A machining device and method using magnetic field and electrolytic auxiliary electric spark wire cutting

Abstract

The application provides a kind of adopts magnetic field and electrolytic auxiliary electric spark wire cutting processing device and method, belongs to electric spark wire cutting technical field, device includes machine tool, working platform on machine tool, power supply and control system, working fluid cooling circulation system, silk transport system and adjustable magnetic field generating device. Among them, working fluid cooling circulation system is used to deliver the working fluid added with sodium chloride-glycol to the processing area, and the adjustable magnetic field generating device is used to form a stable intensity magnetic field in the processing area. The method comprises setting processing parameters, pretreating the workpiece, and processing the workpiece. A prediction model for predicting the contact angle of the microstructure is also provided. The application designs a magnetic field auxiliary device and an electrolytic device in the existing machine tool, applies electrolysis and magnetic field auxiliary technology to the field of electric spark wire cutting, improves the machining precision and surface quality of electric spark wire cutting, and can prepare fine microstructures, providing a basis for improving the hydrophobic performance.

Description

Technical Field

[0001] This invention belongs to the field of wire electrical discharge machining (EDM) technology, and particularly relates to an EDM processing device and method using magnetic field and electrolysis assisted. Background Technology

[0002] Hydrophobic materials are widely used in aerospace, biomedicine, and industrial fields due to their self-cleaning, antibacterial, drag-reducing, and corrosion-resistant properties. The hydrophobicity of the material surface is typically controlled by modifying its microstructure and chemical composition through methods such as laser processing, coating, and chemical etching. Nickel-titanium alloys, with their unique shape memory effect (SME) and excellent biocompatibility, are widely used in automotive, aerospace, and biomedicine. Their good surface hydrophobicity reduces platelet adhesion and activation, thus preventing thrombosis and improving biocompatibility. For applications such as orthodontic wires, their excellent hydrophobicity reduces bacterial adhesion, providing protection. In orthopedic implants, they can also reduce the release of Ni atoms, preventing toxic substances from entering the body. However, traditional machining processes for nickel-titanium alloys can lead to problems such as tool wear, stress, and excessive cutting temperatures. Therefore, special machining processes must be employed to improve their processing characteristics.

[0003] Electrical discharge machining (WEDM) is an unconventional machining method suitable for processing difficult-to-machine materials. In WEDM-LS, the electrode wire does not directly contact the workpiece, thus eliminating problems such as mechanical stress, vibration, and chatter during the machining process. It is not limited by the hardness and toughness of the material and has advantages such as high machining accuracy and good surface quality. Therefore, WEDM plays an important role in modern manufacturing industries such as mold making, instrumentation, aerospace, automotive, and biomedicine. However, due to its working principle, some defects may exist on the workpiece surface, such as pits, recast layers, and microcracks. To solve these problems, this invention proposes a magnetic field-assisted electrolytic wire electrical discharge machining composite process, aiming to improve machining quality and efficiency, while providing an effective way to process hydrophobic surfaces.

[0004] Regarding wire electrical discharge machining (EDM), the following existing technologies exist:

[0005] 1. Chu Xuyang of Xiamen University proposed a method for preparing superhydrophobic metal surfaces by wire electrical discharge machining. With the help of a low-frequency vibration auxiliary device, it is possible to process superhydrophobic metal surfaces with irregular macroscopic shapes in one go, or to process micro-nano double-layer structures with surface morphologies such as square trapezoids, rhomboid trapezoids, and rhomboid cones in multiple times.

[0006] 2. Wang Zhenlong from Harbin Institute of Technology proposed a method for preparing superhydrophobic surfaces by combining electrical discharge machining and spraying. This method solves many problems in terms of process and cost caused by using low surface energy reagents such as fluorinating agents for immersion. The stability of carbon makes the obtained superhydrophobic surface of metal materials wear-resistant and highly stable.

[0007] The existing technologies mentioned above have failed to adequately address the problem of excessive surface roughness generated during wire electrical discharge machining, resulting in insufficient precision of the processed microstructures and consequently affecting hydrophobic properties.

[0008] Therefore, there is a need for a processing device and method that can increase the processing accuracy of microstructures and enable them to have good hydrophobicity, based on existing technologies. Summary of the Invention

[0009] The purpose of this invention is to provide a wire electrical discharge machining device using magnetic field and electrolysis assisted, comprising a machine tool, a work platform mounted on the machine tool, a power supply and control system, a working fluid cooling circulation system and a wire feeding system. The power supply and control system provides power to the machining device, and the work platform is connected to an adjustable magnetic field generator for fixing the workpiece.

[0010] An adjustable magnetic field generator produces a magnetic field of constant and uniform intensity;

[0011] The wire feeding system guides an electrode wire for electrical discharge cutting of the workpiece. The electrode wire is in contact with the workpiece in a magnetic field, and the direction of the magnetic field is perpendicular to the feed direction of the electrode wire.

[0012] The working fluid cooling circulation system injects a working fluid with micro-conductivity and micro-electrolysis properties into the processing area;

[0013] The processing device changes the current density in the working fluid through electrolysis, removing some of the discharge pits and recast layers generated by electrical discharge machining, thereby improving the surface quality of the workpiece.

