Why Do EDM Wires Break?

For wire-EDM users, wire breakage poses a constant threat to their productivity. But with a little knowledge about the wire-EDM process and the behavior of metals when they are subjected to the process, EDM operators can avoid wire breakage and keep their operations running smoothly and efficiently.

In the wire-EDM process, a traveling wire passes in close proximity to an electrically conductive workpiece. The gap between the two is filled with a dielectric fluid, which also serves to flush away the debris generated by the process. A voltage is applied, creating an electrical discharge. The consequence of the discharge is the melting or vaporization of a small volume of the surface of both the workpiece and the wire by the intense heat that is generated in the gaseous envelope the discharge creates.

This envelope quickly collapses under the pressure of the dielectric fluid. The collapse allows the dielectric fluid to rush into the void and quench the liquid/vapor phases of the workpiece and wire that were inside the envelope. The cooled materials form solid particulate that must be flushed away to prevent it from interfering with the electrical discharge.

A Flaw in the Logic

A broken wire will quickly bring the EDM process to a halt. When breakage occurs, EDM operators typically believe it is the low tensile strength of the wire that is at fault. But EDM wires do not break because their tensile strengths are too low. EDM wires break because their fracture toughness is too low to sustain the flaws that are being introduced by the discharges being generated.

Contrary to popular opinion, the tensile strength of an EDM wire needs only to exceed a certain threshold value to prevent mechanical-overload (tensile) failure. Depending on the application and the equipment involved, this threshold value is in the range of 60,000 to 90,000 psi. For optimum cutting performance, the wire should not exceed the threshold value by much. There may be good reasons for desiring high tensile strength for handling performance, but this is a completely different consideration than cutting performance. If operators specify high tensile strength for handling reasons, they must realize that the high tensile strength might also degrade the wire’s cutting performance.

Critical Craters

It is the size and depth of these craters in relation to the wire’s fracture toughness that determines whether or not the wire will break.

Metallurgists tell us that high-strength metals fail because of inadequate fracture toughness. EDM wires are made of high-strength metals; therefore, this principle applies to them. By studying the fracture mechanics of high-strength materials metallurgists have learned that, to prevent breakage, one must focus on crack propagation rather than crack initiation. According to the metallurgists, it does little good to worry about flaws being initiated in real-world materials and structures, since they are already there. That is certainly the case in an EDM wire as each discharge or arc leaves its calling card in the form of a crater on the wire and the workpiece. To a metallurgist, these craters are the same as flaws or cracks, and each represents a point at which a flaw can propagate and cause the wire to break.

Fracture mechanics further teaches that below some critical size, flaws in the surface of a high-strength metal will not cause problems, but above that critical size they will cause catastrophic failure. In the language of fracture mechanics, a high-strength material that allows small flaws to propagate even when the material is subjected to low stresses is said to have low fracture toughness. Those materials that do not allow moderate-size flaws to propagate unless they are subjected to higher stresses are said to posses a high fracture toughness.

In an EDM application, the size of the crater being created in the wire by the electrical discharge is the critical factor. When the crater exceeds the critical flaw size for the given operating conditions, the wire breaks.

Flushing Is the Key

Figure 1: Excessive debris in the gap causes intense arcing, which erodes the part and the wire to a greater degree.

There are a number of variables that influence the size of the flaws an electrical discharge will produce in an EDM wire, and most are related to flushing. Flushing efficiency directly influences the electrical characteristics of the gap between the wire and workpiece, and thus, goes directly to the heart of the process. The conductive debris that can clutter a gap promotes high-energy arcs rather than spot-mode discharges, which erode the workpiece and wire surfaces in a carefully controlled manner. If flushing is inefficient at removing this debris, then the process’ metal-removal rate also will be inefficient, because the arcing that will result from this contamination will reduce the EDM’s cutting speed, deteriorate the quality of the surface finish, and increase the chances that a flaw large enough to cause breakage will be introduced in the wire (Figure 1).

Among the factors that control flushing efficiency is the mechanics of the setup. The workpiece height, dielectric pressure, and the number of free surfaces all can influence the ability of the dielectric-fluid flow to remove debris.

