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Journal of Advanced Research 23 (2020) 151–161 Contents lists available at ScienceDirect Journal of Advanced Research journal homepage: www.elsevier.com/locate/jare Ultrasonic-assisted electrochemical drill-grinding of small holes with high-quality Xiangming Zhu a, Yong Liu a,⇑, Jianhua Zhang b, Kan Wang a, Huanghai Kong a a Associated Engineering Research Center of Mechanics & Mechatronic Equipment, Shandong University, Weihai City 264209, PR China Key Laboratory of High Efficiency and Clean Mechanical Manufacture, Ministry of Education of China, School of Mechanical Engineering, Shandong University, Jinan City 250061, PR China b g r a p h i c a l a b s t r a c t Diamond abrasives Insoluble products Ultrasonic vibration ˉ ˇ Passive film UAECDG ˇ Substrate Theoretical analysis C experimental optimization C (a) Inlet (b) outlet (c) Surface roughness of C-C Cross Section. a r t i c l e i n f o Article history: Received 24 October 2019 Revised 11 February 2020 Accepted 13 February 2020 Available online 15 February 2020 Keywords: Electrochemical drill-grinding Stainless steel Ultrasonic-assisted Surface roughness Small holes a b s t r a c t Electrochemical drill-grinding (ECDG) is a compound machining technology, which combines Electrochemical machining (ECM) with mechanical drill-grinding process. On this basis, a new method of machining small holes which called ultrasonic-assisted electrochemical drill-grinding (UAECDG) is proposed. First, the principle of UAECDG is analyzed through analysis of UAECDG process and electrochemical passivation behavior of materials. Second, the simulation of electrochemical drill-grinding process was studied to illustrate the effect of ball-end electrode on reducing the hole taper and improving the machining accuracy. Afterwards, several groups of experiments are conducted to analyze the influence of electrical parameters, ultrasonic amplitude and matching degree between electrolysis and mechanical grinding on the machining quality of small holes. Finally, small holes with diameter of 1.1 ± 0.01 mm, surface roughness of 0.31 lm and taper of less than 0.6 degree were machined by UAECDG, which revealed UAECDG is a promising compound machining technology to fabricate small holes with high quality and high efficiency. Ó 2020 The Authors. Published by Elsevier B.V. on behalf of Cairo University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review under responsibility of Cairo University. ⇑ Corresponding author. E-mail address: rzliuyong@sdu.edu.cn (Y. Liu). https://doi.org/10.1016/j.jare.2020.02.010 2090-1232/Ó 2020 The Authors. Published by Elsevier B.V. on behalf of Cairo University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 152 X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 Introduction Small holes are widely used in aerospace, automobile, ship and other industries, such as engine blades, combustion chambers, cooling rings, bottom plates and so on [1–4]. Generally speaking, holes with diameter of 0.3–3 mm are called small holes [5,6]. Because the diameter of small holes is relatively small, and the machining materials, such as titanium alloy [7], nickel-based superalloy [8] and stainless steel [9], are usually difficult to machine, therefore, it is hard to realize high machining precision and surface roughness by mechanical methods, in which machining tools also wear a lot [10]. In the field of non-traditional machining, laser machining and EDM will inevitably produce recast layer and micro-cracks on the machined surface, which is prone to stress concentration and do great harm to the structure [11–14]. Conventional ultrasonic-grinding machining can cause great wear and tear to abrasives, which is difficult to process materials with good plasticity and toughness. And Conventional ECM cannot meet the production needs of high machining accuracy and high stability [15,16]. In order to solve the defect of single machining method, many scholars combine different machining methods and put forward various compound machining methods. Among them, electrochemical grinding (ECG) is a compound machining method with many advantages, such as low induced stress, high machining efficiency, large depths of cut, and high machining precision [17–19]. During ECG process, with the feeding of machining tools, some of the substrates are dissolved, some of them are passivated and a thin and brittle passivation film is formed on the surface of the substrate. The abrasive particles at the outer end of the tool contact the passivation film and some electrochemical product which adsorbed on the surface of passivation film, then scrape them off. On the material removal principle of electrochemical drillgrinding, Ge et al. [20] consider that ECM