Electric spark wire cutting machining device and method assisted by magnetic field and electrolysis

Abstract

The invention provides a magnetic field and electrolysis assisted wire cut electrical discharge machining device and method, and belongs to the technical field of wire cut electrical discharge machining. The device comprises a machine tool, a working platform arranged on the machine tool, a power supply and control system, a working liquid cooling circulation system, a wire conveying system and an adjustable magnetic field generating device. Wherein the working solution cooling circulation system is used for conveying a working solution added with sodium chloride-ethylene glycol to a processing area, and the adjustable magnetic field generating device is used for forming a stable intensity magnetic field in the processing area. The method comprises the steps of machining parameter setting, workpiece pretreatment and workpiece machining. The invention further provides a prediction model for predicting the contact angle of the microstructure. A magnetic field auxiliary device and an electrolysis device are designed in an existing machine tool, electrolysis and magnetic field auxiliary technologies are applied to the technical field of wire cut electrical discharge machining, the machining precision and surface quality of wire cut electrical discharge machining are improved, a fine microstructure can be prepared, and a foundation is provided for improvement of hydrophobic performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of wire electrical discharge machining, and particularly relates to a magnetic field and electrolysis assisted wire electrical discharge machining device and method. Background Art

[0002] Hydrophobic materials are widely used in aerospace, biomedicine, industry and other fields due to their self-cleaning, antibacterial, drag reduction, corrosion resistance and other properties. Usually, the surface microstructure and chemical composition of materials are modified by methods such as laser processing, coating, chemical etching, etc. to regulate the hydrophobicity of the material surface. Nickel-titanium alloy is widely used in the fields of automotive, aerospace and biomedicine due to its unique shape memory effect (SME) and excellent biocompatibility. Good surface hydrophobicity can reduce platelet adhesion and activation, thus preventing thrombus formation and improving biocompatibility; for applications such as orthodontic wires, its excellent hydrophobicity can reduce bacterial adhesion and play a protective role; in orthopedic implant medical devices, it can also reduce the release of Ni atoms and prevent toxic substances from entering the body. Nickel-titanium alloy will have problems such as tool wear, stress and excessive cutting temperature under traditional machining processes. Therefore, special machining processes must be used to improve machining characteristics.

[0003] Wire electrical discharge machining (WEDM) is a non-traditional machining method suitable for machining difficult-to-machine materials. In WEDM-LS, the electrode wire does not directly contact the workpiece. Therefore, problems such as mechanical stress, vibration and chatter are eliminated during the machining process, and it is not limited by material hardness and toughness. At the same time, it has advantages such as high machining accuracy and good surface quality. Therefore, wire electrical discharge machining plays an important role in modern manufacturing industries such as molds, instruments, aerospace, automotive, and biomedicine. However, due to its working principle, there may be some defects on the workpiece surface, such as pits, recast layers and microcracks. To solve these problems, the present invention proposes a magnetic field assisted electrolysis wire electrical discharge machining composite process, aiming to improve machining quality and efficiency, and at the same time provide an effective way for machining hydrophobic surfaces.

[0004] Regarding wire electrical discharge machining, 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. Through a low-frequency vibration assisted device, a superhydrophobic metal curved surface with irregular macroscopic shape can be machined at one time, or a micro-nano double-layer structure with surface morphologies such as square trapezoid, rhombic trapezoid, and rhombic pyramid can be machined multiple times.

[0006] 2. Wang Zhenlong from Harbin Institute of Technology proposed a method for preparing superhydrophobic surfaces by combining electric discharge machining and spraying, which solved many problems in terms of process and cost brought about by methods such as soaking with low surface energy reagents like fluorinating agents. The stability of carbon makes the prepared superhydrophobic surface of metal materials have the characteristics of wear resistance and high stability.

[0007] The above prior art fails to fully solve the problem of too large surface roughness generated during wire electrical discharge machining, resulting in insufficient precision of the processed microstructures, and thus affecting the hydrophobic performance.

