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Article

Research on the Ultrasonic Electro-Spark Deposition Process and the Properties of the Deposition Layer

1
Chinese Academy of Agricultural Mechanization Sciences Group Co., Ltd., Beijing 100083, China
2
State Key Laboratory of Agricultural Equipment Technology, Beijing 100083, China
3
Shijiazhuang Zhongxing Machinery Manufacturing Co., Ltd., Shijiazhuang 051530, China
4
Yongkang Xingyuan New Materials Technology Co., Ltd., Yongkang 321300, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(9), 1038; https://doi.org/10.3390/coatings15091038
Submission received: 1 August 2025 / Revised: 20 August 2025 / Accepted: 21 August 2025 / Published: 4 September 2025
(This article belongs to the Section Surface Characterization, Deposition and Modification)

Abstract

The continuous discharge voltage waveform and phenomena between the electrode and substrate were explored in this paper to study the ultrasonic electro-spark deposition process. Additionally, the impact of ultrasonics on the ultrasonic electro-spark deposition process and the properties of the deposition layer were examined. The results show that the charge–discharge frequency of the ultrasonic electro-spark deposition process was commensurate with the discharge frequency of the ultrasonic electro-spark deposition power source, and the voltage waveform was stable. When ultrasonics is introduced, the molten droplet spray trajectory is efficiently guided, resulting in the spark spray trajectory displaying notable directional concentration characteristics. During a single charging and discharging phase, the electrode and substrate made roughly 15 mechanical contacts, 1 of which was discharging, and the remaining 14 were mechanically contacted reinforcement. The surface of the ultrasonic electro-spark deposition layer exhibited a sputtering morphology with no surface cracks. Phase structures such as Co3W3C, Fe3W3C, Fe6W6C, WC, and W2C constituted the majority of the ultrasonic electro-spark deposition layer’s microstructure and showed strong metallurgical bonds with the substrate. The ultrasonic electro-spark deposition layer has a surface roughness of 2.554 μm, a cross-section porosity of 1.3%, and a maximum microhardness of 1038.8 HV0.025. Comparative analysis demonstrates that the addition of ultrasonics can significantly enhance the deposition layer’s quality and performance. When compared to the electro-spark deposition layer, the surface roughness of the ultrasonic electro-spark deposition layer decreases by roughly 61.4%, the cross-sectional porosity decreases by around 57.5%, and the maximum microhardness increases by about 15.5%. Many cracks and much high surface roughness in the conventional electro-spark deposition layer are resolved by the ultrasonic electro-spark deposition technique, which is crucial for cold drawing mold surface strengthening.