dissolves the anode workpiece at high applied voltage which is 20 V and high feed rate which is from 0.5 mm/min to 2.4 mm/min, massive of electrochemical products adsorbed on the surface of the substrate are scraped off by abrasive particles instead of the passive film formed. At the same time, due to the removal of the easily dissolved material, some insoluble components in the anode substrate materials are gradually exposed. All these insoluble components and electrochemical products are removed by grinding. On the tool electrode of ECG, Niu et al. [21] employed an abrasive tool with arrayed holes, and after finish machining with ECG, the surface roughness decreased dramatically, from 1.65lm to 0.648lm. On the abrasives wear in ECG technology, some scholars put forward that diamond is the common abrasive for grinding, compared with mechanical grinding, the tool loss of ECG with diamond is 4 to 15 times smaller [22]. On the practical application of ECG, Wang et al. [23] use it to improve MoP microparticles’ surface states to improve its catalytic activity. The another advantage of ECG is that most of oxide/passivation layer on the workpiece is removed by grinding (5–10% of material removal), so harsh/harmful electrolytes are not needed to dissolve passivation layer [24]. However, ECG will produce lots of electrochemical products and insoluble materials, which will lead to the deterioration of the machining environment and may cause short-circuit phenomenon. In order to solve this problem, this paper proposed ultrasound-assisted electrochemical drillgrinding (UAECDG). It is a kind of compound machining method that uses electrochemical reaction to produce passivation film on the surface of material, and removes the passivation film through mechanical grinding and ultrasonic vibration to expose the machined substrate again, so that it can be machined under the alternating process of electrochemical machining, mechanical grinding and ultrasonic impact. In addition, in this paper, ultrasonic vibration is added to the spindle and it is transmitted to the machining tool to produce periodic vibration, then disturbs the electrolyte, so as to accelerate the renewal of electrolyte in the gap, then improve machining quality. The cavitation effect caused by ultrasound can produce lots of micro bubbles and then collapse between the machining gap, resulting in shock wave generation and accelerate erosion of materials [25,26], which is beneficial to improvement of machining efficiency. On the other hand, ultrasound can renew the electrolyte quickly to bring more stable machining environment [27]. Materials and methods The flow chart of work methodology is shown as Fig. 1. Pre-hole machining is carried out on 304 stainless steel plate firstly, which uses cylindrical spiral electrodes for electrochemical drilling process, the real picture of cathode tool for preparing pre-hole is shown as Fig. 2a. In order to prepare the pre-hole, it is necessary to select reasonable machining parameters, which will significantly affect the hole diameter, hole roundness and hole wall surface roughness of the pre-hole, and will affect the machining allowance during the hole-enlarging process. If the machining allowance of the pre hole is too large, the mechanical grinding will be too strong, resulting in low machining efficiency and poor surface quality of the inner wall of the hole. If the pre hole machining allowance is too small, the mechanical grinding effect in UAECDG process will be weakened, which will not significantly improve the machining quality. Therefore, the parameters for preparing the pre-hole need to be selected reasonably, and the optimized parameters are shown in Table 1. After pre-hole with certain machining accuracy and surface quality is machined, the prehole is enlarged by means of ultrasonic-assisted electrochemical drill-grinding. UAECDG setup and process As shown in Fig. 3a, the small holes are machined by ultrasonicassisted electrochemical drill-grinding set up. While the ball-end electrode rotates continuously, it is accompanied by downward feeding and periodic vibration. It can be seen that the ball-end electrode is equipped with diamond abrasive particles, which are added to the ball-end by electrodeposition as shown in Fig. 2b. The number of diamond abrasives is 1200#. The electrolyte is supplied by side spraying combined with pre-filling in the electrolyte tank. In UAECDG process as shown in Fig. 3b, a ball-end electrode with diamond abrasive particles is used as tool cathode which rotating at a high speed and ultrasonic vibrating along the axis direction. The ball-end has a larger diameter than the premachined hole. Due to the passivation of metals in passive electrolyte, a kind of soft passive oxide film is formed and