ELECTROLYTE FOR THE ELECTROCHEMICAL PROCESSING OF SUPERALLOYS BASED ON GAMMA-GAMMA DOUBLE-PRIME NICKEL

Description

Technical Field

[0001] This invention relates to the field of electrolytes for electrochemical machining (ECM) of nickel-based superalloys of the γ-γ" (gamma-gamma second) type. Prior art

[0002] ECM is an unconventional machining process. This process is primarily aimed at electrically conductive materials. The principle of ECM is based on the anodic dissolution of a workpiece (anode) using a tool called a cathode in the presence of an ionically conductive electrolyte. The inter-electrode gap is defined as the distance between the workpiece and the cathode. It is in the order of 0.1 to 1 mm.

[0003] Precision electrochemical machining (PECM) is based on the same principle of anodic oxidation of the metal. However, it differs from ECM in that it uses a pulsed, rather than a continuous, current. The current is synchronized with an oscillation of the cathode. This oscillation is accompanied by a rectilinear translational movement, but also by a smaller gap (10 to 200 µm). The process thus becomes sensitive to the formation of hydrogen bubbles that form at the cathode and disrupt the efficiency of the process.

[0004] In today's practices, machining of γ-γ type nickel-based superalloys is carried out in a NaNO 3 based electrolyte solution at concentrations between 8% and 20% by weight relative to the total weight of the solution.

[0005] When performing ECM, as well as its derivatives (precision ECM PECM, electrochemical polishing ECG) in NaNO 3 medium, the anodic dissolution of γ-γ" superalloys generates soluble dissolution products but also insoluble products which form a sludge, a large part of which remains stuck on the electrochemically machined surfaces. The latter is mainly made up of oxides and hydroxides which oppose the optimal dissolution reaction. In addition, certain phases constituting these superalloys such as the γ" phases, carbides and nitro-carbides, are insoluble and during machining become loose under the action of the electrolyte flow, which forms a rough surface. All of these consequences lead to a reduction in yield as well as additional operations for reconditioning the machined surfaces (chemical pickling, abrasive polishing).

[0006] This dissolution is also accompanied by the formation of dihydrogen on the surface of the cathode, which causes fluctuation and disruption of the process. These formed gas bubbles cause disruption of the process due to the small distance between the cathode and the workpiece (gap between 10 µm and 200 µm for precision electrochemical machining (PECM), which reduces the yield.

[0007] An electrolyte for the electrochemical machining of certain metals (Fe, Cu, Ni, Al, Sn, Cr and Zn) as well as their alloys is known. This electrolyte is mainly based on the addition of complexing agents which can be EDTA, HEDTA, NTA or citric acid with an inorganic salt mixture based on NaNO 3 , NaCl, NaClO 4 , Na 2 SO 4 , KNO 3 , KCl, KClO 4 , K 2 SO 4 , LiNO 3 , LiCl, LiClO 4 , and Li 2 SO 4 for a concentration range from 100 g / L to 500 g / L. The electrolyte also contains a reducing agent such as ascorbic acid. However, complexing agents do not allow the complexation of Nb, the major element of γ inclusions, especially since NTA was used in a pH range of 2 to 7 which does not allow good complexation action.

[0008] Documents EP2947183, US2016 / 362810 and the article by SURESH et al. (INTERNATIONAL JOURNAL OF RESEARCH IN AERONAUTICAL AND MECHANICAL ENGINEERING, vol.4, No.1, January 1, 2016, pages 72-80) describe an electrolyte for electrochemical machining comprising NaNO3 and a complexing agent.

[0009] Patent application US2009 / 139875 describes an electrolyte for the electrochemical machining of nickel alloys comprising NaNO3 and Nitrilotriacetic acid at a pH less than or equal to 7.

