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.

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.

Half-Hard wire range is start at 500 N/mm²
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!”