adhering to the material surface with the electrochemical anodic dissolution of metal materials. With the feed of ball-end electrode, this passive oxide film which negative to electrochemical reaction is soon removed by the diamond abrasives so that the fresh metal materials can be exposed for the consecutive electrochemical reaction. Therefore, the process of material removal includes both mechanical grinding and electrochemical reaction. Fig. 3c is the schematic diagram of UAECDG. Before the process of UAECDG, a pre-machined hole with a diameter of D0 has been fabricated by ECM. In UAECDG, the material is electrochemically and mechanically removed by the tool’s ball-end with a diameter of d which is larger than the pre-machined hole diameter D0 . As X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 304 stainless steel plate 153 ECM Pre-holes Electrochemical behavior of materials Machining simulation of ECDG UAECDG Influence of ultrasonic vibration Matching of ECM and grinding Final holes with good quality Fig. 1. Flow chart of work methodology. (a) Cylindrical spiral cathode tool for preparing pre-hole. (b) Ball-end cathode tool for UAECDG. Fig. 2. Real picture of cathode tool. shown in Fig. 3c, the process of UAECDG includes the phases of ECM, ECG, and the secondary electrolysis. Because of too large machining gap during the phases of ECM and secondary electroly- sis, the mechanical grinding is not able to remove the passive oxide film effectively which is negative to the electrochemical reaction, so that only a small number of material removal occurs during 154 X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 Table 1 Parameters for preparing pre-hole. Electrochemical behavior of 304 stainless steel Item value Peak voltage Feed rate Electrolyte Rotation speed Pulse period Duty cycle Ultrasound amplitude 7V 0.4 lm/s 10 wt% NaNO3 6000r/min 10 ls 0.25 5 lm the phases of ECM and secondary electrolysis. And the material is mainly removed during the phase of ECG. In addition, the ultrasonic vibration of tool electrode in UAECDG is conducive to update of electrolyte and removing of tiny bubbles and other electrolysis products so that the flow field can be more uniform. Therefore, in order to achieve high machining accuracy, many important factors in UAECDG process, such as the electrochemical behavior of materials, the influence of electrical parameters, ultrasonic amplitude and matching degree between electrolysis and mechanical grinding on the machining quality of small holes should be discussed in the following experiments. AC frequency converter Software interface Control cabinet 304 stainless steel has a passive behavior in passive electrolytes such as NaNO3 solution [28]. In passivation of metal material, a kind of passive oxide film is formed and adhering to material surface, the passivation film on stainless steel surface are mainly chromium and iron oxides / hydroxides [29]. This passive oxide film which in turn affects electrochemical reaction is a link between the electrochemical reaction and mechanical grinding in UAECDG. It has been found that the surface can be well protected from general corrosion by the passive oxide film which formed in passive solution [30]. To select a proper passive electrolyte and obtain a stable passivation during the process, it is essential to research 304 stainless steel’s passivation phenomenon in different electrolyte environments. 304 stainless steel’s polarization curves in different concentration electrolyte are investigated by potentiodynamic method as shown in Fig. 4. As shown in Fig. 4, the passivation performance of 304 stainless steel in 10–20 wt% NaNO3 solution is quite different. 304 stainless steel in 10 wt% NaNO3 solution has no obvious passivation interval, and the passivation performance is weak; the passivation potential range in 15 wt% NaNO3 solution is from 0.58 V to 0.76 V; in 20 wt% NaNO3 solution, the passivation potential range is from
0.15 V to Z axis Ultrasonic motorized spindle Tool electrode Electrolyte tank X, Y axis Lifting platform Granite base Pulse power supply Ultrasonic generator Water chiller (a) Experimental setup Feed direction Rotation direction Vibration direction Auxiliary spray Ultrasonic vibration ˉ Diamond abrasive particle Cathode Anode Ball-end abrasive electrode Diamond abrasives D Passive oxide film Secondary electrolysis Workpiece ECG ECM D0 (b) Sketch of machining area (c) Cross-sectional views of UAECDG process Fig. 3. Sketch of UAECDG process. 155 X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 -2 -3 passive potential range Log(I /A⋅cm-2) -4 -5 -6 -7 10% NaNO3 15% NaNO3 20% NaNO3 -8 -9 -10 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 ESCE /V Fig. 4. Polarization curves in NaNO3 solution with different concentrations. 