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

[0009] The object of the present invention is to provide an electric discharge wire cutting processing device assisted by magnetic field and electrolysis, including a machine tool, a working platform arranged on the machine tool, a power supply and control system, a working fluid cooling and circulating system, and a wire feeding system. The power supply and control system provides power for the processing device, and the working platform is connected with an adjustable magnetic field generating device and is used for fixing the workpiece;

[0010] The adjustable magnetic field generating device forms a magnetic field with a constant and uniform intensity;

[0011] The wire feeding system guides a wire electrode for electric discharge cutting of the workpiece. The wire electrode contacts the workpiece within the magnetic field, and the direction of the magnetic field is perpendicular to the feeding direction of the wire electrode;

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

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

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

[0015] Furthermore, the working fluid cooling and circulating system includes a liquid supply pump and a working fluid tank. The motor of the liquid supply pump is connected to the power supply, and the processing device adjusts the flow rate of the liquid 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 pipeline to realize the extraction of the working liquid. The liquid supply pump is connected to a pipeline with an outlet to transport the working liquid into the processing area;

[0017] The working liquid tank stores the working liquid, and sodium chloride-ethylene glycol electrolyte is added to the working liquid.

[0018] Furthermore, the power supply and control system includes a pulse power supply. The positive electrode of the pulse power supply is directly connected to the workpiece, and the negative electrode of the pulse power supply 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 device, which uses a magnetic field and electrolysis assisted wire electrical discharge machining device, includes the following steps:

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

[0021] S2: Fix the preprocessed workpiece on the working platform through a special fixture and ensure that the workpiece does not contact the electrode wire;

[0022] S3: Install the electrode wire with the set parameter diameter on the wire storage cylinder of the wire feeding system and conduct debugging;

[0023] S4: Install and start the adjustable magnetic field generating device, adjust the current to form a magnetic field with the required intensity in the processing area. At the same time, add sodium chloride-ethylene glycol electrolyte to the working liquid;

[0024] S5: Start the power supply and control system, conduct wire electrical discharge machining, obtain the dimensions of the microstructural boss, and monitor the discharge stability, electrode wire vibration and machining speed in real time;

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

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

[0027] Furthermore, in S1, the preprocessing includes workpiece cleaning, deoxidization layer removal and drying treatment procedures;

[0028] Specifically, the workpiece cleaning is as follows: Place the workpiece to be processed in a special cleaning tank and conduct preliminary cleaning with ethanol to remove surface contaminants. The cleaning time is 10 minutes;

[0029] Specifically, the deoxidization layer removal is as follows: Use dilute hydrochloric acid solution to remove the deoxidization layer of the workpiece to be processed after preliminary cleaning for 2 minutes, and then rinse it with clean water after processing;

[0030] The drying treatment is specifically as follows: Place the workpiece to be processed after the deoxidation layer treatment in a drying oven and conduct the drying treatment at a temperature between room temperature and 80 °C to remove the moisture on the surface of the workpiece to be processed, and the time is 30 minutes.

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

[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 each parameter 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 integrating the Wenzel model and the Cassier - Baxter model. The Cassier - Baxter model is used to analyze the force balance of water droplets on the micro - structure, and the Wenzel model is used to consider the influence of surface roughness on the contact angle when the water droplet directly contacts the micro - structure boss.

[0036] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in: A magnetic field assistance device and an electrolysis device are designed in the existing machine tool, and the electrolysis and magnetic field assistance technologies are applied to the field of electrical discharge wire cutting technology, improving the machining accuracy and surface quality of electrical discharge wire cutting, being able to prepare fine micro - structures, and providing a basis for the improvement of hydrophobic performance. Brief Description of the Drawings

[0037] Figure 1 It is a schematic structural diagram of the magnetic field and electrolysis assisted electrical discharge wire cutting processing device of the present invention.

[0038] Figure 2 It is a diagram of the action mechanism of the magnetic field assisted electrical discharge wire cutting processing workpiece of the present invention.

[0039] Figure 3 It is a diagram of the action mechanism of the magnetic field assisted electrical discharge wire cutting processing workpiece.

[0040] Figure 4 It is a diagram of the double - electric layer model between electrodes of the present invention.

[0041] Figure 5 It is a schematic diagram of the three - dimensional model of the magnetic field assistance device of the present invention.

[0042] Figure 6 It is a schematic diagram of the design principle of the Helmholtz coil of the present invention.

[0043] Figure 7Schematic diagram of the three-dimensional model of the electrolysis device in the present invention.

[0044] Figure 8 Gaussian heat source distribution diagram in the present invention.