In order to create hydraulic efficiency for flushing, a sufficient dielectric pressure and flow rate must be established and maintained. Recent-vintage wire-EDMs are equipped with high-pressure/high-flow-rate pumps to produce the needed pressure and flow rate. However, these pumps will not be able to perform their function and flushing efficiency will suffer dramatically if the volume being flushed is not confined to minimize pressure loss. To maintain pressure, the flushing cups should be directly on top of the workpiece’s surface, and, ideally, this surface should be flat.

Unfortunately, these ideal conditions are not always attainable. For instance, if the wire-EDM is cutting a tall workpiece, the flushing system may be able to achieve efficient hydraulic action near the flushing cups, but the flushing efficiency may be dramatically reduced at the center of the part (Figure 2a). How tall is tall? There is no hard and fast definition one can rely on, but most operators would agree that achieving adequate flushing while EDMing any workpiece over 6" in height can be a challenge.

Figure 2: Tall workpieces (a), pieces with varying height that force the operator to set the cutting heads at some distance from the workpiece (b), tapered cuts (c), and internal cavities and stacked parts (d) all present challenges that can make it difficult to maintain efficient flushing.

It can be equally hard to maintain flushing efficiency while cutting a part with variable height. To keep the flushing cups as close as possible to the part’s surface and avoid a loss of pressure, the cutting-head height should be adjusted to follow the contours of the part as the cut proceeds. But most operators would agree that if they followed this procedure they would run the risk of crashing the delicate and expensive head into the part. Operators are more likely to take the prudent approach and set the head height at a position that will safely clear all obstacles. Unfortunately, when this is done the displaced flushing cups lose much of their high pressure "punch" as they try to direct the water stream into the narrow slot from a distance (Figure 2b).

The EDM’s fluid-delivery system also will have a hard time with taper cuts. Generally, the system directs the fluid flow straight down, but a stream flowing in this direction misses all the action (Figure 2c). Agie USA Ltd., Davidson, NC, addresses this issue with its tilting-head system. With other systems, the larger the angle of taper, the more difficult it is for the fluid to flush out debris along the entire length of the cut. Flushing efficiency will be moderately limited in any tapered cut with a taper angle up to 5°. Cutting a taper with an angle of more than 10° poses a severe limitation on cutting efficiency.

Internal cavities and stacked parts present another challenge to efficient flushing (Figure 2d). The behavior of the stream under these conditions is similar to its behavior when the flushing cups are displaced. Anytime the high-pressure stream encounters a free surface, there is the strong possibility that pressure will be lost as the stream re-enters the slot on the opposite side of the interruption.

A stream flowing through stacked parts will lose pressure because the gap between the parts represents a lineal cavity. This problem is unavoidable with stacked parts, because there will always be a gap between the parts, although its size will vary from stack to stack. This does not mean an EDM shop should avoid cutting stacked parts, however. The gains in productivity that are possible by cutting multiple parts make it a valuable practice. Instead, the shop should select a wire that can tolerate poor flushing conditions.

Metallurgical Problems

Poor flushing, and the wire-breakage problems that it causes, can also be the result of the wire’s heat of sublimation, a property that determines how the wire behaves when it is heated. This property may not be known by name to EDM users, but it is responsible for situations that most users will be familiar with.

All matter exists in one of three phases: as a solid, a liquid, or a gas. In some cases a substance passes through all three phases as it heats up. The rising temperature first transforms it from a solid to liquid, as when ice melts, and then from a liquid to a gas, as when water boils. However, at a high enough temperature, a solid will be transformed directly to a gas. This process is known as sublimation, and the temperature at which a material passes directly from its solid to its gaseous state is called the material’s heat of sublimation.

Figure 3: Water ice forms a liquid phase when it is heated. When refrozen, much of it returns to its solid state. Dry ice turns directly into a gas when heated, and it is not recovered when heat is removed.

The behaviors of materials with different heats of sublimation can be seen in a comparison of a block of water ice to a block of frozen carbon dioxide (dry ice) (Figure 3). As the two heat up, the water ice will melt, and most of it will become liquid water. A small amount of gas will form as well. If we refreeze what is left of the water, we will find that most of it is transformed back into smaller pieces of solid ice. The little bit of the gaseous phase that was formed will end up as a snowflake or two.

