Stray Corrosion Research Articles

A simple method for insulating sidewalls of the cathode tool to enhance precision in ECDM

Conventional micro-turning of metal materials with low rigidity and hardness is highly challenging. Electrochemical Discharge Machining (ECDM) offers an alternative approach for fabricating micro shafts. However, micro-turning by ECDM often leads to severe flaring at the shaft shoulder due to stray corrosion. To mitigate this issue, we propose two novel methods for insulating the sidewalls of the cathode tool: applying insulating tape to the sides of a rectangular cathode tool or attaching aluminum foil along the bottom edge of a rectangular acrylic plate. We evaluated the machining performance of these cathode tools on the 3 mm diameter copper rods. Experimental results demonstrate that using an acrylic plate with attached aluminum foil as the cathode tool significantly enhances machining accuracy in the transition area of the shaft shoulder. Additionally, the simplicity and reliability of this method highlight its advantages.

Titanium Alloy Ti-6Al-4V Electrochemical Dissolution Behavior in NaNO3 and NaCl Solutions at Low Current Density

Jet electrochemical micromilling (JEMM) exhibits significant potential for high-efficiency and high-quality machining of titanium alloy microstructures. However, during the JEMM process, the machined surface of the workpiece inevitably experiences stray current attacks at low current levels. Due to the formation of a dense passive film on the surface of the titanium alloy under electrochemical action, stray corrosion occurs on the machined surface. Hence, the electrochemical dissolution behavior of titanium alloys at low current densities directly impacts both machining efficiency and quality. This study first analyzed the effects of electrolyte composition and current density on the transpassive potential, breakdown of the passive film, current efficiency, and the dissolved surface on Ti-6Al-4V. The transpassive potential and electrochemical impedance of Ti-6Al-4V were found to be lower in NaCl solution than in NaNO3 solution. In addition, lower current densities enabled higher current efficiency and resulted in a more uniform and flat dissolution surface. Subsequent experiments used these two solutions for JEMM of complex-shaped microstructures on Ti-6Al-4V. The findings demonstrated that, compared to the NaNO3 solution, the use of NaCl solution increases the material removal rate by approximately 30%, enhances the aspect ratio by about 26%, and reduces surface roughness by roughly 58%. This indicates that employing NaCl solution can lead to superior machining efficiency and quality.

Electrochemical Properties and Jet Electrochemical Micromilling of (TiB+TiC)/Ti6Al4V Composites in NaCl+NaNO3 Mixed Electrolyte.

Difficult-to-cut titanium matrix composites (TiB+TiC)/Ti6Al4V have extensive application prospects in the fields of biomedical and aerospace metal microcomponents due to their excellent mechanical properties. Jet electrochemical micromilling (JEMM) technology is an ideal method for machining microstructures that leverages the principle of electrochemical anodic dissolution. However, the matrix Ti6Al4V is susceptible to passivation during electrochemical milling, and the inclusion of high-strength TiB whiskers and TiC particles as reinforcing phases further increases the machining difficulty of (TiB+TiC)/Ti6Al4V. In this study, a novel approach using NaCl+NaNO3 mixed electrolyte for the JEMM of (TiB+TiC)/Ti6Al4V was adopted. Electrochemical behaviors were measured in NaCl and NaCl+NaNO3 electrolytes. In the mixed electrolyte, a higher transpassive potential was required to break down the passive film, which led to better corrosion resistance of (TiB+TiC)/Ti6Al4V, and the exposed reinforcing phases on the dissolved surface were significantly reduced. The results of the JEMM machining indicate that, compared to NaCl electrolyte, using mixed electrolyte effectively mitigates stray corrosion at the edges of micro-grooves and markedly improves the uniformity of both groove depth and width dimensions. Additionally, the surface quality was noticeably improved, with a reduction in Ra from 2.84 μm to 1.03 μm and in Rq from 3.41 μm to 1.40 μm.