[0010] There is therefore a need to find a new electrolyte intended for the electrochemical machining of nickel-based superalloys of the γ-γ type and which does not have the disadvantages of the prior art. Statement of the invention

[0011] The inventors have thus surprisingly discovered that an electrolyte based on NaNO 3 and having a particular composition was perfectly suited to the electrochemical machining of nickel-based superalloys of the γ-γ" type without having the drawbacks of the prior art and in particular that this electrolyte makes it possible to reduce the overvoltage of the hydrogen produced on the surface of the cathode, to increase the dissolution efficiency and produces a better effect on the reduction of roughness. The present invention therefore relates to an electrolyte for the electrochemical machining of nickel-based superalloy γ-γ" comprising, advantageously consisting essentially of, in particular consisting of: NaNO 3 in a content of between 10 and 30% by weight relative to the total weight of the electrolyte; a complexing agent which is Nitrilotriacetic acid (NTA) at a pH of between 10 and 14, advantageously at a pH of 10, the complexing agent being present in a content of between 1 and 5%, advantageously between 2 and 3%, by weight relative to the total weight of the electrolyte; optionally an anionic surfactant in a content of between 1 and 5% by weight relative to the total weight of the electrolyte; optionally NaOH in order to obtain the desired pH (depending on the complexing agent chosen); an aqueous solvent, advantageously water.

[0012] Nickel-based γ-γ" superalloys are, for example, Inconel 718 ®< marketed by the company alloy wire international. It is a precipitation-hardenable nickel-iron-chromium alloy with high creep rupture strength at high temperatures of around 700 °C, advantageously having the following composition, in % by weight: Ni 50-55, Cr 17-21, Mo 2.8-3.3, Nb 4.75-5.5, Ti 0.65-1.15, Al 0.2-0.8, C ≤ 0.08, Mn ≤ 0.35, Si ≤0.35, P ≤ 0.015, S ≤ 0.015, Co ≤ 1.00, Ta ≤ 0.05, B ≤ 0.006, Cu ≤ 0.30, Pb ≤ 0.0005, Bi ≤ 0.00003, Se ≤ 0.003 and the balance Fe.

[0013] NTA makes it easier to complex all metal cations such as Ni, Al, Fe, Cr at a pH between 10 and 14, advantageously for an optimal pH of 10.

[0014] An anionic surfactant may be present in the electrolyte. It reduces the surface tension and facilitates the dissolution reaction. It also reduces the overvoltage of the hydrogen produced on the surface of the cathode and which causes disturbances during the machining process and increases the dissolution efficiency. In an advantageous embodiment, the anionic surfactant is chosen from the group consisting of saccharin, sodium dodecyl sulfate, sulfonates, carboxylates, sulfocinates, phosphates and mixtures thereof, advantageously it is chosen from the group consisting of saccharin, sodium dodecyl sulfate and mixtures thereof.

[0015] The electrolyte according to the invention ensures homogeneous anodic dissolution of all phases of the γ-γ nickel-based superalloy. It also guarantees good surface conditions with optimization of the yield and dissolution speed and a reduction in the residues formed.

[0016] The electrolyte according to the invention is prepared by methods well known to those skilled in the art, by simply adding and mixing the various components in the aqueous solvent. If NaOH is necessary, it is added at the end until the target pH is obtained.

[0017] The present invention further relates to the use of the electrolyte according to the invention for the electrochemical machining of γ-γ nickel-based superalloys, in particular the precision electrochemical machining of γ-γ nickel-based superalloys, or the electrochemical polishing (ECG) of γ-γ nickel-based superalloys.

[0018] It further relates to a process for the electrochemical machining of a γ-γ nickel-based superalloy comprising the following successive steps: a- providing a part made of γ-γ" nickel-based superalloy as an anode; b- providing a tool as a cathode; c- providing an electrolyte according to the present invention; d- immersing the anode and the cathode in the electrolyte with an inter-electrode distance of between 0.1 and 1 mm; e- applying a direct current between the anode and the cathode so as to obtain the anodic dissolution of the part made of γ-γ" nickel-based superalloy; f- recovering the machined part obtained in step e).

