Selection of electrode wire and wire breakage of e

In recent years, machining technology has changed with each passing day, and wire cutting EDM, as a special precision machining technology, has also developed rapidly. In particular, slow wire cutting has become an essential processing method to improve processing efficiency and quality in the mold manufacturing and machining industry

the development of wire cutting technology cannot be separated from the synchronous development of electrode wire technology. At present, the internationally popular design concept of wire walking machine tool is to design according to the performance of electrode wire, and the breakthrough of electrode wire technology often drives the innovation of wire cutting machine design. From the initial use of oxygen free copper wire cutting to the current composite wire cutting, the development of slow wire cutting has experienced a long process from low efficiency and low quality to high efficiency, high quality, automation and professional production

China's wire cutting technology has developed from the earliest fast wire walking and medium wire walking processing to the widely used slow wire walking processing. It is based on the gradual introduction and absorption of foreign advanced technology. Since the invention of galvanized electrode wire in the 1970s and 1980s, various electrode wires have appeared in the market, such as ordinary brass electrode wire, galvanized electrode wire, molybdenum wire, tungsten wire, composite wire (the inner layer is steel wire, the outer surface is copper, etc.)

the real slow moving wire technology entered China in the early 1990s, and the first manufacturers to use slow moving wire processing were several wholly foreign-owned enterprises in the Pearl River Delta. At that time, China's non-ferrous metal processing technology was not mature, and the raw materials used depended on imports, so the development of slow moving wire processing in China had been very slow before the 20th century

after 2000, with the development of smelting and machining technology in China, the electrode wire showed a trend of gradually accelerating development. Especially after China's accession to the WTO, more and more types of electrode wires were imported from abroad, which dazzled the majority of users and made it difficult to choose. The correct selection of electrode wire is the key to improve the processing quality, give play to the efficiency of machine tools and reduce labor costs

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

when introducing high-performance wire cutting machines, many users, due to their lack of understanding of the importance of selecting electrode wires, take the cheap ordinary brass electrode wires as the only choice regardless of the model or processing condition

with the continuous improvement of users' skills and the gradual popularization of slow wire cutting technology in the machining industry, people have 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 composite electrode wires such as high-performance brass wires and galvanized electrode wires, and gradually realized that selecting different types of electrode wires under different processing conditions would achieve better results. Not only has the misunderstanding that "electrode wires are made of brass" been broken, but also enterprises that can independently develop and produce electrode wires have begun to appear in China

Figure 1: precision machined parts

characteristics of electrode wire

the introduction of a new technology requires a process of imitation, absorption, digestion and innovation. From the initial H65 ordinary brass (65% copper, 35% zinc), to the current application of more and more h63, H62 and other brass with higher zinc content, people realize that zinc plays a role in the cutting process with a real construction area of more than 10000 square meters. Because the melting point of copper in ordinary brass is 1083.5 ℃, and the boiling point of zinc is only 907 ℃

during the cutting process, due to the high-temperature discharge, the cutting line will instantly reach a high temperature of more than 2000 ℃. The vaporized zinc will bring the energy generated by the discharge during the cutting to the surface of the cutting metal and improve the flushing effect. At the same time, the air pressure generated by the zinc gasification will also blow away the corrosion generated by the discharge. This theoretical study shows that the higher the zinc content in the electrode wire, the better the cutting effect

but in terms of nonferrous metal processing, when the proportion of zinc exceeds 40%, the α The single-phase crystalline structure will become α and β Biphasic crystalline structure. At this time, the material will become too brittle to be processed into filaments with small diameter. Ordinary brass electrode wire in the general pursuit of high-efficiency cutting today is restricted by the processing technology, thus giving birth to electrode wire with other special properties

according to the current domestic and foreign technical research summary, electrode wire materials must have the following characteristics:

1 Mechanical characteristics

in the process of EDM, the electrode wire must be able to withstand a 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 cause multiple cutting line marks. Especially for some difficult metal workpieces, the greater the tension applied during cutting (the strength of the electrode wire material), the easier it is to cut. In addition, the strength of the material also helps to restore the straightness of the electrode wire, and the electrode wire material with certain extensibility will also be beneficial to cutting inclined workpieces

