Experimental study on ultrasonic-assisted electrolyte plasma polishing of SUS304 stainless steel

A new method of ultrasonic-assisted electrolyte plasma polishing (UEPP) is proposed to solve the problem that the oxidation layer generated on the anode workpiece surface in the process of electrolytic plasma and polishing (EPP) cannot be removed completely, which affects the polishing efficiency and surface quality of the workpiece. Firstly, the polishing mechanism of UEPP is described and set up an experimental platform. Taking SUS304 stainless steel as the experimental object, comparative experiments of UEPP and EPP polishing are carried out to verify the beneficial effect of ultrasonic vibration on improving polishing efficiency and polishing quality. The results show that under the same polishing parameters, the introduction of ultrasonic vibration can improve the polishing efficiency by nearly 30%. After UEPP process, the workpiece has better surface texture, the number of surface scratches is greatly reduced, and lower surface roughness can be obtained. In addition, 40 kHz is the best ultrasonic vibration parameter. The optimization experiments of UEPP polishing process parameters are further carried out. For SUS304 stainless steel, the optimal polishing process parameters are power supply voltage of 250 V, polishing solution concentration of 4%wt, polishing solution temperature of 80 ℃ under 40 kHz ultrasonic-assisted conditions. The surface roughness of the experimental workpiece can be reduced from the initial Sa0.5 μm to Sa0.02 μm after 40 min of polishing under the optimized process parameters.


Introduction
As the last process of finish machining, polishing is of great significance to improve the surface microscopic characteristics (roughness) of various parts. At present, the most commonly used polishing methods include mechanical polishing, chemical polishing, electrochemical polishing, etc. The workpiece after mechanical polishing has good flatness and high brightness. However, the surface of the workpiece after mechanical polishing is prone to defects such as stress deformation and metal lattice damage [1], and it is difficult to solve the polishing problem of complex curved surface parts [2]. Chemical polishing and electrochemical polishing can solve the problem of complex workpiece polishing, but the polishing solution is a strong alkaline or strong acid solution, which will produce harmful waste solution and toxic gas to pollute the environment and harm the health of workers [3,4]. Electrolyte plasma polishing (EPP), as a new special polishing method [5], has strong flexibility and can realize the polishing of complex workpieces. Moreover, the polishing solution is a low concentration of inorganic salt solution, and the processing process only produces gas groups dominated by water vapor, which will not endanger human health. In addition, the waste polishing solution generated in the process of EPP will not pollute the environment, and can be reused to reduce the production cost.
The EPP method was first proposed by Duradzhi et al. in 1979 [6]. Once reported, it set off an upsurge of research on EPP polishing for different materials by scholars all over the world. Stanishevsky et al. [7] studied EPP polishing of the aluminum, copper and various alloy steels, and gave specific process parameters and implementation methods. Holger et al. [8] applied the EPP method to the surface pretreatment of magnesium alloy, which can almost completely remove the corrosion impurities in the area near the surface under rolling condition. Aliakseyeu et al. [9] carried out EPP on titanium and titanium alloys, and the workpiece can obtain lower surface roughness after polishing under appropriate process parameters. In addition, the EPP method can also be used to remove coating materials from metal surfaces [10,11]. In the past, researchers have focused on improving polishing efficiency and surface quality by optimizing process parameters. Wang et al. [12] studied the effect of voltage on EPP and found that when the power supply voltage is in the range of 270 ~ 330 V, the surface roughness of the workpiece after polishing is the lowest, and when the power supply voltage is 270 ~ 290 V, the energy consumption is the lowest. Wang et al. [13] further studied the effects of polishing solution temperature, concentration, and workpiece diving depth on surface roughness and material removal rate, and found that the order of the factors affecting the roughness and material removal rate are: Temperature > Concentration > Diving depth. Polishing time is also an important factor affecting the surface roughness in EPP process. Danilov et al. [14] used COMSOL multiphysics® to simulate the relationship between workpiece surface roughness, polishing time and electric field distribution in the EPP process. The results show that the surface roughness decreases exponentially with time, and the current density on the convex position is higher than that in other places, and the removal speed of the convex position is relatively fast, so the purpose of reducing the surface roughness can be achieved.
EPP has the feasibility and ability to realize the precision polishing of workpiece. At the same time, on the one hand, the surface of the workpiece is unavoidably oxidized during polishing process to form dense passive film to inhibit its machining performance. On the other hand, EPP general needs to control a small energy input to prevent the gas discharge from changing from micro arc discharge to arc discharge [15]. The above contradiction leads to insufficient removal of the oxide layer on the surface of the workpiece when the EPP is used for polishing, which affects the machining efficiency and machining accuracy of the workpiece. In order to further improve the polishing efficiency and surface quality of the workpiece, it can be considered to add other energy field assistance during the processing.
Ultrasonic assistance has been applied in electrolytic polishing, electrolytic honing, abrasive flow polishing and other processes [16][17][18], and the results also show that ultrasonic has the ability to improve the machining efficiency and surface quality of the workpieces to a certain extent. In this paper, ultrasonic vibration is introduced into EPP processing, and an ultrasonic-assisted electrolyte plasma polishing (UEPP) method is proposed, which uses ultrasonic enhanced activation to accelerate the removal of anode passivation film and promote the flow of gas layer and electrolyte near the workpiece, so as to improve the polishing efficiency and polishing quality. Firstly, the polishing mechanism of UEPP is explained. The UEPP experimental platform is built and the comparison experiments of SUS304 stainless steel EPP and UEPP polishing are carried out to preliminarily verify the effect of ultrasonic assistance. Finally, the process parameters of UEPP are optimized.

