Pulsed Electrochemical Machining: A Potential Solution to Advanced Material Challenges

Recent advancements in material science and design engineering have helped facilitate a significant increase in the popularity of advanced metal alloys, notably within critical industries such as aerospace and medical device manufacturing.

These advancements are for a variety of purposes: they can exist to improve fuel efficiency in aircraft, or develop safer medical implants, all while simultaneously working to keep manufacturing costs low.

The advanced materials being utilized to develop the next generation of critical parts have unique properties which may include, but are not limited to: high tensile strength, extreme temperature resistance, and corrosion resistance.

Ultimately, advanced materials and improved part designs will allow engineers to develop smaller, more durable components that will serve the next generation of critical engineering applications, from heat exchangers in turbofan engines to spinal cages used in lumbar interbody fusion surgery.

Unfortunately, the increased usage of these advanced materials in complex geometries has presented new manufacturing challenges that have yet to be effectively solved. It is often difficult and expensive to machine these parts with the smaller features, higher surface quality, and higher part volumes commonly found in critical industries. In essence, recent design and material engineering innovations have outpaced the manufacturers’ machining and production capabilities.

What is the scope of these ‘generation-defining’ engineering challenges that advanced materials and improved part designs are attempting to solve? Why are advanced materials and unique part designs difficult to machine and produce in high volumes? How can manufacturers ‘catch up’ with the advancements of design and material engineers in these industries?

This article will seek to answer these questions by introducing a unique, material removal technology called pulsed electrochemical machining (PECM), developed by Voxel Innovations, an American advanced manufacturing company based in North Carolina.

The unique technology they develop is capable of manufacturing unique, tight-tolerance metal parts in high volumes.

Ultimately, this article will seek to explain why manufacturers developing high volumes of complex, advanced-material components should consider PECM as a viable alternative to conventional manufacturing methods.

Let’s first take a brief look at how pulsed electrochemical machining works to get a better understanding of its applicability for critical industries.

How PECM works

Pulsed electrochemical machining, or PECM, is a non-thermal, non-contact material removal method capable of producing small features, superfinished surfaces, and high repeatability on a wide variety of metallic parts, including many advanced metal alloys. It is an improved iteration of electrochemical machining (ECM) that utilizes a pulsed power supply.

A charged electrolytic fluid runs in the microscopic gap between the tool (custom-designed to each project) and workpiece, creating the electrochemical reaction that ultimately dissolves the workpiece to the desired geometry.

There are four key terms central to the technology to understand:

• The cathode, or tool, is a custom-designed part shaped as the inverse of the desired geometry to be machined on the workpiece.
• The anode, or workpiece, takes on many forms. It can be wrought stock, a near-net shape, or even an AM part. Crucially, however, the anode must be a conductive metal material, as the electrochemical reaction will not work on plastics or polymers.
• The electrolytic fluid simultaneously performs two critical roles for the process as it is flushed between the tool and workpiece. The fluid acts both the catalyst for the electrochemical reaction to occur, and the flushing agent that removes the dissolved material.
• The inter-electrode gap (IEG) is the microscopic gap between the cathode and anode where the electrolytic fluid is run. The size of this gap is an extremely important variable impacting the precision of the process; This gap becomes increasingly smaller as PECM technology advances, improving its accuracy. Currently, Voxel Innovations can make this gap as small as 10-100µm Ra (.0004-.004in).

While PECM has historically focused on aerospace applications, in recent years it has been increasingly utilized for other critical environments, such as within the healthcare, energy, and the defense industry.

Voxel’s primary focus for PECM, however, has largely been for the aerospace and medical device industries – both central topics in this article. In the following sections, we’ll discuss three of the most prevalent engineering challenges associated with advanced materials within the aerospace and medical device industries, before discussing how PECM can work to alleviate some of these challenges.

Advanced Material Engineering Challenges

While there are a multitude of engineering challenges related to critical industries, aerospace and medical device manufacturers tend to face these three general obstacles in engineering:

• Miniaturizing critical parts
• Machining high-quality surfaces
• Efficiently producing complex features in high part volumes

Part Miniaturization

The importance of miniaturization

Miniaturization is the ability to decrease the size of a part’s components while simultaneously retaining (or potentially improving) its form, fit, and function. Well-known examples are in the consumer electronics industry, as televisions, game consoles, and mobile phones have significantly decreased in size, improving the device’s mobility, processing power, or simply aligning with social trends. In some industries, such as medical or aerospace, miniaturization can have a direct impact on fuel consumption and emissions or clinical outcomes for patients.

