Research on the Influence of Magnetic Field Assistance on the Quality of an Electro-Spark Deposition Layer

Abstract

Aimed at solving the problems of single control measures in the electro-spark deposition (ESD) process, difficulty controlling the micro-process using heterogeneous materials (for the electrode and matrix), and the unstable quality and reliability of repairs to the deposition layer, a method of magnetic-field-assistance electro-spark deposition (MFESD) was proposed. An MFESD device was built, and a Ni electrode was used for deposition on the surface of 45 steel under the conditions of deposition voltages of 30 V, 60 V, and 90 V, respectively. This study examined the impact of the magnetic field’s intensity and frequency on the microstructure and mechanical properties of electro-spark deposition layers. The results show that the sputtering and protrusion of the electrode material on the surface of the deposition layer gradually decrease with an increase in the magnetic field’s intensity and frequency, defects such as pores and cracks are obviously improved, and the structure is uninterrupted and compact. The surface roughness of the deposited layer decreases with an increase in the magnetic field’s intensity and frequency, and its surface roughness decreases by 44.3%. The cross-section effect of the deposited layer is improved. The thickness of the deposited layer increases with an increase in the magnetic field’s intensity and frequency; the thickness of the deposited layer increases by 13.39%, and its maximum thickness can reach 54.396 μm. At the same time, the microhardness of the deposited layer increases with an increase in the two aforementioned properties of the magnetic field, and its hardness increases by 5.32%. Using a magnetic field to control ESD can effectively control the microscopic process of deposition and obtain high-quality deposition coatings, which have important significance in the surface remanufacturing of key components of high-end equipment.

1. Introduction

Electro-spark deposition (ESD) has become a widely adopted machining method due to its low heat input, simple electrode manufacturing, and minimal residual stress. It can effectively realize high-strength metallurgical bonding between the matrix and the deposited layer [1]. It is widely used in the remanufacturing of failure surfaces of mechanical parts, being especially suitable for the repair and strengthening of failure surfaces of mechanical parts serving in extreme environments such as high temperatures and easy loss [2]. At present, there are still some practical problems, such as the poor stability of the deposition process and single process control measures, in the process of ESD [3]. The molten electrode material splashes when it transitions to the matrix surface, and the time during which the molten heterogeneous material can fuse completely is very short, so it is difficult to control the process. Magnetic-field-assistance ESD can effectively improve the machining process. Scholars have carried out related research domestically and globally on the use of external magnetic-field-assistance processing.