Polishing mechanism of UEPP
UEPP is a special machining method based on the principle of multi energy field coupling, which uses the coexistence of electric, thermal, chemical, and acoustic energy in the process of machining, and realizes metal surface polishing by means of electrochemical oxidation passivation, discharge plasma etching activation and ultrasonic enhanced activation, etc. The schematic diagram of the polishing device is shown in Fig. 1. The device consists of three parts: electrolytic cell, high voltage DC power supply and ultrasonic generator. Among them, four vibrators are evenly distributed at the bottom of the electrolytic cell, and the vibrators are supplied with energy by an ultrasonic power supply, which drives the polishing solution to generate ultrasonic vibration of a fixed frequency. The positive pole of the high-voltage DC power supply is connected with the workpiece to be polished, and the negative pole is connected with the electrolytic cell. The heating device heats the polishing solution so that it is always maintained at a constant temperature.
The technical principle of this polishing method is as follows. The workpiece is dived into the polishing solution at a certain speed, making the polishing solution electrolyzed first. As the polishing solution and the workpiece are in direct contact, resulting in an instantaneous short circuit, a large amount of heat is released and the polishing solution is evaporated to form a vapor gaseous envelope (VGE) around the workpiece [19]. The VGE surrounding the workpiece separates the workpiece from the polishing solution, then the resistance between them is increased, local high voltage will be formed at the VGE. The VGE at the micro convex peak position of the workpiece is relatively thin, which is easy to be broken down under the action of local high voltage to produce discharge plasma, and the high-energy charged particles bombard the surface of the workpiece to dissolve the metal, so as to realize material removal. At the same time, ultrasonic vibration produce cavitation effects in the polishing solution, and convert ultrasonic energy into mechanical energy to remove the passive film produced by the oxidation of the surface of the anode workpiece during the polishing process. Meanwhile, ultrasonic vibration can promote the flow of gas layer and polishing solution near the workpiece, accelerate the discharge of polishing products and heat, so as to achieve the purpose of improving polishing efficiency and polishing quality.