[0014] Furthermore, the adjustable magnetic field generator includes mounting brackets fixed on the same side of the working platform and extending in the same direction to opposite sides of the working area. Helmholtz coils are fixed on opposite sides of the two mounting brackets. The winding directions of the two Helmholtz coils are the same, and the center distance between the two coils is equal to the radius of the center circle of the coil. The strength and direction of the magnetic field are determined by the magnitude and direction of the current, respectively.

[0015] Furthermore, the working fluid cooling circulation system includes a fluid supply pump and a working fluid tank. The motor of the fluid supply pump is connected to a power supply, and the processing device regulates the flow rate of the fluid supply pump by controlling the voltage.

[0016] The inlet of the liquid supply pump is connected to the inside of the working liquid tank through a pipe to extract the working liquid. The outlet of the liquid supply pump is connected to the pipe to deliver the working liquid to the processing area.

[0017] The working fluid tank contains working fluid, which includes sodium chloride-ethylene glycol electrolyte.

[0018] Furthermore, the power supply and control system includes a pulse power supply, the positive terminal of which is directly connected to the workpiece, and the negative terminal of which is connected to the electrode wire through a conductive block.

[0019] A processing method using a magnetic field and electrolysis-assisted wire electrical discharge machining (EDM) device includes the following steps:

[0020] S1: Set the processing parameters and preprocess the workpiece;

[0021] S2: Fix the pre-treated workpiece on the work platform using a special fixture, and ensure that the workpiece does not come into contact with the electrode wire;

[0022] S3: Install the electrode wire with the set diameter onto the wire storage drum of the wire feeding system and perform adjustments;

[0023] S4: Install and start the adjustable magnetic field generator, adjust the current to form a magnetic field of the required intensity in the processing area, and at the same time, add sodium chloride-ethylene glycol electrolyte to the working fluid.

[0024] S5: Start the power supply and control system, perform wire electrical discharge machining to obtain the microstructure boss size, and monitor the discharge stability, electrode wire vibration and machining speed in real time.

[0025] S6: Clean the cut workpiece and let it stand to air dry naturally.

[0026] Furthermore, in S1, the processing parameters are set as follows: current 6-12A, voltage 40V, pulse width 6-15μs, pulse width 6μs, electrode wire diameter 0.2mm, magnetic field strength 0.1-0.4T, and a total of 16 groups are processed.

[0027] Furthermore, in S1, the pretreatment includes workpiece cleaning, oxide layer removal, and drying procedures;

[0028] The specific cleaning process for the workpiece is as follows: the workpiece to be processed is placed in a special cleaning tank and pre-cleaned with ethanol to remove surface contaminants for 10 minutes.

[0029] The process of removing the oxide layer is as follows: After the initial cleaning, the workpiece to be processed is treated with a dilute hydrochloric acid solution to remove the oxide layer for 2 minutes. After treatment, it is rinsed with clean water.

[0030] The drying process involves placing the workpiece after the oxide layer removal treatment in a drying oven and drying it at a temperature between room temperature and 80°C for 30 minutes to remove moisture from the surface of the workpiece.

[0031] Furthermore, in S4, the demand intensity is 0.1T.

[0032] Furthermore, in S6, a contact angle prediction model is used to predict the contact angle of the workpiece after cutting. The calculation formula of this model is expressed as:

[0033]

[0034] By substituting the parameters into the contact angle prediction model, the predicted value of the contact angle is obtained.

[0035] Furthermore, the contact angle prediction model is established by combining the Wenzel model and the Cassier-Baxter model. The Cassier-Baxter model is used to analyze the force balance of water droplets on microstructures, while the Wenzel model is used to consider the influence of surface roughness on the contact angle when water droplets directly contact microstructure protrusions.

[0036] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in the following: The present invention designs a magnetic field-assisted device and an electrolysis device in the existing machine tool, and applies electrolysis and magnetic field-assisted technology to the field of wire electrical discharge machining technology, which improves the processing accuracy and surface quality of wire electrical discharge machining, and can prepare fine microstructures, providing a basis for improving hydrophobic properties. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the magnetic field and electrolysis-assisted wire EDM device of the present invention.

[0038] Figure 2 This diagram illustrates the working mechanism of magnetic field-assisted wire electrical discharge machining of workpieces according to the present invention.

[0039] Figure 3 A diagram illustrating the working mechanism of magnetic field-assisted wire electrical discharge machining (EDM) of workpieces.

[0040] Figure 4 This is a model diagram of the interelectrode double layer in this invention.

[0041] Figure 5 This is a schematic diagram of a three-dimensional model of the magnetic field-assisted device in this invention.

[0042] Figure 6 This is a schematic diagram of the Helmholtz coil design in this invention.

[0043] Figure 7This is a schematic diagram of a three-dimensional model of the electrolysis device in this invention.

[0044] Figure 8 This is a diagram showing the distribution of the Gaussian heat source in this invention.

[0045] Figure 9 This is a two-dimensional model of the pit in this invention.

