Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23H—WORKING OF METAL BY THE ACTION OF A HIGH CONCENTRATION OF ELECTRIC CURRENT ON A WORKPIECE USING AN ELECTRODE WHICH TAKES THE PLACE OF A TOOL; SUCH WORKING COMBINED WITH OTHER FORMS OF WORKING OF METAL
  • B23H1/00—Electrical discharge machining, i.e. removing metal with a series of rapidly recurring electrical discharges between an electrode and a workpiece in the presence of a fluid dielectric
  • B23H1/04—Electrodes specially adapted therefor or their manufacture
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23H—WORKING OF METAL BY THE ACTION OF A HIGH CONCENTRATION OF ELECTRIC CURRENT ON A WORKPIECE USING AN ELECTRODE WHICH TAKES THE PLACE OF A TOOL; SUCH WORKING COMBINED WITH OTHER FORMS OF WORKING OF METAL
  • B23H1/00—Electrical discharge machining, i.e. removing metal with a series of rapidly recurring electrical discharges between an electrode and a workpiece in the presence of a fluid dielectric
  • B23H1/04—Electrodes specially adapted therefor or their manufacture
  • B23H1/06—Electrode material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23H—WORKING OF METAL BY THE ACTION OF A HIGH CONCENTRATION OF ELECTRIC CURRENT ON A WORKPIECE USING AN ELECTRODE WHICH TAKES THE PLACE OF A TOOL; SUCH WORKING COMBINED WITH OTHER FORMS OF WORKING OF METAL
  • B23H9/00—Machining specially adapted for treating particular metal objects or for obtaining special effects or results on metal objects
  • B23H9/006—Cavity sinking