1. Introduction

High hardness and strength, adequate toughness, high heat and wear resistance, and affordability are the benefits of GCr15. In the industrial sector, it is among the most frequently used cold drawing and drawing die steels [1,2]. Due to extended exposure to large loads and other challenging operating circumstances, cold drawing dies are susceptible to localized wear [3,4]. Using cutting-edge surface technology to fix these damaged surfaces instead of replacing dies not only saves a lot of money but also produces better outcomes. Consequently, there is a growing interest in this surface-strengthening remanufacturing technique [5,6]. WC-based metal–ceramic compounds are frequently used for surface strengthening of components like molds and mining gear because of their high hardness and strong wear resistance. The most often used of these are metal–ceramic coatings based on WC-Co [7,8,9]. Die surface strengthening is a common application of physical vapor deposition (PVD) technology abroad; nonetheless, the procedure itself necessitates a high-vacuum environment and precise coating equipment. Energy expenses and the necessary equipment expenditure rise sharply with increasing die sizes. Furthermore, the industrial use of this technology on large dies is restricted due to the requirement for significant tooling adaptation, which raises production costs even further [10,11]. Contrarily, electro-spark deposition technology equipment is straightforward, easy to use, does not require any tooling and can be modified to meet the needs of various die specifications. This allows the performance of the deposition layer to be guaranteed while also drastically lowering equipment investment and unit processing costs, particularly in the field of agricultural wheel rim dies and other large-size part manufacturing, which has demonstrated significant economic advantages [12,13].
According to the discharge processing principle, electro-spark deposition technology is a surface engineering technique that uses the form of pulse discharge and electrode material deposition on the workpiece surface. This means that after the power supply capacitor is fully charged, an instantaneous high-frequency ultrashort pulse width discharge is produced between the electrode and the substrate, causing the electrode and workpiece contact site temperature to reach 8000–25,000 °C. The electrode fusion seepage to the workpiece surface in the plasma state forms the alloying deposition layer [14,15,16]. In addition to creating deposition layers on workpiece surfaces that are resistant to wear, corrosion, and high temperatures, electro-spark deposition technology is an effective remanufacturing technique that can fix local wear, scratches, sand holes, and other flaws [17,18,19]. Luan et al. [20] utilized electro-spark deposition to fabricate a Nb coating on H13 steel, exhibiting a continuous cross-sectional structure. The coating exhibited a microhardness of 642 HV (3.2 times that of the substrate), one-third wear loss of the substrate, and enhanced corrosion resistance in a 3.5% NaCl solution, with a 13 mV increase in self-corrosion potential and reduced corrosion current density. Geng et al. [21] used electro-spark deposition to prepare Ni/Ti (C, N) composite coatings on H13 steel; the coating formed a good metallurgical bond with uniform cross-section. The highest microhardness value of 1420 HV (5.4 times that of the substrate) and the amount of wear of the substrate for the 1/2 can all contribute to extending the die life.
Even though electro-spark deposition technology can successfully increase workpieces’ resistance to wear, issues including excessive surface roughness, limited improvement of hardness qualities, and poor uniformity of the deposition layer still exist [22,23]. Because of this, researchers both domestically and internationally have conducted pertinent studies on ultrasound-assisted machining. Liu et al. [24,25] developed a combined ultrasonic impact and electro-spark deposition technique for Ti-6Al-4V coatings. The method introduced strengthening elements and generated surface compressive stresses, significantly improving corrosion and wear resistance. Shang et al. [26] applied a combined ultrasonic impact and electro-spark treatment to TC4 titanium alloy, producing a dense layer of Ti2O, Al2O3, and Al3Ti compounds. This modification significantly improved corrosion resistance by reducing the corrosion current density and increasing the charge transfer resistance compared to the substrate. Wang et al. [27] developed an ultrasonic-assisted electro-spark deposition technique that achieved metallurgical bonding via Cr3C2 formation, creating net-shape composite layers on TiC particles. Utilizing simultaneous dual-interface discharge, the method produced 400 μm protrusions with cellular crystals and exhibited superior particle retention, demonstrating strong potential for abrasive coatings in aeroengines. Zhao et al. [28] fabricated an in situ TiC-reinforced Ni-based coating on H13 steel via ultrasonic-electro-spark deposition. The metallurgically bonded coating with in situ TiC had a hardness of 1400.5 HV0.05 (2.5 times that of the substrate), reducing the wear rate by two orders of magnitude and the friction coefficient by 46.2%.
Furthermore, a variety of uses for ultrasonic technology have been demonstrated in various coating preparation fields. To create diamond/AlSi coatings on Ti–6Al–4V substrates, Zhang et al. [29] used ultrasonic brazing. This improved wear resistance without causing diamond thermal deterioration by refining the AlSi matrix and encouraging TiC production at diamond interfaces. Chen et al. [30] applied ultrasonic-assisted electrodeposition to achieve dispersive incorporation of TiO2 nanoparticles into Cu coatings, resulting in a dense microstructure with long-term antibacterial efficacy (96.8%) through the continuous release of Cu ions and reactive oxygen species. Using ultrasonic-assisted jet electrodeposition, Cao et al. [31] developed Ni-W-AlO3 nanocomposites, revealing important parameter interactions to achieve 724.9 HV microhardness and outstanding wear resistance.
In conclusion, there has been some advancement in the fields of electro-spark deposition and ultrasonic-assisted coating preparation techniques by both domestic and foreign researchers. Nevertheless, little is understood about how the electro-spark deposition process and the properties of the deposition layer are affected when ultrasonic acts directly on the electrode. In this study, the voltage waveforms between the electrode and the substrate during the ultrasonic electro-spark deposition process were combined with the inter-electrode phenomena recorded by a high-speed camera to analyze the deposition process. The study investigates the effect of ultrasonic on the deposition layer’s properties by describing its surface morphology, surface roughness, cross-sectional morphology, phase composition, and microhardness. This gives researchers information and support for additional research into ultrasonic electro-spark deposition.

2. Materials and Methods

2.1. Materials

With 90%WC and 10%Co as its primary constituents, the electrode material was chosen as WTC-90. The electrode size was 0.9 × 3 × 50 mm for the ultrasonic electro-spark deposition test and Φ 3 × 50 mm for the electro-spark deposition test. Table 1 displays the chemical element compositions of the substrate material, which was chosen as GCr15 and had dimensions of 30 × 20 × 5 mm. It was heat-treated to a hardness of 59 HRC. With a mass percentage of 99.99%, argon was employed as a protective gas.

