A combined electrospark and electrolytic machining process for machining reticular titanium-based composite materials

Abstract

The present invention relates to the technical fields of electric discharge machining and electrolytic machining, and more specifically to a combined electric discharge and electrolytic machining process for machining reticular titanium-based composite materials. The two machining processes can be combined to address the difficult-to-machine characteristics of reticular titanium-based composite materials, the poor surface quality of electric discharge machining, and the problems of stray corrosion in electrolytic machining. When machining the recast layer on the surface of a part, the uneven discharge pits therein inhibit the formation of a passivation film. A high-quality machined surface with a surface roughness as low as 0.1 μm, no surface defects, no stray corrosion, and no recast layer is obtained. The process has broad application prospects in the fields of aerospace, micro-robotics, precision parts, and the like.

Description

Technical Field

[0001] The invention relates to the technical field of electric spark machining and electrolytic machining, and more particularly to an electric spark electrolytic combined machining process for machining a mesh titanium-based composite material. Background Art

[0002] With the continuous development of metal matrix composites, titanium matrix composites have emerged as a material with broad application prospects. They have high strength, excellent ductility, significant wear resistance and superior high-temperature durability. These outstanding properties have led to their widespread application in aerospace and advanced military fields.

[0003] However, research has shown that the reinforcement phase in most current titanium-based composites is relatively uniformly distributed within the matrix, resulting in poor mechanical properties and severe room-temperature brittleness. Furthermore, titanium-based composites are difficult to machine, presenting significant challenges during processing due to the high hardness and brittleness of the reinforcement phase. Conventional machining often results in severe tool wear, fluctuating cutting forces, elevated cutting temperatures, and difficulty in chip removal.

[0004] In order to improve the above defects, the existing technologies also include electric discharge machining and electrolytic machining. However, after traditional electric discharge machining, a recast layer will be formed on the surface of the material, which will cause the following problems: 1. The surface roughness can be machined to a minimum of 0.5μm, which does not meet the use requirements for many microstructure fields. 2. There are defects such as microcracks and micropores on the surface. 3. After the base material is melted at high temperature, it re-solidifies to form a recast layer. The crystal structure of the material changes, and it differs from the base material in terms of hardness, strength, wear resistance, high-temperature durability, etc. Electrolytic machining will also cause a series of problems: 1. Due to the different corrosion rates of the material matrix and the reinforcing phase, stray corrosion is serious and the surface machining quality is poor. 2. Electrolytic machining of titanium-based materials will form a passivation film on the surface of the workpiece, which hinders material removal. Summary of the Invention

[0005] The invention provides an electric spark electrolysis combined machining process for machining a mesh titanium-based composite material, which can obtain a high-quality machined surface that can meet microstructure requirements.

[0006] The above objectives are achieved through the following technical solutions:

[0007] A combined electric spark and electrolytic machining process for machining a reticular titanium-based composite material comprises the following steps:

[0008] Step 1: Perform electrical discharge machining on the surface of the mesh titanium-based composite material;

[0009] Step 2: inspecting the surface of the mesh titanium-based composite material after electrospark machining to obtain the thickness of the recast layer of the mesh titanium-based composite material after electrospark finishing machining;

[0010] Step 3: Electrolytic machining another mesh-shaped titanium matrix composite material;

[0011] Step 4: measuring the removal rate of the electrolytic machining of the mesh titanium-based composite material and the residual degree of the recast layer;

[0012] Step 5: Calculate the electrolytic processing time based on the thickness of the recast layer after EDM;

[0013] Step 6: Electrolytically process the EDM surface of the meshed titanium-based composite material according to the electrolytic processing time and the removal rate during electrolytic processing;

[0014] Step 7: Detecting the surface of the meshed titanium-based composite material electrolytically processed in step 6 to obtain the following data: electrolytically processed surface morphology, surface roughness, and residual degree of the recast layer;

[0015] Step 8: If the data meets the requirements, the processing is terminated; if the data does not meet the requirements, the electrolytic processing is repeated until the requirements are met.