The carbon dioxide ice, on the other hand, will be sublimed by the same amount of heat that turned the water ice to liquid. In other words, it will slowly disappear as heat is applied, because it transforms directly from the solid phase to the gaseous. Materials that behave this way at relatively low temperatures are said to have a low heat of sublimation. If we try to freeze the carbon dioxide again, we find that almost all of it has diffused away. Extreme pressure and cold is required to return this diffused gas to a solid state.

Figure 4: A metal that forms a liquid phase will produce much larger particulate when quenched than a metal that transforms directly to a gas.

Metals exhibit similar differences in behavior. Some metals, when heated, have a tendency to melt first and form very little vapor phase. Those metals that possess a low heat of sublimation, however, will vaporize readily. This explains the different reactions that metals will have when they are heated by the electrical discharge in a wire-EDM operation. Some metals will exist as a liquid while others will exist as a gas inside the envelope created by the discharge. The phase the metal transforms into when heated will determine how much particulate is produced when the plasma envelope collapses and the heated metal is quenched by the dielectric fluid. The quenching will produce a much larger amount of solid particulate if the metal was transformed into a liquid phase than if it was transformed into gas (Figure 4).

The degree of particulate the quenching produces should be of interest to EDM operators, because it is much easier to flush away smaller particulate. Therefore, from a flushability point of view, it is preferable to work with materials that have a low heat of sublimation. A gap that becomes too cluttered with conductive debris will be prone to arcs that can introduce large metallurgical flaws in both the wire and the workpiece. In the workpiece, these flaws deteriorate the surface finish, and in the wire they may cause catastrophic failure.

An operator who knows a material’s heat of sublimation before EDMing it will also have some idea how efficiently the debris can be flushed from the cutting area. Figure 5 lists the heat of sublimation for a number of materials.

Figure 5: The volumetric heat of sublimation for various metals that are commonly EDMed.

The parameter that is listed in Figure 5 is the volumetric heat of sublimation rather than the more commonly quoted gravimetric value. This is because the volumetric value is more meaningful in EDM, where metal-removal rates are characterized in per-unit volume dimensions. The value is given in kilojoules (a unit of energy) per cubic centimeter.

In Figure 5, the elements are listed from lowest to highest values. The elements at the top of the list, therefore, are analogous to dry ice; in other words, they transform directly into a gas and leave little particulate when they are quenched. Consequently, these materials can be expected to flush efficiently. Given the relative position of the elements listed here, it is easy to see why zinc and zinc-enriched brass alloys are so efficient and effective as EDM wire coatings.

The elements at the bottom of the list, on the other hand, are much more difficult to flush because of the amount of particulate produced when they are quenched. Because they make flushing difficult, the elements at the bottom of the list might be expected to cause more wire breakage.

Problems at the Machine

Not all EDM wires break because of mechanical and metallurgical problems. Some break because of problems with the EDM itself or with settings chosen for the operation. For example, worn wire guides or power-feed contacts can cause a machine to cut erratically and break wires. If the wire speed is too slow, there is a greater chance that a flaw introduced by a discharge or arc will propagate to a size big enough to cause a catastrophic failure. Excessive wire tension can cause wire breakage, because the critical flaw size that initiates failure is a function of the tension being applied. A wire under lower tension can tolerate larger flaws.

The voltage imposed on the gap will also influence the size of the flaws being generated. As one might expect, higher power discharges will introduce larger flaws, as will arcing. Some operators may not realize there is a very distinct difference between an electrical discharge and an electrical arc. An electrical arc is much more intense than a discharge and represents a very high energy transfer.

In the real world, we must learn to live with some restricted flushing conditions. Because of part geometries, we may not always be able to achieve the most efficient flow of dielectric fluid. Fortunately, EDM wires are available with varying degrees of fracture toughness, which, as we have discussed, is directly related to the wires’ ability to sustain flaws without breaking. By choosing a wire with a high degree of fracture toughness, an operator can avoid breakage and maintain productivity even when flushing conditions are not ideal.

About the Author
Dandridge Tomalin is a joint-venture consultant associated with the Engineered Wire Products Division of Rea Magnet Wire Co. Inc., Ft. Wayne, IN.