Abstract
Abstract In an appropriate electrochemical environment, the discrete thermal electron emission could be induced in the micro area due to the uneven distribution of electron flux on the anode surface. Thus an oxygen molecule could be ionized at the liquid-solid interface after collision, and then oxygen plasma with distribution characteristics would be formed. The plasma electrolytic oxidation (PEO) could happen at the liquid-solid interface. In this work, the low carbon steel was used to study the deburring process by PEO at a high frequency (70000 Hz) pulse DC mode. Its burr height H from 3.23 mm to 0.04 mm was removed to form a smooth surface within 6 min. The values of corrosion potential and current density for the untreated sample were −0.667 V and 6.735×10
Abstract
−5 A/cm 2 , respectively. But for the treated sample, the corrosion potential and current density were relatively lower, −0.354 V and 1.19×10
Abstract
−7 A/cm 2 . Therefore, PEO was expected to be a new deburring method of carbon steel for the material processing field.
Introduction
Low carbon steels as important structural materials have to meet increasingly stringent requirements for the development of modern industry due to their mechanical and chemical properties. Various methods have been applied to process the steels, such as cutting, forming, blanking, and shearing operations [1, 2] , while one of the most serious shortcomings of those processes is to generate many burrs. A burr is defined as a projection of undesired material beyond the desired machined feature [3] . The noise, the unsafe operation of a machine, the friction and wear of a moving part are caused by the burr, to reduce the fatigue life of components. Various deburring methods that are classified into mechanical, thermal, abrasive and electrochemical processes have been introduced [4] . The most frequently used method is mechanical, which is manually post-processed by a skilled worker, resulting in low productivity and high cost [5] . In thermal deburring, the process requires high capital investment, and has some technical problems [6, 7] , because the workpieces are subjected to a temperature in excess of 1000
Introduction
• C for a few seconds, causing the burrs to ignite and burn off. Due to the high initial cost and lack of adequate technical data, the use of abrasive jet deburring is also limited. As for the traditional electrochemical deburring, it is based on the principle of the anodic dissolution or melting process, such as the electrochemical machining (ECM) and electrical discharge machining (EDM). They may have the possibility of base metal heat damage as well as the high corrosive electrolyte [8] . In the last ten years, plasma electrolytic saturation of steels and titanium alloys with interstitial elements has claimed the attention of researchers. Belkin et al. studied the features of anode plasma electrolytic nitrocarburising of low carbon steel [9, 10] . They found that the continuous motion of the medium in the vapor-gas envelope supplied enough operational saturating components and removed the reaction products, but its risk is overheating or melting of workpieces, as well as increasing the surface roughness in most cases [11] .
Introduction
In this study, we investigated the deburring process for the low carbon steel by plasma electrolytic oxidation (PEO), which is an electrochemical surface treatment to fabricate a ceramic layer on the surface of the valve metals, such as titanium, aluminum, magnesium, and zirconium components [12, 13] . This technique involves the polarization of the valve material at the high voltage in an appropriate electrolyte. A large number of short-lived micro discharges were generated by the dielectric breakdown, and the plasma was formed to modify the coating with the incorporation of species from the electrolyte [14−17] . The corrosion and wear resistance, the high dielectric strength and the heat resistance of the valve metal could be improved by the PEO coating [18, 19] . PEO has been generally considered as an electrochemical surface treatment method only suitable for valve metals. For non-valve metals and alloys like carbon steel, iron, copper, nickel and others, the pretreatment was usually employed to deposit a valve metal layer (usually Al) on the surface before subjecting it to PEO [20, 21] . A barrier layer of Al 2 O 3 or SiO 2 was deposited onto the surface of the iron substrate by the decomposition of the aluminate or silicate electrolyte in the PEO process [22] . Additionally, it was also reported that Fe 3 O 4 coating or Fe 3 O 4 together with other spinel composites in the coating showed excellent anti-corrosion behavior on carbon steels [23, 24] . In the present study, a new method was proposed to deburr the low carbon steels by PEO in a high frequency pulse DC mode. PEO was performed on Q235 carbon steel without any pretreatment in the sodium aluminate system, and ceramic coating was obtained on the surface. The effectiveness of the method as a deburring process and its effect on surface morphology and anti-corrosion behavior were investigated. A possible mechanism for deburring was also proposed.
Experimental details 2.1 Experimental setup
The experimental setup is shown in Fig. 1 . 304 stainless steel was fixed as the cathode, whose shape was a hollow cylinder.
