A new type of surface-modified brass wire recently developed in the United States promises to help users of the wire EDM process. The new wire is less expensive than premium grades but offers significant performance advantages in applications where standard brass wire is most commonly used. The composition and structure of the wire represent a new metallurgical system, which is responsible for the product's performance characteristics.
Typically, users of wire EDM have relied heavily on homogenous brass alloy wires, so much so that these alloys have unquestionably become the predominate wire of choice in EDM. As such, they represent the standard against which all other wire products are measured in terms of their value. To be sure, other products have conclusively demonstrated their worthiness relative to performance, particularly with regard to cutting speed.
However, these high performance wires typically represent a price premium of some three to five times the cost of common brass wire, causing many to question their value. In addition, auto-threading machines have become increasingly more common in recent years, and of the various types of high performance wires, only zinc-coated hard brass has been found to be reliable in auto-thread applications. Hard brass wires, in contrast, have always been found to be reliable in auto-thread applications and thus have become the performance standard in this category.
The new wire offers a favorably altered balance between cost and performance for EDM wire. The concept was developed by Composite Concepts Co., Inc. (Milford,New Hampshire), and the resulting family of wire products, known as GammaWire, is being manufactured by Rea Engineered Wire Products (Fort Wayne, Indiana) for exclusive distribution by Intech EDM (Broadview, Illinois).
The first of this family is tradenamed Deltacut One and has been shown to increase the cutting speed of common wire applications by 15 to 30 percent more than brass using standard machine technologies, and by as much as 80 percent using an optimized machine technology.
This sort of performance is not exactly new to wire EDM, but the cost of this product is. Its price is midway between brass and premium wires. Deltacut One can also match the auto-thread performance of hard brass wires, making it suitable for virtually all applications under 4 inches in height with a taper angle less than 5 degrees, its developers say.
Dr. Dan Tomalin, President of Composite Concepts and author of the patent application which is pending on this new metallurgical concept for EDM wire, explains that the performance of an EDM wire is directly proportional to the zinc content of the eroding surface of the wire, if that surface is thick and tenacious enough to survive the erosion process. Common EDM brass typically has a zinc content of 35-37 percent (by weight) which is the maximum zinc content you can achieve in alpha phase brass, the metallurgical form of homogeneous brass which easily can be cold drawn to fine wire sizes. Although zinc-coated wires have a surface with a 100-percent zinc content, obviously as high as you can get, the low melting point of zinc coatings unfortunately limits their tenacity, because they can be softened easily, distorted and removed by mechanical or hydraulic forces. Currently available diffusion annealed coatings (for example, "X" and "D" type wires) contain beta phase brass which typically has a zinc content of 45 percent and a high melting point thereby providing excellent tenacity. Unfortunately the traditional diffusion anneal heat treatment adds considerable cost to the manufacturing process, hence the higher price.
The GammaWire family of products takes advantage of a low cost method of distributing a layer of gamma phase brass, with a typical zinc content of 65 percent and a high melting point providing excellent tenacity, over the surface of the wire. "It is as simple as that, a higher zinc content on a surface which has excellent tenacity, therefore superior performance," Dr. Tomalin summarizes. He also points out that the phenomena that allows these enriched zinc surface wires to cut fast can also be used to provide better surface finishes at more conventional metal removal rates. Thus the new wire is capable of reducing cycle times by lowering the cost of rough cuts via increased metal removal rates, or by reducing the number of skim passes required to achieve a given surface finish at current rates.
Wire cleanliness, as determined by the presence or absence of foreign materials such as paraffin or the metal oxides commonly associated with traditional diffusion annealed wires, has also become an issue on many machines manufactured in Japan. Dr. Tomalin said this fact was carefully considered in developing the GammaWire process. As a result, sources of contamination were carefully identified and eliminated so that the cleanliness of these wires approaches the best of common brass wire types. Such cleanliness coupled with reliable auto-threading makes this wire especially suitable for Japanese EDM machines.
Dr. Tomalin believes that the advent of a new metallurgical system making EDM wires more productive in a wide variety of applications will have a significant impact on wire EDM usage. The metallurgy of EDM wire has evolved from copper to brass, to zinc-coated brass, to gamma phase brass as the process becomes more economically viable. He expects to see further advances in wire metallurgy and expects these advances to a major force shaping the direction of this technology well into the next century.