[0016] The beneficial effects of the electric spark electrolysis combined machining process for machining a meshed titanium-based composite material of the present invention are:

[0017] like Figure 2 As shown, the composite material uses a network of TiB whiskers (TiBw) as the reinforcement phase and TC4 as the matrix material. Its unique cystic structure allows the hard phase to encapsulate the soft phase, resulting in the composite material exhibiting excellent strength, stiffness, and elastic modulus, while also showing enhanced ductility, wear resistance, and high-temperature durability. Compared with TC4, its operating temperature is increased by 200°C, and compared with commonly used nickel-based superalloys, its specific gravity is lower, thus meeting the industry's demand for lightweight materials.

[0018] This technology addresses the difficult machining of reticular titanium matrix composites, as well as the poor surface quality of EDM and stray corrosion caused by electrochemical machining. By combining two machining processes, the uneven discharge pits during machining of the recast layer on the surface of the part inhibit the formation of a passivation film. The result is a high-quality machined surface with a surface roughness as low as 0.1μm, free of defects, stray corrosion, and recast layers. This technology has broad application prospects in aerospace, micro-robotics, precision parts, and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the process flow chart of EDM combined machining;

[0020] Figure 2 These are optical microscope and electron microscope images of the reticular titanium matrix composite material;

[0021] Figure 3 It is the laser confocal surface topography of EDM rough machining;

[0022] Figure 4 This is an electron microscope image of the recast layer during EDM roughing;

[0023] Figure 5 This is the electron microscope image of surface microcracks during EDM rough machining;

[0024] Figure 6 It is the laser confocal surface topography of EDM finishing;

[0025] Figure 7 This is an electron microscope image of the recast layer during EDM finishing;

[0026] Figure 8 It is the electron microscope image of micro cracks on the surface of EDM finishing;

[0027] Figure 9 This is the XRD analysis diagram of electrolytic processing by-products in NaCl electrolyte;

[0028] Figure 10 This is the laser confocal surface topography image of electrochemical machining in NaCl electrolyte;

[0029] Figure 11 This is the electron microscope surface morphology of electrochemical machining in NaCl electrolyte;

[0030] Figure 12 It is an electron microscope surface morphology image of combined electrospark electrolytic machining. DETAILED DESCRIPTION

[0031] A combined electrospark and electrolytic machining process for machining a mesh titanium-based composite material, Figure 1 , including the following steps:

[0032] Step 1: According to the contour of the part, leave the processing allowance of the part, for example, by writing G code during the CNC programming process. On the other hand, considering the problem of electrode loss, real-time online compensation can be performed to ensure the processing accuracy and stability of the processing quality of the part. Subsequently, the mesh titanium-based composite material (abbreviated as the part) is rough-machined using electric sparks, and a tungsten electrode with a diameter of 0.3 mm is used for processing. The roughness of the part is controlled below 3.0 μm to improve the material removal rate. The rough machining of the part can use large processing energy, with each discharge energy of 1.000-2.000 J, which further improves the material removal rate.

[0033] Step 2: If Figure 3As shown in the figure, laser confocal microscopy was performed on the surface morphology of the parts after rough EDM, and the roughness was measured. Under high processing energy, the surface roughness of the parts was 1.8-2.0 μm. In order to better observe the recast layer, an etching reagent, such as Kroll's reagent (5% HF and 15% HNO3), was used to etch the surface of the parts to expose their crystal structure. Figure 4 As shown, the different crystal structures of the recast layer and the network titanium matrix composite material were observed under an electron microscope.

[0034] Due to the randomness and non-uniformity of discharge, the thickness of the recast layer is different under the same processing parameters. Therefore, the average thickness is used to represent the thickness of the recast layer, as shown in Formula 1:

[0035] (1)

[0036] Where: h is the average thickness of the recast layer;

[0037] n——number of sampling times;

[0038] ——The maximum thickness of the recast layer in the nth electron micrograph;

[0039] ——the minimum thickness of the recast layer in the nth electron microscope image;

[0040] Taking into account the non-uniformity of the recast layer, multiple sampling was performed, and the maximum and minimum thicknesses of the recast layer in all electron microscope photos were taken to calculate their range, which was used to measure the uniformity of the recast layer, as shown in Formula 2:

[0041] (2) R = h max -h min

[0042] Where: R - the range of the thickness of the recast layer obtained by sampling;

[0043] h max —Sampling to obtain the maximum thickness of the recast layer in all photos;

[0044] h min —Sampling to obtain the minimum thickness of the recast layer in all photos;

[0045] The unit of thickness mentioned above is μm.