2.2. Sample Treatment

To obtain a smooth surface, the sample’s surface was first crudely sanded using 180#, 400#, and 600# sandpaper. Next, it was delicately sanded using 1000#, 1500#, and 1800# sandpaper. Finally, it was mechanically polished to create a mirror-like effect. To get rid of any remaining impurities, the surface was lastly ultrasonically cleaned for five minutes using anhydrous ethanol.
During the post-processing stage of the sample, the deposition layer sample was wire-cut to create a cross-sectional sample. The cut surface was then gradually sanded using 400#, 600#, 800#, 1000#, 1500#, and 2000# sandpaper, and the surface smoothness was enhanced by a series of rough and fine grinding steps. This allowed for the observation of the deposition layer’s cross-sectional morphology. In order to meet the quality requirements of the subsequent cross-sectional morphology observation, samples were then ultrasonically cleaned using organic solvents after undergoing a process of rough polishing to fine polishing until the surface displayed a mirror state without visible scratches.

2.3. Preparation Process and Parameters

The power supply used UD100-type ultrasonic electro-spark deposition equipment, and it had an integrated ultrasonic transducer. The ultrasonic transducer was connected directly to the working electrode via a stiff mechanical linkage, which guaranteed that it worked during the entire ultrasonic electro-spark deposition process and that the ultrasonic action time was precisely the same as the ultrasonic electro-spark deposition test. Both the ultrasonic electro-spark deposition test and the conventional electro-spark deposition test can be performed with UD100-type ultrasonic electro-spark deposition equipment (TechnoCoat, Fujieda, Japan). The experimental device is depicted in Figure 1. The deposition gun was operated by a robot to conduct continuous discharge tests on the substrate specimen’s surface. Ultrasonic frequency 20 kHz, ultrasonic amplitude 40 μm, voltage 50 V, capacitance 30 μF and frequency 1360 Hz were the parameters used in the ultrasonic electro-spark deposition test. A TBS2104B digital oscilloscope(Tektronix, Portland, OR, USA) recorded the voltage signal between the electrode and the substrate, and a NEO25M/C high-speed camera (Zhongke Vision, Hefei, China) recorded the inter-pole phenomenon during deposition. The camera took high-speed pictures with its lens axis perpendicular to the specimen’s cross-section and focused to make sure the ultrasonic electro-spark deposition region was visible in the lens. A resolution of 1280 × 320 pixels and a frame rate of 100,000 fps were the settings for the high-speed camera. The electro-spark deposition layer was created in this experiment using the same process settings as previously mentioned, namely voltage 50 V, capacitance 30 µF, and frequency 1360 Hz, in order to examine the impact of ultrasonics on the deposition layer’s properties.

2.4. Characteristic Analysis

The Model Mira3 scanning electron microscope (TESCAN, Brno, Czech Republic) was employed to view the deposition layer’s surface and cross-sectional morphology, and an accompanying energy spectrometer was employed to carry out elemental studies of particular deposition layer regions. The image analysis program Image J (version 1.8.0) was used to process the cross-sectional SEM pictures to determine the porosity. The Zygo NewView 3D white light interferometer (ZYGO, Middlefield, CT, USA) was used to choose any region of the deposition layer’s surface to obtain the area’s arithmetic mean roughness Sa and comprehensive 3D topographic data. Meanwhile, the HUD280+ surface (Shidaihuade, Beijing, China) roughness meter was employed to measure five randomly chosen spots on the surface of the deposition layer to determine the arithmetic mean Ra and assess the general distribution of surface roughness.
Moreover, the Smartlab SE X-ray diffractometer (Rigaku, Tokyo, Japan) was used to investigate the deposition layer’s physical phase composition. The cross-sectional microhardness distribution in the longitudinal region of the deposition layer was measured using the MH-500D Vickers microhardness tester (HENGYI, Shanghai, China). The measuring load was 0.025 kg, and the holding period was 10 s. The isopleth measurement axis was established parallel to the deposition layer surface on the deposition layer cross-section to precisely describe the longitudinal distribution of microhardness in deposition layers. To obtain the microhardness value at a given depth, choose three random locations on the axis for the measurement and take the arithmetic mean of the three measurement values.

