How about Bao Shige's fast wire cutting?

China's WEDM technology is based on the gradual introduction and absorption of foreign advanced technology, from the earliest fast wire cutting and medium wire cutting to the now widely used wire cutting. Since the invention of galvanized electrode wire in 1970s and 1980s, various electrode wires such as common brass electrode wire, galvanized electrode wire, molybdenum wire, tungsten wire and composite wire (inner steel wire and outer copper) have appeared in the market.

The development of WEDM technology is inseparable from the synchronous development of wire electrode technology. At present, the popular design concept of WEDM in the world is to design according to the performance of electrode wire, and the breakthrough of electrode wire technology will often bring innovation in WEDM design. From the initial use of oxygen-free copper wire cutting to the present composite wire cutting, the development of wire cutting has gone through a long process from low efficiency and low quality to high efficiency, high quality, automation and professional production.

The real WEDM technology entered China in the early 1990s, and the earliest WEDM manufacturers were several wholly foreign-owned enterprises in the Pearl River Delta region. At that time, China's non-ferrous metal processing technology was still immature, and the raw materials used depended on imports, so the development of WEDM in China was very slow before the 20th century.

After 2000, with the development of smelting and processing technology in China, the electrode wire showed a trend of gradually accelerating development. Especially after China's entry into WTO, there are more and more kinds of electrode wires imported from abroad, which makes users dazzled and difficult to choose. The correct selection of electrode wire is the key to improve machining quality, give full play to machine tool efficiency and reduce labor cost.

At present, EDM in western developed countries has gradually developed towards high speed and high quality, while China is still at a low level of development. Although in recent years, the parameters of other types of electrode wires except ordinary brass electrode wires have been gradually increased in cutting machines imported from China (including domestic wire cutting equipment), users still often use ordinary brass electrode wires for processing, which makes it difficult to give full play to the efficiency of machine tools.

There are still many users who, due to the lack of understanding of the importance of selecting electrode wires when introducing high-performance wire cutting machines, regard cheap ordinary brass electrode wires as the only choice, regardless of the model or processing conditions.

With the continuous improvement of users' skills and the gradual popularization of wire cutting technology in machining industry, it is found that machine tools using composite electrode wires such as galvanized electrode wires can play a higher efficiency and process workpieces with higher accuracy. After 2000, more and more users began to try to use high-performance composite electrode wires such as brass wire and galvanized electrode wire, and gradually realized that different types of electrode wires would get better results under different processing conditions. Not only the misunderstanding that electrode wire is made of brass has been broken, but also domestic enterprises that can independently develop and produce electrode wire have begun to appear.

Characteristics of electrode wire

The introduction of a new technology must go through the process of imitation, absorption, digestion and innovation. From the original H65 ordinary brass (65% copper, 35% zinc) to more and more brass with higher zinc content, such as H63 and H62, people realize that zinc does play a role in the cutting process. Because the melting point of copper in ordinary brass is 1083.5℃, the boiling point of zinc is only 907℃.

During the cutting process, due to high temperature discharge, the cutting line will reach a high temperature above 2000℃ in an instant. Gasified zinc will bring the energy generated by discharge during cutting to the surface of the cut metal, thus improving the cleaning effect. At the same time, the gas pressure generated by zinc gasification will also blow away the corrosion caused by discharge. This theoretical study shows that the higher the zinc content in the electrode wire, the better the cutting effect.

However, from the point of view of non-ferrous metal processing, when the proportion of zinc exceeds 40%, the α single-phase crystal structure of electrode wire will become α and β double-phase crystal structure, and the material will become too brittle to be processed into small-diameter filaments. Nowadays, ordinary brass wire electrode is limited by processing technology in the process of pursuing efficient cutting, which gives birth to wire electrode with other special properties.

According to the current technical research at home and abroad, electrode wire materials must have the following characteristics:

1. Mechanical characteristics

In the process of EDM, the electrode wire must be able to withstand certain tension and impact force caused by discharge. If the strength of the material is too low or the fracture toughness is too low, it will cause the electrode wire to shake, which will eventually lead to the unsmooth surface of the cutting workpiece and even lead to multiple cutting lines. Especially for some difficult-to-machine metal workpieces, the greater the tension (strength of electrode wire material) applied during cutting, the easier it is to cut. In addition, the strength of the material is also helpful to restore the straightness of the electrode wire, and the electrode wire material with certain ductility is also beneficial to cutting the inclined workpiece.