Mihailov et al. [4], in studying ESD in a magnetic field, showed that under a change in the magnetic field or discharge energy, the alternating accumulation and dispersion of the discharge points will cause quasi-regular oscillation of the discharge indentations. Hao Yuefeng et al. [5] used the test method of composite deposition with a permanent magnet magnetic field and an electric spark, identifying the effect of the magnetic field on the attraction of the ferromagnetic deposited materials, which could increase the thickness of the deposited layer, reduce the surface roughness, and improve the material utilization rate. Peretyatku [6] used electrodes made of different magnetic materials to conduct ESD in a magnetic field. The experiments showed that the transfer mass using ferromagnetic electrode deposition under the magnetic field conditions was twice that without a magnetic field. Yue et al. [7] used a magnetic electrode adsorbed with fine powder to conduct ESD. According to this research, “soft contact” was established between the magnetized powder and the substrate surface in this method, which significantly increased the deposition weight and was more stable than that in the traditional method. X Chen et al. [8] proposed a magnetic-field-assistance ultra-high-speed friction stir welding method. The results show that magnetic-field-assistance ultra-high-speed friction stir welding has the potential to achieve high-quality welding. Zhang et al. [9] studied the influence of the auxiliary transverse magnetic field on the deformation behavior of thin-walled components during the wire-cutting process. Based on theoretical and simulation models, this study revealed the influence mechanism of the magnetic field on distortion. It was found that with an increase in the magnetic field’s intensity, the uniformity of the longitudinal distribution of the discharge point continues to improve. The transverse magnetic field significantly reduces the distortion of the discharge point by 32.77% and the deformation of the recast layer by 22.68%. R Chen et al. [10,11] used an external magnetic field to perform laser–MIG hybrid welding on 316 L stainless steel and discussed the effect of an external magnetic field on the texture changes and its effect on the crack growth rate. The grain refinement and ferrite changes in the alloy were studied using electron backscatter diffraction. It was revealed that the stirring effect caused by the magnetic field reduced the δ-ferrite content, and the effect of the reduction in δ-ferrite and its fatigue crack growth rate was discussed. Muralidharan et al. [12] studied the influence of an external magnetic field on ESD and used a material transport model to study the influence of the magnetic field on the height and weight of the deposited layer. The results showed that the deposition height and weight increased by 23.5% and 10.51%, respectively, under the action of the magnetic field. Zhang et al. [13] designed and developed a rectangular excitation-coil-type magnetic-field-generating device by combining theoretical and simulation analyses, which was symmetrically arranged around the electric spark discharge channel. The simulation results show that the magnetic field form of the magnetic-field-generating device is completely consistent with the magnetic field form of the discharge channel. A single-pulse discharge test in ESD under the different magnetic field parameters was carried out. By changing the relative distance between the deposition current and the coil to the center of the discharge channel, the morphological characteristics of the deposition point after single-pulse deposition were observed and studied. Chelladurai et al. [14] used response surface methodology to study the discharge parameters in the deposition process and analyzed the role of the magnetic field and protective gas in the discharge deposition process. The results show that with an increase in the magnetic field’s intensity, the deposition height increases and the deposition width decreases. The magnetic field effectively mitigates the deposition process challenges, enhancing processing efficiency. Kiryukhantsev et al. [15] combined magnetron sputtering with ESD to study the properties of Ti-C-Ni-Fe, Ti-C-Ni-Al and double-layer Ti-C-Ni-Al/Ti-C-Ni-Fe deposits. The experimental results show that the double-layer Ti-C-Ni-Al/Ti-C-Ni-Fe deposited layer obtained using the two-step MS-ESD technology has better crack resistance, wear resistance, and oxidation resistance, a lower friction coefficient, and enhanced corrosion resistance compared to the single Ti-C-Ni-Al layer. M, Kunieda et al. [16] applied a transverse magnetic field between the electrodes to conduct a single-pulse discharge test. After observation, it was found that the discharge pit changed along the vertical direction of the magnetic field, and the length of the pit increased with an increase in the magnetic field’s intensity. The author believes that the applied transverse magnetic field changing the Lorentz force in the plasma discharge channel may be the main reason. Rouniyar et al. [17] evaluated the recast layer and surface roughness of 6061-aluminum alloy workpieces in magnetic-field-assistance electrical discharge machining (MFEDM) with aluminum powder by the experiments. Oil is used as a dielectric, and aluminum is added. It is concluded that an increase in the powder concentration and magnetic field’s intensity will lead to a decrease in protrusions and holes on the surface, decrease in surface roughness, and decrease in surface crack density. Ming et al. [18] compared the differences in energy efficiency and environmental impact between MFEDM and traditional electrical discharge machining (EDM). It was found that the magnetic field in EDM can increase energy efficiency by 15.2%, tool wear rate by 22.6%, and material removal rate by 21.9%. At the same time, the addition of the magnetic field significantly reduces carbon dioxide emissions. Bui et al. [19] used pure silver and pure copper tools with a diameter of 2 mm to conduct surface modification tests on Ti6Al4V workpieces. The results show that the effect of the magnetic field significantly increases the material content transferred from the tool electrode to the workpiece surface. Additionally, it increases material removal rate and reduces the modified layer thickness. It proves that MFEDM is a potential technology for improving the efficacy of the EDM surface modification. Wang et al. [20] used an EDM milling process that applied a rotating transverse magnetic field to produce the fewest surface cracks, the least “caves”, and the recast layer which is the most uniform and the most continuous. Its impedance value was the highest, self-corrosion potential was the largest, and self-corrosion current density was the lowest according to its electrochemical impedance spectroscopy. In addition, its mass loss per unit area was the least, with the last and the weakest reaction of chemical corrosion of the workpiece surface. Zhang et al. [21] conducted micro electrochemical drilling the experiments with and without a magnetic field and with different magnetic flux densities. Then, the effects of the magnetic field on the machining accuracy of micro electrochemical drilling were analyzed and discussed based on the experimental results. The results showed that an external magnetic field whose direction was perpendicular to the feed direction improved the machining accuracy of micro electrochemical drilling, and the machining accuracy was the best when the magnetic flux density was approximately 0.1 T. Beravala et al. [22] studied the effect of the magnetic field on material removal rate and electrode wear rate. The result shows the magnetic field increased both the material removal rate by 21%–41% and electrode wear rate by 7%–14% in EDM using the liquid–air mixed dielectric. Zabihi et al. [23] found that the use of an auxiliary magnetic field in EDM can significantly improve the MRR, TWR, surface roughness and surface integrity. It was found that maximizing MRR requires an optimal magnetic field’s intensity, such that a deviation from the optimal field’s intensity reduces the MRR.

Based on the above research progress, it can be found that scholars around the world have carried out extensive research in the field of magnetic-field-assistance machining and ESD, but there is still a lack of research on the influence of the magnetic field’s intensity on the quality of ESD layer, which restricts the improvement of the quality of ESD coatings. The appropriate magnetic-field assistance can inhibit the spatter generation during the deposition process and play a role in controlling the molten metal in the molten pool in a timely manner. The self-developed MFESD device was used to carry out the ESD process on the workpiece surface. The surface morphology, roughness, cross-sectional morphology, thickness and microhardness of the deposited layer were used to characterize the effect of ESD machining. It is highly important to explore the influence of different magnetic fields’ intensity and frequency on the micromorphology and mechanical properties of the electro-spark deposited layer.

2. MFESD Machining Device

2.1. MFESD Device Introduction

Aiming at addressing the disadvantages of traditional ESD, such as low accuracy, poor surface quality, and poor bonding of the deposited layer, an MFESD device that can stably and efficiently process high-quality mechanical parts was built. The device was mainly composed of three parts: the overall structure of the device, the gap control system, and the magnetic-field-assistance device. The overall structure of the device consists of a vertical three-axis device, an ESD pulse power supply, and a digital control system. The gap control system ensures real-time adjustment of the electrode–workpiece distance, maintaining discharge stability. The magnetic-field-assistance device can realize the control of the magnetic field’s intensity and frequency. The magnetic-field-assistance device is applied to the lower part of the workpiece and the same axis with the workpiece, which cannot interfere with the relative movement between the electrode and the workpiece, play the effect of magnetic control in the deposition process, improve the transfer process of the electrode molten material, promote the fusion process of the molten electrode material and the matrix material, or improve the surface quality of the deposition layer. The schematic diagram of the overall structure of the MFESD device is shown in Figure 1.