[0046] Figure 10 This is a three-dimensional model and top view of the pit in this invention.

[0047] Figure 11 This is a schematic diagram of the microstructure dimensions in this invention.

[0048] Figure 12 This is a force analysis of the droplet in the Z direction in this invention.

[0049] Figure 13 This is a schematic diagram of the microstructure gaps in this invention.

[0050] Figure 14 This refers to the surface of the microstructured pits in this invention.

[0051] Figure 15 This is a comparison chart of predicted and experimental values ​​under the composite process in an embodiment of the present invention.

[0052] Figure 16 This is a super-depth-of-field microscope image of the microstructure of the processed nickel-titanium alloy surface in an embodiment of the present invention.

[0053] Figure 17 This is a static contact angle (CA) diagram of the hydrophobic metal surface in an embodiment of the present invention.

[0054] The components include: 1. Wire storage drum; 2. Guide wheel; 3. Electrode wire; 4. Conductive block; 5. Power supply and control system; 6. Adjustable magnetic field generator; 7. Guide; 8. Workpiece; 9. Working fluid cooling circulation system; 10. Guide belt; 11. Working fluid; 12. Liquid supply pump; 13. Working fluid tank; 14. Motor; 15. Water inlet; 16. Water outlet; 17. Working platform; 18. Mounting bracket; 19. Silicon steel; 20. Coil. Detailed Implementation

[0055] The present invention will now be described in more detail with reference to the schematic diagrams, which illustrate preferred embodiments of the invention. It should be understood that those skilled in the art can modify the invention described herein while still achieving the beneficial effects of the invention. Therefore, the following description should be understood as being of general knowledge to those skilled in the art and is not intended to limit the invention.

[0056] Example 1

[0057] A magnetic field and electrolysis-assisted wire electrical discharge machining device

[0058] 1. Device Composition

[0059] like Figure 1 As shown, the processing device consists of a work platform 17 (reference). Figure 5 It consists of a power supply and control system 5, a working fluid cooling circulation system 9, a wire feeding system and an adjustable magnetic field generator 6.

[0060] The power supply and control system includes a pulse power supply. The positive terminal of the pulse power supply is directly connected to the workpiece 8, and the negative terminal is connected to the electrode wire via a conductive block 4. The wire feeding system mainly includes a wire storage drum 1, a guide wheel 2, an electrode wire 3, a guide 7, and a guide clamp 10. The wire feeding system is installed on the machine tool and is part of the existing wire EDM machine tool. It is used to drive the electrode wire 3 to move downwards in a unidirectional feed motion. The working fluid cooling circulation system 9 includes a liquid supply pump 12 and a working fluid 11. This invention uses a flushing method to process the workpiece 8. The working fluid 11 is mainly composed of a sodium chloride-ethylene glycol solution at a certain pressure to promote chip removal and cooling, ensuring normal processing. The adjustable magnetic field generating device 6 mainly consists of a pair of adjustable Helmholtz coils (circular in shape). By adjusting the current intensity flowing through the coils 20, a constant and uniform magnetic field can be generated between the coils 20, with a magnetic field strength between 0.1T and 0.4T.

[0061] 2. Working principle:

[0062] refer to Figure 3With the aid of electrolysis and a magnetic field, the workpiece 8 and the electrode wire are connected to the positive and negative terminals of the pulse power supply, respectively. The electrode wire 3 moves unidirectionally at a low speed, guided by the wire storage drum 1, the guide wheels 2, and the guiding device. The workpiece 8 is placed between a pair of adjustable Helmholtz coils (circular in shape). When the distance between these two coils 20 is equal to their radius, a constant and uniform magnetic field can be generated between the coils 20 by adjusting the current intensity flowing through them. The feed direction of the electrode wire 3 is perpendicular to the direction of the uniform magnetic field generated by the Helmholtz coils. The magnetic field causes the electrons in the discharge channel to change their trajectory. During the machining process, spark discharge occurs between the workpiece 8 and the electrode wire 3, with temperatures exceeding 10,000 degrees Celsius. This high temperature causes the workpiece material to be etched away, forming spherical etch products. Simultaneously, the working fluid cooling circulation system 9 continuously supplies working fluid 11 to the machining area. Due to the micro-conductivity and micro-electrolysis characteristics of the sodium chloride-ethylene glycol solution, both electrical discharge machining and electrolytic machining occur simultaneously in the machining area. In the machining area along the feed direction of electrode wire 3, the gap between electrode wire 3 and workpiece 8 is less than the spark discharge threshold, generating an electrical discharge (EDS). Electrolysis also occurs, but material removal is primarily achieved through EDS. In the machining area parallel to the feed direction of electrode wire 3, the gap between electrode wire 3 and workpiece 8 is greater than the spark discharge threshold, resulting in almost no spark discharge. Electrolysis is mainly used to change the current density in the solution, removing some of the discharge pits and recast layers generated by EDS, thereby improving the surface finish.

[0063] 3. Mechanism of magnetic field action

[0064] refer to Figure 2 During the magnetic field-assisted WEDM-LS processing, the electrons in the discharge channel are subjected to the combined action of electric field force and Lorentz force, and their trajectory will no longer be a straight line, thus causing the discharge channel to deflect.