0.7 V, the passivation range is wider, also, as Fig. 4 shows, 304 stainless steel in 20 wt% NaNO3 solution has quite stable current density, and among three of them, it has the lowest current density in passivation state, that is, the passivation reaction is more stable, it indicates formed passive oxide film’s microstructure is denser and more insulating. It is beneficial to mechanical grinding in UAECDG process and reducing stray corrosionIn short, the passivation effect of 304 stainless steel in 10-20 wt% NaNO3 solution is greatly affected by the concentration of NaNO3 solution, the passivation effect of 304 stainless steel in 20 wt% NaNO3 solution is the best. ignores the loss caused by the electrode dynamics and concentration-dependent effect. It assumes that the charge transfer in the electrolyte obeys Ohm’s law. Here, we make two hypotheses: first, the electrolyte is electrically neutral, which counteracts the contribution of current to current density; second, the composition change of electrolyte is insignificant (i.e., uniform distribution), which counteracts the contribution of diffusion to current density, allowing us to consider ion strength as a constant. Furthermore, we believe that the potential drop at the electrode– electrolyte interface will not deviate from the equilibrium value. In other words, there is no activated over-potential. It can be seen that the distribution of primary current depends only on the geometrical structure of positive and negative electrodes. In electrochemical machining, the anode and cathode are good conductors of metal. Therefore, it forms an equal potential surface on the surface of the cathode and the anode. The potential of the anode and the cathode meet the Dirichlet Boundary Conditions: ujB6 ¼ ujB7 ¼ UðtÞ ð1Þ ujB8 ¼ 0 ð2Þ For the electrolyte boundary B1、B2、B3、B4 and B5, the equipotential line in the electrolyte region is approximately vertical to its surface, i.e., the potential derivative along its normal direction on the electrolyte boundary is approximately 0, which meets the Norman Boundary Conditions: @u @u @u @u @u jB1 ¼ jB2 ¼ jB3 ¼ jB4 ¼ jB5 ¼ 0 @n @n @n @n @n ð3Þ Combining Eq. (1), Eq. (2) and Eq. (3), boundary conditions of gap electric field in ECDG process can be obtained: Simulation of ECDG process In UAECDG process, ECDG process is the majority, and electrochemical machining accounts for the vast majority of material removal in ECDG, so it is necessary to study the electric field of this method. In this paper, the electrochemical module of COMSOL Multiphysics is used to simulate hole-enlarging process. Holeenlarging process of 304 stainless steel plate is studied with the condition of primary current distribution. Two-dimensional geometric model of gap’s electric field in ECDG process was established by extracting the contours of prehole and ball-end electrode, as shown in Fig. 5. Among them, the thickness of workpiece is 500 lm, the diameter of tool electrode is 1000 lm, the boundary B1、B2、B3、B4 and B5 are electrolyte boundaries, the boundary B6 and B7 are workpiece boundaries, and the boundary B8 is tool electrode boundary. For the primary current distribution in this simulation, it is only applicable to explain the loss caused by the solution resistance, but B2 Tool electrode B3 B8 8 > < > : @u ujB6 ¼ ujB7 ¼ UðtÞ ujB8 ¼ 0 jB1 ¼ @@nu jB2 ¼ @@nu jB3 ¼ @@nu jB4 ¼ @@nu jB5 ¼ 0 @n The simulation parameters of ECDG process are shown in Table 2. The simulation results for ECDG process are shown in Fig. 6. As shown in Fig. 6(a, b), the current concentrates in the area where the ball-end is very close to the workpiece, while there is almost no current in other areas, which indicates that the localization of ECDG adopted in this paper is very good. As shown in Fig. 6d, it can be seen that the taper of final hole is much smaller after hole-enlarging by UAECDG, compared with the pre-hole. This is because compared with the cylindrical spiral electrode, the disadvantageous electrified area of the ball-end electrode has a larger machining gap with the anode workpiece, which greatly reduces the secondary electrolysis effect of the anode workpiece, which can also be seen in Fig. 6(a, b, c). It is verified that the current density drops sharply in the machining area with machining gap more than 50 lm, which reduces the secondary electrochemical corrosion and decreases the hole taper. It indicates that UAECDG can effectively improve the machining quality of the small hole. B4 B1 B6 Workpiece Table 2 Simulation parameters of ECDG process. B7 Electrolyte Workpiece B5 Fig. 5. Two-dimensional geometric model of gap electric field in ECDG process. ð4Þ Item value Applied voltage Feed rate Conductivity Machining time Initial machining gap 3.3 V 4 lm/s 11.6S/m 280S 20 lm 156 