[0019] The tool usable as a cathode in step b) of the electrochemical machining process according to the invention is mainly made of stainless steel, but can also be made of titanium alloys, platinum alloys, copper-based alloys or tungsten copper.

[0020] The current of step e) may have a voltage ranging from 4 to 30 V. Its current density is advantageously 12.8 A / cm 2 < . The application of direct current in the electrochemical machining process according to the invention makes it possible to circulate the electrolyte continuously between the cathode and the anode during step e) with a sufficient flow rate, in particular obtained using a pressure of between 3.5 and 4 bar (between 350,000 and 400,000 Pa), in order to evacuate the residues of the anodic dissolution (sludge and dihydrogen) and to guarantee an optimal dissolution rate. The dissolution rate is the difference in mass before and after machining divided by the machining time. It is advantageously greater than or equal to 80 mg / min.

[0021] The optimal dissolution efficiency advantageously obtained by the process according to the invention is greater than or equal to 60% by mass. It is calculated as follows: We have considered only nickel as the electroactive element for the sake of simplification. The anodic reaction is then: Ni → Ni 2+< + 2e -<

[0022] The theoretical mass loss ΔWt heo of the samples is established from the following relationship: Δ Wtheo = M × I × t n × F

[0023] M being the atomic molar mass of nickel, I the applied current, t the oxidation time, n the valence of the dissolved ions (n=2) and F the Faraday constant = 96,500 C / mol. The hypothesis that the Faradic efficiency is 100% has been put forward.

[0024] The yield is then defined by: η = Δ Wexp Δ Wtheo

[0025] With ΔW exp the mass difference before and after the test and following a final surface cleaning.

[0026] The present invention finally relates to a method for the precision electrochemical machining of a γ-γ nickel-based superalloy comprising the following successive steps: A- providing a part made of γ-γ" nickel-based superalloy as an anode; B- providing a tool as a cathode; C- providing an electrolyte according to the invention; D- immersing the anode and the cathode in the electrolyte; E- applying a pulsed current between the anode and the cathode, in synchronization with a possible oscillation of the cathode, and accompanied by a possible rectilinear translational movement of the cathode towards the anode making it possible to obtain a minimum inter-electrode distance of 10 to 200 µm, so as to obtain the anodic dissolution of the part made of γ-γ" nickel-based superalloy; F- recovering the machined part obtained in step E).

[0027] The tool usable as a cathode in step B) of the electrochemical machining process according to the invention is mainly made of stainless steel, but can also be made of titanium alloys, platinum alloys, copper-based alloys or tungsten copper.

[0028] The current of step E) may have a voltage ranging from 6 V to 18.7 V. The amplitude of the oscillations may be between 0.2 and 0.8 mm, preferably 0.4 mm. The frequency of the oscillations may be between 27 and 70 Hz and the duration of the pulses may be between 2 and 10 mS.

[0029] Step E) can be implemented with or without cathode oscillation, advantageously with cathode oscillation. If implemented without oscillation, a continuous displacement of the cathode (only with the current pulses) can be present.

[0030] Step E) can also be implemented in dynamic mode (with rectilinear translational movement of the cathode towards the anode) or in static mode (without rectilinear translational movement of the cathode towards the anode), with current pulses and possibly with associated oscillations for polishing or surface reconditioning operations (removal < 1 mm). Thus, step E) of the method according to the invention can be implemented in static mode, without rectilinear translational movement of the cathode towards the anode or step E) of the method according to the invention can be implemented in dynamic mode with rectilinear translational movement of the cathode towards the anode.

[0031] The invention will be better understood in light of the description of the examples which follow. Examples

[0032] Three electrolytes were tested in an ECM process under conditions slightly different from ECM (gap greater than 2 mm and low current density (178 mA / cm 2< )) on Inconel 718 ®<: Solution A: 2.35 M NaNO 3 in water Solution B: 2.35 M NaNO 3 + 0.1 M A. sulfosalicylic acid in water pH 6 Solution C: 2.35 M NaNO 3 + 0.1 M A. nitrilotriacetic acid in water pH 10.