[0046] The results show that under high energy EDM, the average thickness of the recast layer on the surface of the parts is 13μm. At the same time, the thickness of the recast layer varies up to 20μm. Figure 5As shown in Figure 2, the microcrack defects on the surface of the EDM roughened parts were observed using an electron microscope. In summary, after EDM roughening, the surface roughness and average thickness of the recast layer of the parts were large, the recast layer was extremely uneven, and there were many microcracks on the surface of the parts, which seriously affected the surface quality of the parts.

[0047] Step 3: Perform EDM finishing on the parts after EDM rough machining, using a tungsten electrode with a diameter of 0.1mm to control the roughness of the parts below 1.0μm and improve the manufacturing accuracy of the parts. EDM finishing uses low energy processing with a discharge energy of 0.005-0.010J each time, and selects appropriate process parameters such as pulse width, pulse interval and current according to actual conditions, and compensates for the loss of electrode in real time online.

[0048] Step 4: Check the surface of the parts after EDM finishing. Figure 6 As shown in the figure, it is a laser confocal surface topography image of EDM finishing. It can be clearly observed that the diameter of the discharge pit on the surface of the part becomes smaller and its surface roughness is reduced to about 0.8μm. Kroll's reagent (5% HF and 15% HNO3) is used to etch the surface of the part. Figure 7 The results shown are those observed under an electron microscope.

[0049] The results show that under low-energy EDM finishing, the average thickness of the recast layer on the surface of the part is about 6μm. At the same time, the thickness of the recast layer reaches a range of 4μm. Figure 8 As shown in the figure, the micro cracks on the surface of the EDM finished part are significantly reduced. In summary, after EDM finishing, the surface roughness and average thickness of the recast layer of the obtained parts are reduced, the recast layer is extremely uniform, the micro cracks on the surface of the parts are reduced, and the surface quality has been improved to a certain extent. However, it is still not enough to achieve the surface quality requirements of 0.1μm surface roughness, no recast layer, and no surface damage of the microstructure.

[0050] Step 5: Electrochemical machining of the second part was performed, and the byproducts of electrochemical machining of the part in NaCl electrolyte were collected. At the beginning of electrochemical machining, a large number of bubbles were observed on the anode workpiece in the electrolyte. This indicates that oxygen bubbles are generated by the hydrolysis of water on the surface of the anode workpiece, and the electrochemical reaction is shown in Equation 1:

[0051] (1)4OH - →2H2O+O2↑+4e -

[0052] As electrochemical machining proceeds, a large number of dense bubbles are precipitated at the cathode, which indicates that the cathode reduces the hydrogen ions in the electrolyte to hydrogen gas. The electrochemical reaction is shown in Equation 2:

[0053] (2) 2H + +2e- →H2↑

[0054] XRD analysis was performed on the by-products of electrolytic processing of the network titanium-based composite material in NaCl electrolyte. Figure 9 As shown in Figure 3, the byproducts of electrolytic processing of the network titanium-based composite material in NaCl electrolyte are TiO2 and TiB. This is because Cl- ions have an activation effect on titanium-based alloys, and the dissolution of Ti elements is controlled by forming halide complexes. Its electrochemical dissolution behavior is shown in Equations 3 and 4:

[0055] (3)Ti-4e - +4Cl - →TiCl4

[0056] (4)TiCl4+2H2→TiO2+4Cl - +4H +

[0057] Step 6: Observe the surface of the parts electrolytically processed by NaCl electrolyte through laser confocal microscopy. Figure 10 As shown in the figure, it can be found that when electrolytic machining parts, the stray corrosion on the surface of the parts is more serious, and the surface roughness reaches 3.2μm, which is much higher than the surface obtained by EDM. Figure 11 As shown in the figure, electron microscopic observation of the surface morphology reveals that stray corrosion is caused by a mismatch in corrosion rates between the reinforcement phase and the base material. The titanium matrix is ​​first oxidized to titanium ions. As electrolytic machining proceeds, the reinforcement phase gradually protrudes, until the surrounding titanium matrix is ​​completely oxidized to titanium ions, causing the reinforcement phase to fall off. As a result, the reinforcement phase protrudes during the electrolytic process, causing stray corrosion and severely impacting the surface quality of the machined part. In contrast, areas without the reinforcement phase achieve higher surface quality when electrolytically machined.

