UNDERSTANDING EDM WIRE
It is important to understand the technical development and innovation behind an individual brand or product before choosing the perfect wire for your application. All EDM wires are classified into one of two categories: Standard Brass EDM wire and Coated Brass EDM wires.
Standard Brass EDM Wires
All standard brass EDM wires are composed of a homogeneous binary-copper zinc alloy with zinc contents ranging from 35 – 40 weight percent. It is known that EDM electrodes benefit from increased zinc content because of zinc’s low energy of vaporization and to a lesser extent are hindered by lower electrical conductivity. It is also known from metallurgical principles that alpha phase brass is easier to deform than beta phase brass so it is no accident that the most common binary Cu/Zn alloys have 35% (Asian) and 37% (European) zinc contents. They contain the maximum zinc content available in alpha phase brass which has a balance between optimum vaporization and electrical conductivity. The two phase brass (40% Zn) alloy typically cuts 3 - 5 % faster than the single phase alpha brass alloys, but generally costs more because of the added processing costs required by additional process anneals and in some applications can be subject to brittleness issues.
Coated Brass EDM Wires
Zinc Plated Wire was the first attempt to attain higher cutting speed than standard brass EDM wire was to electroplate a thin pure zinc coating onto the surface of a single phase alpha brass alloy core. Such wires became known as “A-Type” wires and are still in use today. Unfortunately the thin coating thickness limited the height of the workpiece to 3 – 4 inches for appropriate applications since the low melting point of the zinc (420⁰C) allows the coating to be softened and literally blown off the surface of the core wire by the intensity of the spark discharges and the hydraulic forces of high pressure flush systems.
Diffusion Annealed Coated Wire increases height limitations. The workpiece height limitation faced by “A-Type” wires was overcome by diffusion annealing which is a well-known metallurgical phenomenon where two elements intermix, driven by a concentration gradient, when heated to an elevated temperature.
The first diffusion annealed wire type introduced was a beta phase brass layer on a copper core which has become known as “X-Type” wire which typically is only used on Charmilles machine tool models due to the low tensile strength of the core. “X-Type” wires have the advantage of a zinc rich surface (approximately 45% zinc), a high conductivity, and the elevated melting point of beta phase brass (approximately 880⁰C).
The second diffusion annealed wire type introduced was a beta phase brass layer on an 80Cu/20Zn core and that product has become known as “D-Type” wire which has found application on a wide variety of machine tool types. It too has a zinc enriched surface (approximately 45% zinc), a higher conductivity core than standard brass, and the elevated melting point of beta phase brass (approximately 880⁰C).
Gamma Phase Coated Wire is a brittle intermetallic alloy (Cu5Zn8) with a high zinc content (approximately 65% Zn) which can also be synthesized by a diffusion anneal. However, when such coatings are wire drawn subsequent to the diffusion anneal, the coating will fracture due to its brittleness and redistribute around the wire circumference creating a discontinuous layer sometimes described as a “porous layer” which promotes turbulent flow enhancing the flushing of debris. However, it is zinc enrichment created at the surface combined with the elevated melting point of CuZn gamma phase (approximately 800⁰C) which are the biggest factors contributing to the outstanding performance of Gamma Phase Brass coated wire types.
Multi-Phase Layered Coated Wire In addition to the three previously brass phases (Alpha, Beta, and Gamma Phase), there is one more brass phase that has unique properties that can be utilized in EDM applications – Epsilon Phase Brass (approximately 85% zinc content). All of these phases can be arranged in various combinations to take advantage of their individual physical and chemical properties. Examples of these combinations are Gamma X, Gamma D, and Epsilon.
