What Is Electrochemical Machining?
Electrochemical machining is a method of machining by passing an electric current through a workpiece in an electrolytic solution to cause a chemical reaction.
It is also called ECM. When an electric current flows between the electrode (negative electrode) and the workpiece (positive electrode), the metal of the workpiece on the positive electrode turns into metal ions, which dissolve into the electrolytic solution.
The dissolved metal ions also produce other products (sludge, etc.) in the electrolyte. Hydrogen gas is produced at the anode. Using this series of electrochemical reactions, it is possible to transfer the shape of the electrode to the workpiece by machining the tool electrode very close to the surface of the workpiece.
Electrolytic machining is used to process parts that are difficult to machine, such as turbine blades in aero engines, and parts such as automobile engine parts, where burrs are not desired.
Uses of Electrochemical Machining
1. Ordinary Electrochemical Machining (ECM)
- Aircraft parts (stainless steel, nickel-based superalloys, titanium alloys)
- Postprocessing of metal additive components
- Medical implants, including orthopedic and some cardiovascular devices
- Mold processing for mint
- Parts for automobile engines (high carbon chromium steel)
- Parts for diesel pumps
- Semiconductor and MEMUS manufacturing
- High-load bevel and strain wave gear profiles
2. Precision Electrochemical Machining Using a Pulsed Power Supply (PECM)
- Aircraft parts (stainless steel, nickel-based superalloys, titanium alloys)
- Medical parts
- Mold processing for mint
- Parts for automobile engines (high carbon chromium steel)
- Parts for diesel pumps
- Semiconductor and MEMUS manufacturing
Principle of Electrochemical Machining
Electrochemical machining can be performed in two ways, depending on the accuracy of the process.
1. Ordinary Electrochemical Machining (ECM)
This is a method of electrochemical machining in which the workpiece metal is the positive electrode and the tool electrode is the negative electrode, and the distance between the electrodes is kept at a very close distance (usually 0.02 to 0.7 mm) in the electrolytic solution. The processing speed is extremely fast, taking only a few 10 seconds.
The process can be applied to hard metals that are difficult to process in the past. Also, complex shapes and holes that have been difficult to cut can be machined with high precision.
2. Precision Electrochemical Machining Using a Pulsed Power Supply (PECM)
Precision electrochemical machining (PECM), which combines the flushing action of the tool electrode with a pulsed power supply, has been put to practical use for even higher precision machining.
In this method, the electrode or workpiece is vibrated up and down at high speed when the electrode is brought close to the workpiece, and products such as sludge are ejected at high speed and a fresh electrolytic solution is constantly supplied. In this process, the distance between the workpiece and the electrode is precisely controlled at around several tens of microns by a pulse power supply.
Characteristics of Electrochemical Machining
1. Complex Shapes Can Be Processed
Electrochemical machining is capable of machining complex contours and cavities that are difficult to achieve with other machining methods, such as thin-walled features or downskin surfaces.
2. High Machining Speed
Difficult-to-cut materials can be machined much faster than with other machining methods. Furthermore, the customization of the tooling allows PECM to add tool surface area and machine multiple features in parallel in a way that conventional machining cannot achieve.
3. Non-contact Machining
Non-contact machining has the advantage of not applying mechanical or thermal stress to the workpiece, allowing tight-tolerance machining to areas sensitive to thermal distortion or tool vibration, such as thin walls.
4. Mirror Finish and Selective Surface Finishing
Because electrochemical machining is a chemical reaction process, it is possible to achieve a mirror finish on the surface, down to less than 0.5 μm Ra, and even down to 0.1μm Ra. Furthermore, on components that may require varied surface quality in different areas, PECM enables selective finishing in one singular machining process.
5. Possible to Process Hard and Brittle Materials
Brittle materials that are difficult to machine can be processed. Brittle materials are hard and brittle materials such as cemented carbide.
- Certain electrically conductive ceramics, including SiC
- Some metal matrix composites (MMCs) that include ceramics can be machined
- Single crystal silicon (only if highly doped)
- Zirconium
Other Information on Electrochemical Machining
1. Disadvantages of Electrochemical Machining
The disadvantage of electrochemical machining is that the electrolytic solution used for electrochemical machining is difficult to maintain uniformity, so the machining accuracy is unstable. It is also difficult to detect the distance between the workpiece and the electrode, and furthermore, the equipment needs to be treated for corrosion resistance. In addition, the product may be toxic.
Furthermore, compared to other conventional process, PECM has higher NRE costs, as developing the ideal cathode (tool) is a complex engineering challenge and therefore requires multiple iterations of part design and testing. This high initial investment, however, is outweighed if the project requires high part volumes.
2. Electrolyte in Electrochemical Machining
Electrochemical machining is performed by using an electrolytic solution, which not only conducts electricity but also cools the workpiece. Additionally, the electrolytic fluid functions as the flushing agent that removes waste material from the workpiece.If the amount of electrolyte is too small, the finished product may vary, while too much electrolyte may cause discoloration. It is necessary to maintain an appropriate amount of liquid for processing.
Three types of electrolytic solutions are used: neutral salt solution, acid solution, and alkali solution.
Neutral Salt Solution
This is the most commonly used electrolyte. It has lower conductivity than alkaline solutions, but is less corrosive and can be used for most metallic materials. Sodium chloride electrolyte is commonly used. This electrochemical machining accuracy can occur because it is difficult to achieve a uniform concentration in the solution.
In some cases, sodium chlorate or sodium nitrate is used as the electrolyte. However, although processing accuracy improves, current efficiency decreases, resulting in increased power consumption.
Acid Solution
Acid solutions are electrolytes that require careful handling and are used in special cases. It has relatively high conductivity, but it is highly corrosive and conductivity decreases after prolonged use.
Alkali Solution
Alkaline solution is an electrochemical machining solution for cemented carbide. It is used for processing tungsten and molybdenum. For cemented carbides, it provides a better quality finish than neutral salt solutions.
It is not used for common metals because insoluble products are generated during machining, preventing the elution of the workpiece.
Citations
https://www.voxelinnovations.com/post/pecm-and-next-gen-manufacturing
https://www.voxelinnovations.com/post/ecm-surface-quality
https://ssw-americas.com/electrochemistry-as-machining-pecm-and-its-applications/