The self-developed MFESD device was used to carry out the deposition experience, and the device is shown in Figure 2. The device can realize the mutual movement of X, Y and Z axes. The Z axis is introduced into the gap control system, and the feed and retreat of the spindle electrode are controlled by the single-chip microcomputer programming to ensure the real-time automatic control of the discharge gap. The X and Y axes are introduced into digital control, and the machining path is planned by Mach digital control software R3.043 to realize the relative movement between the spindle electrode and the workpiece, so as to ensure the continuity of deposition processing.

The specific adjustable ranges of the deposition parameters of the device are as follows: deposition voltage adjustment ranges of 20~100 V, duty ratio adjustment range of 20~100%, and discharge frequency adjustment range of 50~500 Hz. The adjustable range of magnetic field’s intensity is 0 mT to 700 mT, and the adjustable range of magnetic field’s frequency is 0 Hz to 10 Hz.

2.2. Magnetic Field Analysis Calculation

In order to effectively reduce the ejection of charged particles and the sputtering of deposition points in the process of ESdianhuohua D [5], the magnetic-field-assistance device needs to generate a magnetic induction intensity of 100 mT~700 mT on the surface of the workpiece [13]. Therefore, the magnetic field generated by the magnetic-field-assistance device needs to be analyzed and calculated to meet the design requirements. The magnetic field source uses a permanent magnet, and the magnetic field changes periodically when the magnetic field rotates. Therefore, the magnetic induction intensity of the magnetic-field-assistance device at static state is mainly calculated. The permanent magnets of the magnetic-field-assistance device are arranged in symmetrical opposite magnetic poles, so the spatial magnetic field generated by a permanent magnet is calculated first. As shown in Figure 3, the permanent magnet is a cube and is magnetized along the positive direction of the Z axis. For the convenience of calculation, the side length is set to 2a, and point P is any point in the space magnetic field generated by the permanent magnet.

The equivalent magnetic charge method is used to calculate the spatial magnetic field generated by the permanent magnet. The equivalent magnetic charge method is based on the equivalent magnetic charge theory, that is, the spatial magnetic field generated by the magnet is generated by the magnetic charges of the magnets. The generation of the magnetic field can be regarded as magnetic charges distributed according to certain rules [24]. Therefore, the magnetization direction of the magnet is single and uniform, so the magnetic charge density inside the magnet is constant zero, that is,

$${\rho }_m=-{\mu }_0\nabla \cdot M=0$$

(1)

where M is the magnetization vector, and μ₀ is the vacuum permeability.

The surface magnetic charge density is

$${\sigma }_m={\mu }_{0}n\cdot M$$

(2)

where n is the normal unit vector of the surface.

The surface magnetic charge density of the permanent magnet is affected by the size and direction of magnetization. The relationship between scalar magnetic potential and surface magnetic charge density is

$${\varphi }_m={\displaystyle \frac{1}{4\pi \mu }}{\displaystyle \underset{s}{\oint }\frac{{\sigma }_m}{r}}ds+c$$

(3)

where r is the distance between the source point to the field point, μ is the permeability, S is the boundary of the permanent magnet with surface magnetic charge, and c is a constant, which is related to the selected magnetic potential zero.

According to the theory of electromagnetics, the relationship between the magnetic field’s intensity and the scalar magnetic potential is as follows:

$$H=-\nabla {\varphi }_m$$

(4)

From the above formula,

$$H=H_{+}-H_{-}={\displaystyle \frac{1}{4\pi \mu }}{\displaystyle \underset{S_{+}}{\iint }\frac{{\sigma }_m}{r_{+}^3}}{r}_{+}ds-{\displaystyle \frac{1}{4\pi \mu }}{\displaystyle \underset{S_{-}}{\iint }\frac{{\sigma }_m}{r_{-}^3}{r}_{-}ds}$$

(5)

where H₊ and H₋ are the magnetic field’s intensity generated by positive and negative magnetic charges, respectively. S₊ and S₋ are the planes where the positive magnetic charge and negative magnetic charge are located, respectively.

Then, the total magnetic induction intensity is

$$B={\mu }_{0}H$$

(6)

This is obtained by calculating the space magnetic field of a single permanent magnet and then calculating the magnetic field generated by the compound of four permanent magnets on the surface of the workpiece. Four N35 permanent magnets were used, the size was 20 mm × 20 mm × 20 mm, and the center magnetic poles were arranged oppositely. The workpiece was 45 steel, the size was 10 mm × 10 mm × 5 mm, arranged on the upper part of the center of the four permanent magnets, which can meet the requirements of inhibiting the sputtering of the electrode material and spreading the molten heterogeneous materials outward during the ESD process.

3. Experiment Design

3.1. Sample Treatment

3.1.1. Sample Pretreatment

The electrode and workpiece of the experiment were magnetic conductive materials, and the tool electrode material was ERNi-1 with a diameter of 1.6 mm. The electrode material of the workpiece was 45 steel. The matrix samples of 45 steel plates with dimensions of 10 mm × 10 mm × 5 mm were cut by an electric spark NC cutting machine. In order to ensure the accuracy and stability of the deposition processing and reduce the test error, 200#, 400# and 600# sandpaper were used to rough the surface of the sample, and then, 1200#, 1500# and 1800# sandpaper were used to finely grind the surface of the sample. After making sure that the surface of the sample was polished to a flat surface, the surface of the polished sample was polished by a polishing machine, and the surface of the sample was polished to a mirror with no obvious scratches. Then, it was necessary to use alcohol to clean it to remove impurities. After drying, it was sealed and stored in a sealed bag.