Experimental details
The processing object in this test is SUS304 stainless steel with a size of 20 × 15 × 6 mm 3 . Before the experiment, 220 mesh abrasive paper is used to grind the surface of the workpieces repeatedly for 20 min, so that the surface roughness of each workpiece is kept about Sa0.5 μm, and the error is controlled within 0.01 μm. Wash with absolute ethanol and deionized water for 20 min respectively in an ultrasonic cleaner, and then dry them in drying oven for later use. The surface roughness of randomly sampled workpieces are shown in Table 1.
The experimental device for UEPP research is designed and built, as shown in Fig. 2. The output power of the DC power supply used is 10 KW, and the output voltage is adjustable from 0 to 400 V. The electrolytic cell is made of stainless steel, with a volume of 10 L, and is equipped with a temperature control monitoring device, which can control the temperature of the polishing solution in real time. The vibrators are evenly distributed at the bottom of the electrolytic cell, and the vibrators are connected to the adjustable frequency ultrasonic power supply through an external circuit to apply ultrasonic vibration to the polishing solution. The frequency of ultrasonic vibration is 20 kHz, 40 kHz, 80 kHz, and 120 kHz. The workpiece diving process is controlled by the CNC system of the machining center to ensure the accuracy and consistency of the diving speed, diving depth and final spatial position.
In the pre-experiment process, the optimal EPP polishing process parameters for SUS304 stainless steel reported in the literature [12,13,20] are selected, as shown in Table 2. The pretreated workpieces are polished for 0.5 min, 1 min, 2 min, 4 min, 7 min, and 10 min respectively. After the workpieces are cleaned and dried, the high precision electronic balance (FA2004N) is employed to measure the weight, the white light interferometer (Wyko NT9100) is employed to measure the surface roughness, and the scanning electron microscope   where M 1 and M 2 are the weight of the workpiece before and after experiment respectively, ρ is the density of the workpiece, A is the surface area of the workpiece, T is the polishing time.
where PIS a and PIS t are the average surface roughness and maximum surface roughness improvement rate respectively. Figure 3A shows the variation of material removal rate with polishing time. It is indicated that the material removal rate of UEPP and EPP methods for stainless steel polishing has a slight decrease in the first 2 min, and then the material removal rate basically fluctuates around a stable value. There are two reasons for the gradual decrease of MRR in the initial stage of polishing. On the one hand, the thickness of the VEG is thin at the initial stage of polishing, so the current density on the anode surface is relatively large, and the material removal is fast. In addition, in the initial polishing stage, the

Results of material removal rate
surface roughness of the workpiece is large, and there are a lot of micro peaks on the workpiece surface. The current density at the peak location is concentrated, which makes the micro peaks be removed quickly in the initial polishing stage, so the material removal rate is greater than that in the stable stage.
With the progress of polishing, the VEG tends to be stable, the micro peaks on the surface of the workpiece become less and less, and the material removal rate tends to be stable. At the same time, it can be found from Fig. 3b that the addition of ultrasonic assistance can effectively improve the material removal rate. After UEPP polishing for about 7 min, the same material removal amount as EPP polishing for 10 min can be achieved, saving nearly 30% of the polishing time. Figure 4 shows the three-dimensional morphology of workpiece surface measured after EPP and UEPP for different times. It can be clearly seen from the figure that after the same processing time, the workpiece polished by UEPP has lower surface roughness than the workpiece polished by EPP, and the surface texture is also more uniform and smooth. Figure 5 shows the surface profile of the workpiece perpendicular to the scratch direction after pretreatment and polishing for 10 min. It can be seen that the peak-to-valley value of the workpiece surface after polishing are significantly reduced. The number of spikes is also significantly decreased. The peak-to-valley fluctuation of the surface polished by UEPP is smaller than that of the surface polished by EPP. There are two reasons for the above results. On the one hand, the ultrasonic cavitation effect can promote the removal of passivation layer and accelerate the formation of discharge channel at the microscopic convex position, and then the convex position is removed by gas discharge, so that the micro leveling can be realized quickly and obtain a lower surface roughness value. On the other hand, the gas around the workpiece can move rapidly to the liquid surface under the action of ultrasonic vibration, which reduces the gas accumulation on the surface of the workpiece and makes the polishing uniformity of the workpiece better. (a) (b) Figure 6 shows the surface micromorphology of the workpieces after pretreated and polished by EPP and UEPP for 1 min, 4 min, 7 min, and 10 min, respectively. It can be seen from the figure that after pretreatment, the surface of the workpiece has deep scratches and strip shaped debris that has not fallen off. During the first 7 min polishing time, with the progress of polishing, the number and depth of scratches on the surface of the workpiece and the amount of residual debris were rapidly reduced, but the changes were less obvious in the next 3 min. This is due to the fact that during the  Fig.4 Surface topography after EPP and UEPP processing polishing process, the profile of the micro convex position of the workpiece at the beginning of polishing is relatively high, and the current density formed by the concentration of electric field is large, so the material removal rate is fast, the number, depth of scratches on the surface of the workpiece decreased rapidly. However, as the polishing continues, with the continuous reduction of the height of the micro convex position, the current density is relatively reduced, and the surface leveling ability is weakened. In addition, compared the surface morphology of the workpiece after polishing by EPP and UEPP for 10 min, it can be found that the surface of the workpiece polished by the EPP method still has slight machining marks, and the surface still has certain fluctuations. While the mechanical scratches are basically not observed on the surface of the workpiece polished by UEPP, and the surface consistency is also excellent. Because the addition of ultrasonic vibration can reduce the removal time of passivation layer and accelerate the process of gas discharge removal in the UEPP process. On the other hand, it can quickly update the polishing solution around the workpiece and promote the rapid discharge of reaction heat and polishing products.