X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 (a) t=0s (b) t=120s 1.8 profile at initial time profile at end time Y-coordinate (mm) 1.6 1.4 1.2 1.0 0.8 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 X-coordinate (mm) (c) t=240s (d) contours of small hole when t=0s and t=240s Fig. 6. Simulation results of ECDG process. Results and discussion Pre-holes were machined by ECM for preparing for further hole enlargement. Pulse power supply was employed in pre-hole machining process, and the tool is a spiral electrode with a diameter of 0.8 mm. Several pre-holes with good repetition accuracy were successfully machined on 304 stainless steel plate with 0.5 mm thickness. In order to meet the requirement of inner wall surface roughness less than 0.4lm. Therefore, further UAECDG enlargement of pre-holes is a necessary process. To measure diameter of small holes and observe contour of small holes, Nikon SMZ1270 optical microscope was used; to observe Micro-morphology of inner surface of the small hole, FEI Nova Nano-SEM 450 was employed; to measure surface roughness of hole wall, Wyko NT9300 white light interferometer was used. Discussion on electric machining parameters Electrical machining parameters are the controlling factors in UAECDG process. As we know, the material removed by electrochemical machining accounts for about 90% of the combined electrochemical grinding machining. In this paper, the role of ultrasonic vibration is to make the removal and renewal of electrolyte more effectively. In the UAECDG process, electrochemical machining is still the main etching way, so, the selection of electrochemical machining parameters is very important. To explore the influence of machining voltage and duty cycle in UAECDG process, several groups of comparative experiments were carried out. Machining efficiency is characterized by the optimal feed rate, which is the maximum feed rate without short circuit in UAECDG process. The experimental results are shown in Fig. 10a. As shown in Fig. 10a, with increasing of machining voltage, optimal feed rate increases correspondingly, due to the enhancement of electrochemical etching when the voltage increases, and the increase of the etching material at the same time, so the optimal feed rate increases correspondingly. With increasing of duty cycle, optimal feed rate increases, which is similar to the principle of voltage increase, it is caused by the increase of energy density of material etching. When the voltage and duty cycle are maximum, the optimal feed rate is maximum. However, at the same time, stray corrosion becomes more and more serious due to the high energy density of erosion. Moreover, passivation film is easy to be broken down and large pieces of erosion material fall off, resulting in a serious decline in surface quality. X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 Low DC High Pulse Voltage Voltage 157 bilization under low voltage was chosen as the machining parameter of hole-enlarging process in this paper. As shown in Fig. 10a, optimal feed rate is moderate and stable at low voltage of 3– 3.5 V. Finally, 3–3.5 V was chosen as the next step to explore the more suitable voltage range for UAECDG, and the feed rate is 2.5–4.5lm=s. Matching of ECM and mechanical grinding Fig. 7. Machining quality of small hole inlet under different electrical parameters. Therefore, in the case of a certain feed rate, the electrical machining parameters with smaller energy density should be selected. At present, there are two choices, one is high voltage with low duty cycle, the other is low voltage with high duty cycle. When the duty cycle reaches 100%, the pulse signal becomes a DC voltage stabilized signal. As shown in Fig. 7, it is obvious that the shape of small holes produced by DC voltage stabilization at low voltage is superior to that produced by high frequency pulse at high voltage. Although high frequency pulse can enhance the localization of machining, stray corrosion is still serious under high voltage, resulting in worse hole shape, so the hole shape under low voltage DC voltage stabilization machining is better. At last, DC voltage sta- The electrical parameters mentioned above have a great influence on hole-enlarging process. Similarly, if there is no good cooperation with other machining parameters, the advantages of UAECDG cannot be reflected. In this paper, the electrochemical grinding process removes most of the material by electrochemical machining, and a brittle passivation film is formed on the surface of the material. The diamond abrasives attached to the ball-end scrape the passivation film away by the rotation of the spindle, and a new passivation film is produced on the exposed material. Thus, along with the electrochemical corrosion