3. Analysis of the Ultrasonic Electro-Spark Deposition Process

3.1. Deposition Process

Figure 2 shows the voltage waveform of the ultrasonic electro-spark deposition process. It was evident that the pulse discharge time intervals are essentially constant, and the voltage variations are rather stable. The ultrasonic electro-spark deposition power supply had a frequency of 1360 Hz and a period of 736 μs, which corresponded to the length of the time intervals t0 through t3 (see Figure 2). As seen in the expanded view in Figure 2, a certain voltage waveform from many pulse discharges was magnified and examined on the time axis to better understand the deposition process. The voltage between the electrode and the substrate gradually increased to 50 V before time t1, suggesting that the dielectric in the inter-electrode gap had not yet disintegrated. The voltage dropped sharply when the t1 moment was reached, indicating that a discharge channel had formed between the electrodes and that the argon medium in the inter-electrode gap had broken down at the t1 moment. At this moment, the channel’s high-temperature, high-pressure ionized gas expanded instantly, creating an effect known as an explosive shock wave. The discharge stage was the name given to this phase. The discharge process comes to a close at time t2 when all of the electrical energy that has been stored in the power source has been released. An open-circuit condition is restored in the inter-electrode circuit. The de-ionization step was finished when the residual voltage between the poles progressively drops to zero throughout the t2 to t3 time interval, restoring the ionized gas to neutral. A new discharge channel develops between the two electrodes as the voltage progressively increases once again to 50 V. A complete charge–discharge sawtooth waveform was created with another voltage drop. The discharge pathway’s “generation–expansion–extinction” phase is represented by each “tooth”.
Figure 3 shows that the deposition process between the electrode and the substrate pole at a certain point in the ultrasonic electro-spark deposition process was captured by a high-speed camera image. As can be observed, the time interval between Figure 3a,h was 739 μs, which was in line with the ultrasonic electro-spark deposition power supply’s frequency. The time interval from t1 to t3 in Figure 2 was consistent with the period from Figure 3a–g, which was 209 μs. The dielectric between the electrodes broke down and formed a discharge channel when the distance between the electrode and the substrate was close enough, as indicated by the bright white light in Figure 3a. It was evident from Figure 3b,c that the inter-pole white light expanded quickly and increased in volume. This is because ionization-generated electrons constantly bombard the anode electrode’s surface in the discharge channel, creating high temperatures that quickly melt and evaporate the electrode surface material. As a result, the discharge channel’s volume increases significantly and experiences high-temperature expansion. The inter-electrode white light gradually collapsed with noticeable sparking phenomena, as shown in Figure 3d–f. The sparking trajectories also showed notable directional concentration characteristics. Figure 3g shows how the discharge channel suddenly disappeared, the residual ionization progressively diminished, and the dazzling white light was barely perceptible between the poles. A new discharge channel was created when the interpolar medium was punctured once more, as seen in Figure 3h.

3.2. Effect of Ultrasonics on Deposition Process

Figure 4 shows a schematic diagram of the ultrasonic electro-spark deposition process and electrode amplitudes. In contrast to traditional electro-spark deposition technology, the electrode of the ultrasonic electro-spark deposition technology vibrated up and down with regard to the substrate, as seen in Figure 4a. The ultrasonic amplitude was 40 μm, as seen in Figure 4b. The high-frequency mechanical vibration of the electrode caused by ultrasonics causes the gap between the electrode and the substrate to change slightly on a regular basis, resulting in a more uniform discharge channel distribution. This effectively prevents the short-circuit phenomenon caused by the accumulation of corrosion products and lowers the local energy concentration or the phenomenon of stagnation of the discharge. Simultaneously, the periodic variation in the inter-pole gap can shorten the time of de-ionization, speed up the dissipation and reconstruction of the discharge channel, and destroy the stable electric field and thermal balance conditions necessary for the arc’s maintenance. This lowers the risk of anomalous arc discharge and improves the discharge process’s stability. As shown in Figure 4a, the electrode vibrated while exerting a vertical downward force F on the deposition layer. The electrode tip plane then achieved face contact with the substrate plane as a result of the action of F. There may be multiple gap changes between the electrode and the substrate because the ultrasonic frequency was 20 kHz. In other words, during a single charging and discharging phase, the electrode and substrate made roughly 15 mechanical contacts, 1 of which was discharging, and the remaining 14 were mechanically contacted reinforcement.
The localized area between the poles is instantly heated above the melting and boiling points during the electro-spark deposition process, which causes the molten metal to be explosively ejected along with the vaporized metal under the influence of electromagnetic force, electrostatic force, shock wave, gravity, etc. This is the reason for the common spark-splash phenomenon of the electro-spark deposition process. However, when ultrasonics is introduced, mechanical energy is transferred via the electrode, causing the molten metal droplets to preferentially splash in the direction of vibration. The spark spattering trajectory exhibits a notable directional concentration feature, as illustrated in Figure 3e, because the acoustic flow effect of the ultrasound simultaneously efficiently directs and limits the spray trajectory of molten droplets, preventing the random splashing of molten droplets.

