Electrolyte for electrochemical machining of gamma-gamma prime nickel-based superalloys

Abstract

An electrolyte for electrochemical machining of a γ-γ′ nickel-based superalloy includes NaNO.sub.3 at a content of between 10 and 50% by weight relative to the total weight of the electrolyte; an additive chosen from KBr, NaBr, KI, NaI and mixtures thereof, in an additive/NaNO.sub.3 molar ratio of between 1 and 15; optionally an ethylenediaminetetraacetic acid-based complexing agent at a content of between 1 and 5% by weight relative to the total weight of the electrolyte at a pH of between 6 and 12; optionally an anionic surfactant at a content of between 1 and 5% by weight relative to the total weight of the electrolyte; optionally NaOH to obtain the appropriate pH; and an aqueous solvent.

Claims

1. An electrolyte for an electrochemical machining of a γ-γ′ nickel-based superalloy, comprising NaNO.sub.3 in a content of between 10% and 50% by weight relative to the total weight of the electrolyte; an additive selected from the group consisting of KBr, NaBr, KI, NaI and mixtures thereof in an additive/NaNO.sub.3 molar ratio of between 1 and 15; optionally, a complexing agent based on ethylenediaminetetraacetic acid in a content of between 1% and 5% by weight relative to the total weight of the electrolyte at a pH of between 6 and 12; optionally, an anionic surfactant in a content of between 1% and 5% by weight relative to the total weight of the electrolyte; optionally, NaOH to obtain the appropriate pH; an aqueous solvent.

2. The electrolyte as claimed in claim 1, comprising the anionic surfactant.

3. The electrolyte as claimed in claim 2, wherein the anionic surfactant is selected from the group consisting of saccharin, sodium dodecylsulphate, sulfonates, carboxylates, sulfocinates, phosphates, and mixtures thereof.

4. The electrolyte as claimed in claim 2, wherein the anionic surfactant is selected from the group consisting of saccharin, sodium dodecylsulphate and mixtures thereof.

5. The electrolyte as claimed in claim 1, wherein the additive is KBr.

6. The electrolyte as claimed in claims 1, comprising the complexing agent based on ethylenediaminetetraacetic acid.

7. The electrolyte as claimed in claim 6, wherein the complexing agent is ethylenediaminetetraacetic acid.

8. A process for the 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 the electrolyte as claimed in claim 1; 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).

9. A process for a 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 the electrolyte as claimed in claim 1; 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).

10. The process as claimed in claim 9, wherein step E) is implemented in static mode, without rectilinear translational movement of the cathode towards the anode.

11. The process as claimed in claim 9, wherein step E) is implemented in dynamic mode, with rectilinear translational movement of the cathode towards the anode.

12. The process as claimed in claims 9, wherein step E) is implemented with oscillation of the cathode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the efficiency of dissolution of the γ′ phase under ECM conditions for a low current density of 178 mA/cm.sup.2 as a function of the electrolyte used (solution A or B) of comparative example no. 1.

(2) FIG. 2 shows the efficiency of dissolution of the γ′ phase under ECM conditions for a low current density of 178 mA/cm.sup.2 as a function of the electrolyte used (solution A, B or C) of comparative example no. 2.

EXAMPLES

Comparative Example 1

(3) The first step was to separately synthesize the Ni.sub.3Ti hardening phase (γ′ phase). Two electrolytes were then tested in an ECM process under conditions differing slightly from ECM (gap greater than 2 mm and low current density: 178 mA/cm.sup.2):

(4) solution A: 2.35 M sodium nitrate in water

(5) solution B: 2.35 M potassium bromide in water.

(6) The dissolution efficiencies are shown in FIG. 1. They are calculated as follows:

(7) Only nickel was considered as electroactive element for the sake of simplicity. The anodic reaction is then:
Ni.fwdarw.Ni.sup.2.sup.++2e.sup.−

(8) The theoretical mass loss ΔW.sub.theo of the samples is established from the following relation:

(9) $\begin{array}{cc}\Delta \mathrm{Wtheo}=\frac{M\times I\times t}{n\times F},& \left(1\right)\end{array}$
M being the atomic molar mass of nickel, I being the applied current, t being the duration of the oxidation, n being the valence of the dissolved ions (n=2) and F being the Faraday constant=96 500 C/mol.

(10) A Faraday efficiency of 100% has been assumed. The efficiency is then defined by:

(11) $\begin{array}{cc}\eta =\frac{\Delta \mathrm{Wexp}}{\Delta \mathrm{Wtheo}}& \left(2\right)\end{array}$
with ΔW.sub.exp being the difference in mass before and after the test and following a final surface cleaning.

(12) It is observed that the γ′ phase is practically insoluble in nitrate medium (medium A) and goes from 1.4% to 42% efficiency with the use of a potassium bromide medium (medium B).