[0065] To simplify the analysis process, the present invention makes the following reasonable assumptions:

[0066] (1) Ignore the effect of gravity on the trajectory of electrons;

[0067] (2) The interaction between electrons is not considered;

[0068] (3) Assume that the electric and magnetic fields in the discharge region are uniformly distributed.

[0069] The force equilibrium equation for the electron is shown in equation (1):

[0070] F 合 =F E +F B =m e a=qE+qv e ×B (1)

[0071] In the formula:

[0072] m e - The mass of an electron, in kg;

[0073] The acceleration of a-electrons, m·s⁻²;

[0074] The charge carried by the q-electron, C;

[0075] v e - The velocity of electrons, m·s⁻¹;

[0076] E - electric field strength, V·m⁻¹;

[0077] B - Magnetic flux density, T.

[0078] according to Figure 2 And assuming (3) we have: B x =-B,B y =B z =0,E y =-E,E x =E z =0, then expanding equation (1) along the Y and Z directions yields:

[0079]

[0080] Assuming the initial velocity of the electron is 0, i.e., y(0) = z(0) = 0, v y (0)=v z (0) = 0, from which we can obtain the velocity equation of the electron:

[0081]

[0082] Equation of motion trajectory:

[0083]

[0084] 4. Mechanism of electrolysis

[0085] The equivalent electric double layer model between the electrode and workpiece 8. In this model, φ is the electric double layer voltage. u is the voltage between the two electrodes, and C... d and R F These are the double-layer capacitance and resistance, R. e Let be the equivalent resistance of the inter-electrode medium. Before establishing the model, the following reasonable assumptions were made:

[0086] 1. During the pulse, the double-layer capacitance remains constant.

[0087] 2. The double-layer transfer resistance and capacitance at the electrode wire-solution and workpiece-solution interfaces are the same.

[0088] 3. Neglect the adsorption on the electrode surface and the non-uniformity of the electrode surface structure.

[0089] refer to Figure 4 Flow through the double-layer capacitor C d charging current density i c It can be represented as:

[0090]

[0091] According to the Butler-Volmer equation, water flows through resistor R F Faraday current density i F for:

[0092]

[0093] In the formula, i0 is the initial exchange current density; α is the transfer coefficient; n is the valence; F is the Faraday constant; R is the gas constant; and T is the absolute temperature.

[0094] In formula (9), the first term represents the anode current density, and the second term represents the cathode current density. Since the cathode current density is much smaller than the anode current density in actual processing, the above equation can be simplified to:

[0095]

[0096] The current density flowing from the workpiece to the electrode wire can be expressed as:

[0097]

[0098] In the formula, ρ is the resistivity of the electrolyte; d k This is the distance between the workpiece and the electrode wire.

[0099] Equation (11) can be rewritten as:

[0100]

[0101] The double-layer voltage φ can be calculated using formula (12). Therefore, the current density i F The expression for can be determined by formula (10). Electrolysis mainly removes the recast layer on the workpiece surface by changing the current density in the working fluid, ultimately greatly improving the workpiece surface quality and processing efficiency.

[0102] 5. Design of an adjustable magnetic field generator

[0103] The adjustable magnetic field generator includes mounting brackets 18 fixed on the same side of the work platform and extending to opposite sides of the processing area. Silicon steel 19 is fixed on opposite sides of the two mounting brackets 18, and Helmholtz coils are wound around the outer periphery of the silicon steel 19.

[0104] The adjustable magnetic field generator in this invention is designed based on the principle of generating a uniform magnetic field using a Helmholtz coil. A Helmholtz coil consists of two coils 20 with the same winding direction placed parallel to each other, with the center distance equal to the radius of the center circle of each coil 20. Currents in the same direction are passed through the coils 20, and the strength and direction of the magnetic field are determined by the magnitude and direction of the current. At this time, a uniform magnetic field within a certain range is formed along the axis between the coils 20, such as... Figure 5 As shown

[0105] refer to Figure 6 The magnetic field strength along axis O1O2 is formed by the superposition and coupling of the coils at both ends. According to the Biot-Saffar law, the magnetic field strength of a single coil at any point on the axis is:

[0106]

[0107] in,

[0108] μ0 is the free permeability, in H / m;

[0109] I is the current in the coil, measured in amperes (A).

[0110] R is the radius of the coil, in meters (m).

[0111] N is the number of turns in the coil.

[0112] Therefore, the magnitude of the magnetic field strength at any point on the axis can be expressed as:

[0113]

[0114] Considering that the workpiece is placed at point P, the center of the two coils, the magnetic induction intensity at point P can be obtained by combining equation (14):

[0115]

[0116] In this invention, the maximum additional magnetic induction intensity needs to reach 0.4T. According to Equation (15), if winding is used alone, the size of coil 20 will be too large, increasing the cost and causing problems with proper installation on machine tools. If the current is increased, there will be laboratory safety hazards. Considering the above factors, a magnetic medium is added to the center of coil 20 to increase the magnetic induction intensity during the design of the Helmholtz coil. Considering the design cost, cast iron and silicon steel 19 are used as the objects, and the enhanced magnetic induction intensity is theoretically calculated. The relationship between the magnetic induction intensity generated by coil 20 and the magnetic induction intensity generated after adding the magnetic medium can be expressed as:

[0117] B = μ r B0 (16)

[0118] in,

[0119] μ r ρ is the relative permeability of the medium;

[0120] B0 is the magnetic flux density generated by the coil, in tons (T).