4. Analysis of Deposition Layer Properties

4.1. Surface Morphologies of Deposition Layer

Figure 5 shows the surface morphology and 3D morphology of the ultrasonic electro-spark deposition layer and the electro-spark deposition layer. The ultrasonic electro-spark deposition layer had a sputtering morphology characteristic of electro-spark deposition, as illustrated in Figure 5a. The sputtering morphology was flat, and the deposition layer’s surface was free of cracks but had a few micropores. The ultrasonic electro-spark deposition layer’s surface had a high and low undulating morphology, according to the 3D morphology map. Figure 6 displays the surface roughness. A handheld roughness meter indicated that the average surface roughness of the deposition layer was roughly 2.554 μm, which was consistent with the Sa determined by the 3D white-light interferometer in the chosen region. Figure 5b shows the SEM images and 3D morphology of the electro-spark deposition layer surface. It was evident that the surface was considerably rougher, exhibiting a discontinuous bulk material stacking pattern, and had numerous flaws, including microcracks and porosity. The surface morphology of the electro-spark deposition layer exhibited more pronounced high and low undulations, according to the 3D morphology map. In comparison to the ultrasonic electro-spark deposition layer, the electro-spark deposition layer’s surface roughness increased by around 2.42 times to 6.181 μm, as shown in Figure 6.
The ultrasonic electro-spark deposition and electro-spark deposition layers had a characteristic pore-filled “orange peel” sputtering pattern. This is because, during the deposition process, droplets of molten liquid are moved and sputtered onto the workpiece’s surface, where they fuse with the molten material on the substrate to form a deposition point. Innumerable deposition points then fuse with one another, are superimposed, and are connected to form a deposition layer that resembles orange peel. It is simple to wrap and dissolve the gas molecules in the environment when transferring the molten droplets, and some gases are also produced by the metallurgical reaction that occurs when the molten metal solidifies [32]. When the molten droplets cool quickly, these gases are not appropriately released in time, which causes surface pores to form. In comparison to the electro-spark deposition layer, the ultrasonic electro-spark deposition layer had fewer flaws, less surface roughness, virtually no uneven bulk material stacking pattern, and no cracks formed on its surface. This is because the addition of ultrasonics causes the molten droplets on the electrode to accelerate in the direction of the substrate surface. This enhances the droplets’ ability to spread out on the substrate surface and flattens the sputtering morphology, which lowers the deposition layer’s surface roughness. The trajectory of the molten droplets exhibits a directional concentration of spray due to the acoustic flow effect of ultrasonics. This effectively suppresses the random splashing of droplets, allowing the droplets to strike the substrate with sufficient energy to become fully deformed and spread out, and effectively fuses the substrate to reduce the non-uniform stacking of large pieces of material morphology. Because of the cavitation effect of ultrasonics, the molten droplets are smaller, which lowers the likelihood of air trapping during deposition and lessens the formation of pores [33,34]. During the solidification process of the molten metal, the electrode can exert continuous mechanical contact reinforcement thanks to the high-frequency vibration energy introduced by ultrasonics. This allows the electrode to compact the semi-solid metal in real time and contributes to the closure of the pores, which increases the density of the deposited layer. The inhomogeneous shrinkage stress brought on by the molten metal’s rapid cooling and contraction is also lessened by the compressive stress applied by ultrasonics through electrode vibration. This lowers the residual stress following the final cooling of the deposition layer and prevents cracks from forming and spreading [35]. The addition of ultrasonics can significantly reduce surface roughness, suppress the formation of cracks and holes, and enhance the forming quality of the deposited layer.