Influence of ultrasonic vibration frequency
The experimental results at different ultrasonic vibration frequencies are shown in Fig. 7. It can be seen from Fig. 7a, b that the variation of material removal rate and surface roughness with processing time is consistent under different ultrasonic vibration frequencies, but the relationship between polishing effect and ultrasonic vibration frequency is not simple positive or negative correlation. When the ultrasonic vibration frequency is low, with the increase of ultrasonic vibration frequency, the ultrasonic cavitation effect is enhanced. The passivation layer produced by oxidation reaction on the workpiece surface can be removed more quickly, which reduces the time for plasma to remove the passivation layer. Higher material removal and less surface roughness can be achieved in the same polishing time. When the ultrasonic vibration frequency is high, continue to increase the ultrasonic vibration frequency, and the vibration effect of the polishing solution will be enhanced, which may destroy the stability of the original gas layer. At the same time, with the increase of ultrasonic frequency, the number of bubbles in the polishing solution increases, and some bubbles fuse with the gas layer, increasing the thickness of the gas layer, making it not easy to be broken down by local high voltage. The combined effect of the above two conditions lead to a decrease in the material removal rate and a higher surface roughness of the workpiece under the same polishing time. When the ultrasonic vibration frequency is reasonable, the electrochemical, thermal, ultrasonic cavitation and other effects in the process of UEPP are in an optimal dynamic balance state, which can improve the processing efficiency and quality of the polishing process to the greatest extent. According to the experimental results shown in Fig. 7a and Fig. 7b, the optimal ultrasonic vibration frequency for UEPP is 40 kHz. Table 3 and Fig. 7c show the average surface roughness improvement rate and the maximum surface roughness improvement rate under different ultrasonic vibration frequencies after 10 min polishing. It can be clearly seen from the figure that when the ultrasonic frequency is 40 kHZ, the effect is the best. Under this frequency, the average surface roughness improvement rate is about 68%, and the maximum surface roughness improvement rate is about 84%.

