Electrochemical machining stray corrosion inhibition method based on zirconium electrode and bipolar pulse

Abstract

The application discloses an electrolytic machining stray corrosion inhibition method based on a zirconium electrode and a bipolar pulse, and comprises the following steps: adopting metal zirconium as a tool electrode; during machining, a bipolar pulse voltage is applied between the tool electrode and a workpiece; through polarity switching of the bipolar pulse voltage, a surface oxide layer of the zirconium electrode is driven to spontaneously form a structure partition with an end face conducting and a side wall insulating in a positive pulse machining stage, so as to inhibit stray corrosion. The application utilizes the synergistic effect of bipolar pulse alternating polarization and the passivation characteristics of the zirconium electrode, effectively shields the side electric field without a side wall insulating coating, significantly improves machining precision, and fundamentally eliminates the problem of anode dissolution loss of the tool electrode caused by a negative pulse in traditional bipolar pulse electrolytic machining.

Description

Technical Field

[0001] This invention belongs to the field of special processing technology, specifically relating to a method for suppressing stray corrosion in electrolytic machining based on zirconium electrodes and bipolar pulses. Background Technology

[0002] Electrolytic machining is a special machining method that uses the principle of electrochemical anodic dissolution to form parts. It has advantages such as not being limited by the mechanical properties of the workpiece material, producing high-quality machined surfaces, requiring no machining force, and causing no tool wear. Due to these technical characteristics, electrolytic machining has been widely used in aerospace, automotive, mold making, medical device, and other fields.

[0003] Stray corrosion is an important factor affecting the accuracy of electrochemical machining and has long been a key common problem in the field of electrochemical machining. Figure 1 This is a schematic diagram of a typical electrochemical machining process: An electric field 4 is formed within the end-face gap 3 (i.e., the gap between the front of the tool electrode and the workpiece) between the tool electrode 1 and the workpiece 2, causing the material in the ideal machining area (i.e., the area where the tool electrode faces the workpiece) to be electrochemically dissolved and removed. In addition, since an electric field 6 also exists within the side gap 5 (i.e., the gap between the sidewall of the tool electrode and the workpiece), the workpiece material outside the machining area is also dissolved and removed, resulting in stray corrosion and reducing machining accuracy. Currently, the main process measures to reduce stray corrosion are tool electrode sidewall insulation and auxiliary electrodes. The auxiliary anode is an additional electrode placed outside the tool electrode and workpiece. It forms a potential difference with the workpiece, guiding stray current in the side gap into the auxiliary anode, reducing the range of the stray electric field and thus suppressing stray corrosion. Sidewall insulation is a more direct method, using an insulating layer on the sidewall of the tool electrode to shield the electric field in the side gap, concentrating the electric field between the tool electrode and the workpiece within the end-face gap, thereby suppressing stray corrosion.

[0004] However, both auxiliary anode and sidewall insulation methods are generally unsuitable for machining applications using electrodes with complex shapes. Take the electrolytic machining of flexible gears in harmonic reducers as an example: the tool electrode sidewall has hundreds of working teeth, each only a few hundred micrometers in size, evenly distributed circumferentially, resulting in a highly complex shape. For the auxiliary anode method, appropriate shape, position, and potential are required to accurately guide stray currents; however, the electric field distribution in the slit-like machining gap is complex, making the layout design of the auxiliary anode extremely difficult and impractical. On the other hand, the tool electrode sidewall insulation method is also unsuitable. Due to the complex shape of the tool electrode sidewall in flexible gear machining, preparing a uniform and suitable insulating layer on its surface is quite difficult; furthermore, during machining, the insulating layer is subjected to multiple factors such as electrolyte erosion, microbubble cavitation, and Joule heating, making sharp corners and edges prone to damage or detachment, which in turn leads to a deterioration in machining accuracy.

[0005] Replacing DC power with pulsed power is a key method to improve the electrochemical machining process and suppress stray corrosion. Building on this, using bipolar pulsed power can achieve even better stray corrosion suppression. The principle is to remove reaction products adsorbed on the tool during the negative current pulse, allowing the machining gap to be much smaller than in traditional electrochemical machining. This concentrates the electric field within the end-face gap, reducing stray corrosion. However, this stray corrosion suppression principle cannot shield the side electric field; in electrochemical machining where the workpiece's side is formed, tool electrode sidewall insulation is usually still required. More importantly, the introduction of negative pulses causes the tool electrode to intermittently act as an anode, resulting in tool wear due to anodic dissolution of traditional metal electrodes (such as stainless steel and copper), severely affecting the stability and accuracy of the machining process. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention proposes a method for suppressing stray corrosion during electrolytic machining based on zirconium electrodes and bipolar pulses. This method utilizes the alternating anode and cathode polarization principle of bipolar pulses, leveraging the inherent electric field distribution of "strong at the end face and weak on the side face" within the machining gap. This drives a differentiated dynamic evolution of the oxide layer on the zirconium electrode end face and sidewall, causing it to spontaneously form a structural functional partition during the positive pulse machining stage: a loose oxide layer (conductive) at the end face and a dense oxide layer (insulating) on ​​the sidewall. This effectively shields the side electric field and suppresses stray corrosion.