Towards electrochemical machining of powder superalloy René 88DT with high surface integrity in different solvent-based electrolytes

Electrochemical machining (ECM) presents numerous applications in the production of aero-engine components. The effects of current density on surface quality during ECM of René 88DT in NaCl-ethylene glycol and NaNO3-aqueous solutions was investigated here. The results indicated that the René 88DT surface has lower sensitivity to current density and more homogeneous electrochemical dissolution in NaCl-glycol solution than in NaNO3-aqueous solution, which results in the suppression of stray corrosion and consistently high surface quality regardless of current density. These strengths are attributed that C2H5O2- and Cl- ions replace OH– ions and combine with alloy ions to form solubles and complexes, which prevent the formation of obstructive passive film, oxide layer, and insoluble electrolytic products. The electrochemical dissolution model in two solutions was established. Furthermore, the high tensile strength and precision machinability of René 88DT in NaCl-glycol solution were also demonstrated.

Stray current affected zone in electrochemical jet machining

The stray current affected zone (SCAZ) in electrochemical jet machining (EJM) poses a significant and often unavoidable challenge. This study offers an in-depth analysis of the morphological characteristics and formation mechanisms of SCAZ in EJM through a combination of machining experiments and surface characterization. By investigating various workpiece materials and electrolytes, we identify that stray corrosion phenomena are highly dependent on the specific material-electrolyte combinations used. Although applying ultra-short pulses of 750 ns enhances shape accuracy, it does not fully prevent pitting corrosion. Conversely, the bipolar pulse technique effectively reduces pitting, though stray oxidation remains an issue. As a result, post-finishing processes are crucial. Notably, plasma electrolytic polishing is demonstrated to be highly effective in removing stray oxidation, achieving a glossy surface on SUS304 in just 3 s while maintaining dimensional accuracy.

Exploring Electrochemical Direct Writing Machining of Patterned Microstructures on Zr702 with Polyacrylamide Polymer Electrolyte.

Zirconium alloys possess excellent wear resistance, which ensures the durability and longevity of the components, making them widely used in medical and other fields. To enhance the functionality of these materials, it is often necessary to fabricate functional microstructures on their surfaces. Electrochemical machining (ECM) techniques demonstrate excellent machining performance for these metals, particularly in the processing of microstructures on complex curved surfaces. However, ECM often faces challenges due to the fluid nature of the electrolyte, resulting in low machining accuracy and localization. This paper proposes a novel method for fabricating complex patterned microstructures using a maskless electrochemical direct writing technique with a polyacrylamide (PAM) polymer electrolyte. By leveraging the non-Newtonian properties of PAM, this method effectively confines the electrolyte to specific areas, thus addressing the issue of poor localization in traditional ECM and reducing stray corrosion. To elucidate the electrochemical removal mechanism of Zr702 in the presence of PAM, polarization curves, viscosity characteristics, and current efficiency parameters were analyzed. Additionally, an experimental study was conducted using a custom-designed nozzle structure. The results showed that the PAM electrolyte could effectively reduce the EF, positively impacting machining accuracy and localization. By controlling the nozzle's motion trajectory, complex microstructures were successfully fabricated through direct writing, demonstrating promising application prospects.

Advancing the process understanding and localization in jet electrochemical machining with electrolyte confinement through fluid dynamic simulations and experiments

