.g., Cr and Al) lost during the repair processes. 
     Accordingly, it is desirable to restore the lost wall thickness of turbine vanes or airfoils by providing a metal coating layer that includes key elements (e.g., Cr and Al) lost during the repair processes to increase the number of repair cycles for the vanes or airfoils. 
     SUMMARY 
     The present disclosure relates to a composite including nickel-chromium alloy and aluminum, and alloys or compounds formed by nickel, chromium-and aluminum applied to gas turbine components for wall restoration or enhanced bond coat. Specifically, Ni—Cr alloy and Al are sequentially electro-deposited from environmentally benign ionic liquid chemicals. The Ni—Cr—Al composite is subsequently heat-treated to form a diffused Ni—Cr—Al alloy having a composition that mimics the main chemistry of the base alloy, e.g., Ni-based superalloy. The diffused Ni—Cr—Al alloy allows to restore materials lost during the repair processes, and contributes to prolong the lifetime of the turbine parts that arc subject to high temperature environment and repeated repair processes. 
     According to an aspect of the present disclosure, a coated article includes a turbine component and a Ni—Cr alloy and an Al deposit coated on the turbine component, wherein the Ni—Cr—Al composite alloy includes from 2 to 50 wt % chromium, from 0.1 to 6 wt % aluminum, and remaining nickel, and wherein the Ni—Cr—Al composite is heat-treated to form a diffused Ni—Cr—Al alloy that includes an aluminum compound (aluminides) formed by nickel and aluminum and to restore materials lost during repair processes of the turbine component. 
     According to another aspect of the present disclosure, a method for forming a nickel-chromium-aluminum (Ni—Cr—Al) composite and associated alloys on a turbine component is disclosed. The method includes providing a first plating bath for Ni—Cr alloy deposition, which is made from a solution including a solvent, a surfactant, and an ionic liquid (deep eutectic solvent), including choline chloride, nickel chloride, and chromium chloride, wherein a molar ratio of the choline chloride and chromium chloride ranges from 0.5 to 3.5 and the solvent comprises from 5 to 80 vol. % relative to a mixture of the choline chloride and metal chlorides including the nickel and chromium chlorides. 
     The method further includes electrodepositing a Ni—Cr alloy on the turbine component coupled to a cathode by providing an external supply of current to the cathode and an anode in the first plating bath. In addition, the method includes providing a second plating bath made from an ionic liquid including Lewis acidic 1-ethyl-3-methylimidazolium chloride or 1-butyl-3-methylimidazolium chloride and an aluminum compound such aluminum chloride (AlCl3), and electrodepositing an aluminum (Al) onto the Ni—Cr alloy in the second plating bath. The method further includes heat-treating the electrodeposited composite Ni—Cr alloy and Al layer at a high temperature to form a diffused Ni—Cr—Al alloy that includes an aluminum compound primarily formed between nickel and aluminum, and to restore materials lost during repair processes of the turbine component. 
     The details of one or more embodiments of the present disclosure and other benefits are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the present invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates an example of a plating bath filled with an electrolytic solution for electrodepositing either a Ni—Cr alloy or aluminum on a turbine component according to an aspect of the present disclosure. 
         FIG. 2  is a cross-sectional view of a Ni—Cr alloy electrodeposited on a metal substrate in a choline chloride-mixed metal chlorides solution. 
         FIG. 3  is a flow chart of a Ni—Cr—Al composite layer deposition process of the present disclosure. 
         FIG. 4A  is a schematic cross-sectional view of a diffused Ni—Cr—Al composite alloy coated on a turbine component. 
         FIG. 4B  is a micrograph of a diffused Al coated Ni superalloy. 
     
    
    
     The drawings depict various preferred embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example of a plating bath filled with an electrolytic solution for electrodepositing a Ni—Cr alloy or aluminum on a turbine component according to an aspect of the present disclosure. A turbine component  104  which is to be plated with a Ni—Cr alloy and aluminum respectively is pre-treated prior to electrodeposition. A pre-treatment is typically performed to remove grease, oil, oxides and debris from the turbine component by mechanical abrasion, acid or alkaline etching, and/or electro-etching followed by surface activation, but is not specifically limited to the above processing steps and specified sequence. 