[0007] Therefore, the purpose of this invention is to provide a method for suppressing stray corrosion in electrolytic machining based on a zirconium electrode and bipolar pulses, comprising:

[0008] Zirconium metal is used as the tool electrode; During the machining process, a bipolar pulse voltage is applied between the tool electrode and the workpiece; By switching the polarity of the bipolar pulse voltage, the surface oxide layer of the zirconium electrode is driven to spontaneously form a structural partition with conductive end face and insulating side wall during the positive pulse processing stage, so as to suppress stray corrosion.

[0009] As a preferred technical solution, the step of driving the surface oxide layer of the zirconium electrode to spontaneously form a structural partition with conductive end face and insulating sidewall by switching the polarity of the bipolar pulse voltage specifically includes: During the negative pulse phase, the zirconium electrode acts as the anode, and a dense oxide layer is generated on its surface; During the positive pulse phase, the zirconium electrode acts as a cathode. Its end face is damaged by the concentration of electric field, thus restoring conductivity. Its sidewalls maintain the insulating state of the dense oxide layer due to the weaker electric field.

[0010] As a preferred technical solution, the zirconium metal is industrially pure zirconium.

[0011] As a preferred technical solution, the electrolyte is a sodium nitrate electrolyte with a concentration of 10wt.%~30wt.%.

[0012] As a preferred technical solution, the parameter range of the bipolar pulse voltage is: positive pulse amplitude 10~20V, negative pulse amplitude 5~15V, positive pulse width 0.01~1ms, and negative pulse width 0.01~1ms.

[0013] As a preferred technical solution, the initial machining gap between the tool electrode and the workpiece is 0.1~0.3mm, and the machining feed rate is 0.05~0.3mm / min.

[0014] As a preferred technical solution, the electrolyte flow rate within the processing gap is maintained at 5 to 20 m / s during the processing.

[0015] As a preferred technical solution, the workpiece material is 304 stainless steel.

[0016] The method of the present invention includes an electrolytic machining system, the system comprising: Tool electrodes made of zirconium metal; A bipolar pulse power supply is used to provide alternating polarity pulse voltages to the tool electrode and the workpiece during machining. An electrolyte circulation system is used to supply electrolyte to the machining gap.

[0017] As a preferred technical solution, the workpiece is connected to the positive terminal of the bipolar pulse power supply, and the tool electrode is connected to the negative terminal of the bipolar pulse power supply.

[0018] The advantages and positive effects of this invention are: This application utilizes the characteristics of bipolar pulsed alternating positive and negative polarization and the easy passivation properties of zirconium electrodes to drive the oxide layer on the electrode surface to achieve the following during processing: on the electrode end face with a strong electric field, the oxide layer is destroyed and becomes loose and conductive; on the electrode sidewall with a weak electric field, the oxide layer remains dense and insulating. This mechanism can effectively shield stray electric fields from the sides without relying on any additional insulating coating process, making material removal highly concentrated in the area directly opposite the tool end face, thus achieving significant suppression of stray corrosion and a substantial improvement in processing accuracy.

[0019] The dense oxide layer formed on the surface of the zirconium electrode during the negative pulse phase provides reliable protection for itself, enabling it to withstand high-amplitude negative pulses without dissolution and loss. This fundamentally solves the stubborn problem of tool electrode wear due to anodic dissolution in traditional bipolar pulse electrolytic machining.