Functionality-oriented micro-structures, such as micro-dimples and grooves, are widely used in tribology and heat transfer. Jet electrochemical machining (JEM) is effective to fabricate micro-structures on metallic surfaces. However, in traditional JEM, the unrestricted electrolyte flowing can induce stray corrosion on workpiece, and thus, both surface quality and machining localization are reduced. In this paper, a novel electrolyte-confinement technique is proposed for JEM, a high-density liquid (perfluorotripropylamine, FTPA) is used to confine the electrolyte flowing region on workpiece when electrolyte exits nozzle, facilitating reduction in stray corrosion on workpiece and overcut of micro-structures. A multi-physics model including two-phase flow field and electric field is developed to analyze the electrolyte confined by FTPA, and both simulation and observation results show that the area of electrolyte flowing on the workpiece is confined well by FTPA, and the current density distribution becomes concentrated, which enhances the machining localization. Compared to traditional JEM, the etch factor of micro-dimple is improved by 2.5 times and there is no stray corrosion. The material removal rate is increased due to the concentration of current distribution on the workpiece surface. Furthermore, profile evolution of micro-dimples revealed that with feed depth increased, FTPA could flow into the micro-dimple to protect the sidewall from continuous dissolution, thus forming vertical sidewall. Additionally, electrolyte flowing region is still confined during the scanning motion of nozzle, and the etch factor increases from 0.41 to 8.8 compared to traditional JEM. Moreover, increasing inter-electrode gap could reduce electrolyte flowing region on workpiece, further enhancing machining localization.

Electrochemical machining of titanium alloy blades using optimized cathode with built-in insulators

ABSTRACT The passivation characteristics of titanium alloys in electrolytes are often the main reason that impede the electrochemical reaction process. Hence, maintaining uniform electrochemical dissolution is a prerequisite for stable electrochemical machining (ECM) of titanium alloys. In this study, a novel cathode structure with built-in insulator is proposed, whichimproves machining stability of titanium alloys in ECM. Flow dynamics simulation results show that the cathodes with built-in insulator 1 or insulator 3 demonstrate more uniform flow effect. ECM experiments reveal that the optimized cathode can reduce the time of current from transition section to balance section, presenting a optimal transition time of 180 s. Compared with the original cathode, the sample surface machined using the optimized cathode is smooth without defects. Additionally, the optimal width of the non-processing stray corrosion area decreases from 603.47 μm to 214.54 μm, and 1010.47 μm to 349.25 μm. The optimal machining taper angle is 9.85°.

Experimental Research of Ultrasonic Cavitation Evolution Mechanism and Model Optimization of RUREMM on Cylindrical Surface

Micro-pits are widely used in the aerospace and tribology sectors on cylindrical surfaces and electrochemical micromachining which are of great significance for the high material removal rate, absence of tool wear, and mechanical stress, while facing significant challenges such as stray corrosion and low machining efficiency. Aiming at the above problems, this paper proposes a comprehensive method called radial ultrasonic rolling electrochemical micromachining (RUREMM) in which an ultrasonic field has been added onto the cylindrical surface. First, a theoretical model was created to gain the rules of the formation and collapse of bubbles in the liquid medium. Second, to analyze the optimal size of the cathode electrode, the COMSOL5.2 simulation software was proposed to research the influence of the electric field on the different dimensions, and the influences of different parameters in RUREMM on material depth/diameter ratio and roughness are explored through processing experiments. Research results found that the cavitation bubble undergoes expansion, compression, collapse and oscillation, where the max deviation is less than 12.5%. The optimized size was chosen as 200 × 200 μm2 and an electrode spacing of 800 μm through a series of electric field model simulation analyses. Relevant experiments show that the minimum pits with a width of 212.4 μm, a depth of 21.8 μm, and a surface roughness (Ra) of 0.253 μm were formed due to the optimized parameters. The research results can offer theoretical references for fabricating micro-pits with enhanced surface quality and processing precision on cylindrical surfaces.

An Investigation of the Efficient-Precise Continuous Electrochemical Grinding Process of Ti-6Al-4V.