4.2. Cross-Sectional Morphologys of Deposition Layer

Figure 7 shows the cross-sectional morphology SEM images, and energy spectrum analysis results of the ultrasonic electro-spark deposition layer and electro-spark deposition layer. The interface between the ultrasonic electro-spark deposition layer and the substrate was easily apparent, and it was evident that the layer had a few pores and fissures. In contrast to the ultrasonic electro-spark deposition layer, the electro-spark deposition layer had numerous pores, pits, and visible fissures. Additionally, the interface between the deposition layer and the substrate was less apparent. In comparison to the electro-spark deposition layer, the porosity of the ultrasonic electro-spark deposition layer was calculated to be 1.3%, which was a decrease of roughly 57.5%. Both the ultrasonic electro-spark deposition and electro-spark deposition layers included some white granular elements, as shown in the SEM image. From the EDS spectral analysis results of the white particles in the two deposition layers, it can be seen that the white particles distributed within the two deposition layers contain a large amount of the elements W and C. Of them, the white particles in the ultrasonic electro-spark deposition layer contained 86.53 wt.% element W and 9.20 wt.% element C, and the white particles in the electro-spark deposition layer contained 89.02 wt.% element W and 6.67 wt.% element C. Therefore, it can be deduced that the particles were WC or W-rich carbides.
There were fissures where the ultrasonic electro-spark deposition and electro-spark deposition layers met the substrate. This is because the substrate and the deposition layer material have different coefficients of thermal expansion, which tends to create thermal strains inside the deposition layer and cause microcracks in some places [36]. Compared to the electro-spark deposition layer, the ultrasonic electro-spark deposition layer was generally denser, had fewer cracks, and a lower porosity. This is because the ultrasonic acoustic flow effect increases the molten metal’s mobility and decreases the microporous defects brought on by partial local melting. It also improves heat distribution uniformity and effectively reduces the concentration of thermal stress brought on by abrupt temperature changes, which lowers the porosity and cracks in the deposition layer [37]. Additionally, the ultrasonic electro-spark deposition layer’s white particles are smaller than those of the electro-spark deposition layer. This is because, during electro-spark deposition, the deposited layer near the substrate receives a larger energy input during continuous discharge deposition, which causes the WC particles to develop abnormally [38]. But the acoustic flow effect of ultrasonics increases the convection of molten metal, which improves the uniformity of heat distribution and prevents WC particles from clumping together, hence lowering the aberrant growth of WC particles brought on by localized overheating [39].

4.3. Composition Analysis of Deposition Layer

Figure 8 shows the EDS line scan results of the cross-section of the ultrasonic electro-spark deposition layer. It can be seen that elements Fe, W, Co and C were primarily present in the deposition layer, and there was mutual diffusion of the elements between the deposition layer and the substrate. These changes in elemental content were evident along the deposition layer toward the substrate.
The elemental content of Fe contained in the substrate exhibits a fluctuating increase from the deposition layer to the substrate direction, while the elemental content of W and Co contained in the electrode material also exhibits obvious diffusion phenomena toward the substrate. In contrast, the elemental content of C contained in the substrate and electrode exhibits a fluctuating decrease from the deposition layer to the substrate direction. Mutual migration between the atoms of the electrode material and the substrate was demonstrated by the clear elemental diffusion phenomena in the interface region, which were discovered by examining the microstructure and composition of the interface region between the electrode material and the substrate. This kind of interfacial bonding guarantees that the deposition layer has exceptional bond strength, and the elemental interdiffusion behavior verifies that a metallurgical link, not just a mechanical one, has formed between the deposition layer and the substrate.
Figure 9 shows the XRD diffraction pattern of the ultrasonic electro-spark deposition layer. As can be observed, phase structures, such as Co3W3C, Fe3W3C, Fe6W6C, WC, W2C, and so on, constitute the majority of the deposition layer. The results of the earlier microstructure investigation were supported by the XRD data.
The hard phases Co3W3C, Fe3W3C, Fe6W6C, and others in the deposition layer were produced by the high-temperature chemical reaction between the WTC-90 electrode and the chemical elements in the substrate; WC was derived from the material’s initial additive composition in the deposition layer; and W2C was primarily formed by the breakdown of WC under the influence of the high temperature produced during the deposition process. These phases were dispersed throughout the deposition layer. It is obvious that the deposition layer is more than just a simple buildup of electrode and substrate material; rather, it uses spark discharge energy to create a new compound product through a chemical reaction that destroys the initial chemical bond and metallurgical bonding.