     Referring now to  FIG. 1 , there is provided a plating bath  102  containing an electrolytic solution that includes a room temperature ionic liquid including choline chloride, nickel chloride, chromium chloride, solvents, and surfactants like anionic, cationic, or Zwitterionic (amphoteric) surfactants. One of the surfactants includes one of more species of a sodium dodecyl sulfate, fluorosurfactants, cetyl trimethylammonium bromide (CTAB), or cetyl trimethyammonium chloride (CTAC). It is noted that the choline chloride based processing is low-cost and environmentally friendly. In one embodiment, a molar ratio of the choline chloride and chromium chloride ranges from 0.5 to 3.5, and polar aprotic and polar protic solvents are used to adjust the viscosity and conductivity of the plating bath  102  to attain a high quality Ni—Cr alloy coating. 
     Specifically, protic solvents are preferred due to their ability to donate hydrogen bonds. The solvents further include formic acid, citric acid, Isopropanol (IPA), water, acetic acid, and ethylene glycol. In the embodiment, preferred solvent content is from 10 to 80 vol % relative to the mixture of choline chloride and metal chlorides including nickel and chromium chlorides. 
     Referring to  FIG. 1 , an external supply of current is provided to an anode  106  and a cathode which is a turbine component  104  to be plated with Ni and Cr. The current can be a direct current or an alternating current including a pulse or pulse reverse current (not shown). The amount of current supplied can be controlled during the electrodeposition to achieve a desired coating composition, density, and morphology. 
     When the current is supplied, the metal (Ni and/or Cr) at the anode is oxidized from the zero valence state to form cations with a positive charge. These cations, generally forming complexes with the anions in the solution, are reduced at the cathode to produce metallic deposit. The result is the reduction of Ni and Cr species from the electrolytic solution onto the turbine component to be restored. The turbine component  104  is a cathode during electrodeposition. The electrodeposition inevitably decomposes water in the bath  102 , and thus the solution in the bath can be replenished to maintain consistent deposition quality. 
     The anode  106  includes a Ni—Cr alloy anode, a Ni and/or Cr anode, or any combination of these materials that can be chosen to satisfy different requirements. An insoluble catalytic anode (catalyzing oxygen evolution electrode) is preferred, but the type of anode used is not specifically limited to the above anode. A second layer of aluminum is deposited from a different plating bath, where the anode is pure aluminum. Aluminum electrodeposition is conducted in a water free environment and has been known to approach 100% efficiency because both hydrogen evolution and oxygen evolution are avoided. 
     In one embodiment, the Ni—Cr alloy includes from 2 to 50 wt % chromium and a remaining weight percentage of nickel. In a preferred embodiment, the Ni—Cr alloy comprises from 8 to 20 wt % chromium, and a remaining weight percentage of nickel. The electrodeposited Ni—Cr alloy is thicker than at least 10 μm. In a preferred embodiment, the electrodeposited Ni—Cr alloy is thicker than 125 μm. The top aluminum layer can vary in thickness, ranging from 2 μm to more than 125 μm. 
       FIG. 2  is a cross-sectional view of the Ni—Cr alloy  202  formed on a metal substrate  200  in a choline chloride-mixed metal chlorides solution. Referring to  FIG. 2 , a Ni—Cr coating thicker than about  70  um is formed on the substrate  200 . The Ni—Cr coating  202  and aluminum deposit may be applied directly to a surface of a turbine component which is formed from a wide range of metallic materials including, but not limited to, a single crystal nickel-based superalloy, and the copper substrate  200  represents a turbine component. The Ni—Cr aluminum composite  202  coated on a turbine component is subject to a post heat-treatment to homogenize the composition and add wall thickness back to the turbine component and replenish chromium and aluminum lost during the repair of the component. 