Titanium alloys have many excellent characteristics, and they are widely used in aerospace, biomedicine, and precision engineering. Meanwhile, titanium alloys are difficult to machine and passivate readily. Electrochemical grinding (ECG) is an ideal technology for the efficient-precise machining of titanium alloys. In the ECG process of titanium alloys, the common approach of applying high voltage and active electrolytes to achieve high efficiency of material removal will lead to serious stray corrosion, and the time utilized for the subsequent finishing will be extended greatly. Therefore, the application of ECG in the field of high efficiency and precision machining of titanium alloys is limited. In order to address the aforementioned issues, the present study proposed an efficient-precise continuous ECG (E-P-C-ECG) process for Ti-6Al-4V applying high-pulsed voltage with an optimized duty cycle and low DC voltage in the efficient ECG stage and precise ECG stage, respectively, without changing the grinding wheel. According to the result of the passivation properties tests, the ideal electrolyte was selected. Optimization of the process parameters was implemented experimentally to improve the processing efficiency and precision of ECG of Ti-6Al-4V. Utilizing the process advantages of the proposed process, a thin-walled structure of Ti-6Al-4V was obtained with high efficiency and precision. Compared to the conventional mechanical grinding process, the compressive residual stress of the machined surface and the processing time were reduced by 90.5% and 63.3% respectively, and both the surface roughness and tool wear were obviously improved.

Boundary fluid constraints during electrochemical jet machining of large size emerging titanium alloy aerospace parts in gas–liquid flows: Experimental and numerical simulation

Large size titanium alloy parts are widely used in aerospace. However, they are difficult to manufacture using mechanical cutting technology because of severe tool wear. Electrochemical jet machining is a promising technology to achieve high efficiency, because it has high machining flexibility and no machining tool wear. However, reports on the macro electrochemical jet machining of large size titanium alloy parts are very scarce, because it is difficult to achieve effective constraint of the flow field in macro electrochemical jet machining. In addition, titanium alloy is very sensitive to fluctuation of the flow field, and a turbulent flow field would lead to serious stray corrosion. This paper reports a series of investigations of the electrochemical jet machining of titanium alloy parts. Based on the flow analysis and experiments, the machining flow field was effectively constrained. TB6 titanium alloy part with a perimeter of one meter was machined. The machined surface was smooth with no obvious machining defects. The machining process was particularly stable with no obvious spark discharge. The research provides a reference for the application of electrochemical jet machining technology to achieve large allowance material removal in the machining of large titanium alloy parts.

Jet Electrochemical Micromilling of Ti-6Al-4V Using NaCl-Ethylene Glycol Electrolyte.

Titanium alloys are widely used in aerospace and biomedicine because of their excellent mechanical characteristics, but these properties also make such alloys difficult to cut. Jet electrochemical micromilling (JEMM) is based on the principle of electrochemical anodic dissolution; it has some inherent advantages for the machining of titanium alloy microstructures. However, titanium oxidizes readily, forming an oxide film that impedes a uniform dissolution during electrochemical machining. Therefore, a high voltage and an aqueous NaCl electrolyte are usually used to break the oxide film, which can lead to severe stray corrosion. To overcome this problem, the present study investigated the JEMM of Ti-6Al-4V using a NaCl-ethylene glycol (NaCl-EG) electrolyte. Electrochemical testing showed that Ti-6Al-4V exhibits a better corrosion resistance in the NaCl-EG electrolyte compared to the aqueous NaCl electrolyte, thereby reducing stray corrosion. The localization and surface quality of the grooves were enhanced significantly when using JEMM with a NaCl-EG electrolyte. A multiple-pass strategy was adopted during JEMM to improve the aspect ratio, and the effects of the feed depth and number of passes on the multiple-pass machining performance were investigated. Ultimately, a square annular microstructure with a high geometric dimensional consistency and a smooth surface was obtained via JEMM with multiple passes using the optimal parameters.

Fabrication of Dimples by Jet-ECM of Zr-Based Bulk Metallic Glasses with NaCl-Ethylene Glycol Electrolyte.