3.1.2. Sample Postprocessing

Before the observation of the cross-section morphology, the sample of the prepared sedimentary layer was cut by the electric spark NC cutting machine. The cutting surface of the cut sample was coarsely ground and finely ground with 400#, 800#, 1200#, 1500# and 2000# sandpaper, respectively. After smooth grinding, the rough polishing and fine polishing continued, until it was polished to a mirror with no obvious scratches. In order to clearly see the delamination boundary between the matrix and the deposited layer, the corrosion treatment was carried out with 4% nitrate alcohol solution. After corrosion, the surface was cleaned with water, wiped with alcohol, and dried with a fan for testing.

3.2. Experiment Program Design

Considering the operability of the deposition experiment, the magnetic field’s intensity was controlled by adjusting the relative distance between the four permanent magnets at a fixed magnetic field’s frequency of 10 Hz. The magnetic field intensities (the center of the workpiece surface) of 300 mT, 500 mT and 700 mT were selected for deposition test under the deposition voltages of 30 V, 60 V, and 90 V, respectively. The parameters of the other experiment are shown in

Table 1. MFESD Test Parameters.

Parameters	Setting
Deposition voltage	30 V/60 V/90 V
Magnetic field’s intensity	0 mT/300 mT/500 mT/700 mT
Magnetic field’s frequency	0 Hz/3 Hz/6 Hz/10 Hz
Pulse width	45 μs
Deposition frequency	300 Hz
Deposition time	3 min
Spindle motor Angle	30°
Spindle electrode speed	200 r/min
Working medium	Air

. The effects of different magnetic field intensities on the surface morphology, surface roughness, cross-section morphology, thickness and microhardness of the deposited layer were mainly discussed. The fixed magnetic field’s intensity remained unchanged at 700 mT, and the adjusted magnetic field’s frequency (workpiece surface center) was 0 Hz, 3 Hz, 6 Hz and 10 Hz under the deposition voltages of 30 V, 60 V, and 90 V, respectively, and the results were compared with those of the non-magnetic-field-adjusted deposition test.

3.3. Observation Method

3.3.1. Surface Micromorphology Test

The SUPRA 55 field emission scanning electron microscope was used to directly reflect the surface quality of the deposited layer by observing the surface microstructure of the deposited layer. The magnification of the equipment used for shooting is 100×, the extra high tension is 15.00 kV, the detector used is E-T secondary electron detector, the working distance is 10.0 mm, and the microstructure of the surface of the sedimentary layer can be clearly observed.

3.3.2. Surface Roughness Test

The Alicona optical three-dimensional surface topography measuring instrument was used to measure the 3D morphology and roughness of the deposited layer surface. The arithmetic average height Sa of the regional morphology was mainly used as the measurement index to represent the roughness of the two-dimensional morphology of the workpiece surface. In order to ensure the accuracy of the test, three 5 mm × 5 mm measuring areas were selected on the surface of the sample, and the average roughness of the three measuring areas was taken as the test result.

3.3.3. Cross-Section Morphology and Deposited Layer Thickness Measurement/Test

The combination of electrode material and base material can be directly reflected by observing the cross-section morphology of the deposited layer, and whether there are defects such as pores and cracks can be observed. An inverted metallographic microscope Leica DMi8 was used to observe the cross-section morphology of the deposited layer. For thickness measurement, five positions were taken for measurement in parallel on the cross-section, and the average value was taken as the thickness of the deposited layer.

3.3.4. Microhardness Test of Deposited Layer

The microhardness of five different positions on the surface of the deposited layer was measured by HV1000Z microhardness tester of Suzhou Maige Instrument Co., Ltd. Suzhou, China. The loading load was Vickers hardness HV200g, and the loading time was 10 s. The average value of the five measurements was taken as the test result.

4. Effect of Magnetic Field’s Intensity on the Quality of Deposited Layer

4.1. Analysis of Surface Micromorphology Results

Figure 4 shows the effect of the magnetic field’s intensity on the surface morphology of the deposited layer when the deposition voltages are 30 V, 60 V and 90 V, respectively. The results show that under each deposition voltage, with an increase in the magnetic field’s intensity, the surface micromorphology is better, and the defects are gradually reduced. When the deposition voltage was constant, the overall effect of the surface morphology of the deposited layer was obviously improved with an increase in the magnetic field’s intensity. The surface morphology of the deposited layer prepared at a magnetic field’s intensity of 300 mT was not significantly improved. This is because the magnetic field’s intensity is too low and the Lorentz force generated is too small. The small Lorentz force has a weak regulating effect on the ejection process of charged particles in the discharge channel, resulting in a small transition effect on molten droplets during ESD, and it is impossible to control the regular dripping of droplets into the molten pool. As a result, the surface morphology of the coating does not change greatly. However, the surface morphology of the deposited layer prepared under the magnetic field’s intensity of 500 mT and 700 mT was greatly improved. The electrode material sputtering and protrusions on the surface of the deposited layer were less, and the defects such as pores and cracks were significantly improved. The structure was uninterrupted and compact. When the magnetic field’s intensity was 700 mT, the surface morphology of the deposited layer was the best.