Miniaturization in aerospace applications

There are generally two incentives for miniaturizing a component for aerospace-related applications:

• Smaller components can improve an aircraft’s range and fuel efficiency
• Smaller components occupy less space, allowing room for additional functions

The engine in an average passenger jet, for instance, can reach temperatures above 1,400°C (2,500°F), and may need to sustain these extreme temperatures for many hours at a time. This harsh environment requires unique materials, like nickel super alloys, which have a superior temperature, corrosion, and fatigue resistance. There is an increased desire for “small-core” engines which require smaller components and tighter tolerances to help achieve better fuel efficiency.

Engineers may have additional incentives to reduce the size of these components. For example, an ideal aerospace heat exchanger has a large ratio of surface area to volume to achieve the best efficiency. Miniaturizing these fluid channels can have a considerable impact on the functionality of the heat exchanger and reduce the overall weight or size. Thinner walls, tighter spacing and higher aspect ratios can all increase the heat exchanger’s capabilities, but are challenging to manufacture.

Miniaturization in medical applications

Compared to aerospace applications, miniaturizing critical components for the medical device manufacturing industry can have different – but equally important – purposes:

• Like aerospace, smaller components can lightweight medical devices (such as robotic surgery components) improving their dexterity and precision repeatability
• Smaller surgical devices (including both surgical instruments and implants) help facilitate minimally invasive, safer surgical procedures that improve patient convalescence

Pacemakers have evolved significantly over the past half-century as manufacturers work to simultaneously miniaturize the components and improve their capabilities. First-generation pacemakers were composed of a weighty, uncomfortable nickel-cadmium battery worn around the patient’s neck, expected to last a mere six weeks before replacement. Today, however, pacemakers are the size of a small pill, and are capable of lasting for over a decade at a time!

There are similar advancements underway in robotic surgery which is enabling smaller surgical tools and less invasive procedures. Orthopaedic implants are also being miniaturized so they can be used in smaller joints or bones, such as hand surgeries. This requires metallic parts that are smaller and more precise than legacy designs.

Challenges of miniaturization

The many benefits of part miniaturization, however, cannot be achieved without overcoming significant design and material engineering challenges. For example, with additional sensory capabilities and significantly longer lifespans, modern pacemakers require tighter tolerances and advanced materials (usually titanium) to house the battery, circuitry, sensors, and pulse generator. By miniaturizing these parts, they are more susceptible to surface irregularities, such as machining marks or burrs, that are not as critical in larger parts. These surface deformities often occur with conventional heat or contact-based machining methods.

Conventional machining methods also struggle to produce thin-walled features found within microchannel heat exchangers. These features are especially sensitive to thermal or vibratory distortions from the tool, which can occur in heat and/or contact-based processes, such as CNC machining. If the “hardness gap” between the tooling and workpiece material is smaller, including the advanced materials used in many heat exchanger components, there is a higher likelihood the tool (and/or workpiece) will break, resulting in additional time, money, and liability to the manufacturer.

PECM solves miniaturization problems

Pulsed electrochemical machining has unique properties as a material removal method that can be beneficial for manufacturers seeking to miniaturize critical parts, within pacemakers, heat exchangers, and many others.

As PECM does not utilize contact or heat to machine materials, PECM can produce features which are otherwise sensitive to stress in the cutting process and/or high thermal loads, including thin walls with high aspect ratios. Voxel Innovations has created walls of <0.075mm (or <0.0003”) thickness with a 20:1 aspect ratio using PECM (ideal for microchannel heat exchangers).

Another unique benefit of PECM is its disregard for material hardness. PECM is more concerned with the chemical properties of the workpiece material rather than its machinability, allowing the process to create smaller features in otherwise challenging materials without incurring any risks associated with conventionally machining tough parts, such as machining marks or risking other deformities associated with contact- or heat-based machining.

While PECM can machine tight-tolerance, miniaturized parts from raw material or bar stock, PECM is also capable as a secondary machining process by improving part resolution and features from near-net manufacturing process. For example, PECM can be used as a postprocessing operation for smaller metal 3D printed parts, improving part resolution on downskin surfaces, machining thinner walls, and improving surface quality.

Surface quality

The importance of surface quality

Within critical applications in the aerospace and medical device industries, surface quality serves a wide range of important roles, from scramjet engines to x-ray machines.

For instance, surface quality can determine:

• Corrosion resistance
• Resistance to high temperature-flux
• Dexterity of moving components
• Sterility/antibacterial properties

While this list is not exhaustive, let’s review a more specific example of these first two points (corrosion and temperature-flux resistance) and their correlation with surface quality for a critical aerospace component.

