Why do high recrystallization temperatures matter in tungsten, molybdenum, and other wires?
We often get a lot of questions about tungsten wire and dopants. Specifically, people have asked, outside of applications in incandescent lighting, why is tungsten wire still doped? After all, why get a product containing something you don’t need and possibly don’t want?
In the days before LEDs and CFLs, dopants were something everyone wanted in tungsten wire — if not as a manufacturer, then as a consumer/user of incandescent light bulbs. That is because the dopants in the bulbs’ tungsten wire filaments are what enabled these light bulbs to work properly. Without dopants, incandescent bulbs would sag at their white-hot operating temperature, causing arcing and filament failure.
If not for dopants and their non-sag properties, we would not have been able to have incandescent lamps and all of the benefits — or at least not until the advent of the new technologies that are now making incandescent light bulbs obsolete.
How do dopants work?
How do dopants increase non-sag properties in pure (undoped) tungsten wire for use in lamp filaments at high temperatures? Dopants can achieve this effect a few different ways, but basically the tungsten wire is doped, at the powder mixing stage, with potassium and other elements — typically, alumina and silica. The other alumina and silica elements out-gas and the potassium remains, acting as a magic “ball bearing” that lubricates and serves as a kind of buffer between the elongated grain structures of pure drawn tungsten.
The key is that the potassium dopant raises the recrystallization temperature of the tungsten wire, giving it non-sag properties and effectively eliminating the propensity of pure tungsten wire to sag when heated to incandescent temperatures.
As the material is heavily drawn into wire, the interlocking grains get longer and the potassium dopant is spread out; when heated, it volatilizes into a linear array of tiny (submicron-size) bubbles. As the rows of bubbles become finer and longer with increasing deformation, the recrystallization temperature rises, and the interlocking structure becomes more pronounced. It is this structure that gives doped tungsten wire its non-sag properties and effectively eliminates the propensity of pure tungsten to fail when coiled and heated to incandescent temperatures.
In technical terms, commercially pure (undoped) tungsten wire has a recrystallization temperature as high as 1205-1400°C (2201-2552°F). However, tungsten wire doped with alumina, silica, and potassium is characterized by recrystallization temperatures greater than 1800°C (> 3272°F) and beyond, with elevated potassium.
The same recrystallization rule is also true for other refractory metals used for wire. For example, the temperature at which commercially available molybdenum fully recrystallizes in one hour is 1100°C (2012ºF); potassium and silica doped molybdenum recrystallizes at 1200-1800°C (2192-3270°F), depending on how the material was reduced.
Why not add the most dopant possible?
If potassium (and other dopants) are so good for preventing sag and breakage, and high recrystallization is a good thing (more on that later), then why don’t tungsten manufacturers — and the lighting industry that has supported them for so many years — use the maximum amount of potassium in the doping process? After all, potassium as an element is highly abundant in nature and is inexpensive.
The reality is, adding too much dopant can produce excessive breakage in the manufacture of tungsten wire, resulting in poor yields. Paradoxically, even though potassium keeps tungsten wire from breaking during its service life as a light bulb filament, it creates opportunities for breakage during the process of drawing tungsten wire. And breakage interrupts the manufacturing process, causing delays and raising costs.
For those who still care — and we at Metal Cutting do, because we cut, grind, and sell a lot of tungsten, both as wire and in many other forms — managing dopant levels is an important expertise to have. It is a skill that is crucial to producing the highest recrystallization temperature wire at a reasonable cost.
What are the pros and cons of doping for higher recrystallization temperatures?
Processed to have a higher recrystallization temperature than when in its pure state, tungsten, molybdenum, and other wires can remain ductile at room temperature and at very high operating temperatures. The resulting elongated, stacked microstructure also gives the doped wire properties such as good creep resistance, dimensional stability, and easier machining than the pure (undoped) product.
The flip side is that when you recrystallize, you embrittle the tungsten (or other metal) wire, requiring that the wire then be annealed in order to return the tungsten wire to its desired strength. (Annealing alters the properties of a material to increase its ductility and make it more workable; it involves heating the material in an annealing furnace to above the material’s recrystallization temperature, maintaining a suitable temperature, and then cooling.) If you do not anneal the wire, when you subsequently bring the wire to or above its recrystallization temperature during drawing, the wire will break – ultimately, causing the failure of the product in which the wire is used.
So, WHY are there still dopants in tungsten wire?
If the recrystallization properties of tungsten wire are well understood by the dwindling few who use it to make incandescent light bulbs (in itself, another topic), this bring us back to the original questions: Why are dopants still generally used in tungsten wire?
The answer is, there are still product manufacturers for whom elevated temperatures are integral to their applications and their use of tungsten wire. Think about traveling wave tubes or diamond deposition furnaces. For these businesses, the recrystallization temperature of tungsten wire remains critically important, even as their workplaces eliminate the last vestiges of incandescent lighting. Some manufacturers use tungsten wire at temperatures that are elevated but not approaching recrystallization, for products such as electrodes, electronics, and medical applications. For still other manufacturers who use tungsten wire in mechanical applications, such as in probes that operate at room temperature, the presence of dopants and recrystallization properties are (hopefully) irrelevant.
And just how MUCH dopant is used, anyway?
The actual amount of dopant in tungsten, molybdenum, or other wire is part of the “secret recipe” specific to each manufacturer or supplier. As a company that has been providing specialized tungsten and molybdenum wire products for decades, Metal Cutting has its own “secrets.” We can share with you that a dopant “recipe” usually contains 50-90 ppm of potassium. However, there is much more to the recipe — and much more to the expertise that goes into managing dopant levels and recrystallization temperatures.