[0121] R is the magnetic flux density generated after the addition of a magnetically permeable medium, and T is the magnetic flux density generated.

[0122] In Embodiment 1 of the present invention, the setting parameters of the adjustable magnetic field generator are shown in Table 1.

[0123]

[0124] Table 1

[0125] 6. Design of the electrolysis unit

[0126] The working fluid 11 used in the electrolysis and magnetic field-assisted wire EDM process proposed in this invention contains sodium chloride-ethylene glycol electrolyte, which is weakly corrosive. To avoid corrosion of machine tool components and reduce the amount of working fluid 11 used, a working fluid cooling circulation system 9 was designed according to experimental requirements. The self-made working fluid cooling circulation system 9 mainly consists of an 800ml working fluid tank 13, a supply pump 12, and a 6mm diameter connecting pipe. Specifically, the inlet 15 of the supply pump 12 is connected to the working fluid tank 13 via a pipe, and the outlet 16 of the supply pump 12 is located in the working area via a pipe. The motor 14 of the supply pump 12 is connected to a power supply, and the flow rate of the supply pump 12 can be adjusted by controlling the voltage. The supply voltage of the supply pump 12 is 5V, the starting voltage is 1V, and the maximum flow rate is 100L / h. A three-dimensional model schematic diagram of the electrolysis device is shown below. Figure 7 As shown.

[0127] Example 2

[0128] 7. Implement a contact angle prediction model for hydrophobic properties.

[0129] The two main heat sources in electrical discharge machining (EDM) are surface heat sources and volume heat sources. However, volume heat sources account for only 1-2% of surface heat sources. Therefore, the primary form of material removal in EDM is surface heat, which typically refers to the high-temperature heat source formed when heat from the discharge channel is transferred to the electrode surface. The distribution of Gaussian heat sources is as follows: Figure 8 As shown.

[0130] The heat flux density of a single-pulse spark discharge follows a Gaussian distribution, expressed as:

[0131]

[0132] In the formula, q(r,t) is the heat flux density at radius r and time t; q mt is the maximum heat flux density at the center of the plasma channel; r is the radial distance from the plasma center; and R(t) is the time-varying radius of the plasma.

[0133] The formula for the maximum heat flux density at the point of electric spark discharge is as follows:

[0134]

[0135] In the formula, q m ζ represents the maximum heat flux density; U represents the discharge channel voltage; I represents the discharge channel current; and ζ represents the energy distribution coefficient.

[0136] The equation for the discharge channel radius can be expressed as follows:

[0137] R(t)=2.04I 0.43 T on 0.44 (19)

[0138] In the formula, I is the pulse current; T on This refers to the pulse width. When a magnetic field is applied, the radius of the discharge channel becomes 1.5 times its original size, i.e.

[0139] R p (t)=1.5R(t) (20)

[0140] Combining equations (18) to (20), the formula for the heat flux density of the plasma channel is:

[0141]

[0142] The surface morphology of the discharge pit structure is determined by the size parameters of individual discharge micro-pits and the position of the discharge point. Due to the randomness and disorder of the discharge point position during actual discharge, a large number of discharge micro-pits are randomly distributed on the substrate surface, forming the discharge pit structure. Assuming that individual discharge pits are uniformly distributed on the workpiece surface, and that the pits after random discharge partially overlap, an equivalent two-dimensional model is established, such as... Figure 9 As shown.

[0143] The formula for calculating surface roughness is:

[0144]

[0145] In the formula, r is the radius of the pit, r s The radius of the remaining area after the pits are stacked is given, and h is the distance between the initial plane and the bottom of the pit. e This represents the distance between the superimposed image and the surface of the pit.

[0146] According to the Wenzel contact angle model, when the material itself is hydrophobic, the contact angle increases with increasing surface roughness. This is because the workpiece surface is not perfectly smooth; numerous pits increase the solid-liquid contact area. The surface of a workpiece processed by wire electrical discharge machining (EDM) consists of many pits and protrusions. Therefore, a roughness coefficient is introduced, and its value is determined by considering the influence of the magnetic field-assisted electrolytic wire EDM process on the pits. Thus, for ease of calculation, the following assumptions are made regarding the shape and arrangement of the pits:

[0147] 1. All pits are evenly arranged on the machined surface, and the dimensions are taken as the average size;

[0148] 2. The surface of the pit is obtained by rotating a two-dimensional curve, and its shape is similar to a part of a sphere;

[0149] 3. Water droplets completely cover the processed surface.

[0150] Establish a three-dimensional equivalent model, such as Figure 10 As shown.

[0151] To simplify the calculation, assume the shaded area is an isosceles triangle. Then the area of ​​the shaded region is...