reaction, passivation-grinding also takes place. Continuously alternating, the final hole is produced by this compound machining method. However, if electrochemical machining and mechanical grinding do not match, there will inevitably be the following poor machining situation as shown in Fig. 8. As shown in Fig. 8a, when the electrochemical effect is too strong, it can be seen that the inlet of the hole wall has a large stray corrosion, and the diamond abrasives cannot touch the material, and the finishing effect of mechanical grinding cannot be reflected. As shown in Fig. 8b, when the mechanical grinding effect is too strong, although the hole wall is steep, there are obvious scratches on the inner wall caused by mechanical grinding. It is caused by (a) Excessive electrochemical effect (b) Excessive grinding effect Fig. 8. Surface morphology of the inner wall of small holes when ECM does not match mechanical grinding. X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 the direct grinding of the material substrate by diamond abrasives, which lead to increasing of the surface roughness of inner wall. Moreover, due to the direct contact between tool electrodes and (a) Inner wall of pre-hole by ECM the substrate, a lot of scratches are produced. The mechanical grinding force can desorb a large amount of diamond, cause serious electrode wear, and greatly reduce the repeatability. Therefore, (b) Inner wall after hole enlargement by UAECDG Fig. 9. Inner wall of pre-holes and Inner wall after hole enlargement. (a) Feed rate under different voltage and duty cycle. (b) Diameter of small hole under different feed rate and voltage. 8 1.0 Optimal feed rate Surfece roughness Optimal feed rate(Pm/s) 7 0.8 6 5 0.6 4 0.4 3 2 0.2 1 0 0 2.5 5 7.5 10 Ultrasound amplitude(Pm) (c) Surface roughness of inner wall under different applied voltage and feed rate. (d) Feed rate and surface roughness under different ultrasonic amplitude. Fig. 10. Optimization of the influence of machining parameters on small hole quality. 0.0 Surface roughness(Pm) 158 X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 matching of electrochemical machining and mechanical grinding is very important. It is reflected in the matching of machining parameters, for example, the matching of applied voltage and feed rate. Therefore, in order to determine better machining parameters, sev- Table 3 Machining parameters for UAECDG. Item value Applied voltage Feed rate Electrolyte Rotation speed Number of diamond abrasives Ultrasound amplitude 3.3 V 4 lm/s 20 wt% NaNO3 12000r/min 1200# 0–10 lm C eral groups of experiments were conducted. As shown previously, the machining voltage is determined to be 3–3.5 V, and the feed rate is 2.5–4.5lm=s. Firstly, the effect of feed rate and applied voltage on small holes’ diameter machined by UAECDG is discussed. The experimental results are shown in Fig. 10b. The diameter increases with the applied voltage. As the increase of applied voltage, the amount of material eroded by electrochemical effect increase in the same machining time, so the diameter becomes larger. Secondly, the diameter decreases with the feed rate increasing, it results in reduction of erosion per unit time and reduction of diameter. For researching the influence of applied voltage and feed rate on surface roughness of small holes’ inner wall, Wyko NT9300 white light interferometer is used to observe and measure the surface roughness of small holes’ inner wall. The measurement results C (a) Inlet 159 (b) outlet (c) Surface roughness of C-C Cross Section. Fig. 11. Typical small holes machined by UAECDG. 160 X. Zhu et al. / Journal of Advanced Research 23 (2020) 151–161 are shown in Fig. 10c, when applied voltage is 3.3 V and feed rate is 4lm=s, ECM matches the mechanical grinding best. As shown in Fig. 9b, after UAECDG process, the inner wall of the small hole is smooth and the wall is steep as applied voltage is 3.3 V and feed rate is 4lm=s, it verifies that these parameters are the best which match electrochemical machining and mechanical grinding. flow mark and pitting corrosion, and the machining quality has been greatly improved. Therefore, by means of UAECDG, small holes with diameter of 1.1 + 0.01 mm, taper of less than 0.6 degrees and surface roughness of 0.31lm can be machined on 304 stainless steel plate. Effect of ultrasonic vibration on machining quality of small holes In this paper, a brand-new technology UAECDG was proposed, the conclusions can be summarized as follows: In this paper, ultrasound is transmitted to spindle from ultrasonic generator, and then to the tool electrode, which makes the electrode produce periodic up-and-down vibration, then improves the removal and renewal of electrolyte during hole-enlarging process, optimizes the machining environment, and