Selection of electrode wire and wire breakage of wire during wire cutting

In recent years, with the rapid development of machining technologies, wire-cut EDM has been rapidly developed as a special precision machining technology. In particular, slow-cut wire cutting has become an indispensable processing method for improving the processing efficiency and quality of the mold manufacturing and machining industries.

Development and Application of Electrode Wire Technology

The development of wire-cutting technology is inseparable from the simultaneous development of electrode wire technology. Nowadays, the internationally popular design concept of the slow-moving machine tool is based on the performance of the electrode wire, and the breakthrough of the electrode wire technology will often drive the innovation of the wire cutting machine design. From the initial use of oxygen-free copper wire cutting to current composite wire cutting, the development of the slow wire cutting experience has gone through a long process from low efficiency, low quality to high efficiency, high quality, automation and professional production.

China's wire-cutting technology has developed from the earliest fast-moving, medium-stranded wire processing to the nowadays commonly used low-speed wire processing, based on the gradual introduction of foreign advanced technologies. Since the invention of galvanized electrode wire in the 1970s and 1980s, there have been common brass electrode wire, galvanized electrode wire, molybdenum wire, tungsten wire, composite wire (inside layer is steel wire, appearance is copper, etc.) on the market. A variety of electrode wires.

The actual slow wire spinning technology entered China in the early 1990s, and the first manufacturers to use the slow wire processing were several wholly foreign-owned enterprises in the Pearl River Delta region. At that time, China’s non-ferrous metal processing technology was not yet mature. The raw materials are dependent on imports, so the development of silk processing in China before the 20th century has been slow.

After 2000, with the development of smelting and machining technology in China, the electrode wire has gradually accelerated its development. Especially after China's accession to the WTO, the type of electrode wire imported from abroad has also become more and more, making the user dazzled. It is difficult to choose. The correct selection of electrode wire is the key to improving the quality of processing, the efficiency of machine tools, and reducing labor costs.

At present, EDM in western developed countries has gradually developed toward high speed and high quality, but China is still at a low level of development. Although in recent years China's introduction of cutting machine tools (including domestically produced wire cutting equipment) has gradually added the use of other types of electrode wire in addition to ordinary brass wire, but users often use common brass. Electrode wire processing, resulting in the machine tool efficiency is difficult to play better.

There are also many users who introduce high-performance wire saws. Because of the lack of understanding of the importance of wire selection, ordinary brass electrode wire is the only choice, regardless of model or processing conditions.

With the continuous improvement of the user's skills and the gradual popularity of the slow thread cutting technology in the machining industry, it has been found that the machine tool using a galvanized electrode wire and other composite electrode wires can exert higher efficiency, and the accuracy of the processed workpiece can also be improved. higher. After 2000, more and more users began to try to use composite electrode wires such as high-performance brass wire and galvanized electrode wire, and gradually realized that different types of electrode wires would receive better results under different processing conditions. effect. Not only did the misunderstanding that "electrode wire is made of brass" was broken, but companies in China that could independently develop and produce electrode wires also began to appear.

Figure 1: Precisely machined parts

Electrode wire characteristics

The introduction of a new technology has to go through the process of imitating, absorbing, digesting, and innovating. From the original H65 common brass (65% copper, 35% zinc) to more and more H63, H62 When brass with a higher zinc content is used, it is recognized that zinc actually plays a role in the cutting process. Because the melting point of copper in ordinary brass is 1083.5°C, the boiling point of zinc is only 907°C.

During the cutting process, due to the high temperature discharge, the cutting line will reach a high temperature above 2000°C in an instant. After gasification, the zinc will bring the energy generated in the cutting discharge to the surface of the cutting metal, and improve the flushing effect. At the same time, zinc gasification The pressure generated will also blow away the eroded material from the discharge. This theoretical study shows that the higher the zinc content in the electrode wire, the better the cutting effect.

However, from the standpoint of non-ferrous metal processing, when the proportion of zinc exceeds 40%, the α-phase single-phase crystal structure of the electrode wire will change into a two-phase crystalline structure of α and β, and the material will become too brittle and unsuitable for processing. Small diameter filaments. Ordinary brass wire electrode has been restricted by the processing technology in today's pursuit of high-efficiency cutting, which has led to the birth of electrode wire with other special properties.