       FIG. 3  is a process flow chart of applying a Ni—Cr aluminum composite layer described in the present disclosure. Typically, a turbine component to be coated with a Ni—Cr—Al composite layer is pre-treated prior to the electrodeposition to remove foreign materials like debris, oxides and grease/oil from its surface. A method for electrodepositing a nickel-chromium-aluminum (Ni—Cr—Al) alloy on a turbine component begins at step  300  where a first plating bath filled with a solution is provided. The solution includes a solvent, a surfactant, and an ionic liquid including choline chloride, nickel chloride, and chromium chloride, wherein a molar ratio of the choline chloride and chromium chloride ranges from 0.5 to 3.5, and the solvent comprises from 5 to 80 vol. % relative to a mixture of the choline chloride and metal chlorides including the nickel and chromium chlorides, as disclosed above with reference to  FIG. 1 . 
     At step  302 , electrodepositing a Ni—Cr alloy on the turbine component is performed. An external supply of current is provided to a cathode and an anode in the first plating bath. The turbine component is the cathode, and a metal source is the anode. The component coated with Ni—Cr alloy is then rinsed and dried and prior to aluminum deposition. Additional surface preparation required for aluminum deposition is also performed. At step  304 , a second plating bath filled with an ionic liquid including Lewis acidic 1-ethyl-3-methylimidazolium chloride or 1-butyl-3-methylimidazolium chloride and an aluminum salt is provided for aluminum deposition on the Ni—Cr alloy coated component. At step  306 , electrodepositing aluminum (Al) onto the Ni—Cr alloy is performed in the second plating bath to form a Ni—Cr—Al composite on the turbine component. Once the Ni—Cr—Al composite is formed on the turbine component, at step  308 , a post heat-treatment of the Ni—Cr—Al alloy at 1100° C. or at a higher temperature is applied to the coated article to homogenize the composition, to form alloys and intermetallic compounds, and to restore key materials lost during previous repair processes or service of the turbine component, as shown in  FIGS. 4A and 4B . 
       FIG. 4A  is a cross-sectional view of a diffused Ni—Cr—Al alloy coated on a turbine component. The coated article  400  comprises a turbine component  402  which is typically made of Ni-based superalloy, a Ni—Cr alloy  404 , a Ni—Cr—Al zone  406 , an Al coating  408 , and a bond coat  410  which is typically re-applied after the dimensional restoration of the turbine component. 
     The coated article  400  is subject to a post heat-treatment at a high temperature as described above to form a diffused Ni—Cr—Al alloy  404 / 406 / 408 . Referring to  FIG. 4 , aluminum (Al) diffuses from Al coating  408  to Ni—Cr alloy  404  to form a Ni—Cr—Al zone  406 , chromium (Cr) diffuses from the Ni—Cr alloy  404  to the Al coating  408 , and Ni and/or Cr from the Ni—Cr alloy  404  diffuses into bond coat  410  and turbine component  402 , respectively, to homogenize the composition, to form an aluminum compound between nickel and aluminum, and to restore materials lost during previous repair processes of the turbine component.  FIG. 4B  is a micrograph of an Al deposit  420  on a Ni superalloy  422  before heat-treatment, and a diffused Al coated Ni superalloy  424  after heat-treatment at a high temperature. 
     In one embodiment, the Ni—Cr—Al composite includes from 2 to 50 wt % chromium, from 0.1 to 6 wt % aluminum, and a remaining weight percentage of nickel. In the embodiment, the electrodeposited Ni—Cr—Al alloy is thicker than 10 μm. In a preferred embodiment, the Ni—Cr—Al alloy includes from 8 to 20 wt % chromium, from 0.1 to 6 wt % aluminum, and a remaining balance of nickel. In the preferred embodiment, the electrodeposited Ni—Cr—Al composite is thicker than 125 μm. The coated article includes turbine vanes, rotor blades, or stators. 
     It is to be understood that the disclosure of the present invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible to modification of form, size, arrangement of parts, and details of operation. The disclosure of the present invention rather is intended to encompass all such modifications which are within its spirit and scope of the invention as defined by the following claims.