The aerospace manufacturing industry requires advanced processes to enable the efficiency and durability required under stringent size, weight and power (SWaP) requirements and current machining techniques can be insufficient, giving rise to less conventional methods such as pulsed electrochemical machining (PECM).

In PECM, a part and a tool in its inverted form are positioned with a small gap in between. An electrolyte solution flows through the gap to remove machining waste. The reverse shape tool moves towards the part and a pulsating direct current is applied – dissolving the material of the part until the desired shape is achieved.

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Turbines, propellers, hard drives

Turbomachinery components are subjected to high forces, require complex 3D surfaces, and demand tight tolerance characteristics. Components such as bladed discs (blsks) and integral bladed rotors (IBRs) are a growing but demanding application that embodies many of these challenges. Normally manufactured by forging a large disc from a nickel or titanium alloy followed by 5-axis CNC milling of the complex airfoil geometry, as airfoils become thinner or include advanced 3D features or geometries. tip, high CNC milling speeds become difficult due to blade vibration or access between airfoils. Similar geometric challenges also exist with non-rotating components such as stator and vane segments.

Alternative methods such as casting, powder metallurgy, friction welding or additive manufacturing (AM) all have the potential to reduce material use, but present their own challenges. Casting involves differential cooling rates for thin airfoils versus thick hub walls, creating material stresses that can cause cracks in large components. Some AM and powder metallurgy techniques also have scale limitations due to thermal distortions. In general, techniques close to the net also come with additional material risks that have not been fully taken into account for critical components. Finally, in most cases, near-thread options require final machining to correct any tolerance distortion or improve surface finish, especially in applications with high Mach number airflow.

Many rotary and static turbine engine components, including vanes / IBRs, turbines, and impellers, have common design and manufacturing challenges. They are all very sensitive to geometric deviations and material properties and any variation can compromise the efficiency of the component and lead to imbalances in use.

Fortunately, PECM is an interesting alternative value proposition. PECM is neither a thermal nor a contact process, which means that cooling speeds and cutting vibrations are non-existent. Instead, PECM dissolves material atom by atom to the desired shape while allowing high aspect ratios and maintaining an attractive material removal rate. The precise shape and smooth surfaces made possible by PECM greatly improve the efficiency of turbomachines. PECM is also flexible and can be used for bulk material removal or the final finish of a part made in a quasi- lattice.

(Left): Pulsed electrochemical machining (PECM) can create mirror-like finishes, depending on the material. (Right): Unlike traditional machining methods, Pulsed Electrochemical Machining (PECM) does not use contact or heat to shape materials, allowing for the integrity of thin walls and intended geometry.

Metal AM, then PECM

With the increasing prevalence of AM in the aerospace industry, it should be noted that PECM can also be used as a finishing process, not just a primary machining method.

Metal AM has found its place in aircraft engines (combustion devices, casings, heat exchangers), rocket engines (injectors, combustion chambers, turbines) and a range of satellite applications. However, surface roughness is a common problem in 3D printing, especially the remnants of supporting structures. These surface irregularities can lead to improper fit, increased friction, corrosion and cracking. However, PECM can deal with surface roughness in post-processing. Although PECM has similarities to electropolishing, it differentiates by using a locally small space between the electrode and the workpiece. Custom PECM methods developed by Raleigh, NC-based Voxel Innovations make it particularly effective in removing macro-level roughness associated with AM support structures, down surfaces, powder bed fusion by beam fusion. electron (EBM) or free form AM methods.

Metal AM also has limitations on wall thickness and aspect ratios and may experience distortions due to the inherently thermal nature of the process. Instead of compromising the design to operate within these limits, PECM creates thin-walled or critical features in a secondary machining step.

Another potentially cost-effective solution can be to add additional stock material to the surface of the AM part and then use the PECM to dissolve the additional material within the tolerance. Likewise, the AM metal construction parameters could be adjusted to speed up deposition rates while using PECM to process the resulting geometry more efficiently. As metallic AM expands into increasingly volume production applications, a hybrid approach to metallic AM followed by PECM may be a more efficient and effective manufacturing approach.

High temperature applications

The choice of materials is a careful balance, especially in the aerospace industry, and design engineers must optimize bonding and assembly issues, corrosion, system mass, and cost, while taking resistivity into account. thermal and corresponding expansion coefficients.

In high temperature applications in liquid rocket engines or hypersonic systems, these needs cannot be met by standard material choices. Ceramics and composite ceramics with high thermal conductivity and corrosion resistance may seem like an ideal solution, however, their cost, bonding challenges and forming limitations preclude a wider implementation. This leaves high temperature metal alloys, including nickel superalloys, titanium aluminides, refractory metals, and high entropy alloys, which can be difficult to machine conventionally due to resistance to heat and resistance. wear – exactly why they are used in high temperature applications. The difficulty in machining these materials is particularly problematic in heat exchangers, since these components only perform at their maximum efficiency with high aspect ratios, thin walls and tight spacing. Many conventional machining processes rely on heat or contact to form features, which can warp thin walls and result in irregular channel sizes (and uneven heat flow) or improper fit with other components. Many electronic applications require a different set of material choices. Chip-level cooling and high-power radio frequency (RF) or microwave devices in airborne applications may require a low coefficient of thermal expansion (CTE) or corresponding thermal expansion of another material (for example, the silicon). This can include materials such as metal matrix composites, refractory metals, or low CTE Invar and Kovar, all of which are difficult to fabricate. Additionally, these electronic applications often require precise control of depth and wall thickness to handle heat transfer and packaging requirements. Cryo-refrigerators used to cool infrared camera sensors are another example of difficult-to-machine geometries, given the typical requirement for high aspect ratio cooling functions.

High aspect ratios, high feature density and thin walls can be machined efficiently with Pulsed Electrochemical Machining (PECM).
Pulsed Electrochemical Machining (PECM) finishing of additively fabricated nickel alloy tensile bar samples with as-printed ends.
Pulsed Electrochemical Machining (PECM) is compatible with a wide range of materials, including stainless steels and nickel alloys.

Traditional CNC or EDM milling can be too slow, too expensive, or lead to compromised designs. In contrast, PECM is insensitive to the mechanical properties of materials and can machine nickel superalloys as quickly as copper or aluminum. Plus, the low forces make it an especially good fit for thin walls and high aspect ratios.

Ultimately, cost is one of the most critical issues in any manufacturing process. Parallel production, made possible by PECM, creates many slots, pockets, or other features simultaneously in the same vector, enabling higher density features, faster production, and lower cost per part.

PECM excels in smooth surfaces, high aspect ratios, high feature density and thin walls, even in difficult-to-machine materials. All of this can be done in large quantities and at a relatively low cost, which is why aerospace companies should consider taking advantage of PECM.

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