[0152]

[0153] The area of ​​the spherical cover is

[0154] S2=2πRa (24)

[0155] Where R is the radius of a sphere with the same size as the pit.

[0156] The figure shows a top view of the intermediate section, from which the equation for calculating the radius can be obtained.

[0157]

[0158] Combining equations 23 and 24, the area of ​​S2 is:

[0159]

[0160] The area of ​​the flattened surface before processing is

[0161]

[0162] The surface roughness coefficient of a solid surface is the ratio of the surface area of ​​a rough surface to that of a smooth surface. Therefore, the value of λ is expressed as...

[0163]

[0164] This invention employs magnetic field-assisted electrolytic wire cutting to process microstructure surfaces. Considering the air force exerted on the water droplets in the grooved areas, the force balance of the water droplets on the microstructures is analyzed based on the Cassie-Baxter contact model. Furthermore, since the microstructure protrusions are in direct contact with the water droplets, their surface roughness affects the contact angle. Therefore, a secondary microstructure contact angle prediction model is established by combining the Wenzel model and the Cassie-Baxter model.

[0165] To facilitate calculation, the model is simplified, and the following assumptions are considered.

[0166] 1. Only the hydrophobicity of the droplets on the surface of the microstructure needs to be considered.

[0167] 2. When the droplet is in a rectangular gap, due to the upward surface tension of the air on the droplet, the droplet takes on an arc shape and its size remains unchanged.

[0168] 3. The cohesive force inside the water droplet remains constant.

[0169] Assuming the droplet remains stationary on the microstructure, the size diagram is as follows: Figure 11 As shown, an equilibrium state is reached. Force balance analysis is performed on it, and a system of force balance equations is established based on the relationship between the droplet's geometry and the three-phase contact line. α is solved, and finally, the static apparent contact angle is obtained. The analysis of force balance is mainly carried out from the Z direction.

[0170] The force on the droplet in the Z direction is as follows Figure 12 As shown, the forces include: the droplet's gravity, the supporting force on the microstructure, the surface tension at the gaps in the microstructure, and the component of the surface tension between the droplet and the workpiece surface. The force balance equation is as follows:

[0171] G = W1 + W2 + W3cosα (29)

[0172] In the formula, G is the gravity of the droplet; W1 is the supporting force on the microstructure; W2 is the surface tension at the gap of the microstructure; and W3 is the surface tension between the droplet and the workpiece surface.

[0173] The formula for calculating gravity is:

[0174]

[0175] To calculate the supporting force of the microstructure on the droplet, according to Newton's third law, the magnitude of the force exerted by the microstructure on the droplet is equal to the weight of the liquid column on the microstructure. The volume of the liquid column supported by each microstructure is expressed by the following formula.

[0176]

[0177] And among them l, R n The formula is as follows

[0178]

[0179] l=Rsinα (34)

[0180] R n =Rcosα (35)

[0181] Substituting equations (32) to (35) into (31), the formula for calculating W1 is:

[0182]

[0183] To calculate the surface tension of the droplet caused by the gaps in the microstructure, we assume that the air in the gaps pushes the droplet up in a circular arc. For ease of calculation, we establish a Cartesian coordinate system for this assumption, such as... Figure 13 As shown

[0184] according to Figure 13 The radius of the arc can be obtained as

[0185]

[0186] The corresponding central angle is

[0187]

[0188] The supporting force of the droplet at the microstructure gaps is

[0189]

[0190] Overall, the contact area of ​​the workpiece surface supporting the droplet is equal to the area of ​​the base radius R. n The circle is given by the formula for calculating W3.

[0191] W3=2πRcosαγ GL (40)

[0192] Therefore, the equilibrium equation in the Z direction can be written as:

[0193]

[0194] Water droplets do not perfectly conform to the Cassie-Baxter model; Wenzel wetting exists in the local submicrostructures of the droplet. Therefore, the length of the contact line on the microstructure changes, and the proportion of this increase is related to surface roughness. For example, on rough surfaces... Figure 14 As shown, there is no solid surface at the gap, so the gap value is assumed to remain unchanged. After considering the surface roughness, the dimensional parameters become...

[0195] a′=a (44)

[0196] b'=λb (45)

[0197] In the formula, λ is the surface roughness coefficient of the solid, reference Figure 14 .

[0198] Substituting the equivalent dimensions, which take into account the changes in structural dimensional parameters after considering surface roughness, into the formula, we obtain the contact angle prediction model for the secondary structure as follows:

[0199]

[0200] The required parameters are: radius R, density ρ, gravitational acceleration g, microstructure dimensions a and b, roughness coefficient λ, central angle β corresponding to the droplet model at the gap between the bosses, and solid-liquid surface tension γ. SL and gas-liquid surface tension γ VL Substituting the values ​​into the secondary microstructure prediction model, the value of α is obtained, and then the contact angle is calculated.

[0201] Example 3

[0202] A processing method using a magnetic field and electrolysis-assisted wire electrical discharge machining device includes the following steps:

[0203] Step 1: Equipment Preparation

[0204] The magnetic field and electrolysis-assisted wire EDM machining device described in this invention includes a machine tool bed, a power supply and control system, a spindle, a work platform, a working fluid cooling circulation system and a wire feeding system, as well as a magnetic field generating device.

