WEDM Technical Advice & Articles - The Metallurgy of Brass EDM Wire

The Metallurgy of Brass EDM Wire – Part #3  Continued from “The Role of Metallurgy in EDM Wire”

  Continuing with our exploration of Metallurgy of EDM wire & work pieces from part 2, we move on to the bedrock of Wire EDM!

We know brass works well as an EDM wire material because it is a stable conductor. Today brass represents the most popular wire type by a very wide margin. Undoubtedly the reason people first tried brass is because it is stronger than copper and they assumed it would resist wire breakage. Although it is indisputable that brass is stronger than copper, it turns out that has little to do with the real reason it works better in WEDM. In order to understand the true value of brass in this application, we need to spend a few minutes understanding some of the basic metallurgy of brass.

Brass is an alloy of copper and zinc. If one takes a block of zinc and presses it against a block of copper while heating the two, one will form brass as the copper atoms diffuse into the zinc and the zinc into the copper. However metallurgy tells us there are many different forms, or in its language different phases of brass, and each phase can be uniquely defined by its chemical, physical, and mechanical properties. In the picture below we see that in a diffusion couple formed between copper and zinc, we have developed three distinctly different phases of brass -- alpha phase, beta phase, and gamma phase. There has also been a new development Epsilon Phase it is more of a coating than a true phase though.

https://www.gip-edmwire.com/wedm-tech-wize Brass Metallurgy Review

    Below, I have listed some of the distinguishing characteristics of these brass phases as they relate to EDM. Common EDM brass wire is alpha phase brass with a zinc content of 35% to 37% zinc. The choice of this alloy composition is no accident. The zinc content of 35% to 37% is as high as one can safely achieve in a commercial Brass alloy and still have the alloy remain alpha phase brass. What is so important about alpha phase brass? Alpha phase brass is very ductile and can very easily be drawn to fine diameter wire at room temperature. Higher zinc content phases of brass become increasingly brittle which either prevents them form being cold drawn altogether, or significantly raises the cost to produce them.

Beta phase brass has a higher zinc content than alpha phase brass 45% versus 35% to 37% zinc and has only a slightly lower melting point than alpha phase. Although it will work harden faster and become more brittle and therefore more difficult to draw cold than alpha phase, one can play a metallurgical trick on it. Used as a relatively thin coating on a copper or alpha phase brass core, it can be successfully cold drawn. The trick is to have the thin coating metallurgically bonded to the core so it can be forced to deform and therefore cold drawn into fine wire. Hence its use as the coating of “X-Type” and “D-Type” wires.

   Unfortunately, gamma phase brass is less ductile and completely brittle because of the complex crystal structures that are formed, but its very high zinc content and relatively high melting point make it extremely attractive as a candidate coating material. There are metallurgical tricks that can also be played on it to bond the discontinuous particles of gamma phase to the core wire, and so it has become the basis of the recently introduced “Gamma-Type” wires. We will talk more on Gamma & Epsilon in the future.

    https://www.gip-edmwire.com/wedm-tech-wize Brass Metallurgy Review

  Getting back to brass EDM wire, the tensile strength of brass wires range from 54,000-173,000 PSI, (373 N/mm² - 1200 N/mm²) depending upon the composition of the alloy and how its tempered.

Brass wires are polished gold in color.

Brass wires with a matte finish or with discoloration are indications of oxidation or contamination.

Brass wire is an all-around value product that can be used by almost all machines.

Brass wire is cost-effective cutting of tool steels & most materials is possible with plain brass wire.

Available in elongation ranges from <2% to over 30%, wires with low percentages of elongation will thread reliably but are limited in tapering ability. Half-Hard & Soft wires with high elongation can taper-cut to 45° in some WEDM’s but with much reduced threading reliability if you don’t have an annealing unit on your WEDM! 

What is the difference between Half Hard & Soft brass wire you might ask? 

Well the tensile strength is less in soft wire. 

1. Half-Hard wire range is start at 500 N/mm²

2. Soft wire range start at 440 N/mm² and go down

Soft wire – Can make this type of high taper cuts! 

The last thing I will touch on is the negative of brass wire!

Copper is the best conductor of electricity.

So brass being 65% to 63% copper should be pretty good too right?

Not really if you look at copper at 100% conductive, brass with 35% to 37% zinc in it is only 25% to 28% as conductive as copper!

What does that mean to operators? Basically It will cut slower than a copper core wire will.

That is not the only negative in my experience brass wire is not suited well for two materials carbide 

& PCD.