But at the same time, it can be seen that under the different deposition voltages, there are different degrees of material sputtering, protrusions, pores and cracks on the surface of the deposited layer prepared in the non-magnetic field. With the increase in deposition voltage, the surface morphology of the deposited layer prepared in the non-magnetic field gradually deteriorates. This is because, with the increase in deposition voltage, the energy generated during discharge increases, so that the melting amount of the electrode and the matrix material increases. When the droplets contact the surface of the matrix, the irregular sputtering increases. Due to the increase in droplet transfer, the release of residual heat is not sufficient and the solidification rate is fast, which leads to the formation of material protrusions, pores and sputtering phenomena on the surface of the deposited layer.

The selection of an appropriate magnetic field’s intensity can significantly reduce the sputtering of metal materials during the deposition processing and can improve the fluidity of molten materials in the molten pool. With an increase in the magnetic field’s intensity, the magnetic field has a magnetic control effect on the molten pool, and the inhibition on the defects during the processing process is enhanced, so that the electrode and the matrix surface material are more fully fused, the metallurgical bonding effect is more significant, and the surface morphology of the deposition layer is more uniform.

4.2. Analysis of Surface Roughness Results

Figure 5 shows the influence of the changes in the magnetic field’s intensity on the 3D surface morphology of the deposited layer when the deposition voltages are 30 V, 60 V, and 90 V, respectively, and the surface roughness of the deposited layer is measured. The influence of the magnetic field’s intensity changes on the surface roughness of the deposited layer under the different deposition voltages is plotted according to the surface roughness values, as shown in Figure 6.

It can be seen from Figure 5 that under the same deposition voltage, the surface roughness of the deposited layer gradually decreases with an increase in the magnetic field’s intensity. When the deposition voltage is 30 V, the surface roughness of the deposited layer without the magnetic field is the highest, which is 16.6 μm, and the surface roughness of the deposited layer under the 700 mT magnetic field’s intensity is the lowest, which is 9.3 μm, which is relatively reduced by 44.3%. When the deposition voltage is 60 V, the surface roughness of the deposition layer without the magnetic field is the highest, which is 26.9 μm, and the surface roughness of the deposition layer under the 700 mT magnetic field’s intensity is the lowest, which is 20.0 μm, which is relatively reduced by 25.66%. When the deposition voltage is 90 V, the surface roughness of the deposited layer without the magnetic field is the highest, which is 29.0 μm, and the surface roughness of the deposited layer under the 700 mT magnetic field’s intensity is the lowest, which is 22.4 μm, which is relatively reduced by 22.52%. The higher the magnetic field’s intensity, the lower the surface roughness of the deposited layer. This is because an increase in the magnetic field’s intensity has a stronger inhibitory effect on the sputtering generated by the molten droplet of the electrode material when it transitions to the matrix surface.

At the same time, the control effect of Lorentz force generated by the external magnetic field controlling is enhanced, which makes the molten material rotate around the center of the molten pool and effectively reduces the material protrusion caused by the rapid solidification of the molten droplet. The droplet transits to the surface of the matrix and spreads rapidly, which leads to the decrease in the surface roughness of the deposited layer with an increase in the magnetic field’s intensity.

4.3. Analysis of Cross-Section Micromorphology Results

Figure 7 shows the influence of the changes in the magnetic field’s intensity on the cross-sectional morphology of the deposition layer when the deposition voltage is 30 V, 60 V and 90 V, respectively. The results show that under each deposition voltage, with an increase in the magnetic field’s intensity, the cross-section micromorphology effect is better, and the defects decrease gradually. When the deposition voltage is constant, the overall effect of the cross-sectional morphology of the deposition layer is significantly improved with an increase in the magnetic field’s intensity. The cross-sectional morphology of the deposited layer prepared at the 300 mT magnetic field’s intensity is roughly the same as that of the deposited layer prepared in the non-magnetic field. This is because the attraction of the electrode material droplets caused by the low magnetic field’s intensity is too low, which cannot effectively reduce the irregular sputtering formed by the impact of the electrode material droplets hitting the body surface due to gravity, electric field force, centrifugal force, etc., and the fusion between the droplets is not sufficient. However, under the magnetic field’s intensity of 500 mT and 700 mT, the cross-sectional morphology of the deposition layer is improved, the cross-sectional defects are reduced, and the deposition layer is closely combined with the matrix. When the magnetic field’s intensity is 700 mT, the surface morphology of the deposition layer is the best, and there are no obvious defects such as sputtering, protrusions and pores. At the same time, in the absence of the magnetic field, with the increase in deposition voltage, there are many obvious defects in the cross-section morphology of the deposited layer after processing, such as large pores and other obvious defects. It is further verified that an increase in the auxiliary magnetic field makes the electrode and the surface material of the matrix more fully fused, the metallurgical bonding effect is better, the cross-section effect of the deposition layer is improved, and the deposition processing is promoted.

4.4. Analysis of Deposited Layer Thickness Results

Figure 8 shows the influence of the changes in the deposition voltage on the thickness of the deposited layer. The results show that under the same deposition voltage, the thickness of the deposited layer increases gradually with an increase in the magnetic field’s intensity. When the deposition voltage is 30 V, the thickness of the deposition layer is the largest at the 700 mT magnetic field’s intensity, which is 32.064 μm, and the thickness of the deposition layer is the smallest without the magnetic field, which is 30.274 μm, with a relative increase of 5.91%. When the deposition voltage is 60 V, the thickness of the deposition layer is the largest at the 700 mT magnetic field’s intensity, which is 48.385 μm, and the thickness of the deposition layer is the smallest without the magnetic field, which is 42.671 μm, with a relative increase of 13.39%. When the deposition voltage is 90 V, the thickness of the deposition layer is the largest at the 700 mT magnetic field’s intensity, which is 54.396 μm, and the thickness of the deposition layer is the smallest without the magnetic field, which is 48.932 μm, with a relative increase of 11.17%.