Definitions

• the field of the invention is that of the machining of an imprint in an electrically conductive material by the technique of electroerosion (or EDM, acronym for “Electro Discharge Machining”).
• EDM electroerosion
• the invention relates in particular to the production by EDM of sealing slots in high pressure (HP) turbine rings for aircraft, made of ceramic matrix composite (CMC) material.
• SiC/SiC rings are made from a fibrous reinforcement in the form of wefts woven in three dimensions from SIC (silicon carbide) fibers sheathed with BN (boron nitride). The reinforcement is then densified so that the fibers are embedded in a predominantly SIC matrix. Densification is obtained by several successive steps and the SiC/SiC material thus obtained has specific physico-chemical properties such as its high hardness, which make it difficult to machine with a cutting tool.
• SIC silicon carbide
• BN boron nitride
• the shapes and widths of the sealing slots of less than 1 mm make it necessary to use small tools to machine them, which also substantially reduces the machinability rate.
• the sealing slots are produced by EDM for metal aeronautical parts. It is for all these reasons that it was decided to make the slots of the CMC rings by the EDM technique, having previously checked that the material was sufficiently electrically conductive.
• machining by electroerosion or EDM has as its general principle the removal of material by thermal erosion generated by a succession of electric discharges between a tool electrode 1 and the workpiece 2, all two being immersed in an electrically insulating machining liquid, called dielectric 4.
• the part to be machined must necessarily be electrically conductive (conductivity greater than 10' 2 S/cm) and is generally connected to the negative pole of a current source, while the tool electrode is connected to the positive pole of the current source.
• the dielectric has the functions of reducing the temperature of the machining area, this area can sometimes reach a temperature between 8,000°C and 12,000°C, of evacuating the residual particles (slag) produced during the electroerosion and allow the creation of the spark.
• the breakdown voltage is the minimum electrical voltage which renders a portion of an electrical insulator electrically conductive and which allows the ignition of the spark in the dielectric; this breakdown voltage depends on:
• the target surface condition requirement is a roughness (Ra) of less than or equal to 5 ⁇ m.
• the inventors used the technologies and strategies commonly used on metallic materials for the EDM machining of the sealing slots on the CMC SiC/SiC composite material. They therefore used, as electrode-tools, blades machined in solid material, in graphite or copper alloy.
• the first electrodes used are copper electrodes of the Cu-Al type (AFNOR standard (or Cu-ETP (ISO standard)) 0.5 mm thick for a targeted sealing slot width of 0.8 mm.
• the machining strategy used consists in carrying out two descents of the electrode-tool (roughing and finishing) in the part immersed in a dielectric hydrocarbon, the second descent taking place after resharpening at the end of the electrode-tool to clear wear and best approach geometric requirements.
• a vertical sinking along the Z axis is carried out up to the programmed dimension in depth.
• a planetary displacement in X and in Y with an eccentricity radius (also called eccentricity vector) of approximately 0.15 mm is carried out to best approach the geometric ribs of the slots d seal of 0.8 mm in the desired width.
• the inventors chose to machine the first samples with low energy parameters.
• the material removal rate (in mm 3 /min) is significantly lower on the CMC material than on a metallic material (ratio ⁇ 1/3);
• the roughnesses measured on the surfaces of the sides of the sealing slots are between 10 and 12 ⁇ m; this roughness obtained is essentially linked to the roughing regime (direct sinking) with a machining gap of approximately 0.15 mm and therefore, with a tool electrode 0.5 mm thick, the dimension of 0.8 mm is already reached in roughing and, consequently, the machined surfaces are very little taken up with the planetary displacement of the electrode-tool in the finishing regime;
• the sparking during the roughing regime is not constant (that is to say stable and repeatable); in fact, arcing is detected which causes fracturing in the SiC/SiC ceramic and generates large particles of material, which are difficult to evacuate with respect to the machining gap; surface and sub-surface cracks, crazing, as well as the formation of scattered craters on the machined surfaces are also observed;
• the performances are very closely linked to the renewal of the dielectric and the evacuation of the eroded particles and the gases produced; in fact, the material torn from the electrodes is in the form of small spherules whose dimensions range from a few micrometers in the finishing regime to a few hundred micrometers in the roughing regime; these particles accumulate in the machining gap and end up quickly by creating conductive bridges between the electrode-tool and the workpiece; these phenomena have already been observed for the machining of metallic materials, but they are amplified when “refractory” ceramics are machined in EDM; indeed, the removal of material resulting from the thermal erosion of “refractory” ceramics is generated essentially by fracturing and very little by the melting of the material, hence the formation of larger debris to be evacuated; this is why the circulation of the dielectric between the electrode-tool and the part is essential for the EDM machining of CMC materials;
• the inventors have sought to design a tool electrode capable of being used to machine a CMC material by EDM erosion, in particular for producing sealing slots in a SiC/SiC material, while substantially increasing the performance of the current EDM process and by improving the surface states obtained.
• the subject of the invention is an electrode-tool for machining, in a part made of electrically conductive material, an impression by electroerosion of the sinking type
• the electrode-tool comprising a body made of an electrically conductive material having a lower face and an upper face, opposite to each other, and two main side faces, opposite to each other, the body having a lower zone and an upper zone, which are superimposed in a direction of sinking of the electrode-tool, the lower zone comprising the lower face and part of the main lateral faces and the upper zone comprising the upper face and the other part of the main lateral faces
