The Role of Metallurgy In EDM Wire - The Technology of Flushing
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The Role of Metallurgy in EDM Wire
The metallurgical properties of the wire and the workpiece are equally important to the mechanics of flushing in EDM, and as most of you are aware, EDM needs great flushing.
Thus the role of metallurgy in EDM is quite extensive. Not only do the physical properties of surfaces influence flushing by determining the particulate size of the eroded debris, but the electrical properties of the wire and workpiece control the energy transfer process at the gap.
The fracture mechanic properties of the wire, control its breakage frequency which in turn controls the electrical circuit integrity.
As everyone here knows, when the wire electrode breaks in WEDM, no current flows, and no metal can be removed as the only thing that vaporizes is profits.
Which type of wire cuts better?
Keep reading it will be worth the time!
Critical EDM Wire Properties
•Zinc Concentration/Melting Point of Eroding Surface
•Fracture Resistance (a.k.a. Fracture Toughness)
•Electrical Conductivity
•Handling Characteristics, Particularly For AWT
We can now focus on the critical wire properties in WEDM, from the metallurgical perspective. We will be looking at zinc concentration and melting point of the eroding surface, fracture resistance, electrical conductivity, and handling characteristics.
The handling characteristics, more specifically the wire straightness, is critically important to those machine without annealing units as part of there automatic wire threaders, so older machines! As you might suspect, this property is also related to metallurgical phenomenon, but this really in the domain of wire manufacturers, so we will not be digging into it here.
The Annealing unit have made this much less an issue as they have solved the problem of the wire not threading EVERYTIME!
The Technology of Flushing
If you take nothing more than this away from my article, you will be well on your way to understanding the metallurgy of EDM wires.
Earlier I said there is no such thing as a vaporization temperature for metals. However vaporization as a process is critically important to EDM, and the property that controls it is known as the heat of sublimation. Do not be intimidated by this complicated sounding name; it is quite simple concept which we will consider momentarily. In the meantime the fact is, the heat of sublimation determines the flush-ability of wire and workpiece.
Low values yield good flush-ability; high values yield poor flush-ability. Zinc and zinc alloys have low values and therefore provide good flush-ability.
The following two sentences tell you much of what you need to know about wire selection.
When considering EDM wires, the one with surface with the highest zinc content will cut the fastest, IF, that surface with the high zinc content is thick and tenacious enough to survive the erosion process.
The tenacity of the surface is directly proportional to its melting point with a minimum 550oC required for superior performance.
Notice I capitalized the “if” because it is a very “big” if. Thick and tenacious are relative terms that can only be fully judged after the fact. Thickness is important because there must be enough of the eroding surface available to make it through the gap.
Tenacity is important because the large values of the physical and hydraulic forces that exist in the gap of wire EDM are capable of deforming and/or removing some of the surface. It is literally possible for the surface to be “blown away.
Hopefully not like this!
The Melting Point
Hence the importance of melting point. Higher melting points prevent the surface from being softened and distorted by the large hydraulic and mechanical forces imposed on the wire. Why 550oC? Because we know it works. The application of these principles will become more obvious when we review the metallurgy of brass in a few minutes.
In the meantime let’s look at this concept of heat of sublimation to see if we can make some sense of it. All matter exists as one of three phases: as a solid, as a liquid, or as a gas (vapor). You are no doubt familiar with the process which describes the transformation of a solid to a liquid, i.e. melting, or a liquid to a gas or vapor, i.e. boiling. However it is also possible to transform a solid directly to a gas or vapor, and that process is called sublimation. Although the terminology may not be familiar to you, the process may well be.
Consider for a moment two blocks of ice, one wet ice (frozen water ) and the other dry ice (solid carbon dioxide).
Phase Changes of Ice
Consider what will happen if we apply heat to the two blocks of ice. The wet ice will melt forming droplets of water. It turns out there will also be some water vapor in equilibrium with the liquid water as there will always be a gaseous phase in equilibrium with a liquid phase although the amount of gas may be minuscule. If we were then to withdraw the heat, and go so far as to freeze what remains, we would find smaller pieces of solid ice and a snowflake or two formed from the vapor phase.
In the case of the dry ice, the application of heat will cause the dry ice to vaporize without forming any liquid phase (hence the term dry ice). The block would slowly disappear as it sublimed. If one were to then refreeze that which now existed, one would have absolutely nothing since the carbon dioxide gas would have diffused away and could only be returned to a solid state if it were subjected to extreme pressure and cold.
Now let us take one step back toward the world of EDM by considering two solid blocks of “Metal M” and a “Metal V.”
Let us say “Metal M” is similar to H2O and has a tendency to melt first and form very little vapor phase. Such a metal would be characterized as having a high heat of sublimation, i.e. it would take a lot of energy to transform it directly from a solid to a vapor. Let us further say “Metal V” is similar to dry ice (CO2) and has a tendency to vaporize very readily. Such a metal would be characterized as having a low heat of sublimation, i.e. it would take a relatively small amount of energy to transform it directly from a solid to a vapor.
Just as we did with the ice, suppose we were to now to withdraw heat form these examples and were to freeze, or in metallurgical parlance, to quench these systems. In the case of “Metal M,” we would get relatively large pieces of solid from the liquid and relatively small pieces of solid form the vapor in a manner similar to the way the wet ice behaved. In the case of “Metal V,” we would get the same relatively small pieces of solid as we got from the vapor phase of “Metal M,” but none of the larger pieces of solid that previously came from the liquid phase.
If you were wondering what the point of this discussion might be, consider the fact that the sequence we have just described is analogous to what goes on during the EDM process.
The electrical discharge in the EDM process generates an intense heat locally and melts and/or vaporizes a small volume of both the wire and the workpiece. This all occurs in a plasma envelope which eventually collapses under the pressure of the dielectric fluid, thereby quenching the liquid/vapor phases that are present. The solid particulate thus formed is the debris in the gap which must be disposed of (flushed away) in order to maintain the electrical conditions necessary to reform additional discharges. Let me assure you it is much easier to flush the smaller particulate than it is to flush the larger particulate, as you can well imagine. In addition the larger particulate can form a conductive path to initiate a D.C. arc which we will see later can cause instantaneous wire breakage.
Returning full cycle to where we started, it is very helpful to have some knowledge of the relative heats of sublimation of the materials we must deal with in WEDM because that will give us an idea of their relative flush-abilities.
The next edition of WEDM Tech Wise we’ll provide just that sort of information. “May your Sparks always be ON-TIME!”
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