2. Gasification characteristics

In the process of machining, the heat generated by discharge is increasing. If the heat can't be released in time through other media, the electrode wire with high melting point will be burned out, so the material that makes up the electrode wire must be able to take away the heat.

3. Geometric features

The geometric characteristics mentioned here usually refer to dimensional accuracy and dimensional stability. The outer diameter of the electrode wire directly determines the surface dimension accuracy of the cut workpiece, and the current design of the electrode wire machine tool also requires that the electrode wire has a small geometric error. However, the final dimensional accuracy of the wire electrode can only be guaranteed by the dimensional accuracy of the wire drawing die. At present, the cutting machine tools in the world do not have the function of automatically adjusting the cutting accuracy according to the outer diameter of the electrode wire. If the dimensional accuracy is unstable, the final cutting accuracy is also unstable.

4. Physical properties

The physical characteristics of electrode wire are the key to improve cutting efficiency, which usually refers to the melting point of electrode wire. This is mainly determined by the basic composition ratio of electrode wire material and the metal properties of core material. Because the jitter of the electrode wire during cutting will produce an instantaneous short-circuit effect, the cutting process will slow down at this time, so that there is no gap between the electrode wire and the cutting metal due to electric discharge ablation, and the cutting waste will not be taken out by washing water, thus burning the electrode wire.

5. Electrical characteristics

EDM cuts metal by instantaneous discharge corrosion. The shorter the instantaneous discharge gap, the higher the surface smoothness of the cut metal. Therefore, the electrode wire must have the ability to withstand instantaneous high pulse current and large cutting current.

Classification of electrode wire

At present, the popular electrode wire on the market is an organic combination of the above five characteristics, and some other particularly obvious characteristics will be added, such as high strength and strong extensibility. According to the different use conditions and processing conditions, electrode wires can be divided into the following categories:

1. Ordinary brass electrode wire

This electrode wire is a binary alloy of copper and zinc. Due to the limitation of zinc content, its cutting speed is limited, and it is generally used by ordinary domestic users. The phenomenon of copper powder loss and wire breakage often occurs in the use of this electrode wire.

2. Galvanized electrode wire

The core material is ordinary brass and the outside is galvanized. Because of the gasification of zinc in the cutting process, the discharge of this electrode wire will be more stable, and the cutting surface will be smoother than that of ordinary brass wire. Mature manufacturing countries in the world are mainly distributed in Europe, and some domestic manufacturers can produce them at present. Powder dropping is a common problem in the use of electrode wire.

3. Diffusion annealing electrode wire

It is found that the electrode wire with uniform pores on the surface will improve the discharge cleaning effect of the electrode wire, and on this basis, the electrode wire with porous structure on the surface is produced. The core material of diffusion annealing electrode wire is oxygen-free copper, and a layer of copper-zinc alloy is coated by diffusion annealing (the ratio of copper to zinc is 1: 1). Because the core material is oxygen-free copper and the surface layer is porous structure formed by diffusion annealing, the cleaning effect of this electrode wire is better than that of conventional electrode wire. But the tensile strength of oxygen-free copper itself is low, and the strength of electrode wire made of oxygen-free copper is also very low, only 500MPa. At present, this kind of electrode wire is only suitable for special machining of special machine tools.

4. Rapid electroplating of electrode wires

The core material is common brass, and the coating is copper-zinc alloy, which is slightly thicker than the common galvanized electrode wire. This kind of coated electrode wire has faster cutting speed than common galvanized electrode wire and is suitable for efficient processing. At present, this kind of electrode wire can only be produced abroad, and China is still in the development stage.

5. Fine electrode wire

Generally, the minimum diameter of electrode wire is 0.07mm, and for finishing with small α angle, the diameter of α angle should reach 0.02-0.10 mm. At this time, tungsten or molybdenum must be selected to manufacture ultra-fine electrode wire, but because tungsten or molybdenum is a rare metal, it is difficult to process, so the following piano wire is often used instead.

