All About Refractory Metals, and Where PECM Fits

Kirk Abolafia

10 minutes ago

6 min read

Tungsten, Molybdenum and related metals known as "Refractory Metals" are adept at solving issues in extreme-temperature environments-- but their capabilities go only as far as the manufacturing processes machining them.

The group of metals on the periodic table known as Refractory Metals are a relatively small group known for two key attributes: high melting points and incredible resistance to other forms of deformation. This allows them to operate in some of the most crucial operating environments on the planet, including semiconductor tools, X-ray machines, and aerospace turbine components. As usual, however, their incredible resistance to heat and other forms of deformation acts as a double-edged-sword: these metals are incredibly difficult to cut, or drill into, or even finish, without causing severe damage to the part or tool...whether it's a molybdenum plate with a microhole pattern or perhaps a tungsten flow restrictor plate.

But what defines a refractory metal, making them so different? How are they specifically used? What makes them challenging to machine, and how can Voxel's PECM methodology work on them?

Key Takeaways

By definition, the basic refractories are niobium, tungsten, molybdenum, tantalum, and rhenium -- all of these with a melting point above 2000 degrees celcius (for reference, the photosphere of the sun is around 5500 celcius).
This list is technically longer including Titanium, Zirconium, Chromium and more
CNC and other contact-based methods encounter room-temperature brittleness in refractory metals, while laser-based processes struggle with the high thermal conductivity of these materials
A key capability of these materials is their ability to form stable surface oxide films: enabling corrosion resistance but also protections against certain material removal methods
Voxel has experience machining Molybdenum, Titanium, Zirconium, and Niobium with PECM, but we strongly recommend an application-fit review first.

So, What Makes a Metal Refractory?

As mentioned, a key aspect of what makes a refractory metal special is its incredible resistance to heat: the gap is so large that temperatures that otherwise do not affect, say, Tantalum (its melting point is above 3,000 celcius) can easily begin to melt most stainless steels (SS420 is known to have a melting point around 1500 celcius). In fact, Tungsten melts at 3422 celcius, the highest melting point of any metal.

So why can these metals survive such extreme heat? The answer lies in their molecular structure, more specifically: their electrons. A metal melts when heat shakes its atoms hard enough to break the bonds holding them in place. In most metals, each atom contributes one to three outer electrons to a shared "pool" that holds the lattice together like a glue. Aluminum, for instance, contributes about 3 electrons per atom to this "pool", whereas Tungsten or Molybdenum contribute about six.

It's also about their crystalline structure: refractory metals have a body-centered cubic crystal structure, surrounding each atom with 8 close neighbors, and 6 more beyond them, all bonded at once. Pulling that network apart into a liquid takes a truly enormous amount of thermal energy.

It's also important to note that the cubic crystalline structure of many refractories (like tungsten) also make them brittle. We will discuss manufacturing challenges later in this article, but in short: a metal's ductility is largely governed by this crystalline structure due to atoms' ability to "slip over one another", akin to cards sliding in a deck. In these face-centered cubic metals (aluminum, copper, certain stainless steels), these lattices slide easier. However, in body-centered cubic lattices (say, Tungsten) there is an extra atom in the middle, making the layers "bumpier". In Tungsten's case this also translates to considerable brittleness.

Classification of Refractories

The "big five" refractories we discussed earlier earn their place because they clear the heat threshold of 2,000 C with plenty of margin. Niobium, the "coolest" of the group, still melts at about 2 477 °C, and tungsten tops out near 3 422 °C. Most definitions agree with this logic. They also share a set of working traits: high density, extreme hardness, and strength that holds up under sustained heat and load.

Titanium, zirconium and chromium, though, fall short of that bar. Titanium melts at about 1668 C, zirconium near 1855 C and chromium around 1907 C, which is why they only appear under wider definitions that lower the cutoff to about 1 850C-ish. However, these materials are by no means inferior: they're chosen for different reasons. Titanium is prized for its incredible strength-to-weight ratio and biocompatibility, and zirconium for its unique corrosion resistance. Ti-6Al-4V (sometimes called Ti64) is an extremely common implant alloy, and Cobalt-Chrome (CoCr) is also used in orthopedic fixtures for its excellent corrosion-resistant properties.