Among them, the thickness of the deposited layer prepared at the 300 mT magnetic field’s intensity has no significant change compared with the thickness of the deposited layer prepared without the magnetic field. The reason is that although the controlling of the magnetic field reduces the sputtering of the electrode material generated during the deposition process to a certain extent, the magnetic field’s intensity is too low to effectively inhibit the molten electrode material with large throwing force, resulting in an increase in the electrode material thrown off the matrix surface. Therefore, the thickness of the deposited layer prepared at the 300 mT magnetic field’s intensity and the 90 V deposition voltage is smaller than that of the deposited layer prepared in the non-magnetic field. The thickness of the deposited layer prepared at the 500 mT and 700 mT magnetic field’s intensities is greater than that prepared without the magnetic field, and the thickness of the deposited layer is proportional to the magnetic field’s intensity. This is because an increase in the magnetic field’s intensity effectively reduces the material waste caused by the electrode material throwing off the matrix surface under the combined action of the centrifugal force and so on.

It is further concluded that with the increase in the magnetic field’s intensity, the Lorentz force generated in the charged particles in the discharge passband becomes larger, which reduces the phenomenon of the electrode material being removed from the matrix surface under the action of gravity, electric field force, centrifugal force, etc., increases the thickness of the deposited layer, reduces the waste of electrode material, and improves the material utilization rate.

4.5. Analysis of Deposited Layer Microhardness Results

Figure 9 shows the effect of the changes in the deposition voltage on the hardness of the deposited layer. The results show that the hardness of the deposited layer increases with an increase in the magnetic field’s intensity under the same deposition voltage. When the deposition voltage is 30 V, the deposited layer hardness under the 700 mT magnetic field’s intensity is the highest, which is 172.41HV0.2, and the deposited layer hardness under the non-magnetic field is the lowest, which is 166.22HV0.2, with a relative increase of 3.72%. When the deposition voltage is 60 V, the deposited layer hardness at the 700 mT magnetic field’s intensity is the highest (197.39HV0.2), and the deposited layer hardness at the non-magnetic field’s intensity is the lowest (191.47HV0.2), with a relative increase of 3.1%. When the deposition voltage is 90 V, the hardness of the deposited layer at the 700 mT magnetic field’s intensity is the highest (213.60HV0.2), while the hardness of the deposited layer at the non-magnetic field’s intensity is the lowest (202.82HV0.2), with a relative increase of 5.32%.

Among them, the microhardness of the deposited layer prepared at the 300 mT magnetic field’s intensity has a positive and negative difference with that without the magnetic field. This may be because the magnetic field’s intensity is too low, resulting in poor magnetic field regulation, resulting in sputtering affecting the stability of subsequent deposition discharge. At the same time, the melting quality of the molten droplets of the electrode material increased, and the thickness of the deposited layer increased because of the stacking of layers. Because the magnetic field control device was arranged at the lower part of the matrix, the magnetic field’s intensity decreased, and the Lorentz force generated became smaller, which was not enough to spread the electrode and the substrate material before solidification in the molten state, resulting in insufficient release of internal residual heat and air inclusion. This further led to porosity, cracks and other defects in the deposited layer, and reduced the microhardness of the deposited layer. At the same time, under the magnetic field’s intensity of 500 mT and 700 mT, the hardness of the deposited layer was higher than that of the non-magnetic field. The problems existing in the process of EDM deposition were effectively improved, so that the prepared sediment layer had fewer defects, and a more continuous, dense and highly binding sediment layer was obtained. Thus, the microhardness of the sediment layer was improved.

5. Effect of Magnetic Field’s Frequency on the Quality of Deposited Layer

5.1. Analysis of Surface Micromorphology Results

Figure 10 shows the surface topography of the deposited layer under the magnetic field’s frequency when the deposition voltage is 30 V, 60 V, or 90 V. The results show that under the different deposition voltages, with an increase in the magnetic field’s frequency, the surface microstructure defects decrease gradually, and the overall effect becomes better. It can be observed from the figure that under the different deposition voltages, sputtering, protrusion, porosity and other phenomena exist on the surface of the deposited layer prepared in the non-magnetic field state. As the deposition voltage increases, the surface topography of the deposited layer prepared in the non-magnetic field state gradually deteriorates.

When the magnetic field’s intensity is constant, the overall effect of the surface morphology of the deposited layer under the corresponding deposition voltage is obviously improved with an increase in the magnetic field’s frequency. Among them, when the magnetic field’s frequency is 10 Hz, the surface topography of the deposited layer is the best, without obvious sputtering, protrusion and pore defects. However, it can be seen from the figure that the surface topography of the deposited layer prepared at the magnetic field’s frequency of 0 Hz is not improved, and the phenomenon of material protrusion and sputtering still exists. This is because although the 0 Hz magnetic field’s frequency reduces the irregular sputtering phenomenon caused by the impact of electric field force and centrifugal force on the matrix surface of the electrode material in the deposition process, the magnetic field at 0 Hz magnetic field’s frequency is static, which does not effectively inhibit its sputtering, but only increases its attraction. In addition, the magnetic field’s frequency of 0 Hz cannot be magnetic control of the molten pool, so that the metal material in the molten pool is solidified without spreading, the residual heat and gas are not released, and the stacking of deposited layers leads to sputtering, protrusion and porosity of the surface electrode material.