[0205] Step 2: Setting processing parameters

[0206] The current was set to any value between 6-12A, the voltage to 40V, the pulse width to any value between 6-15μs, the inter-pulse width to 6μs, the electrode wire diameter to 0.2mm, and the magnetic field strength to any value between 0.1-0.4T. A total of 16 sets of different parameters were processed.

[0207] Step 3: Workpiece preparation

[0208] 3.1) Workpiece Cleaning: First, place the nickel-titanium alloy workpiece to be processed in a dedicated cleaning tank. Use ethanol to perform a preliminary cleaning of the workpiece to remove surface grease, dust, and other contaminants. The cleaning time is generally 10 minutes to ensure the workpiece surface is clean and free of impurities.

[0209] 3.2) Oxide De-oxidation: After cleaning, the workpiece is treated with a dilute hydrochloric acid solution to remove the oxide layer. The purpose of this step is to remove the oxide layer formed on the surface of the workpiece due to oxidation, so as to ensure good contact between the electrode wire and the workpiece during processing. The oxide de-oxidation treatment time is generally 2 minutes, followed by rinsing thoroughly with clean water.

[0210] 3.3) Drying treatment: Place the cleaned and deoxidized workpiece in a drying oven and dry it at an appropriate temperature (e.g., room temperature to 80°C) to thoroughly remove moisture from the workpiece surface. The drying time is generally 30 minutes to ensure that the workpiece surface is completely dry and free of moisture.

[0211] Step 4: Fix the workpiece

[0212] After drying, a nickel-titanium alloy workpiece measuring 50mm × 50mm × 2mm is fixed on the work platform, ensuring that the workpiece does not contact the electrode wire. A special fixture is used to firmly fix the workpiece on the work platform to prevent it from moving or deforming during processing. The fixed workpiece extends out of the work platform, with one end extending into the processing area.

[0213] Step 5: Electrode wire installation and adjustment

[0214] Install a 0.2mm diameter brass wire onto the wire storage drum and adjust it.

[0215] Step 6: Magnetic field and electrolysis auxiliary settings

[0216] The adjustable magnetic field device is placed perpendicular to the electrode wire. The magnetic field generator is activated, and the current is adjusted to produce a magnetic field of 0.1T in the processing area. At the same time, an appropriate amount of sodium chloride-ethylene glycol working solution is added to the working fluid.

[0217] Step 7: Processing

[0218] The power supply and control system are started, and the wire electrical discharge machining is initiated. The resulting microstructure boss has a size of 400μm, a height of 200μm, and a groove gap of 200μm. The discharge stability, electrode wire vibration, and machining speed are monitored in real time.

[0219] Step 8: Post-processing treatment

[0220] After cutting, the workpiece is cleaned in anhydrous ethanol for 10 minutes at room temperature using an ultrasonic cleaner, and then left to air dry naturally.

[0221] Step 9: Microstructure Detection and Hydrophobicity Study

[0222] The microstructure of the processed nickel-titanium alloy surface was examined using a super depth-of-field microscope, revealing a highly precise microstructure. Simultaneously, the contact angle was measured using a JC2000D1 contact angle meter, yielding a range of 120°–145°, indicating excellent hydrophobicity.

[0223] 9. Comparison of experimental and predicted values

[0224] like Figure 15 The figure shows a comparison between predicted and experimental contact angle values ​​under composite processes. As can be seen, the contact angle increases with increasing pulse width and peak current. The predicted and experimental values ​​show basically the same trend, with the predicted values ​​generally being lower. This is because the workpiece surface is assumed to be a uniformly pitted surface; the actual surface is rougher and more complex than the model, and the width of the machined pits is larger than the theoretical pit width, thus increasing the actual contact angle. From the experimental values, the average error between the experimental and predicted contact angle values ​​is only 9.98%, verifying the reliability of the theoretical model proposed in this invention.

[0225] Figure 16 The image shows the overall three-dimensional morphology of the workpiece surface microstructure observed under a super depth-of-field microscope. It can be seen that the microstructure surface is relatively smooth and flat, without obvious burrs. Under the assistance of magnetic field and electrolysis, the surface roughness of the workpiece is reduced, and the processing accuracy and surface quality are greatly improved.

[0226] Figure 17 The contact angle of the workpiece surface was measured to be 137.79° under a pulse width of 12μs, a current of 10A, and a magnetic field strength of 0.1T. Figure 16 This shows that, under the effect of composite process, the workpiece not only improves its surface quality, but also further obtains good hydrophobic properties.

[0227] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the present invention. Any equivalent substitutions or modifications made by those skilled in the art to the technical solutions and content disclosed in the present invention without departing from the scope of the present invention shall be deemed to have remained within the protection scope of the present invention.