Experimental details 2.1 Experimental setup
Its internal diameter and external diameter were 1.5 mm and 2.5 mm respectively. The anode was Q235 carbon steel specimens with burrs, which were generated spontaneously after cutting by a milling machine. Table 1 shows the chemical compositions of the Q235 carbon steel. Before cutting, they were first mechanically-polished by a semi-automatic polishing machine (MoPao2DE, Weiyi), and then cleaned using ethanol.
Experimental details 2.1 Experimental setup
The cathode could be moved up and down along the sliding rail by a servomotor, whose control precision was 500 µm, while the anode could be also moved planarly on a workbench. The electrolyte cell was made of polypropylene with a size of 30 cm× 30 cm× 30 cm. The electrolyte solution was prepared by mixing 2 g/L NaOH, 20 g/L NaAlO 2 , and 10 g/L Na 2 HPO 4 ·12H 2 O into the distilled water. The electrical conductivity and pH were 33.1×10 −3 S/cm and 12 respectively. The electrolyte was circulating from the cathode to the anode, and then the electrolyte cell by the circulating pump.
Experimental details 2.1 Experimental setup
During the whole deburring treatment, the temperature of the electrolyte was always kept below 40 o C by the recirculation of the cold tap water through a heat exchanger situated in a 60 L electrolyte tank. 
Experimental process
The plasma-deburring process was run at a voltage of 150-260 V. A power supply (MAP, peak power: P p = 500 kW, peak voltage: U p = 550 V, peak current: I p = 500 A, pulse width: 5-30 µs, pulse frequency: 25000-70000 Hz) developed by Xi'an University of Technology was used to conduct the process. The output power was modulated at a constant current mode.
Experimental process
The average current density was thomaswagner@example.org A/dm 2 . The pulse frequency and pulse width were kept at 70000 Hz and 10 µs respectively, especially the gap distance is about 0.5-1 mm. After the treatment, an optical microscope (Mshot, China) and scanning electron microscopy (SEM, JSM-6700, Japan) were used to analyze the surface morphology of the workpiece. The phase composition of the treated sample surface was investigated via X-ray diffraction (XRD, SHIMADZU LIMITED XRD-7000 s). The corrosion tests were evaluated by potentiodynamic polarization through a CHI604 electrochemical analyzer (Shanghai, China) in 3.5% NaCl solution.
Experimental process
3 Results and discussion
Deburring effect by plasmadeburring
The side edge angle and the effective rake angle on the top burr are popularly used to evaluate the edge quality of miniature components [25] . In this work, it needs to ensure that the proposed plasma-deburring process can remove all burrs and can fillet the steel edge. Therefore, the burr height H is the index of the edge quality of the steel. Fig. 2 presents the optical micrographs of Q235 carbon steel specimens before and after plasma-deburring. As shown in Fig. 2(a) and (b) , the burrs are clearly observable on the steel edge after cutting by the milling machine. Especially in Fig. 2(a) , they are distributed along the edge promiscuously, where the peak H was measured to be 3.23 mm. Fig. 2(b) shows a larger version of the salient point in Fig. 2(a) , where the burr height of the salient point was about 0.15 mm. After 3 min of plasma-deburring, it was found that plasma-deburring had an actual effect on the burrs, and the maximum burr height was removed to a large extent as shown in Fig. 2(c) and (d) . The top of the burrs were almost stripped. To further remove the burrs, the process was extended for three more minutes. It was interesting to note that the burrs were almost removed from the surface (Fig. 2(e) , (f)), and the peak H of the remaining burrs was found to be 0.04 mm. It showed a considerable reduction in the peak burr height from 3.23 mm to 0.04 mm after 6 min of plasma-deburring. Fig.2 Optical micrographs of Q235 carbon steels specimens: (a, b) before plasma-deburring (c, d) after 3 min plasma-deburring, and (e, f) 6 min plasma-deburring
Phase composition and morphologies of treated sample surface
The X-ray diffraction spectra of the surface layer after 3 min and 6 min of plasma-deburring treatment are plotted in Fig. 3 . It shows the appearance of Al 2 O 3 , Fe 3 O 4 and Fe 2 O 3 in the coating after 6 min plasmadeburring treatment. However, the Al 2 O 3 and Fe 2 O 3 phases are not detected in the pattern of the 3 min sample. In addition, the diffraction spectra show the possible presence of iron diffraction peaks. It may be explained by that the oxide layer is relatively thin at the first 3 min. The presence of iron and aluminum oxides in the deposited layers could be explained by the decomposition of electrolyte components on the surface of oxidized samples, and by anodic oxidation of the iron within the sample. Fig. 