Where Are Refractory Metals Used?

In essence, refractory metals tend to appear wherever extreme heat or radiation would ordinarily destroy any other alloys or materials. The most common applications include:

Medical imaging: tungsten anodes, radiation shielding and collimators for CT scanners. (Our X-ray components article goes deeper on these parts).
Semiconductor equipment: molybdenum and tungsten structural components, heaters and sputtering targets. Molybdenum microhole plates and tungsten flow restrictors fall here, where particle risk and surface integrity requirements for high-purity gas hardware are strict.
High-temperature furnaces: heating elements, reflectors and heat shields for vacuum and reducing atmospheres.
Medical implants and instruments: titanium, plus zirconium- and niobium-bearing alloys.
Aerospace: rhenium, molybdenum, and tungsten as strengthening additions in superalloys.

Why Conventional Machining Struggles With Refractories

Each conventional process has its unique set of problems it encounters when machining refractory metals, making switching processes difficult as it essentially just trades one problem for another. Let's walk through a few:

CNC milling and drilling are often fighting the brittle properties of refractory metals we discussed earlier-- the crystalline structure of these metals allows incredible resistance to heat, but also makes them brittle. Essentially, at lower temperature-machining, edges and thin walls will often chip (notably tungsten). On a microhole array plate of Tungsten, for example, micro-drill breakage and constant tool changes add significant amounts to cycle times.

On the opposite end, laser drilling will fight these materials due to their high thermal conductivity. Put simply, machining these high-temperature-resistant materials with high temperatures can cause adverse effects. Tungsten, for instance, conducts heat at about 173 W/m·K and molybdenum at about 138 W/m·K, compared with roughly 11 W/m·K for Inconel 718. Essentially, in laser drilling, the beam will spread into the surrounding material so much more of it is needed, and the melt will often cause a recast.

Side-note: Titanium encounters the opposite problem-- its low conductivity concentrates heat, which is why laser drilled holes also carry HAZ concerns, especially in fatigue-sensitive parts.

Then we have EDM - it works well at times on refractory metals because they conduct electricity, but it is still fundamentally a thermal process. Research on micro wire-EDM of molybdenum shows that recast layer thickness changes with process parameters, and recast on brittle metals from EDM is a common starting point for microcracks. Where recast isn't allowed on semiconductor vacuum hardware, every EDM feature needs a secondary step.

PECM's Unique Relationship With Refractories

So where does that leave PECM? At times, refractory metals may resist ECM for the same reason they resist corrosion: they form stable, protective oxide films, and unlike EDM or laser drilling, ECM removes material by anodic dissolution-- providing benefits like no recast or HAZ, but also meaning the metal must dissolve evenly first.

Titanium is a good example of this challenge. A study of Ti-6Al-4V in sodium nitrate electrolyte found almost no anodic current until the oxide film broke down at about 5,24 V, and machining didn't begin until about 6,6 V. Once the film broke, the alloy dissolved unevenly because its phases dissolve at different rates. We discussed this microstructure effect in a recent article: What Truly Determines Whether PECM Handles an Alloy Well?

Pulse timing, gap control and electrolyte selection all affect how an oxide film forms and breaks down. That's why refractory metals need process development specific to each alloy and feature. Voxel's electrolyte and cathode approaches are proprietary, and they account for much of the difference in results between electrochemical machining providers.

But Voxel has had key successes in recent years through dedicated R&D: we can now utilize proprietary methodologies to reliably machine tungsten, molybdenum, cobalt-chrome, and many titanium alloys in repeatable, high-volume, tight-tolerance environments, with some other refractory metals also machinable under specialized settings.

Working on a molybdenum plate, a tungsten flow restrictor or a titanium implant feature? Contact us at info@voxelinnovations.com.

Material	Status	Notes
Tungsten	Machined with PECM	Challenging; feasibility depends heavily on geometry
Molybdenum	Machined with PECM	Established; suited to plates, arrays and internal features
Titanium and Ti alloys	Machined with PECM	Difficult at times; alloy phase behavior matters
Zirconium	Machined with PECM	Evaluated per application
Niobium	Machined with PECM	Evaluated per application
Cobalt-chromium	Machined with PECM	Established across medical components
Other refractory metals	Feasibility review	Send material and geometry for evaluation