(13) However, in a similar ECM process but implemented on the Udimet 720 γ-γ′ nickel-based superalloy from Waspaloy in medium B, a deterioration in volume of the material was noticed, the material becoming porous. It appeared to be necessary to find an alternative electrolyte having a compromise between the effect of bromides and that of nitrates.

Example 1

(14) Electrolyte C=1.6 M NaNO.sub.3+2.35 M KBr (KBr/NaNO.sub.3 molar ratio=1.5), was tested in an ECM process identical to that of comparative example 1 but implemented on the Udimet 720 γ-γ′ nickel-based superalloy from Waspaloy, and good results are obtained, without deterioration in volume of the material.

(15) Different molar ratios of NaNO.sub.3 and KBr (R) were also tested as shown in table 1 below.

(16) TABLE-US-00001 TABLE 1 $R=\frac{\left[\mathrm{KBr}\right]}{\left[\mathrm{NaNO}3\right]}$   0   0.2   0.7   1   1.6   2.35   ∞ [KBr] mol/L 0 0.5 1.6 2.35 2.35 2.35 2.35 [NaNO.sub.3] mol/L 2.35 2.35 2.35 2.35 1.5 1 0

(17) The microstructure was monitored after each test as a function of the different ratios. The results of greatest interest are obtained for molar ratios between 1 and 2.35.

(18) Thus in NaNO.sub.3 medium and in the absence of KBr (KBr/NaNO.sub.3 molar ratio=0), the dissolution occurs inhomogeneously. Specifically, the gamma matrix is preferentially dissolved with respect to the gamma prime precipitates which become dislodged while leaving a rough surface.

(19) By introducing KBr, the size and also the dispersion of the gamma prime precipitates (gray particles) are reduced and the dissolution+dislodging phenomenon observed in NaNO.sub.3 medium becomes a phenomenon of co-dissolution of the two phases (gamma and gamma prime).

(20) However, for a proportion of less than 1 (KBr/NaNO.sub.3 molar ratio <1), the desired effect of co-dissolution is not ensured.

(21) On the other hand, the use of the medium based on 100% KBr was rejected because of the deterioration of the surface after the anodization tests.

Comparative Example 2

(22) In addition, the following electrolyte solutions were tested under the same conditions as in comparative example 1:

(23) Solution A: 2.35 M NaNO.sub.3 in water.

(24) Solution B: 2.35 M KBr in water.

(25) Solution C: 2.35 M KCl in water.

(26) The dissolution efficiencies are shown in FIG. 2. It is observed that solution B, which contains bromides, is more effective for the dissolution of the gamma prime phase because the bromides, in addition to being a pitting agent, are also complexing agents and their complexing power is greater than for the chlorides, which are nevertheless part of the same family as the bromides (halogen family).

Example 2

(27) In addition, the following electrolyte solution was tested under the same conditions as in example 1:
Electrolyte=1.6 M NaNO.sub.3+2.35 M KBr (molar ratio=1.5)+0.1 M EDTA+NaOH in water (pH 6).

(28) The disappearance of the metal oxide layer from the machined surface is observed, but in contrast no problem of porosity on the material appeared. The results obtained are better than those of example 1. The presence of EDTA therefore has a positive effect on the electrolyte and hence on the machining.

Example 3

(29) Metallographic sections were also created after testing on a PECM machine under the same conditions as comparative example 1 and compared in NaNO.sub.3 medium (solution A of comparative example 2), NaNO.sub.3+KBr (KBr/NaNO.sub.3 molar ratio (R)=2.35 of example 1) and NaNO.sub.3+KBr+0.1 M EDTA (KBr/NaNO.sub.3 molar ratio (R)=2.35) (electrolyte=1 M NaNO3+2.35 M KBr+0.1 M EDTA).

(30) In NaNO.sub.3 medium (KBr/NaNO.sub.3 molar ratio (R)=0) a layer of dissolution residue product is formed at the surface. The removal of this layer requires additional operations such as chemical pickling or mechanical removal (sandblasting). In the presence of KBr (KBr/NaNO.sub.3 molar ratio=2.35), this layer is almost absent.

(31) ICP analyses were also carried out on the spent baths after machining operation in order to determine the content of dissolved elements according to the different media.

(32) Table 2 below summarizes the percentage by mass of the major elements of the material.

(33) A clear enrichment was observed in a medium having a KBr/NaNO.sub.3 molar ratio=2.35 for the elements Ni, Ti and W, compared to the NaNO.sub.3 medium. This enrichment is more pronounced in the presence of EDTA used at a level of 0.1 M as complexing agent.

(34) TABLE-US-00002 TABLE 2 Ni Cr Co Mo Al Ti W (% by weight) (% by weight) (% by weight) (% by weight) (% by weight) (% by weight) (% by weight) NaNO.sub.3 46.9 22.5 16.3 4.6 0.0 1.2 3.9 R = 2.35 50.7 21.1 14.9 5.0 0.7 3.0 4.8 R = 2.35 49.9 14.3 13.6 5.7 1.1 4.4 6.5 with EDTA