[0058] Step 7: Develop a combined EDM and electrolytic machining process: Based on the surface morphology and surface roughness of the electrolytically machined parts, the calculated electrolytic machining part removal rate, and the electrolytic machining time calculated based on the thickness of the recast layer after EDM machining, perform electrolytic machining on the surface of the parts obtained by EDM machining.

[0059] Step 8: Inspect the surface of the part obtained by electrolytic processing in step 7. If the part meets the requirements of surface roughness, residual thickness of the recast layer, surface defects and stray corrosion, then the processing is terminated; if the surface quality does not meet the requirements, then continue processing according to the requirements of surface roughness, residual thickness of the recast layer, surface defects and stray corrosion until the requirements are met. Figure 12The image below shows the result of EDM combined machining. The part surface is even and flat, with no obvious defects, and the recast layer is completely removed. Laser confocal microscopy shows that the surface roughness of the meshed titanium-based composite material produced by EDM combined machining is as low as 0.08μm, meeting the machining requirements for micro-parts.

Claims

1. A combined electric spark and electrolytic processing process for processing a mesh titanium-based composite material, characterized in that: The following steps are involved: Step 1: Perform electrical discharge machining on the surface of the mesh titanium-based composite material; Step 2: inspecting the surface of the mesh titanium-based composite material after electrospark machining to obtain the thickness of the recast layer of the mesh titanium-based composite material after electrospark machining; Step 3: Electrolytic machining another mesh-shaped titanium matrix composite material; Step 4: measuring the removal rate of the electrolytic machining of the mesh titanium-based composite material and the residual degree of the recast layer; Step 5: Calculate the electrolytic processing time based on the thickness of the recast layer after EDM; Step 6: Electrolytically process the EDM surface of the meshed titanium-based composite material according to the electrolytic processing time and the removal rate during electrolytic processing; Step 7: Detecting the surface of the meshed titanium-based composite material electrolytically processed in step 6 to obtain the following data: electrolytically processed surface morphology, surface roughness, and residual degree of the recast layer; Step 8: If the data meets the requirements, the processing is terminated; if the data does not meet the requirements, the electrolytic processing is repeated until the requirements are met.

2. The combined electric spark electrolysis machining process for machining a reticular titanium-based composite material according to claim 1, characterized in that: During EDM, real-time online compensation is performed for electrode loss.

3. The combined electric spark electrolysis machining process for machining a reticular titanium-based composite material according to claim 1, characterized in that: During EDM, tungsten electrodes with diameters of 0.3 mm and 0.1 mm were used successively.

4. The combined electric spark electrolysis machining process for machining a reticular titanium-based composite material according to claim 1, characterized in that: When using a 0.3mm tungsten electrode, the discharge energy is 1-2J, and when using a 0.1mm tungsten electrode, the discharge energy is 0.005-0.010J.

5. The combined electric spark electrolysis machining process for machining a reticular titanium-based composite material according to claim 1, characterized in that: After EDM, the roughness of the parts is controlled below 1.0 μm.

6. The combined electric spark electrolysis machining process for machining a reticular titanium-based composite material according to claim 1, characterized in that: In step 2, a corrosive agent is used to corrode the machined surface of the mesh titanium-based composite material, and a recast layer of the machined surface is obtained by observation under an electron microscope.

7. The combined electric spark electrolysis machining process for machining a reticular titanium-based composite material according to claim 1, characterized in that: In step three, byproducts of electrolytic processing of the network titanium-based composite material in a NaCl electrolyte are collected.

8. The combined electric spark electrolysis machining process for machining a reticular titanium-based composite material according to claim 7, characterized in that: The processed surface obtained by electrolytic machining in NaCl electrolyte was observed by laser confocal microscopy, and its surface morphology was observed under an electron microscope.

9. The combined electric spark electrolysis machining process for machining a reticular titanium-based composite material according to claim 1, characterized in that: The surface roughness of the mesh titanium-based composite material processed by combined electrospark and electrolytic machining is controlled below 0.1 μm.

10. A mesh titanium-based composite material, characterized in that: The TiB whiskers distributed in a network are used as the reinforcement phase, and TC4 is used as the matrix material.

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