The Impact of Heat-Affected-Zones (HAZ) on Scalable Medtech

And how Voxel's PECM process mitigates this issue, at-scale.

Heat-affected zones (HAZ) are not merely cosmetic "burn" regions, but rather, localized areas of thermal damage that are generated when a process melts (or strongly heats) a workpiece and then cools it rapidly. In laser cutting, EDM, welding, and certain grinding operations, that thermal cycle can inherently affect a material's properties:

• Grain structure

• Hardness

• Residual stress

• Oxide chemistry & corrosion behaviour

In this article, we'll identify why HAZ is a prohibitively expensive and challenging problem to solve at scale, and how Voxel's PECM process, by avoiding HAZ, can enable new designs and scalability.

These changes can create significant performance differences, even if they are in a few micrometers-worth of material. For medtech leaders, where small devices can be heavily inspected and extremely fatigue-limited or corrosion-sensitive, these altered properties have considerable effects.

HAZ, fundamentally, is often manageable at the prototype scale but is clearly unmanageable and prohibitively expensive at production scale. Representative studies show laser-cut nitinol HAZ lasers can become substantially large fractions of the very parts themselves. While lower-energy EDM capabilities have been shown to reduce HAZ, it does not eliminate the possibilities of HAZ entirely: even narrow HAZs just a few micrometers wide can change hardness, pitting behaviour, crack susceptibility and more for critical medical devices for 316L and CoCr components.

HAZ matters more than ever, as FDA/ASTM frameworks increasingly evaluate the susceptibility of finished devices for corrosion, nickel release, and dimensional integrity as opposed to simply evaluating their chemistry. The FDA's nitinol guidance, in fact, explicitly references how thermal processing and surface finishing changes corrosion and "nickel-leach" behaviour.

How HAZs Form

Small fixture devices like these are inherently susceptible to small surface defects.

When certain materials have extremely localized energy input followed by rapid cooling, their very material structures can change. The material closest to the energy source can melt and re-solidify (such as in recast layers), but the surrounding material also undergoes transformation: recrystallization or coarsening of the grain structure can still occur away from the direct site of machining.

HAZs occur in a wide variety of processes: laser cutting, EDM, and welding all have literature showcasing their susceptibility to producing thermal-created surface defects, most notably HAZs.

Nitinol is especially sensitive to the effects of HAZs, as the alloy's very functionality relies on its "thermomechanical phase transformation", as noted from the FDA paper. It is a "shape memory alloy" and its unique ductility and corrosion resistance can be directly affected by the presence of HAZs, not to mention increased susceptibility of nickel-leaching. Nickel ion release is limited to 0.5 µg/kg/day, an extremely small amount that allows no room for surface irregularities in nitinol devices.

As one study noted comparing CoCr, Nitinol and 316L, "Across all three alloy systems, the recurring material-specific lesson is the same: HAZ does not merely “roughen” the edge. It perturbs the very microstructural and surface states that medtech alloys are chosen for in the first place..."

Why Are HAZs Harmful?

In short, we can identify 4 key reasons:

The first is fatigue. Medical devices operate under millions to hundreds of millions of cycles, often in corrosive physiological environments. Tensile residual stress, microcracks, recast layers, and local hardness mismatches all act as crack initiators or crack-growth accelerants. FDA’s stent guidance ties fatigue failure to loss of radial support, thrombosis, restenosis, and vessel perforation risks, and it explicitly warns that manufacturing flaws not fully removed by polishing can contribute to clinical complications.

Fatigue is a key issue for medical devices like this bone fixture.

The second is corrosion/biocompatibility. Metallic implant surfaces are treated specifically to improve corrosion resistance, and that embedded foreign matter or post-marking surface disturbance can require additional treatment. Where HAZs change, say, oxide thickness, composition, segregation, contamination, etc., it lowers pitting resistance and increases ion release: especially important for nickel-heavy materials such as Nitinol.

The third is tolerance and inspection burden. If a thermally damaged layer must be removed to recover performance, then the true manufacturing problem becomes a compounded issue of the thermal process and its re-passivation before inspection. Microfeatures (like a 10 µm-class HAZ) can consume a large portion of the tolerance budget for manufacturers: the amount of material removed in cleanup and the inspection effort both rise with feature count.