[0020] Furthermore, this method is not limited by the complex geometry of the tool electrode, and it is highly compatible and easy to implement. It can be implemented simply by changing the electrode material and configuring a bipolar pulse power supply, without requiring modifications to existing machine tools and tooling. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the principle of a typical electrolytic machining process; Figure 2 It is a bipolar pulse voltage waveform; Figure 3 This is a schematic diagram of the method for suppressing stray corrosion in electrolytic machining based on zirconium electrodes and bipolar pulses according to the present invention. Figure 4 This is the processing result achieved in Example 1 using a zirconium electrode and a bipolar pulse power supply; Figure 5 It is the result of processing using zirconium electrodes and a unipolar pulse power supply; Figure 6 It is the result of processing using stainless steel electrodes and a unipolar pulse power supply; Figure 7 It is the result of processing using stainless steel electrodes and bipolar pulse power supply; Figure 8 This is the processing result of using zirconium electrodes and a bipolar pulse power supply in Example 2; Figure 9 This is the processing result of using zirconium electrodes and a bipolar pulse power supply in Example 3; Figure 10 It is the micro-gear electrolytic machining tooling in Example 4; Figure 11 The fine tooth profile is fabricated using a zirconium electrode and a bipolar pulse power supply as described in Example 4; Figure 12 It is a micro-tooth profile processed using zirconium electrodes and a unipolar pulse power supply.

[0022] 1. Tool electrode; 2. Workpiece; 3. End face gap; 4. Electric field; 5. Side gap; 6. Electric field; 7. Negative pulse; 8. Positive pulse; 9. Zirconium electrode; 10. Dense oxide layer; 11. Loose pore structure; 12. Machining fixture. Detailed Implementation

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below; obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1: Electrolytic machining of circular pits: The method for suppressing stray corrosion in electrolytic machining based on zirconium electrodes and bipolar pulses in this embodiment includes the following steps: S1. A 2mm diameter round bar made of industrial pure zirconium ZR702 is selected as the zirconium electrode. The workpiece made of 304 stainless steel is clamped on the electrolytic machining tool. The workpiece is electrically connected to the positive terminal of the bipolar pulse power supply, and the zirconium electrode is electrically connected to the negative terminal of the bipolar pulse power supply. The initial machining gap between the end face of the tool electrode and the surface of the workpiece to be machined is adjusted to 0.1mm. S2. Start the electrolyte circulation system and continuously pump the pre-prepared sodium nitrate electrolyte with a concentration of 15wt.% into the processing gap between the tool electrode and the workpiece at a flow rate of 10m / s to ensure that the processing area is completely wetted and that the electrolytic products can be flushed away in time.

[0025] S3. Set the output waveform parameters of the bipolar pulse power supply. Specifically, set the positive pulse amplitude to 15V and the positive pulse width to 0.1ms; the negative pulse amplitude to 10V and the negative pulse width to 0.1ms. Further set the tool electrode to feed vertically downwards at a constant speed of 0.1mm / min. S4. Simultaneously activate the bipolar pulse power supply and the machine tool motion mechanism to begin electrolytic machining. The area of ​​the workpiece directly opposite the electrode end of the tool undergoes continuous and controllable anodic dissolution under the influence of a strong electric field, resulting in efficient material removal. Meanwhile, the adjacent non-machined areas are almost unaffected due to the effective shielding of the sidewall electric field. Finally, once the preset machining depth is reached, stop the pulse power supply and electrode feed, and remove the workpiece.

[0026] Specifically, during the negative pulse phase, the zirconium electrode acts as the anode, rapidly generating a dense oxide layer with high impedance across the entire immersion surface. When the pulse voltage switches to the positive pulse phase, the tool electrode becomes the cathode. Due to the extremely small gap between the end face and the workpiece and the highly concentrated electric field, a violent cathodic hydrogen evolution reaction occurs. The newly generated active hydrogen destroys the dense oxide layer in this area, transforming it into a loose, porous structure and restoring good conductivity. Meanwhile, the sidewall area, due to the relatively large gap between it and the workpiece and the weaker electric field, exhibits a weak hydrogen evolution reaction, allowing the dense oxide layer structure to be maintained and a high-resistance insulating state to be preserved. Thus, within each positive pulse machining cycle, the tool electrode spontaneously forms a functional partition with a conductive end face and insulating sidewalls, forcibly confining the anodic dissolution electric field of electrolytic machining to the workpiece area directly opposite the tool end face, effectively shielding stray electric fields from the sides. When the pulse flips back to a negative pulse, the tool electrode end face repairs and generates a dense oxide layer again, while the sidewalls, protected by a certain thickness of insulating layer, essentially stop growing, and the electrode returns to its initial passivation state.

[0027] By cyclically switching between the positive and negative pulses, the surface of the tool electrode can continuously and stably suppress stray corrosion without any insulating coating.