• the electrode-tool being characterized in that the body is provided, at least in the lower zone, with a plurality of openings chosen from channels or slots, a channel being an opening which opens onto at least one of the upper and lower faces, a slot being an opening which opens onto the lower face and onto the two main lateral faces, each channel having a diameter less than or equal to twice the sinking machining gap and each slot having a width, defined by
• the sinking machining gap is for example between 0.02 mm and 0.3 mm.
• the electrode-tool comprises channels in the lower zone and these channels extend into the upper zone, each channel opening out both on the lower face and on the upper face of the body.
• the distance between two adjacent channels is greater than or equal to twice the sinking machining gap.
• the electrode-tool comprises openings only in the lower zone, these openings being slots.
• the upper zone is therefore free of openings.
• the distance between two adjacent slots is greater than or equal to twice the sinking machining gap.
• the tool-electrode comprises slots in the lower zone and channels in the upper zone, each channel of the upper zone leading to a slot of the lower zone.
• each channel is superimposed on a slot in the sinking direction.
• the body of the electrode-tool is made of a CuCrZr copper alloy, preferably CuCriZr 0 ,i.
• the height of the lower zone in the sinking direction is at least twice the height of the upper zone in the sinking direction.
• the cavity to be machined having to have a given depth, length and width
• the body of the electrode has a thickness which is less than a value corresponding to the width of the cavity, from which two times the sinking machining gap (that is to say e ⁇ I - (2 x G)).
• the invention also relates to a machining device for machining an impression by electroerosion of the sinking type, the device comprising a tool electrode as described above and means cooperating with the openings of the tool electrode to create a circulation of machining liquid in the bottom of the cavity being formed, these means being chosen from means for injecting pressurized machining liquid into the plurality of openings of the electrode-tool, when the openings are channels, and machining liquid injection means around the electrode, when the openings are slots.
• the invention also relates to a process for forming an imprint in a part made of electrically conductive material by electroerosion machining, implementing a machining device as described above, and comprising the steps of:
• the method comprises rough machining, followed by finishing machining.
• the gap machining in the roughing regime, can be between 0.10 mm and 0.3 mm; in the finishing regime, the machining gap can be between 0.02 mm and 0.10 mm.
• the step of injecting machining liquid into the plurality of openings of the electrode-tool is concomitant with the step of finishing the impression and with the step of finishing the impression.
• the cavity to be machined is a sealing slot and the part is made of SiC/SiC composite material.
• the cavity to be machined is a sealing slot for a turbine ring.
• FIG. 1 is a schematic representation in section and in a front view showing the general principle of spark erosion by sinking along the Z axis;
• FIG. 2 is a schematic representation according to a perspective front view of the electrode-tool according to a first embodiment of the invention
• FIG. 3 is a sectional view along plane A of the tool electrode of Figure 2, the tool electrode being shown in the cavity of the workpiece;
• FIG. 4 is a schematic representation in section and according to a top view showing the eccentricity of the electrode-tool of Figure 3 in the cavity along the X-Y plane;
• FIG. 5 is a sectional view according to a front view of the electrode-tool of Figure 3, the electrode-tool being represented in the cavity of the part and showing the circulation of the machining liquid;
• FIG. 6a is a schematic representation according to a front view of the electrode-tool according to a second embodiment of the invention.
• - Figure 6b is a sectional view along the line BB of the electrode-tool of Figure 6a;
• - Figure 7a is a schematic representation according to a front view of the electrode-tool according to a third embodiment of the invention;
• Figure 7b is a sectional view along line C-C of the tool electrode of Figure 7a.
• the debris of material located at the bottom of the cavity d the machining cannot be evacuated and strongly penalize the advance of the machining according to the depth machined.
• the electrode-tool according to the invention makes it possible to improve the distribution of the dielectric in the machining cavity.
• Figures 2 to 7b show preferred embodiments of tool electrodes according to the invention, namely a configuration with through channels (Figure 2), a configuration in the form of a comb ( Figure 6a) and a configuration comprising both through channels in the upper part and a comb shape in the lower part (FIG. 7a).
• the plurality of openings in the electrode-tool according to the invention makes it possible to create a forced circulation of machining liquid in the machining gap G , which has the effect, on the one hand, of favoring the renewal of the dielectric in the machining gap and which makes it possible, on the other hand, to evacuate from the machining zone the degraded dielectric, as well as the residues resulting from material removal.
• the forced circulation of machining liquid in the machining gap can be created by injecting the machining liquid into the channels at their end close to the electrode holder.
• the electrode-tool 1 is a block of parallelepipedic material in which are made through channels 8 each having a diameter "d" less than or equal to twice the value of the machining gap in sinking (that is to say in roughing mode).
• a block of material having a height “H” of approximately 15 mm and a thickness “e” of 0.4 mm.
• I being the width of the sealing slot that it is desired to obtain.
• the length “L” of the electrode-tool can be a few millimeters or centimeters.
• a multitude of through-channels 8 are produced having for example a diameter "d" of 0.3 mm, which are arranged parallel to each other in the sinking direction Z (in the direction of the height ) and in a single row.
• the channels are spaced from each other by a value "c" equal to 2 times the machining gap in roughing mode G.
• FIG. 3 there is a cross-sectional view of FIG. 2 along plane A, the electrode-tool 1 being represented inside the machining cavity of the part 2 during the roughing regime.
• the tool electrode is shown inside the machining cavity during the finishing regime; the displacement of the tool-electrode in the X-Y plane in the finishing regime (after sinking the latter into the bottom of the machining cavity in the direction Z) is shown schematically by arrows in bold lines.
• R represents the eccentricity radius in the finishing regime and is greater than the machining gap in the roughing regime G.