[0045] Figure 9 Two-dimensional model of the pit in the present invention.

[0046] Figure 10 Three-dimensional model and top view of the pit in the present invention.

[0047] Figure 11 Schematic diagram of the microstructure size in the present invention.

[0048] Figure 12 Force analysis of the droplet in the Z direction in the present invention.

[0049] Figure 13 Schematic diagram of the gap between microstructures in the present invention.

[0050] Figure 14 Surface of the microstructure pit in the present invention.

[0051] Figure 15 Comparison chart of predicted values and experimental values under the composite process in the embodiment of the present invention.

[0052] Figure 16 Ultra-depth-of-field microscope image of the surface microstructure of the processed nickel-titanium alloy in the embodiment of the present invention.

[0053] Figure 17 Static contact angle (CA) diagram of the hydrophobic metal surface in the embodiment of the present invention.

[0054] Among them, 1. Wire storage cylinder; 2. Guide wheel; 3. Electrode wire; 4. Conductive block; 5. Power supply and control system; 6. Adjustable magnetic field generating device; 7. Guide; 8. Workpiece; 9. Working fluid cooling and circulation system; 10. Guide and clamp; 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. Specific embodiments

[0055] The following will describe in more detail a magnetic field and electrolysis assisted wire electrical discharge machining device and method of the present invention with reference to the schematic diagrams, which show the preferred embodiments of the present invention. It should be understood that those skilled in the art can modify the present invention described herein while still achieving the advantageous effects of the present invention. Therefore, the following description should be understood as a broad guidance for those skilled in the art and not as a limitation to the present invention.

[0056] Embodiment 1

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

[0058] 1. Device composition

[0059] As Figure 1 shown, the machining device consists of a working platform 17 (refer to Figure 5 ), a power supply and control system 5, a working fluid cooling and circulation system 9, a wire feeding system, and an adjustable magnetic field generating device 6.

[0060] The power supply and control system 5 only includes a pulse power supply. The positive pole of the pulse power supply is directly connected to the workpiece 8, and the negative pole is connected to the electrode wire through a conductive block 4. The wire feeding system mainly includes a wire storage cylinder 1, guide wheels 2, an electrode wire 3, a guide 7, and a guide clip 10. The wire feeding system is arranged on the machine tool and is part of an existing wire electrical discharge machining machine tool, used to drive the electrode wire 3 to move downward for a one-way feeding motion. The working fluid cooling and circulation system 9 includes a liquid supply pump 12 and a working fluid 11. In this invention, the flushing type is adopted to machine the workpiece 8. The working fluid 11 is mainly composed of a sodium chloride - ethylene glycol solution with a certain pressure, which promotes chip removal and temperature reduction to ensure the normal progress of machining. The adjustable magnetic field generating device 6 is mainly composed of a pair of adjustable Helmholtz coils (circular in shape). By adjusting the current intensity flowing through the coil 20, a constant and uniform intensity magnetic field can be generated between the coils 20, and the magnetic field intensity is between 0.1T - 0.4T.

[0061] 2. Working principle:

[0062] Refer to Figure 3, with the assistance of electrolysis and magnetic field, the workpiece 8 and the electrode wire are respectively connected to the positive and negative poles of the pulse power supply. The electrode wire 3 moves in a unidirectional and low-speed manner under the guidance of the wire storage cylinder 1, each guide wheel 2 and the guiding device. The workpiece 8 is placed between a pair of adjustable Helmholtz coils (circular in shape). When the distance between the two coils 20 is equal to their radius, by adjusting the current intensity flowing through the coils 20, a magnetic field with a constant and uniform intensity can be generated between the coils 20. The feeding 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 movement trajectories. During the machining process, a spark discharge will occur between the workpiece 8 and the electrode wire 3, and its temperature is as high as over ten thousand degrees. This high temperature will erode the workpiece material and form spherical erosion products. At the same time, the working fluid cooling and circulation system 9 continuously supplies the working fluid 11 to the machining area. Due to the micro-conductivity and micro-electrolysis characteristics of the sodium chloride-ethylene glycol solution, both electric discharge machining and electrolytic machining exist in the machining area. In the machining area in the feeding direction of the electrode wire 3, the gap between the electrode wire 3 and the workpiece 8 is less than the critical value of spark discharge, resulting in spark discharge. At the same time, the electrolysis process also exists, but the material removal is mainly based on electric discharge machining. In the machining area parallel to the feeding direction of the electrode wire 3, the gap between the electrode wire 3 and the workpiece 8 is greater than the critical value of spark discharge, and almost no spark discharge occurs. The current density in the solution is mainly changed through electrolysis to remove some discharge pits and recast layers generated by electric discharge machining, thereby improving the machining surface quality.

