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In many aspects of life, the characteristics of strength and toughness are thought to be synonymous, and indeed that is often the case. However in the world of wire EDM, considering strength and toughness to be synonymous can result in a fair measure of disappointment when one is struggling with wire breaks caused by less than ideal flushing conditions. However, a new EDM wire from Rea Magnet Wire Co. (Fort Wayne, Indiana) has been developed with the optimum ratio of strength to toughness required to resist breaking.
This wire supplier finds that, in wire EDM, strength has only a minimal impact on wire breakage while toughness controls its frequency. They believe that favoring strength over toughness can be a serious mistake. That's because the strength of a wire is usually characterized by its tensile strength, which is a simple, but arbitrary, measurement that has very little in common with the EDM application.
The company's research shows that, once a wire's tensile strength exceeds an application's threshold value (usually less than 90,000 psi), additional "strength" offers little value to cutting performance and will not reduce wire breakage.
However, the material characteristic that metallurgists' call fracture toughness has been shown to control wire breakage frequency. Simply stated, an EDM wire will break when a discharge (or DC arc) introduces a flaw in the wire which is greater than the critical flaw size necessary to produce catastrophic failure under the preload tension that has been applied.
Many high strength materials, including EDM wires, are notorious for their low fracture toughness, that is, their inability to withstand relatively small flaws without failing. Each and every discharge in the EDM process makes a crater, which metallurgists would term a defect or flaw, in both the wire and the workpiece. As flushing conditions deteriorate, those flaws tend to become larger and larger, eventually causing catastrophic failure of the wire. In short, it breaks.
In the past, an operator had two options for coping with wire breakage when flushing conditions deteriorated, for example, the conditions often encountered when cutting tall parts, or when hydraulic conditions are compromised by internal cavities or displaced flushing cups. One could lower the wire tension (preload) which allows larger flaws to be tolerated, or one could lower the power setting thereby introducing smaller flaws. Although either of these adjustments allow one to continue cutting with fewer breaks, the speed of the cut is typically reduced, often dramatically, raising costs and/or reducing profits.
More recently, a third option has been available. By using a composite wire in which the core is made from a material with high fracture toughness, such as plain carbon steel, one could continue to cut at higher power settings without having to reduce wire tension. The new wire being introduced by Rea Magnet Wire Co. features a steel core that allows an operator to increase the wire tension while still maintaining higher power settings in deteriorated flushing conditions. This wire has been engineered to meet the needs of demanding applications, including very tall parts (the company reports that it has been tested successfully in cuts 20 inches and taller), interrupted cuts, variable height workpieces where the flush cups are displaced from some sections, and difficult to machine materials such as stainless steel, nickel based superalloys, graphite, tungsten carbide, polycrystalline diamond, and others.
The wire is similar to earlier types of steel-core wire in that it is formed around a core of plain carbon steel which composes close to 40 percent of its cross sectional area. According to its developers, the core gives this composite wire more than adequate tensile strength (125,000 psi), but more important, it provides the highest fracture toughness of any commercially available EDM wire.
The steel core is surrounded by a layer of high purity copper that enhances the composite wire's conductivity (32 to 35 percent of the conductivity of copper). In comparison, standard brass EDM wires typically have conductivities in the range of 20 to 22 percent and are therefore less efficient at carrying the high electrical currents desired for increased cutting speeds. Finally, the wire is coated with a thick layer (1,200 microinches) of zinc-enriched brass by a proprietary process.
The coating thickness can be critical in some applications, particularly taller parts where coatings can be completely consumed and their benefit lost. For difficult flushing applications, the company says, it is important to have a zinc-rich alloy coating because high zinc alloys have been shown to significantly improve the flushability of composite wire. The zinc content of Rea's coating ranges between 40 and 44 percent which is fractionally less than the 44-45 percent range of diffusion annealed types. Other steel-core wires have had one or more of the above characteristics, but Rea believes that its new wire is the first time this set of properties has been available in one general purpose product.
The advantage is that it allows operators greater freedom in choosing the EDM operating conditions. A less-experienced operator who may not make the best choice of settings can run more conventional jobs without continual wire breaks. Experienced operators can attempt more challenging jobs that may have been previously thought to be too difficult or unprofitable.
The Rea steel-core wire is made in the United States and is available on various European (Din 125, 160, 200) and Japanese (P3, P5, P15) spool types in spool weights, ranging from 6.6 to 50 pounds. It is available in standard diameters of 0.010 inch (0.25 mm), 0.012 inch (0.30 mm), and 0.013 inch (0.33 mm) with other sizes available on request. MDA MMS