Why is that? Because it will actual start electro plating the material in the skim passes.

The materials after finishing will have a gold color and the surface stays that way after cleaning and it 

is soft. Not what you want in a cutting tool edge. So you don’t want to use brass wire in those applications.

 Then what wire do you want to use for Carbide & PCD? We will address this in the future! 

The Next edition of WEDM Tech Wize we will look at why EDM Wire Breaks! 

“May your Sparks always be ON-TIME!”

The Physical Properties of Eroded Surfaces Encountered In Wire EDM - Part #2

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The Physical Properties of Eroded Surfaces Encountered In Wire EDM – Part #2  Continued from “The Role of Metallurgy in EDM Wire”

 It might be easiest to start this discussion by considering what are the potential physical surface properties which have been suggested to control the wire flushing efficiency in WEDM.  Flushing in the WEDM application involves removing the solid debris created by each individual discharge which in fact is created one at a time in rapid succession but never two simultaneously.  The discharge envelope collapses under the pressure of the liquid dielectric medium (D.I. Water in most cases) after the ON-time and OFF-time cycles are completed by quenching the debris converting it to solid particulate.  

Those candidate surface properties controlling the wire’s contribution to flushing are:

1. “Vaporization Temperature” (represented by eroded surfaces’ Boiling Point) as suggested by the traditionally accepted theories of  WEDM  Heat of Sublimation as characterized by the energy per unit volume needed to transform the material affected by each individual discharge. If you will recall the Post of “Five Myths of EDM” the first candidate property cannot be the surfaces’ “vaporization temperature” because, as discussed in that Post, there is no such animal since metals vaporize over a very wide range of temperatures and not at some arbitrary fixed temperature. 

So that leaves 

2. the Heat of Sublimation as the only viable candidate. You need to know, as illustrated in the previous Post, that sublimation is defined as the process whereby a substance transitions directly from the solid to the gas state, without passing through the liquid state.  The Heat of Sublimation is the heat required to accomplish that transition and has the correct units, e.g. KJ/cm3, which relate to the WEDM process where one is trying to remove unit volumes of the workpiece.  

This Skim pass shows an example of the Discharge during the WEDM process: This is easy Flushing! www.gip-edmwire.com

  Each discharge introduces a finite amount of energy into finite units of volume at both ends of the discharge envelope, i.e. at the wire and the workpiece. So, what is the relationship of Heat of Sublimation to flushing efficiency?  We just said that each discharge introduces a finite amount of energy into the discharge envelope in the gap between the wire electrode and the workpiece, some fixed portion of that energy will be delivered to the wire and the balance to the workpiece.  We will have the most efficient flushing of the wire debris if the maximum proportion of wire debris is from solidified vapor, i.e. smaller sized particulate.  That maximum will be determined by the Heat of Sublimation of the eroded wire volume, i.e. the lower the energy required to accomplish the transition the higher the probability it will happen.  The same is true of the workpiece but unfortunately one has little control over that since the workpiece is a given and not a choice.  As we will see later, that does not mean this parameter cannot be used to understand the performance of the workpiece. However, we do have control over the physical properties of the wire surface as determined by one’s choice of wire type.

   The following chart lists the volumetric heats of sublimation of the metallic elements.

Volumetric Value Chart for Metallic Elements www.gip-edmwire.com

Unfortunately, the volumetric heats of sublimation of metallic alloys are not readily available but one can assume they logically would be similarly ranked in the same order and magnitudes as those of the predominate alloying element of a given alloy system, e.g. iron in the case of tool steels or zinc in the case of brass alloys.  We will have more to say about the metallurgy of the brass alloy system which is critical to the WEDM application in a subsequent Post, but for now just consider the ranking of the metallic elements.  We are not so interested in the absolute values of the heats of sublimation but rather to their relative magnitudes.  For example, the relatively low value of zinc explains its predominant role in the WEDM application.  Although we have and will be focusing on the implications of this chart to the wire debris contributing to the total debris being flushed, note that the chart can also be used to better understand the simplicity or difficulty of using WEDM to erode various workpiece materials.  Note that aluminum has about half the value of iron (Tool Steel).  Do you suppose that has anything to do with the faster cutting speed of eroding aluminum parts as compared to tool steel parts?  The problem is aluminum flushes so efficiently it also obstructs the filtration system with its very fine debris particulate.  Guess why that might be!  Now that you understand the heart of our concept of approaching wire type selection, we will move on to the metallurgy of brass wire in the next edition of WEDM Tech Wize. “May your Sparks always be ON-TIME!”

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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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