Experiment design
Under the optimal ultrasonic frequency parameter of 40 kHz obtained from the pre-experiments, the influence of process parameters such as power supply voltage, polishing solution concentration, and polishing solution temperature on the polishing results in the UEPP process is studied. The polishing time is fixed at 10 min. The diving depth and spatial direction have little influence on simple workpiece, so it is not studied here. The orthogonal experiments consist of three factors voltage, concentration of polishing solution and temperature of polishing solution at four different levels shown in Table 4. L16(3 4 ) table is chosen for the experiments. The specific parameters and results of each group of experiments are shown in Table 5. Table 6 lists the average material removal rate and surface roughness of the workpieces under the experimental conditions of different factors and levels. Figure 8 shows show that when the power supply voltage is 250 V, the material removal rate is the highest and the surface roughness is the lowest. When the voltage is too low, plasma is not easily generated in the VGE, and the discharge process is easily interrupted, so the material removal rate is low, and the surface roughness of the workpiece after polishing is also large. When the voltage is too large, the temperature of the polishing solution increases rapidly, and the thickness of the VGE increases, which hinders the normal discharge of the gas layer. Therefore, as the voltage increases, the material removal rate decreases, while the surface roughness of the workpiece increases. In addition, when the voltage is too high, there may be a risk of burns on the surface of the workpiece due to the excessive discharge process. Figure 9 shows the effect of the polishing solution temperature on material removal rate and surface roughness. It is indicated that the material removal rate decreases with the increase of the temperature. When the temperature of the polishing solution is lower than 80 ℃, the surface roughness of the workpiece decreases with the increase of the temperature. When the temperature of the polishing solution   is higher than 80 ℃ the surface roughness of the workpiece increases. The reason is that when the temperature is too high, the gas layer is difficult to maintain its original shape and size, and the polishing process tends to be unstable, and the micro convex peak positions of the surface cannot be effectively removed.

Experimental result and analysis
The effect of concentrations on the material removal rate and surface roughness are shown in Fig. 10. It can be found that when the concentration of (NH 4 ) 2 SO 4 solution is in the appropriate low concentration range, the material removal rate will also increase with the increase of the concentration. When the concentration exceeds 4wt%, the material removal rate will decrease with the increase of slurry concentration. It also can be seen from the Fig. 10, the surface roughness of the workpiece decreases with the increase of the concentration, but the change range is not large. In addition, with the increase of the concentration, a few of reactants will remain on the surface of the workpiece, resulting in the decrease of the brightness of the workpiece surface after polishing. Considering the economy and processability, the optimal concentration of UEPP is about 4wt%.
According to the experimental results, the optimal process parameters of UEPP are as follows: voltage 250 V, polishing solution temperature 80 ℃, polishing solution concentration about 4wt%, and ultrasonic frequency 40 kHz. Figure 11a shows the variation of surface roughness of the workpiece with the polishing time under the optimal process parameters. The surface roughness of the workpiece decreased greatly in the first 10 min, with the further extension of the polishing process, the variation range of the roughness gradually decreases. The surface roughness of the workpiece can be reduced from Sa0.5 μm to Sa0.02 μm after being processed by UEPP for 40 min, which reaches the sub nanometer level. Figure 11b shows the comparison of the workpiece before and after UEPP polishing, and the workpiece can achieve mirror effect after polishing.

Conclusion
Compared with EPP, the workpiece polished by UEPP can obtain lower surface roughness, more uniform and smooth surface texture, and the processing efficiency is increased by about 30%, which proves that the introduction of ultrasonic vibration can effectively improve the polishing efficiency and quality of EPP. The main reasons that ultrasonic introduction can improve the polishing effect of EPP are as follows: 1) The ultrasonic cavitation effect promotes the removal of the passive film on the surface of the workpiece. 2) Ultrasonic vibration accelerates the movement of bubbles on the surface of the workpiece, reduces the accumulation of gas on the surface of the workpiece, and improves the uniformity of VGE. 3) Ultrasonic vibration accelerates the renewal speed of polishing solution around the workpiece, and promotes the rapid discharge of reaction heat and polishing products.
The optimal process parameters of SUS304 stainless steel UEPP processing are ultrasonic vibration frequency of 40 kHz, power supply voltage of 250 V, polishing solution temperature of 80 ℃, and polishing solution concentration of about 4wt%. Under the optimal polishing parameters, the surface roughness of the specimen can be reduced from Sa0.5 μm to Sa0.02 μm after 40 min polishing, reaching sub-nanometer level.