What Truly Determines Whether PECM Handles an Alloy Well?
- Kirk Abolafia
- 2 days ago
- 5 min read
PECM fundamentally views materials through an entirely different lens than conventional material removal methods, which alters what material properties govern machinability.
Conventionally, hardness and toughness make a given alloy difficult to machine (be it cutting, drilling, or grinding). The source of many of those properties come from how the atoms are actually arranged in the structure: and how easily a dislocation may move through that lattice when put under some form of mechanical load. PECM, however, is pulling those atomic bonds apart on the surface of the workpiece through anodic dissolution: an atom-by-atom material removal process that has different physical properties than the macro-world of conventional machining. This allows PECM to enjoy relatively easy material removal capabilities on otherwise tough materials: Inconel, certain titanium alloys, and cobalt-chrome to name a few.
However, as we'll explain, this does not render microstructures irrelevant in the PECM conversation-- just that they matter for different reasons, as we will discuss in this article.
If an alloy's temper condition, or processing history, or microstructure is part of what dries a process evaluation for you, send Mike or Kirk the material and feature geometry- a short technical discussion is enough to know whether the microstructure changes the picture for your part specifically. Use our booking link.
Key Takeaways
Cutting, drilling, and grinding resistance comes from crystal structure and hardness, but for PECM, this depends on electrochemical potential: an entirely different material property
Grain boundaries (and precipitates within a grain) don't always dissolve at the same rate as the surrounding matrix
Multi-phase alloys (duplex stainless steel's ferrite-austenite structure, for instance) contain regions with different electrochemical potentials...making real galvanic effects in the areas where those regions meet
Studies (like those on Hastelloy orTi64) suggest that dissolution behavior does in fact change with microstructure-- IE, two samples of the same nominal alloy may respond differently in PECM depending on how they were processed or heat treated
When Hardness Isn't The Sole Limiting Factor Anymore
The crystalline structure of a metal (whether that is face-centered cubic, or hexagonal, etc) is one of the key means in which dislocations or fractures actually move through the material under stress: and this is, ultimately, what determines hardness and machinability for conventional processes. Technically speaking, it's cobalt-chrome's dense, work-hardening microstructures that are what wears down cutting tools on CNC microdrilling equipment.
The atomic makeup of a lump of coal and a shining diamond are not dissimilar... but it's the crystalline bond structure between those atoms that makes the two items distinct.
Think of the key differentiator being a geometric one, not a chemical one. When viewing a diamond's metallic structure, every carbon atom forms four identical covalent bonds arrange in a tetrahedron: so the whole crystal is one continuous, uniform, lattice with no natural plane along which it can shear or slip....making diamond the hardest naturally-occurring material on earth.
Coal, however, is a disorganized mess in comparison. While still comprised of carbon, some regions might have flat or layered sheets (similar to graphite), held together by weaker forces rather than covalent bonds. The layers slide past each other much easier (making graphic soft enough to write with) because there are more geometric opportunities for the atoms to detach from one another.
Please note that while serving as an excellent example of metallurgical property differences, as they are not electrically conductive PECM cannot machine diamonds.
Now we turn our attention back to PECM: anodic dissolution is an entirely separate framework. Current passes from the workpiece (anode) through an electrolytic-fluid-filled micro-gap, and metal atoms leave the surface as ions based on the alloy's electrochemical behaviour. A material like aluminum readily exchanges its electrons to create a more stable aluminum hydroxide, which is then flushed away by the same electrolytic fluid it dissolves in. Material removal rate in PECM follows Faraday's Law of Electrolysis, where the charge passed and the metal's atomic weight and valence (not hardness) sets the pace of material removal. Read our recent article on Faraday's Law to learn more.
Where Microstructure DOES Matter in ECM: Grain, Precipitates, Multi-Phases & Galvanic Effects
The conversation becomes complicated when dealing with alloys: they are rarely perfect uniform lattices and generally contain grain boundaries with different atomic packing...ultimately changing how easily specific areas dissolve relative to the bulk material during conventional ECM methods (But not at Voxel, as we'll explain).
One study on Inconel 738 found that large precipitates on grain boundaries had some level of resistance during ECM and remained on the surface, even under otherwise optimal processing conditions, thereby limiting surface finishing capabilities. Smaller intermetallic phases inside of those grains, by contrast, dissolved more easily. Voltage and process time, as well, were important determinants in how well this machining went. Similar studies on Hastelloy and Ti64 also found that microstructure measurably changed dissolution behaviour-- analyzing that two coupons of the same nominal alloy specification dissolved different if their underlying grain or precipitate structure differed.
But what about alloys that have two distinct phases that coexist throughout its microstructure, such as duplex stainless steels? In these materials, they are roughly built from equal-part ferrite and austenite structures-- and each of those have distinct
electrochemical potentials.
Notably, where those ferrite/austenite regions meet can create galvanic regions (with one phase acting anodic relative to the other). Technically, this should mean that those two phases do not dissolve under the same rate of ECM, which could result in differing textures or uneven surface qualities after machining.
What does Voxel's PECM Do Differently?
Importantly, the aforementioned differentiators in material do not actually pose significant difficulties for Voxel's PECM methodology. We regularly machine Inconel, CoCr, and hardened stainless steel in similar material removal rates to that of copper, and we are even able to machine surface qualities of those tough alloys down below 0.1um Ra. We have years of internal R&D and process IP experience to fine-tune the following properties (and more) to allow us unique machinability on these tough alloys:
Pulse strategy
cathode design
electrolyte formulation
Temperature and voltage control
Inter-electrode gap (IEG) control
Cathode coating techniques
Electrolyte flow methodologies
And much more
While we generally don't publish the specifics of how, the practical result is measurable and easy to share: we regularly machine complex-alloy components in unique, small features with fine surface qualities at high-volume scale.
If an alloy's temper condition, or processing history, or microstructure is part of what dries a process evaluation for you, send Mike or Kirk the material and feature geometry- a short technical discussion is enough to know whether the microstructure changes the picture for your part specifically. Use our booking link.
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