• FIG. 5 is a cross-sectional view showing the electrode-tool of FIG. 2 in the machining cavity, the large downward black arrows representing the clean dielectric circulating in the channels, the small horizontal black arrows representing the clean dielectric in the exit from the channels and the large rising white arrows representing the "dirty" dielectric which is evacuated by the sides of the electrode-tool.
• the channels 8 must not have a diameter greater than 2 times the machining gap in the roughing regime, otherwise there is a risk of generating "carrots" of material inside them in roughing regime (direct sinking of the electrode-tool along a vertical axis Z). Any surface irregularities at the bottom of the machining cavity linked to the injection channels will be leveled by the planetary displacement of the tool-electrode in the finishing regime.
• the injection channels will make it possible to improve the evacuation of the slag created during the sinking of the electrode-tool and, consequently, to increase its speed of advance.
• the electrode is moved, on either side of its central position, by at least 0.2 mm in X, and by at least 0.2 mm in Y to obtain a sealing slot of 0.8 mm wide using a 0.4 mm wide tool electrode.
• the finishing gap is set to 0.05 mm.
• wear on the sides of the electrode is estimated at 0.05 mm.
• the thickness of the worn electrode at the end of the finishing operation is thus approximately 0.3 mm.
• the finishing operation makes it possible to ensure a recovery of the surfaces of the impression and to reduce the roughness of the surfaces obtained in roughing.
• the electrode-tool has a comb-shaped configuration, obtained by making a series of slots 9 in the lower part (ie the lower zone) of the body of the electrode- tool, the space between two adjacent slots forming a tooth of the comb.
• Each slot has a width "a" which is less than or equal to 2xG and a height "h” which is greater than or equal to 3xP, P being the depth of the sealing slot to be machined.
• the spacing width “b” between two adjacent slots is here chosen equal to 2 ⁇ R.
• FIG. 6b A sectional view along line B-B of Figure 6a is shown in Figure 6b.
• This comb-shaped configuration will, when sinking it, allow the formation of spaces between the teeth (i.e. in the slots), which will improve the distribution of the dielectric at the bottom of the cavity (by immersion of the workpiece and by external spraying around the tool-electrode causing circulation of the machining liquid between the slots).
• the width “a” of the slots must not be greater than twice the value of the machining gap in the roughing regime G, so as to be able to machine the CMC material located at the level of the slots. It is possible that with this configuration surface irregularities in the form of domes are created at the bottom of the machining cavity plumb with the slots, but these irregularities will be easily removed with the displacement of the electrode-tool in X and Y in finishing mode.
• a third embodiment is shown in Figure 7a and is a combination of the first and second configurations.
• the combination of channels and slots allows a better distribution of the dielectric as close as possible to the spark in order to obtain better machining stability and to optimize the evacuation of the slag and therefore to gain in performance.
• the inventors chose to machine them in thick plates of CuCrZr type (AFNOR standard), for example in CuCriZr 0 ,i alloy. They selected several machining processes according to the shapes of the openings to be made.
• a wire EDM cut can be used with a 0.15 mm diameter wire instead of the 0.25 mm diameter wire used in standard wire EDM.
• the electrode-tool To obtain the 0.4 mm thickness of the electrode-tool, it is possible to produce a draft in wire EDM cutting, and to carry out a recovery of the faces in grinding (finishing) to ensure the geometrical requirements such as the precision of the dimensions and the flatness of the reference faces.
• These shapes of electrodes can also be developed in additive manufacturing or built in 3D by powder bed fused by laser beam.
• the electrode-tool according to the invention with its various possible configurations is relatively easy to obtain by the various machining methods described above.
• the additional manufacturing cost after an optimization is largely amortized by the contribution of a significant gain during the realization of the impressions with reduced cycle times and a better quality obtained.
• the electrode is preferably made of an alloy of the CuCrZr type, preferably CuCriZr 0 ,i which has better rigidity and better resists wear and erosion compared to a Cu-al electrode (AFNOR standard (or Cu-ETP (ISO standard) But other materials can be used, such as all copper-based alloys, graphites, brass, tungsten carbide or cupro-tungsten.
• the electrode-tool according to the invention was designed to solve a problem of EDM machining of sealing slots in a composite material with a SiC/SiC ceramic matrix, but it can be used for the production of other guys impressions by EDM on other types of CMC materials and other electrically conductive materials, in particular metallic materials.
• the electrode-tool comprises channels
• the distribution of the dielectric in the channels of the electrode-tool takes place under pressure, preferably between 2 and 50 bars; spraying is thus obtained from the inside of the tool-electrode.
• an external coolant at low pressure for example between 0.1 and 2 bar
• an external coolant at low pressure for example between 0.1 and 2 bar
• the improvement of the circulation of the dielectric in the shapes machined using the electrode-tool according to the invention, through the channels inside the electrode-tool and/or through the teeth of the comb, iso machining parameters, makes it possible to evacuate the residues resulting from erosion more quickly.
• the fact of not re-machining on the debris improves the sinking feed rate and therefore the machining yield.
• the use of the electrode-tool according to the invention in an EDM process makes it possible to optimize the process, and can therefore be applied to industrial production for mass-produced parts.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
• Spark Plugs (AREA)

Applications Claiming Priority (2)

Publications (1)

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ID=80999303

Family Applications (1)

Country Status (5)

Family Cites Families (3)

• 2021
  • 2021-12-08 FR FR2113156A patent/FR3129860A1/fr active Pending
• 2022
  • 2022-11-28 CN CN202280088090.7A patent/CN118524902A/zh active Pending
  • 2022-11-28 EP EP22840238.4A patent/EP4444495A2/de active Pending
  • 2022-11-28 WO PCT/FR2022/052183 patent/WO2023105137A2/fr not_active Ceased
  • 2022-11-28 US US18/717,347 patent/US20250041957A1/en active Pending

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