[0063] 3. Magnetic field action mechanism

[0064] Reference Figure 2 , during the magnetic field-assisted WEDM-LS machining process, the electrons in the discharge channel are jointly affected by the electric field force and the Lorentz force, and their movement trajectories will no longer be straight lines, thus causing the discharge channel to deflect.

[0065] For the purpose of simplifying the analysis process in the present invention, the following reasonable assumptions are made:

[0066] (1) Neglect the influence of gravity on the movement trajectory of electrons;

[0067] (2) Do not consider the interaction between electrons;

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

[0069] The force balance equation of electrons 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 - Mass of the electron, Kg;

[0073] a - Acceleration of the electron, m·s-2;

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

[0075] v e - Movement speed of the electron, m·s-1;

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

[0077] B - Magnetic induction intensity, T.

[0078] According to Figure 2 and assumption (3), there is: B x =-B, B y =B z =0, E y =-E, E x =E z =0, then expand Equation (1) along the Y and Z directions to get:

[0079]

[0080] Assume the initial velocity of the electron is 0, that is, y(0)=z(0)=0, v y (0)=v z (0)=0, and the velocity equation of the electron can be obtained:

[0081]

[0082] Trajectory equation:

[0083]

[0084] 4. Electrolysis mechanism

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

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

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

[0088] 3. Ignore the adsorption on the electrode surface and the inhomogeneity of the electrode surface structure.

[0089] Reference Figure 4 , the charging current density i flowing through the double-layer capacitance C d can be expressed as: c can be expressed as:

[0090]

[0091] According to the butler-volmer equation, the faradaic current density i flowing through the resistance R F is: F is:

[0092]

[0093] Wherein, i 0 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; T is the absolute temperature.

[0094] In formula (9), the first term represents the anodic current density, and the second term represents the cathodic current density. Since the cathodic current density is much smaller than the anodic 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] Wherein, ρ is the resistivity of the electrolyte; d k is the distance between the workpiece and the electrode wire.

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

[0100]

[0101] According to formula (12), the double-layer voltage φ can be obtained. Therefore, the expression of the current density i F 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 surface quality and machining efficiency of the workpiece.

[0102] 5. Design of the adjustable magnetic field generating device

[0103] The adjustable magnetic field generating device includes mounting brackets 18 fixed on the same side of the working platform and extending towards opposite sides of the processing area respectively. On the opposite sides of the two mounting brackets 18, silicon steel 19 is fixed, and around the outer periphery of the silicon steel 19, Helmholtz coils are wound.

[0104] The adjustable magnetic field generating device in the present invention is designed based on the principle of generating a uniform magnetic field by Helmholtz coils. The Helmholtz coils are two coils 20 with the same winding direction placed parallel to each other, the center distance is equal to the radius value of the center circle of the coil 20, the coils 20 are passed with the same-direction current, and the magnetic field strength and direction are determined by the magnitude and direction of the current. At this time, a certain range of uniform magnetic field will be formed on the axis between the coils 20, as Figure 5 shown

[0105] reference Figure 6 , the magnetic induction intensity on the axis O1O2 is formed by the superposition and coupling of the two end coils. Combining with the Biot - Savart law, the magnetic induction intensity of a single coil at any point on the axis is:

[0106]

[0107] Among them,

[0108] μ 0 is the vacuum permeability, with the unit H / m;

[0109] I is the current in the coil, with the unit A;

[0110] R is the radius of the coil, with the unit m;

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

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

[0113]

[0114] Considering that the workpiece is placed at the center P point between the two coils during the actual processing process, combining with Equation (14), the magnetic induction intensity at point P can be obtained:

[0115]