Aerospace example: Turbofan blades and surface quality

There are a range of design and material considerations for developing parts within a turbofan engine. For instance, the extreme temperature flux and fatigue wear within these engines require materials (notably nickel superalloys) capable of withstanding these extremely harsh environments (turbofan engines can spin at over 3,000RPM with temperatures of over 2,000°C (3632°F)).

However, an equally important design consideration for many critical aerospace components is surface quality. As one example: the level of surface quality on a given turbine blade or vane is directly correlated with its ability to reduce drag and thereby improve fuel efficiency. Most notably, studies have confirmed that the blade’s leading edge and the front half of the suction surface had more influence on the part’s overall aerodynamic capabilities than any other part of the blade, and these features can be directly improved by reducing surface roughness.

“Suder et al found that the roughness distribution at the blade leading edge and in the front half of the suction surface contributed to most of the aerodynamic loss. Roughness on these regions accounted for more than 70% of the performance degradation found in fully coated blades with a surface finish of 2.5–3.2 μm (rms).”

Surface quality is a crucial aspect of critical aerospace engineering for other reasons, as well, as rough or porous surfaces can create notches or pits and poor surfaces can introduce microcracks on the blade. This not only affects the fatigue life of a turbine blade but can also initiate corrosion that may lead to drag and, potentially, outright part failure. Ultimately, prioritizing high surface quality in critical components makes a considerable difference in an aircraft’s lifespan and fuel efficiency, but simultaneously presents considerable engineering challenges.

Medical example: Bone fixture devices

At first glance, engineering objectives for components in the medical device manufacturing industry run parallel with the objectives of aerospace engineers. For example, many choices related to material, tolerances, and design of many orthopaedic devices are made to enable high levels of fatigue resistance and anti-corrosive properties.

However, smooth surface quality has a much wider range of benefits for the medical device manufacturing industry that are central to patient safety, specifically within implantable devices. For example:

• Smooth surface quality on a given implant reduces the possibility of cytotoxic molecules being released in the patient’s bloodstream, such as cobalt ions discharged from cobalt-chrome
• Smooth surface quality allows improved antibacterial properties to protect the implant from harmful biofilm formation, most notably Staphylococcus Aureus (staph infections)
• Smoother surfaces within moving parts help eliminate unnecessary friction which may improve mobility and patient comfort

Challenges associated with surface quality

While adequate surface quality can be achieved on many non-critical parts via conventional manufacturing methods (such as grinding, honing, or polishing), it is challenging and expensive for manufacturers to create superfinished surfaces using conventional machining processes for parts with complex features such as contoured surfaces (e.g. turbine blades or orthopaedic joint surfaces). These features can pose challenges for conventional CNC milling processes, given the need for small tools and step-overs during the machining process.

Many conventional processes also introduce their own surface irregularities. For example, thermal machining processes such as electrical discharge machining (EDM) or laser cutting can create surface defects such as recast layers, and milling, grinding or drilling can create micro burrs that must be removed in a secondary operation.

A recast layer occurs when the removed material re-solidifies itself on the cut surface. The recast layer has a different metallurgical composition than the base metal and can introduce microcracks that impact the integrity of the part. While recast layers are specific to thermal-based machining processes, burrs are specific to contact-based processes. These irregularities occur when slivers of material are pushed out of the way by the cutting tool without being removed. Furthermore, burrs can sometimes be compacted against the workpiece and hidden from sight.

While the aforementioned surface irregularities do not occur in additive manufacturing, AM introduces its own inherent issues. For example, metal-AM may have larger layer lines, support structure remnants, and low resolution on downskin surfaces. For these reasons, many conventional (and some advanced) machining and finishing processes are problematic for critical applications, including aerospace and medical device engineering applications.

PECM overcomes surface challenges

Pulsed electrochemical machining is capable of both machining and producing superfinished surfaces on a variety of parts: including a bar stock, a conventionally machined near-net shape, or even additively manufactured components, producing a mirror-like surface quality on tough materials. Voxel has demonstrated PECM’s ability to reduce surface finish down to 0.005 – 0.4μm Ra (0.2-16μin) on advanced materials serving the aerospace and medical device industries.

Furthermore, as PECM is not contact or heat-based, it leaves no machining marks, recast layers, burrs, or any other surface irregularities on a part, which can be especially critical for tight-tolerance parts in extreme environments.

One of PECM’s best advantages is its capability of machining complex features, such as contoured surfaces, with faster speeds. This is owed in part to PECM’s ability to machine the entire surface simultaneously with a single tool. PECM’s rate of machining is not necessarily tied to the size of the surface and can be significantly more advantageous than conventional processes when finishing parts with complex features. PECM also machines and finishes parts simultaneously, which can further speed up the process.