[0028] The final processing result is as follows Figure 4 As shown, the circular recess has steep sidewalls and sharp edges. Under the same processing conditions, Figure 5 It is the result of unipolar pulse machining of zirconium electrodes. Figure 6 This is the result of unipolar pulse machining of traditional stainless steel electrodes. Figure 7 This invention is the result of bipolar pulse processing of traditional stainless steel electrodes. Compared with the above-mentioned traditional methods, the sidewalls of the circular pits processed by this invention are more vertical, the side processing gap is greatly reduced, and it exhibits obvious stray corrosion inhibition ability.

[0029] Example 2: Electrolytic machining of circular pits: Compared with Example 1, this embodiment adjusts the parameters of steps S2 and S3, while the remaining steps are exactly the same as in Example 1. Specifically, in S2, a 10 wt.% sodium nitrate electrolyte is circulated into the processing gap at a flow rate of 5 m / s. In S3, the output waveform of the bipolar pulse power supply is set as follows: positive pulse amplitude 10V, positive pulse width 0.01 ms; negative pulse amplitude 5V, negative pulse width 0.01 ms. Under these parameter conditions, although the amplitude and width of both the positive and negative pulses are relatively small, a dense oxide layer can still be rapidly formed on the surface of the zirconium electrode within the extremely short negative pulse period. When switching to a positive pulse, hydrogen evolution occurs at the end face due to the concentrated electric field, resulting in a loose conductive layer, while the sidewall retains an insulating oxide layer due to the weaker electric field distribution. Because the electrolyte concentration is low and the pulse energy is low, the electrochemical dissolution rate of the end face material is relatively slow. Therefore, the tool electrode is fed vertically downwards at a slower speed of 0.05 mm / min to ensure the stability and morphological accuracy of the processing.

[0030] The final processing result is as follows Figure 8 As shown, even under conditions of low electrolyte concentration, pulse amplitude, and width, the obtained circular pits have clear outlines, good sidewall verticality, and significantly suppressed stray corrosion.

[0031] Example 3: Electrolytic machining of circular pits: Compared with Example 1, this embodiment adjusts the parameters of steps S1, S2, and S3. Specifically: in S1, the initial machining gap between the tool electrode end face and the workpiece is adjusted to 0.2 mm; in S2, the electrolyte circulation system pumps a 20 wt.% sodium nitrate electrolyte into the machining gap at a high flow rate of 20 m / s to match the enhanced electrochemical reaction dosage and chip removal requirements. In S3, the bipolar pulse power supply parameters are set as follows: positive pulse amplitude 20 V, positive pulse width 1 ms; negative pulse amplitude 15 V, negative pulse width 1 ms. Due to the improved material removal rate, the tool electrode is correspondingly fed vertically downwards at a faster speed of 0.3 mm / min.

[0032] Under higher pulse energy and electrolyte concentration, the zirconium electrode forms a dense oxide layer more rapidly and with increased thickness during the negative pulse phase. Upon entering the positive pulse phase, the oxide layer on the end face undergoes more thorough destruction due to the concentrated electric field and stronger hydrogen evolution reaction, forming a loose structure with good conductivity, while the dense oxide layer on the sidewalls remains in a high-resistivity state due to the electric field shielding effect. During processing, the high dissolving current acts only on the area directly opposite the end face, effectively suppressing the risk of stray corrosion that might be exacerbated by increased electrolyte concentration and voltage. The final result is as follows: Figure 9 The circular pit shown.

[0033] Compared with the results of Examples 1 and 2, although the side clearance of the pit is slightly increased, it still shows a significant effect in inhibiting stray corrosion and the processing efficiency is significantly improved.

[0034] Example 4: Electrolytic machining of micro-gears: In this embodiment, a toothed zirconium electrode was manufactured using industrial pure zirconium ZR702. Its tooth shape is complex and has many edges and corners, representing an extremely complex surface that traditional sidewall insulation coating processes cannot handle at all.

[0035] S1. The toothed tool electrode and the 304 stainless steel workpiece are arranged according to... Figure 10 The positional relationship shown is assembled within a dedicated machining fixture 12, and the fixture is then mounted entirely on an electrochemical machining machine. The initial machining clearance between the tooth tip and the workpiece surface to be machined is adjusted to 0.1 mm. The machining fixture 12 is a traditional gear electrochemical machining fixture used in industrial production, and will not be described in detail here.

[0036] S2. Start the electrolyte circulation system, pumping a 15wt.% sodium nitrate electrolyte at a flow rate of 10m / s along the axial direction of the toothed tool electrode into the narrow and complex tooth-like machining gap, ensuring a uniform and stable flow field within each micro-tooth cavity. S3. Set the bipolar pulse power supply output waveform as follows: positive pulse amplitude 15V, positive pulse width 0.1ms; negative pulse amplitude 10V, negative pulse width 0.1ms.