[0116] In the present invention, the maximum additional magnetic induction intensity needs to reach 0.4T. Combining with Equation (15), it can be known that if only winding is adopted, the size of the coil 20 will be too large, resulting in increased costs and problems with unreasonable installation on the machine tool. If the method of increasing the current is adopted, there will be potential safety hazards in the laboratory. Considering the above factors, a magnetic conductive medium is added to the center of the coil 20 during the design of the Helmholtz coil to increase the magnetic induction intensity. Considering the design cost, taking cast iron and silicon steel 19 as the objects, the enhanced magnetic induction intensity is theoretically calculated. The relationship between the magnetic induction intensity generated by the coil 20 and the magnetic induction intensity generated after adding the magnetic conductive medium can be expressed as:

[0117] B = μ r B 0 (16)

[0118] Wherein,

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

[0120] B 0 is the magnetic induction intensity generated by the coil, T;

[0121] R is the magnetic induction intensity generated after adding the magnetic conductive medium, T

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

[0123]

[0124] Table 1

[0125] 6. Design of the electrolysis device

[0126] The working fluid 11 used in the electrolysis and magnetic field assisted wire electrical discharge machining process proposed by the present invention is added with sodium chloride-ethylene glycol electrolyte and has weak corrosiveness. In order to avoid corrosion of machine tool components and reduce the usage amount of the working fluid 11, a working fluid cooling and circulation system 9 is designed according to the test requirements. The self-made working fluid cooling and circulation system 9 mainly consists of a 800 ml working fluid tank 13, a liquid supply pump 12 and a connecting pipe with a diameter of 6 mm. Specifically, the liquid inlet 15 of the liquid supply pump 12 is connected to the inside of the working fluid tank 13 through a pipe, and the liquid outlet 16 of the liquid supply pump 12 is arranged at the working area through a pipe. The motor 14 of the liquid supply pump 12 is connected to the power supply, and the flow rate of the liquid supply pump 12 can be adjusted by controlling the voltage. The supply voltage of the liquid supply pump 12 is 5V, the starting voltage is 1V, and the maximum flow rate is 100 L / h. The three-dimensional model schematic diagram of the electrolysis device is as Figure 7 shown.

[0127] Embodiment 2

[0128] 7. Contact angle prediction model for achieving hydrophobicity

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

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

[0131]

[0132] where q(r,t) is the heat flux density at a radius of r and time t; q m is the maximum heat flux density at the center of the plasma channel; t is the time; 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 electrical discharge point is as follows

[0134]

[0135] where q m is the maximum heat flux density; U is the discharge channel voltage; I is the discharge channel current; and ζ is the energy distribution coefficient.

[0136] The discharge channel radius equation can be expressed as

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

[0138] where I is the pulse current; T on is the pulse width. When a magnetic field is applied, the discharge channel radius becomes 1.5 times the original, i.e.,

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

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

[0141]

[0142] The surface topography of the discharge pit structure is determined by the size parameters of individual discharge micro-pits and the positions of the discharge points. Due to the randomness and disorder of the discharge point positions during the actual discharge process, 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 evenly distributed on the workpiece surface and there is partial overlap of the pits after random discharges, an equivalent two-dimensional model is established, asFigure 9 as shown

[0143] The calculation formula for surface roughness is

[0144]

[0145] In the formula, r is the radius of the pit, and r s is the remaining part of the radius after the pits are superimposed, h is the distance between the initial plane and the bottom of the pit, and h e is the distance from the superimposed part to the ground of the pit

[0146] According to the Wenzel contact angle model, it can be seen that when the material itself is hydrophobic, the contact angle increases with the increase of surface roughness. This is because the workpiece surface is not absolutely smooth, and there are many pits that increase the solid-liquid contact area. The surface of the workpiece processed by wire electrical discharge machining consists of many pits and protrusions. Here, a roughness coefficient is introduced, and the influence of the magnetic field-assisted electrolytic wire electrical discharge machining process on the pits is combined to determine the value. Therefore, for the convenience of calculation, the shape and arrangement of the pits are assumed as follows:

[0147] 1. All pits are evenly arranged on the processing plane, and the size is taken as the average size;

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

[0149] 3. The water droplet completely covers the processed surface.