Zr-based bulk metallic glasses (BMGs) possess unique mechanical and biochemical properties, which have been widely noticed. Jet electrochemical machining (jet-ECM), characterized by a high-speed jet, is a non-contact subtractive method with a high resolution and a high material removal rate (MRR). Past work on the electropolishing of Zr-based BMGs has indicated the feasibility of the NaCl-Ethylene Glycol (EG) electrolyte. In this research, the jet-ECM of Zr-based BMGs in the NaCl-EG electrolyte was studied to explore the dissolving mechanisms and surface integrity according to the voltage, pulse-on time and effective voltage time. The diameter, depth and surface morphologies of dimples were evaluated. The results showed that using this alcohol-based electrolyte led to a desirable surface morphology. The diameter and depth of the dimples varied with the voltage and the effective voltage time in a significantly positive proportional manner. Additionally, cases based on multiple parameter sets exhibited different stray corrosion severity. Afterward, machining performance can be enhanced in the next stage by tuning machining parameters to obtain microscale dimples with better quality.

Control of corner arc radius in wire electrochemical micromachining

Wire electrochemical micromachining (WECMM) technology is commonly used to process micro parts. Some thick parts contain many sharp structures, and have high machining accuracy requirements for the arc radius at the corner, such as jet sheet, micro gear, etc. However, when machining thick workpiece, it is easy to form a large arc radius at the corner due to stray corrosion and difficult discharge of electrolytic products. In order to solve this problem, this paper proposes a corner path optimization method, which reduces the arc radius of the sharp structure by adjusting the processing path of the wire electrode, thereby improving the machining accuracy of the sharp structures. The model of the relationship between the inclination of the machining path and the angle of the sharp structure is established. The inclination and offset distance of the processing path are obtained through theoretical calculation, and further optimized through experiments to avoid overcut or undercut at the corner. In the experiment, tungsten wires with different diameters were used as wire electrodes to process acute angle, right angle and obtuse angle structures on 510 μm thick 9Cr18Mo stainless steel. The influence of wire electrode processing path on the corner arc radius of different angle sharp structures was explored. The experimental results show that compared with the original path, the arc radius of the sharp structure could be reduced from tens of microns to <5 μm by using the optimized processing path.

Fabrication of high-quality surface and stray corrosion suppression mechanism with magnetic field assistance electrochemical micro-machining

Magnetic field assistance electrochemical micro-machining strategy was used to produce 304 stainless steel with a high-precision surface. The surface roughness was measured and the micro-morphology was observed. The results showed that the optimal surface morphology with a roughness of about 0.30 μm was obtained after magnetic field assistance electrochemical micro-machining for 10 min. The magnetic field assistance electrochemical micro-machining produced a lower surface roughness than that of electrochemical micro-machining at the same processing time, indicating that magnetic field assistance was beneficial to improving the machining efficiency. The surface microstructures proved that the magnetic field assistance electrochemical micro-machining provided a relatively flat and smooth surface. The stray corrosion was significantly reduced. The generation of the electromagnetic force regulated the charge transfer in processing and affected the behavior of the anode dissolution reactions. The physical models describing the mechanism of stray corrosion suppression were established.

A sustainable, high-quality method for machining film cooling holes: Cryogenic airflow-assisted electrochemical discharge drilling

Film cooling hole is an essential part of aero-engines, necessitating without recast layer, no stray corrosion, and high machining efficiency. Its sustainable manufacturing has been at the forefront of industrial focus. Existing machining methods have problems such as low efficiency and environmental hazards. In order to achieve the production requirements of precision, quality, and green simultaneously, an external cryogenic airflow-assisted electrochemical discharge drilling (ECDD) is proposed. Compared with the current electrochemical drilling (ECD) or electrical discharge drilling (EDD), a glycol working fluid with ultra-low conductivity neutralizing salt replaces the high conductivity of acidic solution or spark oil to realize green machining without a recast layer. Compared to existing ECDD, the introduction of cryogenic gas with glycol can significantly reduce stray corrosion and achieve exceptionally high surface quality. In the theoretical part, the influence mechanism of a low-temperature environment on the electrochemical interface was analyzed by using the electrochemical analysis technique. In addition, a multi-physical field simulation model was established, which was finally verified by experiments. Finally, the surface characteristics were analyzed by using TEM. The results demonstrate that the cryogenic environment enhances the passivation effect, as evidenced by a 35% reduction in taper at 258 K and a 28.2% decrease in stray corrosion. The hole side has nano-roughness (Ra 0.512 μm). Notably, the material removal rate does not significantly diminish at low temperatures and achieves 0.1 mm/s, superior to EDD. As there is no need for an additional machining step after drilling, this method proves its characteristic to attain sustainability, efficiency, and high quality, indicating promising application prospects.