6. Piano electrode wire

This kind of electrode wire is also called piano wire, because the core material is high carbon steel used to make piano. The core material of piano electrode wire is high carbon steel, and the surface layer is plated with brass, which is also galvanized again. High carbon steel is cheap, so this kind of electrode wire often replaces the fine electrode wire above. After many processing and heat treatment, the strength of high carbon steel wire can be equivalent to that of tungsten wire or molybdenum wire.

7. Steel cored wire

The core material is steel wire, and the surface layer is plated with pure copper and then brass, and some are directly plated with brass. Because the core material of this electrode wire is steel, the strength of this electrode wire at high temperature is much higher than that of the above four kinds of electrode wires, and it is especially suitable for processing under the difficult conditions of ultra-thick, large inclination, multi-layer, poor washing, graphite, monocrystalline silicon and other difficult materials.

Broken wire of electrode wire

At present, precision mould and mechanical precision manufacturing in China is still a new industry, and the application of many new technologies is still in the trial stage. The third, fourth, fifth, sixth and seventh kinds of electrode wires mentioned above are rarely used in China, and only two kinds of electrode wires are often used, namely brass wire and galvanized electrode wire. The following is an analysis of the wire breakage problems that often occur in the use of these two kinds of electrode wires.

1. Misunderstanding of disconnection

In the process of using electrode wire, users or cutting masters will mistakenly think that the electrode wire is broken because of its low strength. In fact, the real reason for wire breakage is not the low strength of electrode wire, but the lack of toughness.

At present, a large number of electrode wires with tensile strength of only 450MPa have appeared in the market. Is it often broken due to low strength? That was not the case. As mentioned above, the essence of WEDM is to cut metal through discharge corrosion, and the discharge corrosion pit formed on the surface of electrode wire during discharge is the fundamental reason for wire breakage during cutting.

From the operational level, the higher the tensile strength, the better, because the higher the strength, the better the straightness of the electrode wire, and the easier it is to thread. However, if the tensile strength is improved from the operational level, the discharge performance will be reduced. At present, some electrode wire manufacturers are developing electrode wires with high tensile strength and no damage to discharge performance.

2. The nature of disconnection

From the point of view of fracture mechanics, fracture occurs after the crack source expands to a certain extent. In the process of discharge, many pits will be formed on the surface of the electrode wire due to discharge corrosion, and the electrode wire will also be impacted by electric spark or chip removal. In the process of cutting, due to the lack of fracture toughness, there will be several small pits with the least stress, that is, broken wires.

3. The problem of cheap electrode wire

At present, more than 95% of electrode wires used in China are ordinary brass containing 60~65% copper, and China is a country with a serious shortage of copper resources. With the continuous rise of copper price in recent two years, some domestic manufacturers began to use recycled copper scrap to produce electrode wire buses, which often contained harmful impurities such as iron, nickel, tin and lead, which reduced the performance of electrode wires.

At present, the domestic copper refining process level is not high, and it is difficult to remove these impurities, which leads to the uneven internal structure of the produced electrode wire, which reduces the toughness and end-use performance of the electrode wire and becomes the chief culprit of wire breakage. This will not only reduce the machining efficiency, but also affect the machining quality of the cut workpiece and even damage the machine tool. But ordinary users can't detect the existence of these impurity elements. In practice, wire breakage is often reduced by reducing cutting speed, and some users even attribute it to the change of machine tool performance or the low skill of cutting master.

4. The role of washing in EDM of electrode wire.

The pits formed in the process of discharge are related to many factors, among which the most important factor is washing, and the effect of washing directly affects the gasification characteristics of the gap between the electrode wire and the workpiece. Of course, washing can also easily remove the waste generated by electric corrosion and take away the heat generated during discharge.

Water plays an important role in EDM. WEDM usually uses resin, filter, electrode wire and other fragile materials. Resin and filter are mainly used to clean the water recovered from the filter bed to ensure that the gasification characteristics of the electrode wire are not reduced.

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To sum up, when choosing to use electrode wire, we should first start with the model to see what type of electrode wire the model is suitable for; Secondly, processing conditions should be considered, such as hardness of processing materials, surface quality requirements, precision requirements of processing parts, quality of supporting consumables selected by machine tools and, of course, water quality. Thirdly, it is necessary to consider whether machine tools and equipment often run at full capacity; Finally, the performance differences of different electrode wires should be considered.