The proper magnetic field control can effectively reduce the sputtering of electrode materials during deposition and improve the transfer of molten materials and the fusion of heterogeneous materials. When the magnetic field’s intensity is constant, with an increase in the magnetic field’s frequency, the Lorentz force generated increases the limiting frequency of charged particles in the discharge channel, effectively reducing the irregular range thrown out. In addition, in the fusion stage between the electrode and the matrix material, the fusion process of metal materials in the molten pool is accelerated, the residual heat and gas are accelerated, and the temperature gradient in the heterogeneous fusion process is reduced. The electrode and the matrix material are fused more fully, the metallurgical bonding effect is more significant, and the surface defects of the deposited layer are less.

5.2. Analysis of Surface Roughness Results

Figure 11 shows the 3D topography of the surface of the deposited layer under the different magnetic field’s frequencies when the deposition voltage is 30 V, 60 V and 90 V, and the surface roughness of the deposited layer is measured. According to the surface roughness values, the cylindrical chart of the surface roughness of the sedimentary layer under the different deposition voltages and the magnetic field’s frequencies is drawn, as shown in Figure 12.

It can be seen from Figure 12 that under the same deposition voltage and the magnetic field’s intensity, the surface roughness of the prepared sediment layer gradually decreases with an increase in the magnetic field’s frequency. When the deposition voltage is 30 V, the surface roughness of the deposited layer is the highest (16.6 μm) without the magnetic field, and the surface roughness of the deposited layer is the lowest (9.3 μm) at the magnetic field’s frequency of 10 Hz, which decreases by 44.3%. When the deposition voltage is 60 V, the surface roughness of the deposited layer is the highest (26.9 μm) without the magnetic field, and the surface roughness of the deposited layer is the lowest (20.0 μm) at the magnetic field’s frequency of 10 Hz, which decreases by 25.66%. When the deposition voltage is 90 V, the surface roughness of the deposited layer in the non-magnetic field state is the maximum, which is 29.0 μm, and the surface roughness of the deposited layer in the magnetic field’s frequency of 10 Hz is the minimum, which is 22.4 μm, which is relatively reduced by 22.52%.

The higher the magnetic field frequency, the lower the surface roughness of the deposited layer. This is because the higher the magnetic field’s frequency, the faster the control rate of the charged particles in the discharge channel per unit time, and the spatter generated by the impact on the substrate surface has an inhibiting effect, so that the irregular range of the spatter will not expand due to the increase in the deposition voltage, and the impact on subsequent deposition discharge is reduced. In addition, due to high temperatures, high pressure and other reasons, the electrode and the base material melt during the deposition process. At this time, the external magnetic field regulation will generate Lorentz force in the molten electrode and the base material, and generate magnetic force regulation under the action of Lorentz force to accelerate the mixing of metal materials in the molten pool and make it rotate around the center of the molten pool. The defects such as material protrusion caused by rapid solidification of molten droplet of electrode material are effectively reduced.

5.3. Analysis of Cross-Section Micromorphology Results

Figure 13 shows the cross-sectional topography of the deposited layer under the different magnetic field’s frequencies when the deposition voltage is 30 V, 60 V and 90 V. It can be seen that under the different deposition voltages, with an increase in the magnetic field’s frequency, the cross-section defects gradually decrease, and the morphology effect becomes better.

When the magnetic field’s frequency is constant, the overall effect of the cross-sectional topography of the deposited layer under the corresponding deposition voltage is obviously improved with an increase in the magnetic field’s frequency. When the magnetic field’s frequency is 10 Hz, the cross-section topography of the deposited layer has the best effect without the obvious defects. However, the effect of cross-sectional morphology of the deposited layer prepared at the 0 Hz magnetic field’s frequency is not greatly improved. The static magnetic field of the 0 Hz magnetic field’s frequency only plays a downward gravitational effect. After the deposition voltage increases, the irregular range of its sputtering expands, and the static magnetic field cannot effectively control it. However, the appearance of the section of the deposited layer prepared at the magnetic field’s frequency of 3 Hz, 6 Hz and 10 Hz has significantly improved, and the magnetic field’s frequency has increased, which not only controls the irregular range of the electrode material splashing and makes the deposition and discharge more stable, but also promotes the full fusion between the droplets, so that the air and heat can be released timely and effectively. The cracks, holes and other defects existing in the section of the deposited layer are effectively reduced, and the combination of the deposited layer and the matrix is more tight and reliable.

5.4. Analysis of Deposited Layer Thickness Results

As can be seen from Figure 14, when the magnetic field’s intensity is constant, the thickness of the deposited layer prepared under the different magnetic field’s frequencies gradually increases with the increase in the deposition voltage. When the deposition voltage is 30 V, the thickness of the deposited layer at the 10 Hz magnetic field’s frequency is maximum, which is 32.064 μm, and the thickness of the deposited layer at the non-magnetic field’s frequency is minimum, which is 30.274 μm, with a relative increase of 5.91%. When the deposition voltage is 60 V, the thickness of the deposited layer under the magnetic field’s frequency of 10 Hz is maximum, which is 48.995 μm, and the thickness of the deposited layer under the magnetic field’s frequency is minimum, which is 42.671 μm, with a relative increase of 13.39%. When the deposition voltage is 90 V, the thickness of the deposited layer under the magnetic field’s frequency of 10 Hz is maximum, which is 54.396 μm, and the thickness of the deposited layer under the magnetic field’s frequency is minimum, which is 48.932 μm, with a relative increase of 11.17%. It can be seen that the higher the magnetic field’s frequency, the greater the thickness value.