[0150] A three-dimensional equivalent model is established as Figure 10 shown

[0151] To simplify the calculation, assume that the shaded part is an isosceles triangle, then the area of the shaded part is

[0152]

[0153] The area of the spherical cap is

[0154] S 2 = 2πRa (24)

[0155] where R is the radius of the sphere with the same size as the pit

[0156] The figure is the top view of the middle cross-section, and the calculation equation for the radius can be obtained as

[0157]

[0158] Combining Equation 23 and Equation 24 gives S 2 The area is

[0159]

[0160] The area of the flat surface before processing is

[0161]

[0162] The roughness coefficient of the solid surface is the ratio of the surface area of the rough surface to that of the smooth plane. Then the value of λ is expressed as

[0163]

[0164] The present invention uses a magnetic field-assisted electrolytic wire electrical discharge machining process to machine the microstructured surface. Considering the force of air on the water droplet in the groove part, the force balance of the water droplet on the micro-structure is analyzed based on the Cassie-Baxter contact model. Also, since the micro-structure boss is in direct contact with the water droplet and its surface roughness will affect the contact angle, a secondary micro-structure contact angle prediction model is established by integrating the Wenzel model and the Cassier-Baxter model.

[0165] For the convenience of calculation, the model is simplified, considering the following assumptions

[0166] 1. Only the hydrophobicity of the liquid droplet on the micro-structured surface needs to be considered.

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

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

[0169] Assume that the liquid droplet remains stationary on the micro-structure. The dimensional schematic diagram is as Figure 11 shown. When it reaches the equilibrium state, the force balance analysis method is used for it. According to the relationship between the geometric dimensions of the liquid droplet and the three-phase contact line, a force balance equation set is established to solve for α, and finally the static apparent contact angle is For the analysis of the force balance, it is mainly carried out in the Z direction.

[0170] The forces acting on the liquid droplet in the Z direction are as Figure 12 shown, which are respectively: the gravity of the liquid droplet, the supporting force received on the micro-structure, the surface tension at the micro-structure gap, and the component force of the surface tension between the liquid droplet and the workpiece surface. The force balance equation is

[0171] G = W 1 + W 2 + W 3 cosα (29)

[0172] In the formula, G is the gravity of the liquid droplet; W 1 is the supporting force received on the micro-structure; W 2 is the surface tension at the micro-structure gap; W3 is the surface tension between the droplet and the workpiece surface.

[0173] The calculation formula for 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 water droplet is the gravity of the liquid column on the microstructure. The volume of the liquid column supported by each microstructure is expressed as follows

[0176]

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

[0178]

[0179] l = Rsinα (34)

[0180] R n = Rcosα (35)

[0181] Substitute equations (32) to (35) into (31), then W 1 The calculation formula for

[0182]

[0183] To calculate the surface tension of the microstructure gap on the droplet, we assume that the air in the gap lifts the droplet into a circular arc. To facilitate the calculation, we establish a rectangular coordinate system for it, as Figure 13 shown

[0184] According to Figure 13 the radius of the circular arc can be obtained as

[0185]

[0186] The corresponding central angle is

[0187]

[0188] Then the supporting force of the microstructure gap on the droplet is

[0189]

[0190] Generally speaking, the contact area of the supporting force of the workpiece surface on the droplet is a circle with a bottom radius of R n Therefore, the calculation formula for W 3 is

[0191] W 3 = 2πRcosαγGL (40)

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

[0193]

[0194] The water droplet does not fully conform to the Cassie - Baxter model, and there is a Wenzel wetting state in the local sub - microstructure of the droplet. Therefore, the length of the contact line of the water droplet on the microstructure will change, and the increased ratio is related to the surface roughness. For a rough surface such as Figure 14 As shown, there is no solid surface at the gap, so it is considered that the value of the gap remains unchanged. After considering the surface roughness, the dimensional parameters become

[0195] a′ = a (44)

[0196] b' = λb (45)

[0197] where λ is the solid surface roughness coefficient, refer to Figure 14 .

[0198] Substitute the equivalent dimensions after the change of the structural dimension parameters considering the surface roughness into the formula, and the contact angle prediction model for the secondary structure is obtained as

[0199]

[0200] Through the required parameters: 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 boss gap, solid - liquid surface tension γ SL and gas - liquid surface tension γ VL Substitute into the secondary microstructure prediction model, solve for the value of α, and then obtain the contact angle as

[0201] Example 3

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

[0203] Step 1: Device preparation

[0204] Set up the magnetic field and electrolysis - assisted wire - cut electrical discharge machining device described in the present invention, including a machine tool bed body, a power supply and control system, a main shaft, a work platform, a working fluid cooling and circulation system, a wire feeding system, and a magnetic field generating device.

