Copper Tungsten Properties & Increased Thermal Conductivity
If you’ve been reading the past few posts on the use of copper tungsten for Electrical Discharge Machining (EDM) electrodes, you know by now that the success of the process is largely influenced by both workpiece and electrode material properties.
Accordingly, choosing an electrode material ultimately depends on its ability to interact productively with your workpiece material, as well as on your particular production goal (e.g., cost savings? surface finish? improved wear resistance?).
There are several common metrics for measuring the success of your EDM production job:
- EWR (electrode wear ratio),
- MRR (material removal rate) and
- Ra (surface finish).
So far we’ve discussed how copper tungsten can improve EWR both because of its innate structural integrity as derived from both copper and tungsten, and because of its resistance to DC arcing–a common EDM related frustration.
This post is all about Material Removal Rates. MRR, or cutting speed, is largely dependent on the interaction between material properties and machining parameters.
Material Properties of Copper Tungsten
Copper with its excellent thermal conductivity, is the portion of the CuW composition that drives cutting speed. This begs the question: “Why not use a pure copper electrode then?” The answer is that electrolytic (pure) copper poses enormous challenges in manufacturability and wear rates. But, as you may have guessed, alloying it with Tungsten improves both conditions dramatically producing a better overall performance.
Interestingly, the addition of tungsten to copper often generates a recast layer, commonly referred to as “black layer,” during the EDM process which can actually improve wear resistance. However the additional build up slightly lowers thermal conductivity and ultimately reduces the MRR.
EDM Process Parameters to Improve MRR
Because EDM is a thermal process one would logically infer that increasing thermal conductivity will increase MRR. But material removal rate is only one of three interdependent variables listed above. In reality, it’s a bit more of a “Goldilocks” problem; there is a “just-right” solution where conductivity is high enough to improve cutting rates, but not so high as to leave the spark gap empty of heat. The material lends itself to a kind of process parameter sweet spot that can produce on spec parts, incredibly efficiently. And luckily, there are many really smart people who have developed empirical models for determining process parameters, as they relate to material properties.
And more crucially, studies have shown that thermal conductivity, alone, does not influence MRR, but the combination of copper tungsten’s thermal conductivity and peak current does–the direct correlation being a stronger, more impactful spark makes for fast (albeit not pretty) material removal. (Note: Just increasing peak current without also increasing conductivity, compromises surface finish because of the more explosive, uneven burn.) More specifically, the influence of thermal conductivity is realized only in conjunction with peak current.
To find out more about copper tungsten and why you might choose for your EDM related applications, download our free white paper!