improves the machining stability and quality. The main controlling factors of ultrasonic machining are ultrasonic amplitude and frequency, in which the resonant frequency obtained by sweeping the tool electrodes is generally 24.9– 25.1 kHz, and the ultrasonic amplitude is the controlling factor in this section. In order to explore the influence of ultrasonic vibration on the optimal feed rate in UAECDG, this paper used the parameters of Table 3 as the machining parameters, in which the ultrasonic amplitude is varying from 0 to 10lm. The machining results are shown in Fig. 10d. As shown in Fig. 10d, with increasing of ultrasonic amplitude, optimal feed rate increases correspondingly. This is because the increase of amplitude leads to the enhancement of mass transfer effect of electrolyte and the improvement of electrochemical machining environment, which reduces the probability of short circuit and increases the optimal feed rate accordingly. And then, optimal feed rate increases sharply and reaches a relatively stable state when ultrasonic amplitude changes from 0lm to 2.5lm, that is, from no ultrasonic to ultrasonic. The increased optimal feed rate shows that the machining efficiency of hole-enlarging process has been significantly improved by ultrasonic vibration. In addition, it can be seen from Fig. 10d, when the ultrasonic amplitude is 5lm, the optimal feed rate is 5.5lm=s. For researching the influence of ultrasonic vibration on surface roughness of small holes, experiments which employ machining parameters in Table 3 are carried out in this section. As shown in Fig. 10d, with increasing of the ultrasonic amplitude, surface roughness of small holes’ inner wall decreases continuously. When the ultrasonic amplitude reaches 5lm, it reaches the minimum of 0.31lm. In addition, it can be seen that when the ultrasonic amplitude changes from 0lm to 2.5lm, i.e. from nonultrasonic to ultrasonic vibration, the surface roughness of small holes’ inner wall decreases from 0.65lm to 0.35lm, and reaches a relatively stable state, which indicates that the surface roughness of the hole-enlarging process has been significantly improved by ultrasonic vibration. Typical machining results Combined with the previous experiments and analysis, by choosing the following machining parameters of UAECDG, the best quality holes can be obtained on 304 stainless steel plate. The applied voltage is 3.3 V, the feed rate is 4lm=s, the electrolyte is 20 wt% NaNO3, the electrode rotation speed is 12000r/min, the number of diamond abrasives is 1200#, and the ultrasonic amplitude is 5lm. The typical machining results are shown in Fig. 11. As shown in Fig. 11, the holes obtained by UAECDG has good dimensional consistency, good surface quality and minimal taper. Compared with the pre-holes, there are almost no electrochemical Conclusions (1) Through the study of the anode polarization curve of 304 stainless steel, it is concluded that 20 wt% NaNO3 solution can produce the most stable passivation reaction and reduce the stray corrosion, which is most conducive to holeenlarging by UAECDG. (2) The electric field simulation results of the ECDG process revealed that the ball-end electrode used in this paper can effectively improve the machining localization and reduce the secondary electrochemical corrosion. (3) Experimental study of ultrasonic amplitude in UAECDG process proved that combined the ECDG technology with reasonable ultrasonic vibration can effectively improve the machining efficiency and the surface roughness of small holes. (4) Influence of electrical parameters, ultrasonic amplitude and matching degree between electrolysis and mechanical grinding on the machining quality of small holes are discussed experimentally, which demonstrated that the small holes with the hole diameter of 1.1 ± 0.01 mm, the taper of less than 0.6 degree, the surface roughness of 0.31 lm can be obtained on 304 stainless steel plate by UAECDG with the optimal parameters. (5) For further research, UAECDM technology in this paper could be rapidly applied to the field of metal additive manufacturing, in order to significantly improve the machining accuracy and surface quality of small hole structures in metal additive manufacturing parts. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Authors acknowledge financial support from the National Key R&D Program of China (No.2018YFB1105900), the Key R&D Program of Shandong Province (No. 2019GGX104023), the Natural Science Foundation of Shandong Province (No. ZR2018MEE018), the China Postdoctoral Science Foundation (Nos. 2018M630772, 2019M662347), and the Young Scholars Program of Shandong University, Weihai (No. 2015WHWLJH03). References [1] Gurav MM, Gupta U, Dabade UA. 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