[0205] Step 2: Processing parameter setting

[0206] Set the current to any value between 6 - 12 A, the voltage to 40 V, the pulse width to any value between 6 - 15 μs, the inter-pulse width to 6 μs, the wire electrode diameter to 0.2 mm, the magnetic field strength to any value between 0.1 - 0.4 T, and process a total of 16 groups with different parameters.

[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 that the workpiece surface is clean and free of impurities.

[0209] 3.2) Removal of oxide layer: After cleaning, use a dilute hydrochloric acid solution to treat the workpiece for oxide layer removal. The purpose of this step is to remove the oxide layer formed on the workpiece surface due to oxidation to ensure good contact between the wire electrode and the workpiece during the processing. The oxide layer removal treatment time is generally 2 minutes, and then rinse it thoroughly with clean water.

[0210] 3.3) Drying treatment: Place the workpiece after cleaning and oxide layer removal in a drying oven and perform drying treatment at an appropriate temperature (such as room temperature to 80 °C) to completely remove the moisture on the workpiece surface. The drying time is generally 30 minutes to ensure that the workpiece surface is dry and free of moisture.

[0211] Step 4: Workpiece fixation

[0212] Fix the nickel-titanium alloy workpiece with dimensions of 50 mm × 50 mm × 2 mm after drying on the working platform, ensuring that there is no contact between the workpiece and the wire electrode. Use a special fixture to firmly fix the workpiece on the working platform to prevent the workpiece from moving or deforming during the processing. The fixed workpiece extends beyond the working platform, and one end extends into the processing area.

[0213] Step 5: Installation and debugging of the wire electrode

[0214] Install the brass wire with a diameter of 0.2 mm on the wire storage cylinder and perform debugging.

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

[0216] Place the adjustable magnetic field device perpendicular to the wire electrode, start the magnetic field generating device, and adjust the current to generate a magnetic field of 0.1 T in the processing area. At the same time, add an appropriate amount of sodium chloride - ethylene glycol working fluid to the working fluid.

[0217] Step 7: Processing process

[0218] Start the power supply and control system, and begin wire electrical discharge machining. A micro-structured boss with a size of 400 μm, a boss height of 200 μm, and a groove gap of 200 μm is obtained. Meanwhile, the discharge stability, wire vibration, and machining speed are monitored in real time.

[0219] Step 8: Post-processing after machining

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

[0221] Step 9: Micro-structure detection and hydrophobicity study

[0222] Use a super-depth-of-field microscope to detect the micro-structure on the surface of the processed nickel-titanium alloy. It is observed that a micro-structure with high precision is formed on its surface. At the same time, a JC2000D1 contact angle measuring instrument is used to measure the contact angle, and the measured contact angle is 120° - 145°, indicating its excellent hydrophobicity.

[0223] 9. Comparison between experimental values and predicted values

[0224] As Figure 15 shown in the comparison chart of the predicted and experimental values of the contact angle under the composite process, it can be seen from the figure that the contact angle increases with the increase of the pulse width and peak current. The change trends of the predicted values and the experimental values are basically the same, but the predicted values are generally on the low side. This is because the workpiece surface is assumed to be a surface with uniform pits, and the actual surface is rougher and more complex than the model, and the width of the processed pits is larger than the theoretical pit width, resulting in a larger actual contact angle. From the experimental values, the average error between the experimental values and the predicted values of the contact angle on the workpiece surface is only 9.98%, verifying the reliability of the theoretical model proposed in the present invention.

[0225] Figure 16 is the overall three-dimensional morphology diagram of the micro-structure on the workpiece surface observed under the super-depth-of-field microscope. It can be seen that the micro-structure surface is relatively smooth and flat, without obvious burrs. With the assistance of magnetic field and electrolysis, the surface roughness of the workpiece is reduced, and both the machining accuracy and surface quality are greatly improved.

[0226] Figure 17 is the contact angle size on the workpiece surface under a pulse width of 12 μs, a current of 10 A, and a magnetic field intensity of 0.1 T, which is measured as 137.79°. Combining Figure 16 , it can be seen that under the action of the composite process, while improving the surface quality of the workpiece, good hydrophobic properties are further obtained.