4.4. Microhardness of Deposition Layer

Figure 10 shows the cross-sectional microhardness distribution curves of the ultrasonic electro-spark deposition and electro-spark deposition layers, and it can be seen that the two deposition layers’ microhardness had a gradient along the direction of thickness. The microhardness changed abruptly when the deposition layer and substrate were combined, and it was highest in the deposition layer region. The ultrasonic electro-spark deposition layer’s microhardness varied smoothly and was generally higher than the electro-spark deposition layer. The maximum microhardness of the ultrasonic electro-spark deposition layer can reach 1038.8 HV0.025, which is about 15.5% higher than that of the electro-spark deposition layer.
The two deposition layers’ microhardness was noticeably greater than the substrate’s. This is because both microstructural and XRD investigations showed that the deposition layer contained a considerable number of W-rich carbides. These carbide particles can significantly increase the hardness of the deposition layer [40]. The electro-spark deposition layer’s microhardness exhibited notable oscillations. When the SEM results were analyzed, it became evident that this is because of the unequal distribution of WC particles in the electro-spark deposition layer. Additionally, the deposition layer near the substrate receives a higher energy input during the deposition process, which causes the WC particles to coarsen and results in a greater variation in microhardness. The ultrasonic electro-spark deposition layer had a higher microhardness than the electro-spark deposition layer because ultrasonics effectively decreases the deposition layer’s porosity and increases its densification, which can disperse the external load and lessen stress concentration, increasing the layer’s microhardness. During the solidification of the molten metal, ultrasonics allows the electrode to continuously reinforce mechanical contact, which causes plastic deformation of the semi-solid deposition layer’s surface, and the development of a work-hardening layer, all of which raise the deposition layer’s microhardness.

5. Conclusions

The process of ultrasonic electro-spark deposition and the properties of the ultrasonic electro-spark deposition layer were studied and conclusions are summarized as following:
(1)
The results show that the charge–discharge frequency of the ultrasonic electro-spark deposition process was commensurate with the discharge frequency of the ultrasonic electro-spark deposition power source, and the voltage waveform was stable. During a single charging and discharging phase, the electrode and substrate made roughly 15 mechanical contacts, 1 of which was discharging, and the remaining 14 were mechanically contacted reinforcement. The addition of ultrasonics successfully directs the molten droplet spray trajectory, preventing haphazard splashing and causing the spark spray trajectory to exhibit notable directional concentration features.
(2)
The addition of ultrasonics improves the spread ability of the molten droplets on the substrate surface by decreasing their size and quickening their motion toward the surface. Real-time compression of semi-solid metal by the mechanical contact strengthening produced by ultrasonics helps to lessen the shrinkage stress brought on by fast cooling, which prevents cracks from starting and spreading in the deposition layer. The ultrasonic electro-spark deposition layer’s surface exhibited sputtering morphology, with no surface cracks and a surface roughness of 2.554 μm, which was around 61.4% less than that of the electro-spark deposition layer.
(3)
By increasing the fluidity of the molten metal, the introduction of ultrasonics reduces microporosity defects brought on by incomplete local melting, relieves the concentration of thermal stress, lessens the occurrence of cracks in the deposited layer, lowers its porosity, and prevents the aberrant growth of WC particles. The cross-section porosity of the ultrasonic electro-spark deposition layer was 1.3%, or 57.5% less than that of the electro-spark deposition layer. Phase structures such as Co3W3C, Fe3W3C, Fe6W6C, WC, and W2C constituted the majority of the ultrasonic electro-spark deposition layer’s microstructure and showed strong metallurgical bonds with the substrate.
(4)
Simultaneously, the semi-solid deposition layer’s surface experiences plastic deformation due to the mechanical contact strengthening produced by ultrasonics, creating a work-hardened layer that raises the deposition layer’s microhardness. At a maximum of 1038.8 HV0.025, the microhardness of the ultrasonic electro-spark deposition layer was roughly 15.5% more than that of the electro-spark deposition layer.
In conclusion, the ultrasonic electro-spark deposition technology effectively resolves the long-standing problems of high roughness and cracking connected to traditional electro-spark deposition. The surface strengthening of crucial parts, such as cold drawing dies, extrusion tools, and other wear-prone parts, where dependability and service life are essential, would benefit greatly from this study. Furthermore, this technology offers a realistic route for the high-quality and efficient repair and additive fabrication of metal components. Optimizing the ultrasonic parameters, assessing durability in realistic wear and corrosion scenarios, and performing comparison analyses with alternative coating methods will be the main goals of future research.

Author Contributions

Methodology, B.L. and X.M.; validation, B.L.; investigation, B.L., X.M. and Y.L.; data curation, B.L. and H.W.; supervision, R.W., Y.L. and H.W.; writing—original draft preparation, B.L.; writing—review and editing, X.M., Y.L. and R.W.; project administration, M.B., validation, Y.L. and H.W.; visualization, B.L. and M.B.; resources, R.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key R&D Program of China (2024YFB3714100).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Authors Bihan Li, Xiaobin Ma, Manyu Bao and Ruijun Wang were employed by the company Chinese Academy of Agricultural Mechanization Sciences Group Co., Ltd. Author Yongwei Liu was employed by the company Shijiazhuang Zhongxing Machinery Manufacturing Co., Ltd. Author Hanqi Wang was employed by the company Yongkang Xingyuan New Materials Technology Co., Ltd.

