Patent Application
20070295695
The features and advantages of this invention will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying figures, wherein:
In general, EDM wire will cut more efficiently with a higher zinc content on the eroding surface. For instance a zinc coated brass alloy wire will cut more efficiently than an uncoated brass alloy wire. However, the melting point of the coating is an important factor in determining the efficiency of any given coating's performance. Since unalloyed zinc has a relatively low melting point of 420° C., alloyed coatings with higher melting points (e.g. beta, gamma, or epsilon phase brass alloy coatings) but with lower zinc contents can outperform unalloyed zinc coatings. The higher melting points of these alloys delays them from being removed from the eroding surface by the mechanical and hydraulic forces imposed upon it, and therefore a higher zinc content is available on the surface when it is needed for further erosion. Unfortunately these higher zinc content alloy phases tend to be brittle and therefore are difficult and/or expensive be included on hard drawn EDM wires as continuous coatings.
The brass alloy phases commonly applicable to EDM wires are alpha phase, beta phase, gamma phase, and epsilon phase. Of the brass alloy phases, alpha phase has the highest melting point (approximately 910° C. at its highest commercially feasible zinc content of 35-37 weight percent), beta phase has the next highest melting point (approximately 890° C. in a diffusion annealed coating with a typical 45 weight percent zinc content), gamma phase has the next highest melting point (approximately 800° C. in a diffusion annealed coating with a typical 65 weight percent zinc content), and epsilon phase has the lowest melting point (approximately 550° C. in a diffusion annealed coating with a typical 85 weight percent zinc content).
As the zinc content of these alloy phases increases, the ductility of the phases decreases proportionately and hence the resulting wire becomes more difficult to draw without damaging the coating. The ability to cold draw EDM wire is important because EDM wire needs to have an elevated tensile strength to sustain the tensile loads that are imposed on the wire to keep them accurately located as the process proceeds. Because of their relatively low zinc content, beta phase coatings have been successfully applied to EDM wires, even though they are brittle enough that a full sectioned beta phase wire would be difficult to cold draw. Gamma phase coatings are even more brittle than beta phase coatings, and in point of fact, they are often so brittle that they produce discontinuous coatings where islands of gamma phase become embedded in the wire surface after being cold drawn. However even though the coating does not cover the full wire surface, the increased zinc content of the surface is enough that gamma phase coatings have been shown to outperform beta phase coatings. Conventionally processed, epsilon phase coatings are even more brittle than gamma phase. In addition to the brittleness limitation, epsilon phase is very unstable making it difficult to control the process of converting a zinc coating to epsilon phase in a manner similar to that used for converting a zinc coating to gamma phase.
The invention herein disclosed provides a process that allows the zinc to be converted to epsilon phase in a controlled manner thereby allowing the higher zinc content of the alloy phase coating to be taken advantage of. Furthermore by processing the wire at lower diffusion anneal temperatures than previously attempted, it has been discovered that zinc coatings can be converted to epsilon phase at very low temperatures where the epsilon phase is very stable. At these low temperatures, the process can be precisely controlled such that the metallurgical structure (and therefore the mechanical properties) of the wire is not modified.
The lower melting point of the epsilon phase is generally considered to be a disadvantage of epsilon phase coatings when compared to beta or gamma phase coatings. However, the higher zinc content of the epsilon phase has been found to offset that disadvantage such that epsilon phase coatings have been found to match the performance of beta phase coatings while being competitive with the performance of gamma phase coatings. Therefore, the epsilon phase coating provides similar cutting performance while having a lower cost to manufacture than either beta or gamma phase. Infiltrating the porous epsilon phase coating with graphite, e.g. by drawing the wire in a lubricant composed of a suspension of fine graphite particles in an aqueous medium, can further improve the performance of an epsilon phase coating.
In the following example, EDM wire was produced with a finish diameter of 0.25 mm and at a starting size and heat treatment as described.
Core: 65Cu/35Zn; electroplated 10 μm of zinc at 0.9 mm diameter
Cold drawn from 0.9 mm to 0.25 mm
Annealing Temperature: 70° C.
Annealing Time: 20 hours (air cool)
Referring to
It is also believed that the ductility of the epsilon phase formed at such low temperatures is ductile enough to allow the heat treated wire to be drawn again to a finish diameter while maintaining a substantially continuous coating of epsilon phase, thereby further improving the effectiveness of the coating. The added drawing step may introduce some discontinuities in the coating.
As can be seen from the foregoing description, drawing a zinc coated, copper bearing core wire to its finish size and then heat treating the wire at very low temperature provides an EDM wire with a substantially continuous epsilon phase brass coating while maintaining the mechanical properties of the core wire. The coating resulting from the diffusion anneal may be porous, allowing it to be infiltrated with graphite to further enhance its discharge properties. The resulting EDM wire electrode can equal the cutting speed of beta phase coatings and remain competitive with the cutting speed of gamma phase coatings at a lower manufacturing cost than either of the other high zinc phase coatings. It is also believed that the epsilon coating is ductile enough to allow cold drawing of the heat treated wire while maintaining a substantially continuous or discontinuous coating of epsilon phase brass.
While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. The appended claims are therefore intended to cover any variations, uses, or adaptations of the invention using its general principles as well as any departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