In situ X-ray imaging of electrochemical dissolution behavior at low current density of as-built additively manufactured AlSi10Mg alloy

Despite extensive research on electrochemical machining (ECM) for improving the surface quality of additively manufactured (AMed) metals, common problems of stray corrosion and uneven dissolution remain. Here, in-situ synchrotron X-ray imaging was firstly used for observing the dissolution behavior of as-built AlSi10Mg alloy at low current density. The bubble evolution, surface roughness and dissolved depth was investigated. The results demonstrated that surfaces with higher roughness exhibited a greater dissolution rate and consumed a larger proportion of electrons compared to smooth surfaces. The study also identified three distinct stages of surface roughness change at low current density, providing valuable insights for the development of ECM techniques for AMed parts.

Investigation of the stray corrosion of Inconel 718 at low current density in NaNO3 solution

Investigation of the stray corrosion of Inconel 718 at low current density in NaNO3 solution

A Shunt-Assisted Silicon Electrode for Micro Electrochemical Machining

Abstract Stray current causes undesired material dissolution in micro-electrochemical machining (micro-ECM). The reduction of stray corrosion, caused by stray current, continues to be a major challenge for accuracy improvement. To limit the distribution of stray current, a shunt-assisted silicon electrode, with an auxiliary anode sharing stray current, is proposed in this study. The auxiliary anode is arranged outside the insulating layer of the sidewall-insulated electrode. It is proved in simulation that the auxiliary anode can help reduce the average material removal rate on the machined surface by 55% and improve processing accuracy. A fabrication process of shunt-assisted silicon electrode by bulk silicon process and thin film deposition process are presented. Microgrooves and holes are machined in ECM experiments. The angle between each side-wall and the vertical plane is less than 10 deg. The gap between the sidewall of the machined structures and electrode-outer-contour is about 30 μm±6 μm for the grooves and 45 μm±10 μm for the holes. These long-term experiments and consistent processing results show the shunt-assisted electrode is reliable in ECM process. But due to the stray corrosion induced by DC power supply and conservative feed method, the effect of the shunt-assisted silicon electrode in inhibiting stray corrosion is not significant. In the future, a micro-ECM system with novel power supply and active control methodologies is expected to better utilize the effect of the shunt-assisted silicon electrode.

Electrochemical machining of blades by using tapered cathode sheet with micro-grooves structure

The combined electrode with cathode body and cathode sheet is widely used in the electrochemical machining of ruled surface parts. Due to the thickness of the cathode sheet, however, stray corrosion of the side wall of the cathode sheet will affect the machining accuracy and surface quality of the workpiece. In order to suppress stray corrosion, a tapered cathode sheet with a micro-grooves structure is proposed in this paper, which reduces the intensity of the stray electric field by increasing the distance between the side wall of the cathode sheet and the workpiece. According to the electric field distribution, the current density on the surface of the machined anode workpiece decreases as the cathode taper angle increases. In addition, to enhance the liquid-phase mass transfer, a micro-grooves structure was fabricated in the side wall of the tapered cathode sheet. Through the numerical simulation analysis of the flow field, the micro-grooves structure reduces the flow resistance of the electrolyte and suppresses stray corrosion of the dispersed electrolyte. Finally, the experimental results show that the ruled surface blade with a machining taper of 0.93° and blade surface roughness of 0.566 μm can be successfully obtained using a 50° tapered cathode sheet with a micro-grooves structure. This study provides an effective process that can be used for the ECM of ruled surface parts with high precision and good quality, and in the future this process can be extended to machining space complex surface parts.