[0227] The above are only the preferred embodiments of the present invention and do not impose any restrictive effect on the present invention. Any person skilled in the art, within the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification and other changes to the technical solution and technical content disclosed by the present invention, which are all within the content of the technical solution of the present invention and still fall within the protection scope of the present invention.

Claims

1. A device for wire-cutting electric discharge machining using a magnetic field and electrolysis, characterized in that: It includes a machine tool, a working platform arranged on the machine tool, a power supply and control system, a working fluid cooling circulation system and a wire transport system, wherein the power supply and control system provide power for the processing device, and the working platform is connected to an adjustable magnetic field generating device and is used to fix the workpiece; The adjustable magnetic field generating device forms a magnetic field with constant and uniform strength; The wire feeding system guides an electrode wire for electric spark cutting of a workpiece, the electrode wire contacts the workpiece in a magnetic field, and the direction of the magnetic field is perpendicular to the feeding direction of the electrode wire; The working fluid cooling circulation system injects working fluid having micro-conductivity and micro-electrolysis properties into the processing area; The processing device changes the current density in the working fluid through electrolysis, removes the discharge pits and recast layers produced by EDM, and improves the surface quality of the workpiece.

2. The device for wire-cutting electric discharge machining using magnetic field and electrolysis as aid according to claim 1, characterized in that: The adjustable magnetic field generating device includes a mounting bracket fixed on the same side of the working platform and extending in the same direction to the opposite sides of the working area. Helmholtz coils are fixed on the 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 center circle radius of the coils. The strength and direction of the magnetic field are respectively determined by the magnitude and direction of the current.

3. The device for wire-cutting electric discharge machining using magnetic field and electrolysis as aid according to claim 1, characterized in that: The working liquid cooling circulation system comprises a liquid supply pump and a working liquid tank, the motor of the liquid supply pump is connected to a power supply, and the processing device adjusts the flow rate of the liquid supply pump by controlling the voltage; The water inlet of the liquid supply pump is connected to the inside of the working liquid tank through a pipeline to extract the working liquid. The liquid supply pump is connected to the pipeline to set a water outlet to transport the working liquid to the processing area. The working liquid tank stores working liquid inside, and the working liquid contains sodium chloride-ethylene glycol electrolyte.

4. The device for wire-cutting electric discharge machining using magnetic field and electrolysis as aid according to claim 1, characterized in that: The power supply and control system comprises a pulse power supply, the positive electrode of the pulse power supply is directly connected to the workpiece, and the negative electrode of the pulse power supply is connected to the electrode wire through a conductive block.

5. A machining method using a magnetic field and electrolysis assisted wire electric discharge machining device, using the magnetic field and electrolysis assisted wire electric discharge machining device as claimed in any one of claims 1 to 4, characterized in that: The following steps are involved: S1: Set processing parameters and pre-process the workpiece; S2: Fix the pre-treated workpiece on the working platform by a special fixture and ensure that the workpiece does not contact the electrode wire; S3: Install the electrode wire with the set parameter diameter on the wire storage drum of the wire transport system and debug it; S4: installing and starting the adjustable magnetic field generating device, adjusting the current to form a magnetic field of required strength in the processing area, and at the same time, adding sodium chloride-ethylene glycol electrolyte to the working fluid; S5: Start the power supply and control system, perform wire EDM, obtain the size of the microstructure boss, and monitor the discharge stability, electrode wire vibration and processing speed in real time; S6: Clean the cut workpiece and let it stand to dry naturally.

6. The processing method using a magnetic field and electrolysis assisted wire electric discharge machining device according to claim 5, characterized in that: In S1, the processing parameters are set as: 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 electric 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 is specifically as follows: placing the workpiece to be processed in a special cleaning tank and performing preliminary cleaning with ethanol to remove surface pollutants, and the cleaning time is 5-10 minutes; The deoxidation layer is specifically: the workpiece to be processed after preliminary cleaning is treated with a dilute hydrochloric acid solution to remove the oxidation layer for 1-2 minutes, and then rinsed with clean water; The drying treatment is specifically as follows: placing the workpiece to be processed after the oxide layer removal treatment in a drying oven, and performing a drying treatment at room temperature-80° C. to remove moisture on the surface of the workpiece to be processed for 10-30 minutes.

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

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

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

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