A Guide To Electrochemical Machining, Part 1

Manufacturing trends signal rising demand for parts produced in tougher metals and machined with finer precision. Designers and engineers for industries like aerospace and medical devices have ambitious plans for their next groundbreaking development, like compact rocket engines and complex medical implants. However, limitations in the typical manufacturing processes may restrain their progress.

Standard machining processes, such as CNC milling and electrical discharge machining (EDM), offer various advantages but may include important drawbacks. For example, manufacturing time and cost can be greatly affected by the hardness or toughness of the workpiece material and the durability of the cutting tool in a machining operation. Components that require high-accuracy machining of titanium and stainless steel may present difficulties when the direct contact, heat, and wearing of the tool reduces the precision of machining. EDM achieves impressive precision in challenging materials but it also involves slow production speeds and electrode wear, which limit its economic viability in high-volume programs.

For scalable manufacturing projects requiring tough alloys to be machined at high precision — electrochemical machining (ECM) is an ideal, but often overlooked, solution.

ECM has four primary features:
-  The tool (also called the electrode or cathode);
-  The workpiece (also called the anode);
-  The conductive fluid (also called the electrolyte); and,
-  The power supply.

The cathode tool is designed to be approximately the inverse of the intended shape or pattern to be machined on the anode. This means it is specific and matched to a part model, and therefore better suited to volume production. 

While each cathode is machined to be unique to the project, it can a have very long service life because there is no direct contact with the workpiece, as well as because of the absence of any thermal erosion.

The anode (workpiece) can theoretically be any conductive metal, but ECM’s value proposition is best for tough-to-machine metals such as iron, nickel, and chrome-based alloys, as well as more exotic materials like nitinol, titanium aluminide, or high-entropy alloys. The anode could be a raw plate or bar stock but ECM is often used on cast, rough machined, stamped, or otherwise near-net-shape part.

The electrolyte is used both as a conductive agent for the electrochemical reaction to happen, as well as a flushing solution to remove the waste products of dissolved metal, some heat produced while current flows, and gases produced due to electrochemical reactions at the cathode and anode surface. This fluid is continually pumped between the gap of the cathode and anode to maintain a stable process.

The power supply provides the current necessary for the electrochemical reactions and is directly related to the speed of the process (i.e., more current = faster material removal rate.)

ECM is a low voltage (5-50V), high current (25-150 A/cm² or 160-970 A/in²) process. Traditional power supplies apply a continuous DC current while more advanced, “pulsed” processes provide pulses of power.

ECM offers many unique benefits that are not present in other manufacturing methods. For example:

Non-contact machining. The cathode and anode are separated by a microscopic space, also known as the interelectrode gap (IEG). The size of this gap is indicative of the precision of the ECM process — a smaller gap around 10-100 μm can achieve higher feature resolutions and accuracy. A primary benefit of no-contact machining is how the cathode remains unaffected by tool wear throughout the process. Not only does this reduce tool replacement costs but the repeatability of the ECM process remains high, as there are no dimensional changes to the cathode tool.

Pristine surface finishes. Because there is no direct contact or heat generation, the resulting part displays a smooth, burr-free finish. ECM research has demonstrated surface finishes of 0.005 - 0.4 μm R_a — saving manufacturers the time of implementing any secondary finishing processes. The lack of heat generation also means the surfaces are the same as the bulk material – no recast layer or heat affected zones. The lack of thermal stress also can enable thin-walled features, down to 10-100 micrometers in thickness.

Tough materials. Unlike traditional machining processes, mechanical properties such as hardness are an irrelevant factor in electrochemical machining. ECM cuts through tough alloys such as Inconel at an equivalent rate as for aluminum. So long as the alloy is conductive, ECM can machine with similar speeds across a range of materials.

ECM’s speed is dependent on the geometry of the desired workpiece, and the ability to flow electrolyte fluid through the part’s features. Sinking rates can vary from <1 μm/s for blind, high aspect ratio, difficult-to-reach features up to 30+ μm/s when sufficient flushing can occur. A unique attribute of ECM is its ability to process multiple features, surfaces, or parts simultaneously. If the electrode tool doubles in size, the sinking speed can stay the same, which leads to double the volumetric removal rate (and double the amperage and electrolyte flow).

While the idea of using electrolytic reactions for manufacturing had been discussed since at least 1920, thanks to the Russian chemist Evgeny Shpitalsky, it was not until 1959 that ECM became a commercialized process. The Anocut Engineering Co., Downers Grove, Ill., implemented ECM as a new way to shape high-strength alloys.

As the need for aerospace innovations skyrocketed thanks to commercial air travel and the space race, manufacturers needed to research new materials and new machining methods to create aerospace parts that matched new expectations for lightweight, high-durability and highly temperature-resistant components.

To this day, however, much of the innovation relating to the ECM process has been centered in Europe, and only a handful of North American companies are aware of electrochemical machining possibilities, much less providing services in this field.

Achieving high performance with ECM takes expertise, and given the lack of U.S. service providers, CNC machining or other processes have displaced some legacy ECM operations. But with new ECM suppliers and consistent innovations during the past few decades, including the development of pulsed ECM (PECM), it has become an increasingly viable option for domestic manufacturers across multiple industries to consider this production technology option.

The next installment of this series will explore practical applications for electrochemical machining, including for aerospace technology and medical device manufacturing.

Kirk Gino Abolafia is the Technical Marketing Manager for Voxel Innovations, Inc., Raleigh, N.C.