An excellent example of PECM’s advantages is with finishing additive Inconel turbine vanes. PECM is capable of fulfilling the role of both a secondary machining process and finishing operation on these parts by reducing wall thickness and improving Ra on both the blade walls and complex downskin surfaces on additive superalloy parts. With this improved resolution and surface quality, AM turbine blades postprocessed with PECM can allow a longer lifespan, reduced fatigue, improved corrosion resistance, and improve the aircraft’s fuel efficiency and range.

Repeatability

The importance of repeatability at high volume

As part volumes increase alongside demand for engines, aerospace components and medical devices, manufacturers must be increasingly conscientious of machining and production costs. Let’s briefly review examples of how manufacturers are making adjustments to production to accommodate high part volumes, and their associated challenges.

Surgical instruments

Consider how a variety of social and market factors can influence the part volumes required of certain components. For example, consider how the increased prevalence of musculoskeletal issues, motor vehicle injuries, and a growing geriatric population warrant increased part volumes of medical devices treating traumatic injuries. In addition, these advanced surgical tools must be able to withstand more pressure, have improved torque resistance, and (notably with reusable surgical instruments) have improved corrosion resistance – all potentially accomplished by utilizing more advanced, tough-to-machine materials.

In addition to these factors, one must also consider how the increased popularity of disposable, single-use surgical instruments are impacting part volumes for manufacturers. As disposable instrumentation can improve patient convalescence by reducing many of the risks associated with corrosion, microcracks, and harmful biofilm formation on reusable instrumentation, part volumes have dramatically increased for manufacturers. These part volumes are further inflated by the fact that these instruments are single-use.

The challenge of high-volume production

Producing higher part volumes of critical parts creates significant cost and engineering challenges. For instance, 316 stainless is a durable, versatile, corrosion-resistant material, but is challenging to machine in unique geometries without incurring significant tool wear from conventional machining processes. Furthermore, conventional machining processes have inherent risks that may prolong manufacturing including the aforementioned recast layers, burrs, and machining marks.

Alongside these conventional machining challenges, additive manufacturing companies are also experiencing difficulties adjusting to demand for higher part volumes. As AM companies seek to produce higher part volumes of complex metal-AM parts, they are simultaneously sacrificing part resolution by using thicker layer lines, larger powders, and faster re-coating methods, ultimately to cut costs.

PECM solves repeatability problems

Fortunately, PECM is capable of producing quality components to accommodate increasing part volumes without sacrificing surface quality, tolerances, or resolution, for two primary reasons.

Firstly, as PECM involves no contact or heat, there is little-to-no tool wear involved in the process, allowing a single PECM cathode to be capable of machining thousands, or potentially tens of thousands of identical parts without incurring tool wear and tool replacement costs. For example, PECM can machine higher part volumes of hardened stainless steel without wearing down its tooling, which could be useful for accommodating higher part volumes for the disposable surgical instrumentation market.

Secondly, a single PECM tool can potentially machine multiple features in tandem, or machine multiple parts out of a single piece of bar stock with a single plunging motion. For example, PECM can machine multiple microchannel features on a heat exchanger part simultaneously. This uniquely scalable approach allows high throughput without a decrease in part quality.

PECM’s postprocessing and finishing capabilities may also be a good fit to solve the aforementioned ‘lower-resolution, higher-volume’ issue within the metal-AM industry. With a single tool, PECM can improve both part resolution and surface quality on a metal 3D-printed part without wearing down the tool, including on rougher downskin areas of the part. Crucially, PECM is capable of repeatedly producing these features in thousands, or potentially tens of thousands, of identical AM parts. Using PECM can also potentially reduce the steps necessary for manufacturing a component by machining and finishing simultaneously.

Conclusion

While most conventional machining methods are capable of machining advanced materials, the primary issue lies in their ability to efficiently and economically machine these tough materials in complex geometries, especially exacerbated by rising part volumes.

More specifically, conventional machining methods struggle to accommodate decreasing critical part sizes, the need for superfinished surfaces, and increased demand for parts serving critical functions within the aerospace, medical device, and energy industries.

Fortunately, pulsed electrochemical machining may be capable of alleviating some of these advanced engineering challenges. By machining almost any conductive material with no contact or friction, PECM has unique advantages that may ultimately assist manufacturers in developing complex critical components in high part volumes – such as machining thermally sensitive features and superfinishing surfaces.

If complex components in critical environments can be machined with improved efficiency and tolerances, a wide variety of industries will benefit. Increasing fuel efficiency for aerospace components has major economic (and ecological) advantages. Smaller, stronger medical devices can mitigate risk and prolong human lives. Ultimately, the capabilities of advanced engineering hinges on both the ability of the design engineers and the manufacturing processes that help those ideas come to fruition.