The fourth is regulatory risk. The FDA’s Nitinol guidance makes finished-form corrosion and surface finish key regulatory hurdles. A supplier cannot rely on conventional alloy guidances alone if the manufacturing route has created an inherently different material surface.

The Scalability Problem

Laser manufacturing, for instance, is attractive for its highly automated, digitized capabilities, able to cut stents and other fine geometries at scale. But when geometries become denser, with tighter tolerances and harder-to-reach internal areas, scalability becomes a nightmare. Consider geometries: as features continue to shrink (miniaturization is a prevalent trend across nearly all medtech devices), a fixed HAZ thickness occupies a larger fraction of the geometry, and this only worsens at-scale. Laser machining is inherently a serial process, meaning each feature adds incremental cycle time. At the same time, each feature introduces its own thermal history. Variability in beam interaction, assist gas flow, and local heat accumulation leads to feature-to-feature inconsistency in HAZ formation, even within the same part.

High-density feature devices like these are challenging to scale.

For EDM, there's an opposite trade-off, as those processes are capable of accurate geometries, but the economic appeal degrades very quickly with both high-volume components and high-feature-density components (such as stents, or microhole arrays in drug delivery devices). Debris accumulation in the spark gap, electrode wear, and flushing limitations introduce variability that becomes more difficult to control as geometries become deeper, denser, or more complex. In practice, this means that scaling EDM to high-density arrays or high-throughput production environments introduces nonlinear increases in cycle time, maintenance burden, and process variability.

Ultimately, HAZs force manufacturers into a difficult position: either accept altered material properties or remove the affected layer through secondary processing, which directly impacts dimensional control and yield.

These scaling limitations create three key issues:

1. Yield and scrap rates: Variability in HAZ thickness or removal leads to out-of-tolerance features, especially in micro-scale geometries
   

2. Process complexity: Additional steps (polishing, passivation, inspection) become mandatory
   

3. Validation burden: Regulatory expectations require demonstration that surface integrity (corrosion resistance and fatigue performance, for instance) is consistent across production batches

PECM: Eliminating HAZ and Changing the Equation

Processes that avoid thermal interaction like PECM entirely remove this constraint at its source. By eliminating HAZ formation, PECM fundamentally changes the relationship between geometry, material behavior, and scalability. Instead of managing or removing a damaged layer, the process preserves the base material state throughout machining. This has several downstream effects that are particularly relevant for medtech applications:

• Feature size is no longer coupled to thermal damage: Microfeatures and thin walls can be produced without a proportionally large affected zone
  

• Feature-to-feature consistency improves: Without localized thermal variability, arrays of features behave more uniformly across the part
  

• Post-processing requirements are reduced: The need for aggressive secondary removal processes diminishes, simplifying the manufacturing chain
  

• Internal geometries become more viable: Since no thermally altered layer needs to be removed, inaccessible features are no longer penalized

To reinforce that point, it’s important to recognize that eliminating HAZ essentially removes an entire failure mechanism from the process chain. In thermal machining, even well-optimized processes still rely on downstream steps to “repair” the surface: be it recast layer removal, electropolishing, passivation, or chemical etching. Each of those steps introduces its own variability and, critically, its own interaction with complex geometries.

When those steps are no longer required, the manufacturing process becomes inherently more stable, impacting yield (as fewer parts fall out of tolerance due to over-removal), uneven finishing, or inaccessible regions. At production scale, that translates into more predictable output, tighter process windows, as well as reduced dependence on operator intervention.

There is also a design-level implication that becomes more apparent as device geometries evolve with new designs. When engineers are no longer constrained by the need to avoid or later remove thermally damaged material, they can begin to treat internal surfaces and microfeatures as genuine design elements, rather than liabilities. This is particularly relevant for emerging device categories (such as microfluidic drug delivery systems, high-density microhole arrays, or especially AM additively manufactured implants) where performance is dictated, largely, by internal geometry and surface condition. In these cases, avoiding HAZ enables a more direct path from design intent to production reality, reducing the need for compromise between geometry and long-term device performance.