According to current domestic and foreign technical studies, electrode wire materials must have the following characteristics:

1. Mechanical properties

In the EDM process, the electrode wire must be able to withstand a certain tensile force and impact due to discharge. If the material strength is too low or the fracture toughness is too low, the electrode wire will be shaken, and eventually the surface of the cut workpiece will not be smooth, and even multiple cut lines will be traced. Especially for some difficult-to-machine metal workpieces, the greater the tension applied when cutting (the strength of the constituent wire material), the easier it is to cut. In addition, the strength of the material also contributes to the straightness of the electrode wire, and the wire material having a certain extensibility is also useful for cutting a workpiece having a slope.

2. Gasification characteristics

In the process of processing, the heat generated by the discharge is constantly increasing. If the heat cannot be released in time through other media, the electrode wire with a higher melting point will also be blown. Therefore, the materials that make up the electrode wire must be able to take this heat out.

3. Physical characteristics

The physical properties of the electrode wire are the key to improving the cutting efficiency and usually refer to the melting point of the electrode wire. This is mainly determined by the ratio of the basic composition of the electrode wire material and the nature of the composition of the core material. Because the electrode wire will have a momentary short-circuit effect when it is cut, the cutting process will slow down. In this case, the electrode wire will not produce gaps between the ablation and the cutting metal, and the cutting waste will not be flushed. Bring out, then the electrode wire will be blown.

4. Geometric characteristics

The geometric properties mentioned here usually refer to dimensional accuracy and dimensional stability. The outer diameter of the electrode wire directly determines the surface dimensional accuracy of the cut workpiece. The current electrode wire machine tool design also requires the wire electrode to have a small geometric error in the shape. The final dimensional accuracy of the electrode wire can only be guaranteed by the dimensional accuracy of the wire drawing die. Today's cutting machine tools in the world do not have the function of automatically adjusting the cutting accuracy based on the outer diameter of the electrode wire. If the dimensional accuracy is not stable, the final cutting accuracy is also Unstable.

5. Electrical characteristics

EDM relies on instantaneous discharge corrosion to cut metal. The shorter the instantaneous discharge gap, the higher the surface finish of the cut metal, so the electrode wire must have the ability to withstand high instantaneous pulse currents and large cutting currents.

Classification of electrode wire

The electrode wires that are currently on the market are all organic combinations of the above five characteristics, and will also incorporate other particularly significant properties, such as high strength and strong extensibility. According to different conditions of use and processing conditions, electrode wire can be divided into the following categories:

1. Ordinary brass wire electrode

This kind of electrode wire is a copper-zinc binary alloy. Due to the limitation of zinc content, the cutting speed is limited. It is generally used by ordinary domestic users. The copper powder and wire breakage are common phenomena in the process of using this electrode wire.

Figure 2: Brass wire

2. Galvanized electrode wire

The core material is ordinary brass, and the outside is coated with a layer of zinc. Due to the gasification of zinc during the cutting process, the discharge of the electrode wire is relatively stable, and the cutting surface is smoother than ordinary brass wire. The more mature manufacturing countries in the world are mainly distributed in Europe. At present, there are also individual manufacturers in the country that can produce powder. This is a common problem in the use of the electrode wire.

Figure 3: Galvanized wire

3. Speed ​​type plating electrode wire

The core material is common brass, the coating is copper-zinc alloy, and the coating is slightly thicker than ordinary galvanized electrode wire. The plated electrode wire has a faster cutting speed than an ordinary galvanized electrode wire, and is suitable for high-efficiency machining. At present, this electrode wire can only be produced abroad, and China is still in the development stage.

4. Diffusion annealing type electrode wire

It has been found that electrode wires with a uniform pit on the surface will improve the discharge rinsing effect of the electrode wire. Based on this, electrode wires having such a porous structure are produced. The diffusion annealing type electrode wire core material is oxygen-free copper, and is coated with a copper-zinc alloy (a ratio of copper and zinc is 1:1) through diffusion annealing. Since the core material is oxygen-free copper, the surface layer is formed by diffusion annealing. Due to the porous structure, the electrode wire has a better flushing effect than the conventional electrode wire. However, the oxygen-free copper itself has low tensile strength, and the strength of the electrode wire made of the core material is also very low, only 500 MPa. At present, this type of electrode wire is only suitable for special machining of special machine tools.

5. Steel core wire

The core material is steel wire, and the surface is plated with pure copper and then plated with brass. There are also direct brass plating. Because the electrode wire core material is steel, the strength of this electrode wire is much higher than the above four electrode wires at high temperatures. Particularly suitable for processing under difficult conditions, such as ultra-thick, large slope, multi-layer, poor flushing, graphite, single crystal silicon and other difficult-to-machine materials.

