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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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