Claims

1. A device for wire electrical discharge machining using magnetic field and electrolysis assisted, characterized in that, It includes a machine tool, a work platform mounted on the machine tool, a power supply and control system, a working fluid cooling circulation system, and a wire feeding system. The power supply and control system provides power to the processing device, and the work platform is connected to an adjustable magnetic field generator and is used to fix the workpiece. The adjustable magnetic field generator generates a magnetic field of constant and uniform intensity; The wire feeding system guides an electrode wire for electrical discharge cutting of a workpiece. The electrode wire is in contact with the workpiece in a magnetic field, and the direction of the magnetic field is perpendicular to the feed direction of the electrode wire. The working fluid cooling circulation system injects a working fluid with micro-conductivity and micro-electrolysis characteristics into the processing area; The processing device changes the current density in the working fluid through electrolysis, removing discharge pits and recast layers generated by electrical discharge machining, thereby improving the surface quality of the workpiece.

2. The wire electrical discharge machining apparatus using magnetic field and electrolysis assisted as described in claim 1, characterized in that, The adjustable magnetic field generating device includes mounting brackets fixed on the same side of the working platform and extending in the same direction to opposite sides of the working area. Helmholtz coils are fixed on opposite sides of the two mounting brackets. The winding direction of the two Helmholtz coils is the same, and the center distance between the two coils is equal to the radius of the center circle of the coil. The strength and direction of the magnetic field are determined by the magnitude and direction of the current, respectively.

3. The wire electrical discharge machining apparatus using magnetic field and electrolysis assisted as described in claim 1, characterized in that, The working fluid cooling circulation system includes a fluid supply pump and a working fluid tank. The motor of the fluid supply pump is connected to a power supply, and the processing device adjusts the flow rate of the fluid supply pump by controlling the voltage. The inlet of the liquid supply pump is connected to the inside of the working liquid tank through a pipe to extract the working liquid. The outlet of the liquid supply pump is connected to the pipe to deliver the working liquid to the processing area. The working fluid tank contains a working fluid, which contains sodium chloride-ethylene glycol electrolyte.

4. The wire electrical discharge machining apparatus using magnetic field and electrolysis assisted as described in claim 1, characterized in that, The power supply and control system includes a pulse power supply, the positive terminal of which is directly connected to the workpiece, and the negative terminal of which is connected to the electrode wire through a conductive block.

5. A processing method using a magnetic field and electrolysis-assisted wire electrical discharge machining (EDM) device, comprising using the magnetic field and electrolysis-assisted wire EDM device as described in any one of claims 1-4, characterized in that, Includes the following steps: S1: Set the processing parameters and preprocess the workpiece; S2: Fix the pre-treated workpiece on the work platform using a special fixture, and ensure that the workpiece does not come into contact with the electrode wire; S3: Install the electrode wire with the set diameter onto the wire storage drum of the wire feeding system and perform adjustments; S4: Install and start the adjustable magnetic field generator, adjust the current to form a magnetic field of the required intensity in the processing area, and add sodium chloride-ethylene glycol electrolyte to the working fluid at the same time. S5: Start the power supply and control system, perform wire electrical discharge machining to obtain the microstructure boss size, and monitor the discharge stability, electrode wire vibration and machining speed in real time. S6: Clean the cut workpiece and let it stand to air dry naturally.

6. The processing method using a magnetic field and electrolysis-assisted wire electrical discharge machining device according to claim 5, characterized in that, In S1, the processing parameters are set as follows: current 6-12A, voltage 40V, pulse width 6-15μs, pulse width 6μs, electrode wire diameter 0.2mm, magnetic field strength 0.1-0.4T, and a total of 16 groups are processed.

7. The processing method using a magnetic field and electrolysis-assisted wire electrical discharge machining device according to claim 5, characterized in that, In S1, the pretreatment includes workpiece cleaning, oxide layer removal, and drying procedures; The workpiece cleaning process specifically involves placing the workpiece to be processed in a dedicated cleaning tank and using ethanol for initial cleaning to remove surface contaminants. The cleaning time is 5-10 minutes. The deoxidation process specifically involves treating the pre-cleaned workpiece with a dilute hydrochloric acid solution for 1-2 minutes, followed by rinsing with clean water. The drying process specifically involves placing the workpiece to be processed after the oxide layer removal treatment in a drying oven and drying it at a temperature between room temperature and 80°C to remove moisture from the surface of the workpiece for 10-30 minutes.

8. The processing method using a magnetic field and electrolysis-assisted wire electrical discharge machining device according to claim 5, characterized in that, In S4, the demand intensity is 0.1T.

9. The processing method using a magnetic field and electrolysis-assisted wire electrical discharge machining device according to claim 5, characterized in that, In step S6, a contact angle prediction model is used to predict the contact angle of the workpiece after cutting. The expression of this model is: By substituting the parameters into the contact angle prediction model, the predicted value of the contact angle is obtained.

10. The processing method using a magnetic field and electrolysis-assisted wire electrical discharge machining device according to claim 5, characterized in that, The contact angle prediction model is established by combining the Wenzel model and the Cassier-Baxter model. The Cassier-Baxter model is used to analyze the force balance of water droplets on microstructures, while the Wenzel model is used to consider the influence of surface roughness on the contact angle when water droplets directly contact microstructure protrusions.

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