4 shows SEM images for plasma-deburring samples coated by Al 2 O 3 , and iron and iron oxides. The surface morphology of the sample treated with 6 min is similar to the surface morphology of valve metal treated by PEO (Fig. 4(a) ). There were many micro-pores on the coating surface which were just like volcanic craters. There were also many irregularly shaped sintered belts piled on the sample surface, which were formed by the quenching effect of the electrolyte during the discharge reaction. The thickness of the remaining layer was from 3 µm to 5 µm. While in Fig. 4(b) , several pits formed after fusion or stripping of oxide are observed clearly on the sample surface, which were obviously different from volcanic craters or corrosion spots on the surface morphology of the valve metal treated by PEO. These pits showed the 'footprints' of the oxide stripping after the burr oxidation. The average surface roughness of 1.543 µm, slightly higher than that of the valve metal oxide layer (0.75-1.5 µm) [26] , was detected by a TR200 roughness tester (Shenzhen, China). The higher surface roughness was attributed to the oxide stripping, which had apparently caused larger pits on the specimen surface.
Phase composition and morphologies of treated sample surface
3.3 Corrosion resistant properties of treated sample surface Fig.5 Polarization curves of the untreated and treated Q235 carbon steel in 3.5% NaCl Fig. 5 shows the polarization curves of the samples before and after plasma-deburring in 3.5% NaCl. The polarization curve of the treated sample shows a shift towards the region of lower current density and the higher potential compared to those of the untreated sample. The values of corrosion potential and corrosion current density for the untreated sample were −0.667 V and 6.735×10 −5 A/cm 2 , respectively. For the treated sample, they were correspondingly −0.354 V and 1.19× 10 −7 A/cm 2 . The corrosion resistance of the steel surface was significantly improved by a shift of corrosion potential in the noble direction, more than 300 mV, and the corrosion current density was also decreased by two orders of magnitude. The better corrosion resistance of the treated steels might be attributed to the chemical stability of Al 2 O 3 , and iron oxides coating which acts as a barrier between the corrosion environment and the substrate.
Possible mechanism of plasmadeburring
Fig . 6 shows the treatment process in a dynamic system where the electrolyte was supplied at a velocity of 3-5 L/min. At the first step, the high potential difference between the electrodes results in the accumulation of ions and gas, typically O 2 ( Fig. 6(a) ). They are always present in the electrolyte, in close vicinity to the anode, chiefly on the surfaces of gas bubbles. As a result, the high electric field intensity is strongly localized between the cathode and the working surface. The next stage is the PEO process ( Fig. 6(b) ). During this process, the electric field intensity of the plasma layer can be as high as 105 V/m or higher [27, 28] . When such a high electric field is reached, the gas space inside bubbles will be ionized, and the plasma discharge will be initiated on the surface of the workpiece, especially near the burr due to the point effects [29] . Thus the oxide layer would be formed on the tip of the burr preferentially. While during the next stage (Fig. 6(c) ), the plasma temperature can be as high as 2000 o C and the duration of plasma discharge is about 10 −6 s [27] , thus hot bubbles are surrounded with a relatively cool electrolyte (about 100 o C). It causes the quick cooling of plasma [30] . As a result, bubbles are burst on the working surface, and then the kinetic energy which like the impact wave is transferred to the liquid layer as well as the working surface, which results in the oxide being stripped from the working surface ( Fig. 6(d), (e) ). 
Conclusions
In this paper, we investigated the deburring process for the low carbon steel by plasma electrolytic oxidation. It was found that in the high frequency and narrow pulse width electrochemical environment, thermal electron emission could be induced preferentially near the salient point due to the uneven distribution of electron flux on the anode surface. It caused the oxygen molecule at the liquid-solid interface of the salient point to ionize preferentially after collision, and then oxygen plasma would be formed. The burrs on the steel could be removed successfully. The corrosion resistance of the steel surface was significantly improved due to the chemical stability of the Al 2 O 3 and iron oxides coating. In view of the above, PEO might be an effective method to pave the way for high-speed, high-automation and high-reliability machining steel and other alloys. 

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