Custom Copper Tungsten Compositions Influence Performance
Part Four of our series on Copper Tungsten for Electrical Discharge Machining (EDM) electrodes, is all about composition. As we’ve previously discussed, much of the value of the material is the result of two unique sets of mechanical and physical properties. Copper’s high electrical conductivity and arc erosion resistance, combined with the excellent thermal conductivity and wear resistance of Tungsten make for an optimal combination of both performance and manufacturability.
Copper Tungsten Composite Material
Composite materials like Copper Tungsten are not true alloys, but pseudo-alloys made up of a composite (as the name suggests) of two chemically or physically disparate materials. Traditional alloy methods, which require the pure materials to be soluble, will be ineffective for materials like Copper Tungsten where the melting point of Tungsten is approximately 3,400ºC, and the melting point of Copper is near 1,083ºC. The Copper would evaporate before the Tungsten even began to melt.
As EDM has continued to become an invaluable non-traditional machining process for making dies and molds–especially in Tungsten carbide and tool steel workpieces–powder metallurgists have experimented with a variety of fabrication methods for combining Copper Tungsten including liquid phase sintering, metal injection molding, hot press, vacuum plasma spray, mechanical alloying and other methods.
Infiltration and Porosity
Most commonly, Copper Tungsten electrode material is fabricated through a powder metallurgy process, wherein a porous pre-sintered Tungsten ‘skeleton’ is infiltrated by a liquid copper. A key concern for electrode manufacturers and for EDM machinists is porosity, as it can cause a bump in the EDM cavity. Generally, the press-sinter-infiltrate process reduces the risk of porosity, but a fully dense Copper Tungsten composite is almost impossible to fabricate, as the difference in thermal contraction between solid tungsten and molten copper during cooling, post-infiltration can still cause some residual porosity.
Some studies have shown that the use of nanoscale Tungsten particles coated onto Copper powder, and a lower sintering temperature has improved the density of the final material, thus reducing the risk of pores, copper lakes, and tungsten agglomerates. Equally, hot-press, a simpler and more cost effective process involving simultaneous heat and pressure has been noted to improve density.
In addition to fabrication methods for improving density and microstructure, manipulating the composition ratio can produced specialized performance properties for the EDM application. Generally the higher the percentage of Tungsten, the greater the EWR (electrode wear resistance) and cutting stability, but at the expense of slower cutting speeds. Conversely, the more Copper, the better the surface finish and MRR (material removal rate), but with reduced EWR.
Special Compositions for EDM Applications
The most standard composition is 30% Cu, 70%. However, the composition can be customized based on application. Other typical compositions include 50% W, 50% Cu for sealed switches, and 89% W, 11 % Cu for spot welding electrodes.
To find out more about copper tungsten and why you might choose it for your EDM related applications, download our latest white paper!