[0033] Solution B is not in accordance with the invention.

[0034] SEM analyses of the surface conditions obtained after machining have shown a considerable reduction in corrosion products on the surface by using solutions B and C compared to the sodium nitrate medium (solution A). These products become less adherent in the presence of the complexing agent and are easily removed by the electrolyte flow.

[0035] Chemical analyses (mg / L concentration) of the dissolved elements in the used electrolytes show that in the presence of the sulfosalicylic acid-based complexing agent (solution B), niobium is 7 times more soluble than in nitrate medium alone. Similarly, the concentrations of Ti and Mo also increased in the electrolyte, which shows that their passage into solution was facilitated in the presence of the complexing agent. The results are summarized in Table 1 below. [Table 1] Neither Fe Cr Mo Nb You Solution A 11,9 4,6 4,7 0,6 0,1 0,0 Solution B 11,5 5,1 5,0 1,0 0,7 0,1

Claims

1. An electrolyte for the electrochemical machining of a γ-γ" nickel-based superalloy, comprising - NaNO3 in a content of between 10% and 30% by weight relative to the total weight of the electrolyte; - a complexing agent which is nitrilotriacetic acid at a pH of between 10 and 14, the complexing agent being present in a content of between 1% and 5% by weight relative to the total weight of the electrolyte; - optionally, an anionic surfactant in a content of between 1% and 5% by weight relative to the total weight of the electrolyte; - optionally, NaOH in order to obtain the desired pH; - an aqueous solvent.

2. The electrolyte as claimed in claim 1, characterized in that it comprises an anionic surfactant, advantageously selected from the group consisting of saccharin, sodium dodecylsulfate, sulfonates, carboxylates, sulfocinates, phosphates, and mixtures thereof, more advantageously selected from the group consisting of saccharin, sodium dodecylsulfate, and mixtures thereof.

3. The use of the electrolyte as claimed in any one of claims 1 or 2 for the electrochemical machining of a γ-γ" nickel-based superalloy, in particular the precision electrochemical machining of a γ-γ" nickel-based superalloy or the electrochemical grinding of a γ-γ" nickel-based superalloy.

4. A process for the electrochemical machining of a Y-Y" nickel-based superalloy, comprising the following successive steps: a- providing a γ-γ" nickel-based superalloy workpiece as an anode; b- providing a tool as a cathode; c- providing an electrolyte as claimed in any one of claims 1 or 2; d- immersing the anode and the cathode in the electrolyte with an inter-electrode distance of between 0.1 and 1 mm; e- applying a continuous current between the anode and the cathode so as to achieve the anodic dissolution of the γ-γ" nickel-based superalloy workpiece; f- recovering the machined workpiece obtained in step e).

5. A process for the precision electrochemical machining of a γ-γ" nickel-based superalloy, comprising the following successive steps: A- providing a γ-γ" nickel-based superalloy workpiece as an anode; B- providing a tool as a cathode; C- providing an electrolyte as claimed in any one of claims 1 or 2; D- immersing the anode and the cathode in the electrolyte; E- applying a pulsed current between the anode and the cathode, synchronized with a possible oscillation of the cathode, and accompanied by a possible rectilinear translational movement of the cathode towards the anode making it possible to obtain a minimum inter-electrode distance of 10 to 200 µm, so as to achieve the anodic dissolution of the γ-γ" nickel-based superalloy workpiece; F- recovering the machined workpiece obtained in step E).

6. The process as claimed in claim 5, characterized in that step E) is implemented in static mode, without rectilinear translational movement of the cathode towards the anode.

7. The process as claimed in claim 5, characterized in that step E) is implemented in dynamic mode, with rectilinear translational movement of the cathode towards the anode.

8. The process as claimed in any one of claims 5 to 7, characterized in that step E) is implemented with oscillation of the cathode.