[0037] Finally, the toothed tool electrode is further fed vertically downwards at a speed of 0.1 mm / min, while the bipolar pulse power supply and machine tool motion mechanism are activated. The workpiece material is selectively dissolved and removed in the area directly opposite the tooth tip of the tool electrode, and the complex tooth profile is gradually copied onto the workpiece with high precision. During the machining process, in the positive pulse machining stage, for each tiny tooth tip of the toothed tool electrode, due to the smallest gap with the workpiece and the strongest electric field, the dense oxide layer in this area is fully destroyed to form a loose conductive structure, ensuring machining efficiency. For the side surface of each tooth, the root arc surface, and the outer wall of the non-machined area, due to the relatively large gap with the side surface of the workpiece and the weaker electric field, the dense oxide layer is preserved in situ, forming a perfect conformal insulating layer.

[0038] After processing, the following is obtained: Figure 11 The micro-tooth structure shown. Under the same processing conditions... Figure 12 This is the result of processing using the same zirconium electrode but applying a unipolar pulse power supply. Compared with this embodiment, the processed micro-tooth profile has a flat top, extremely distinct edges and corners, and steep tooth sidewalls. Almost no rounded corners or overcut marks formed by stray corrosion can be observed on the bottom and sidewalls. The geometric integrity and dimensional accuracy of the micro-tooth profile are qualitatively improved, proving that this method has broken through the technical bottleneck that complex-shaped tool electrodes cannot effectively implement sidewall insulation.

[0039] The embodiments given above are preferred examples for implementing the present invention, and the present invention is not limited to the above embodiments. Any non-essential additions or substitutions made by those skilled in the art based on the technical features of the present invention are within the protection scope of the present invention.

Claims

1. A method for suppressing stray corrosion during electrolytic machining based on zirconium electrodes and bipolar pulses, characterized in that, include: Zirconium metal is used as the tool electrode; During the machining process, a bipolar pulse voltage is applied between the tool electrode and the workpiece; By switching the polarity of the bipolar pulse voltage, the surface oxide layer of the zirconium electrode is driven to spontaneously form a structural partition with conductive end face and insulating side wall during the positive pulse processing stage.

2. The method for suppressing stray corrosion during electrolytic machining according to claim 1, characterized in that, The method of driving the surface oxide layer of the zirconium electrode to spontaneously form a structural partition with conductive end face and insulating sidewall by switching the polarity of the bipolar pulse voltage specifically includes: During the negative pulse phase, the zirconium electrode acts as the anode, and a dense oxide layer is generated on its surface; During the positive pulse phase, the zirconium electrode acts as a cathode. Its end face is damaged by the concentration of electric field, thus restoring conductivity. Its sidewalls maintain the insulating state of the dense oxide layer due to the weaker electric field.

3. The method for suppressing stray corrosion in electrolytic machining according to claim 1, characterized in that, The zirconium metal is industrially pure zirconium.

4. The method for suppressing stray corrosion during electrolytic machining according to claim 1, characterized in that, The electrolyte is a sodium nitrate electrolyte with a concentration of 10 wt.% to 30 wt.%.

5. The method for suppressing stray corrosion in electrolytic machining according to claim 1, characterized in that, The parameters of the bipolar pulse voltage are: positive pulse amplitude 10~20V, negative pulse amplitude 5~15V, positive pulse width 0.01~1ms, and negative pulse width 0.01~1ms.

6. The method for suppressing stray corrosion in electrolytic machining according to claim 1, characterized in that, The initial machining gap between the tool electrode and the workpiece is 0.1~0.2mm, and the machining feed rate is 0.05~0.3mm / min.

7. The method for suppressing stray corrosion in electrolytic machining according to claim 1, characterized in that, During the processing, the electrolyte flow rate within the processing gap is maintained at 5 to 20 m / s.

8. The method for suppressing stray corrosion in electrolytic machining according to claim 1, characterized in that, The workpiece is made of 304 stainless steel.

9. A method for suppressing stray corrosion during electrolytic machining as described in any one of claims 1 to 8, characterized in that, Including electrolytic machining systems, including: Tool electrodes made of zirconium metal; A bipolar pulse power supply is used to provide alternating polarity pulse voltages to the tool electrode and the workpiece during machining. An electrolyte circulation system is used to supply electrolyte to the machining gap.

10. The method for suppressing stray corrosion in electrolytic machining according to claim 9, characterized in that, The workpiece is connected to the positive terminal of the bipolar pulse power supply, and the tool electrode is connected to the negative terminal of the bipolar pulse power supply.

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