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Figure 1. Schematic diagram of the overall structure of experimental device.
Figure 1. Schematic diagram of the overall structure of experimental device.
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Figure 2. Waveform of continuous deposition voltage.
Figure 2. Waveform of continuous deposition voltage.
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Figure 3. Photographs of the ultrasonic electro-spark deposition process taken at high speed. (a) 1 μs; (b) 13 μs; (c) 25 μs; (d) 38 μs; (e) 50 μs; (f) 87 μs; (g) 209 μs; (h) 740 μs.
Figure 3. Photographs of the ultrasonic electro-spark deposition process taken at high speed. (a) 1 μs; (b) 13 μs; (c) 25 μs; (d) 38 μs; (e) 50 μs; (f) 87 μs; (g) 209 μs; (h) 740 μs.
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Figure 4. Schematic diagram of the ultrasonic electro-spark deposition process and electrode amplitudes (a) the ultrasonic electro-spark deposition process; (b) electrode amplitudes.
Figure 4. Schematic diagram of the ultrasonic electro-spark deposition process and electrode amplitudes (a) the ultrasonic electro-spark deposition process; (b) electrode amplitudes.
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Figure 5. Surface SEM images and 3D morphology map of ultrasonic electro-spark deposition and electro-spark deposition layers: (a) ultrasonic electro-spark deposition layer; (b) electro-spark deposition layer.
Figure 5. Surface SEM images and 3D morphology map of ultrasonic electro-spark deposition and electro-spark deposition layers: (a) ultrasonic electro-spark deposition layer; (b) electro-spark deposition layer.
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Figure 6. Surface roughness of ultrasonic electro-spark deposition and electro-spark deposition layers.
Figure 6. Surface roughness of ultrasonic electro-spark deposition and electro-spark deposition layers.
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Figure 7. The cross-sectional morphology SEM images and energy spectrum analysis results of the ultrasonic electro-spark deposition and electro-spark deposition layers: (a) ultrasonic electro-spark deposition layer; (b) electro-spark deposition layer.
Figure 7. The cross-sectional morphology SEM images and energy spectrum analysis results of the ultrasonic electro-spark deposition and electro-spark deposition layers: (a) ultrasonic electro-spark deposition layer; (b) electro-spark deposition layer.
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Figure 8. The EDS line scan results of the cross-sectional view of the ultrasonic electro-spark deposition layer.
Figure 8. The EDS line scan results of the cross-sectional view of the ultrasonic electro-spark deposition layer.
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Figure 9. The XRD diffraction pattern of the ultrasonic electro-spark deposition layer.
Figure 9. The XRD diffraction pattern of the ultrasonic electro-spark deposition layer.
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Figure 10. The cross-sectional microhardness distribution curves of the ultrasonic electro-spark deposition and electro-spark deposition layers.
Figure 10. The cross-sectional microhardness distribution curves of the ultrasonic electro-spark deposition and electro-spark deposition layers.
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Table 1. Main chemical components of GCr15 (mass fraction) %.
Table 1. Main chemical components of GCr15 (mass fraction) %.
CMnSiSCrFe
0.95–1.050.20–0.400.15–0.35≤0.0201.30–1.65Bal.
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Li, B.; Ma, X.; Liu, Y.; Wang, H.; Bao, M.; Wang, R. Research on the Ultrasonic Electro-Spark Deposition Process and the Properties of the Deposition Layer. Coatings 2025, 15, 1038. https://doi.org/10.3390/coatings15091038

AMA Style

Li B, Ma X, Liu Y, Wang H, Bao M, Wang R. Research on the Ultrasonic Electro-Spark Deposition Process and the Properties of the Deposition Layer. Coatings. 2025; 15(9):1038. https://doi.org/10.3390/coatings15091038

Chicago/Turabian Style

Li, Bihan, Xiaobin Ma, Yongwei Liu, Hanqi Wang, Manyu Bao, and Ruijun Wang. 2025. "Research on the Ultrasonic Electro-Spark Deposition Process and the Properties of the Deposition Layer" Coatings 15, no. 9: 1038. https://doi.org/10.3390/coatings15091038

APA Style

Li, B., Ma, X., Liu, Y., Wang, H., Bao, M., & Wang, R. (2025). Research on the Ultrasonic Electro-Spark Deposition Process and the Properties of the Deposition Layer. Coatings, 15(9), 1038. https://doi.org/10.3390/coatings15091038

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