2. Gasification characteristics

in the processing process, the heat generated by discharge is increasing. If the heat cannot be released in time through other media, the electrode wire with higher melting point will also be burned out. Therefore, the material constituting the electrode wire must be able to bring out these heat

3. Physical properties

the physical properties 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 composition metal properties of core material. Because the jitter of the electrode wire during cutting will produce an instantaneous short-circuit effect, at this time, the cutting process will slow down, so that there is no gap between the electrode wire and the cutting metal due to discharge ablation, the cutting waste will not be brought out by flushing, and then the electrode wire will burn out

4. Geometric characteristics

the geometric characteristics 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 cutting workpiece. The current design of the electrode wire machine also requires that the electrode wire has a small geometric error. The final dimensional accuracy of the electrode wire can only be guaranteed by the dimensional accuracy of the drawing die. The cutting machine tools in the world today 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

5. Electrical characteristics

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

classification of electrode wire

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

1 Ordinary brass electrode wire

this electrode wire is a copper zinc binary alloy. Due to the restriction of zinc content, its cutting speed is limited. It is generally used by ordinary domestic users now. Copper powder dropping and wire breaking 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 in the cutting process, the discharge of this electrode wire will be relatively stable, and the cutting surface is smoother than ordinary brass wire. Internationally, more mature manufacturing countries are mainly distributed in Europe. At present, some domestic manufacturers can produce it. Powder dropping is a common problem in the use of this electrode wire

Figure 3: galvanized wire

3 Speed coated electrode wire

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

4. Diffusion annealed electrode wire

the study found that the electrode wire with uniform small holes on the surface will improve the discharge flushing effect of the electrode wire. Based on this, this kind of electrode wire with porous surface structure is produced. The core material of diffusion annealing electrode wire is oxygen free copper, and a layer of copper zinc alloy (the ratio of copper to zinc is 1:1) is wrapped by diffusion annealing. Because the core material is oxygen free copper, and the surface layer is a porous structure formed by diffusion annealing, the flushing effect of this electrode wire is better than that of conventional electrode wire. However, the tensile strength of oxygen free copper itself is low, and the strength of electrode wire made of it as core material is also very low, only 500MPa. At present, this kind of electrode wire is only suitable for special processing of special machine tools

5. Steel core electrode wire

the core material is steel wire, and the surface is plated with pure copper and then brass. Some are directly plated with brass. Because the core material of this electrode wire is steel, the strength of this electrode wire is much higher than the above four electrode wires at high temperature, which is especially suitable for processing under difficult conditions, such as ultra thick, high inclination, multi-layer, poor flushing, graphite, monocrystalline silicon and other difficult to process substances

6. Fine electrode wire

generally, the minimum diameter of electrode wire is 0.07mm, for small α Fine machining required by corners, α The diameter of the corner is required to reach 0 At this time, tungsten or molybdenum must be used to make ultra-fine electrode wires. However, because tungsten or molybdenum are rare metals and difficult to process, the following piano wires are often used instead

7. Piano electrode wire

this kind of electrode wire is also called piano wire because its core material is high carbon steel used to make piano. The core material of piano electrode wire is high carbon steel, the surface is plated with brass, and some are galvanized again. The price of high carbon steel is low, so this kind of electrode wire often replaces the above fine electrode wire. After multiple passes of processing and heat treatment, the strength of high carbon steel wire can be equivalent to that of tungsten wire or molybdenum wire

wire breakage problem of electrode wire

at this stage, China's precision mold and mechanical precision manufacturing and processing is still an emerging industry, and the use of many new technologies is still in the trial stage. The above-mentioned third, fourth, fifth, sixth and seventh electrode wires are rarely used in China. There are only two kinds of electrode wires that are often used: 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 wire breakage

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

nowadays, a large number of electrode wires with tensile strength of 450MPa appear on the market. Are they often broken because of their low strength? This is not the case. As mentioned above, the essence of slow wire EDM cutting is to cut metal by discharge corrosion. When discharging, the electrode wire is cutting when the discharge corrosion pit is formed on the surface of the electrode wire