6. Fine wire electrode

The diameter of the general electrode wire is at least 0.07mm. For the fine processing with a small α angle, the diameter of the α-angle is required to be 0.02-0.10mm. At this time, tungsten or molybdenum is required to make the ultra-fine electrode wire, but because tungsten Or molybdenum is a rare metal and it is difficult to process. The following piano wire is often used instead.

7. Piano wire

This kind of electrode wire is also called piano wire because the core material is high carbon steel for piano. Piano wire core material is high carbon steel, brass plated surface, but also re-galvanized. High carbon steel prices are low, so this type of electrode wire often replaces the fine wire electrode described above. High-carbon steel wire after multi-pass processing and heat treatment, strength can be equivalent to tungsten or molybdenum wire.

Wire breakage problem

At this stage, precision manufacturing and processing of precision molds and machinery in China is still a new industry, and the use of many new technologies is still at an experimental stage. The actual use of the third, fourth, fifth, sixth, and seventh types of electrode wires mentioned above is very few in China. The commonly used electrode wires are only two kinds of brass wire and galvanized electrode wire. The following two wire electrodes are used in the process. An analysis of the problem of disconnection that often occurs.

1. Misunderstanding of disconnection

In the process of using the electrode wire, the user or the cutting master mistakenly believes that the wire breakage is due to the low strength of the electrode wire. In fact, the real reason of the wire breakage is not that the electrode wire has low strength but insufficient toughness.

Nowadays, a large number of wire electrodes with a tensile strength of only 450 MPa are frequently broken due to their low strength. The actual situation is not the case. As mentioned above, the essence of EDM is to cut metal by electric discharge corrosion. The formation of discharge corrosion pits on the surface of electrode wire during discharge is the root cause of wire breakage during cutting.

On the operational level, the higher the tensile strength, the better, because the higher the strength of the wire is, the better the wire straightness is, and the easier it is to thread, but if the tensile strength is increased from the operation level, the discharge performance will be reduced. . There are also some wire electrode manufacturers that are developing electrode wires that have high tensile strength and do not suffer from discharge performance at the same time.

The nature of the disconnection

From the point of view of fracture mechanics, fractures occur after the source of cracks has extended to a certain extent. During discharge, many pits are formed on the surface of the electrode wire due to discharge corrosion, and the electrode wire is also subjected to the impact of electric discharge or chip removal. In the cutting process, several pits with the least stress are fractured due to insufficient fracture toughness. , that is broken.

3. The role of flushing in electrode wire electrical discharge machining

The role of water in the EDM process is very important. The commonly used fragile materials for the slow wire machine tools are resin, filters, electrode wire, etc., while the resin and filter are mainly used for cleaning and filtering on the machine tool for reciprocating recycling. Water to ensure that the electrode wire gasification characteristics are not reduced.

The small pit formed during discharge is related to many factors. The most important factor is flushing. The effect of flushing directly affects the gasification characteristics of the gap between the electrode wire and the workpiece. Of course, flushing can also easily remove waste generated by electro-erosion and take away the heat generated during discharge.

4. Cheap electrode wire problem

At this stage, more than 95% of electrode wires used in China are made of common brass containing 60 to 65% of copper, and China is a country with a severe shortage of copper resources. With the rising prices of copper in the past two years, individual domestic manufacturing companies have begun to use recycled scrap copper to produce electrode wire busbars, and these scrap copper often contain harmful impurities such as iron, nickel, tin, lead, etc. that reduce the performance of the electrode wire.

Because China's current level of refined copper technology is not high, it is difficult to remove these impurities, resulting in the uneven internal structure of the electrode wire produced, thereby reducing the electrode wire toughness and end-use performance, has become the culprit causing disconnection. This will not only reduce the processing efficiency, but also affect the machining quality of the cut workpiece or even damage the machine. The general user does not detect the existence of these impurity elements. In actual operation, the line breakage is often reduced by reducing the cutting speed, and some users even attributed to machine tool performance changes or the cutting master's technique is not high.

Figure 4: Diagram of the production process of ordinary cutting lines

Conclusion

To sum up, when selecting the electrode wire, it is necessary to start with the model to see what type of electrode wire the model is suitable for; secondly, consider the processing conditions, such as the hardness of the processing material, the surface quality requirements, the precision requirements of the processed parts, and The quality of the consumables used in the machine tool selection, of course, must also consider the quality of the water quality; once again consider whether the machine tool equipment is always running at full capacity; in the end, the performance differences of various wire electrodes must also be considered.