Among them, at the magnetic field’s frequency of 0 Hz and 3 Hz, the thickness of the deposited layer changes very similarly with the deposition voltage. This is because the deposition voltage increases, the energy of interelectrode discharge increases, and the centrifugal force generated by the rotation of the spindle electrode increases the droplet throwing output of the electrode material, and even part of it is thrown away from the substrate surface. At this time, the magnetic field’s frequency is too low, which weakens the droplet inhibition effect on the electrode material thrown away from the surface of the substrate. Hence, the thickness of the deposited layer exhibits no substantial variation under the two magnetic field’s frequencies. Subsequently, the thickness of the deposited layer changes in direct proportion to the magnetic field’s frequency. This is because with an increase in the magnetic field’s frequency, the mass of the molten droplet of the electrode material thrown off the substrate surface caused by deposition, discharge, centrifugal force and other factors decreases, so that it is thrown into the substrate surface as much as possible. Therefore, the thickness of the deposited layer increases with an increase in the magnetic field’s frequency.

It is further concluded that when the magnetic field’s intensity is constant, the frequency of charged particles in the discharge channel per unit time increases with an increase in the magnetic field’s frequency, which effectively reduces the mass of the molten electrode material thrown off the substrate surface under the action of gravity, electric field force, centrifugal force, etc., reduces the electrode material waste, improves the material utilization rate, and further increases the thickness of the sediment layer.

5.5. Analysis of Deposited Layer Microhardness Results

Figure 15 shows the variation in the hardness of the deposited layer with the magnetic field’s frequency under the different deposition voltages. It can be seen from the figure that when the deposition voltage and the magnetic field’s intensity are constant, the hardness of the prepared sediment layer gradually increases with an increase in the magnetic field’s frequency. When the deposition voltage is 30 V, the sediment layer hardness at the 10 Hz magnetic field’s frequency is the highest, which is 172.41HV0.2, while the sediment layer hardness at the non-magnetic field’s frequency is the lowest, which is 166.22HV0.2, with a relative increase of 3.72%. When the deposition voltage is 60 V, the deposition layer hardness at the 10 Hz magnetic field’s frequency is the highest, which is 197.39HV0.2, while the deposition layer hardness at the non-magnetic field’s frequency is the lowest, which is 191.47HV0.2, with a relative increase of 3.1%. When the deposition voltage is 90 V, the deposition layer hardness at the 10 Hz magnetic field’s frequency is the highest, which is 213.60HV0.2, while the deposition layer hardness at the non-magnetic field’s frequency is the lowest, which is 202.82HV0.2, with a relative increase of 5.32%.

This is because the magnetic field’s frequency is increased, which attracts the molten droplet of the electrode material, and effectively inhibits the sputtering effect of the material and reduces the irregular range of the material thrown out. In addition, the magnetic control rate of the molten electrode and the substrate material becomes faster, and the number of controls increases in the same time, which promotes the rotational motion rate of the molten material with the magnetic field center as the axis, effectively accelerates the fluidity of the molten material, and improves the quality of the electrode molten material transfer and the fusion using heterogeneous materials (for the electrode and matrix). The combination of the deposited layer and the matrix is more reliable, and the hardness of the deposited coating is increased.

6. Conclusions

A self-developed MFESD device was used to carry out the ESD experiments under the conditions of the deposition voltages of 30 V, 60 V and 90 V, and the effects of the magnetic field’s intensity and frequency on the surface morphology, surface roughness, cross-section morphology, thickness and hardness of the deposited layer were discussed. The test results show the following:

• The study demonstrates that increasing the magnetic field’s intensity and frequency significantly enhances the quality of electro-spark deposition layers by reducing defects and improving material bonding, the surface roughness of the deposited layer decreases with an increase in the magnetic field’s intensity, its optimal value can be reduced by 44.3%, and the minimum can reach 9.3 μm.
• With an increase in the magnetic field’s intensity and frequency, the inhibition effect on the electrode material sputtering generated in the deposition process is enhanced, and the magnetic control effect on the molten pool is improved, the metallurgical bonding effect is better, and the cross-section effect of the deposited layer is improved.
• The thickness of the deposited layer increases with an increase in the magnetic field’s intensity and frequency, and the maximum increase is 13.39%; its maximum thickness can reach 54.396 μm. The microhardness of the deposited layer increases with an increase in the magnetic field’s intensity and frequency, and its maximum increase is 7.64%, and its hardness up to 213.60 HV_0.2.

At the same time, by comparing the deposited layers prepared without the magnetic field, it is proved that the addition of an appropriate magnetic field control can effectively improve the quality of the deposited layer, the surface topography and cross-section topography of the deposited layer, reduce the surface roughness, and increase the thickness and microhardness of the deposited layer. The use of the magnetic field to control ESD to obtain more efficient deposition coatings effectively solves the key problems of low ESD efficiency and poor reliability, breaks through the bottleneck limiting the application and development of ESD repair technology, and has important significance in the surface remanufacturing and industrial application of key components of high-end equipment.