1, brass wire brass wire is the first generation of professional electrode wire in the field of wire cutting. From 65438 to 0977, brass wire began to enter the market. This kind of electrode wire once brought a breakthrough in cutting speed. At that time, the cutting speed was increased from 12mm2/ min to 25mm2/ min for the workpiece with a thickness of 50mm. What doubled the speed? Brass is an alloy of copper and zinc. The most common proportions are 65% copper and 35% zinc. At that time, it was found that zinc in brass wire can improve the washing resistance because of its low melting point (420℃, while copper is 1080℃). In the cutting process, zinc is gasified due to high temperature, which reduces the temperature of the electrode wire and transfers heat to the machined surface of the workpiece. Theoretically, the higher the proportion of zinc, the better. In the manufacturing process of brass wire, when the proportion of zinc exceeds 40%, the α single-phase crystal structure of electrode wire becomes α and β double-phase crystal structure. At this time, this material becomes too brittle to be drawn into small-diameter filaments. Brass wire can have different tensile strength to meet different equipment and applications. This is achieved through a series of wire drawing (quenching) and heat treatment (annealing) processes. The tensile strength of ordinary brass wire is between 490 and 900 N/mm2. Main disadvantages of brass wire:
(1) The machining speed cannot be improved: because the proportion of zinc in brass remains unchanged, the energy conversion efficiency during discharge cannot be further improved; Taking 0.25mm brass wire cutting 30-60mm thick steel as an example, the main cutting speed of many domestic users is around 120mm2/ min.
(2) Poor surface quality: copper powder on the surface of brass wire and copper particles brought out by gasification on the surface of electrode wire during discharge will accumulate on the machined surface of workpiece to form surface copper. At the same time, due to poor washability, a thick metamorphic layer is produced on the surface of the workpiece, which will affect the surface hardness and roughness of the workpiece;
(3) Low machining accuracy: especially when machining thick workpieces, due to poor washability, large straightness errors (up and down size errors and drum shape differences) will occur. In addition, due to price competition, there are various quality problems in domestic low-priced brass wires at present, such as poor copper blank material, wire drawing equipment and process, which lead to more copper powder on the surface of brass wires, which will lead to the decline of discharge stability and seriously affect the processing speed and quality. At the same time, it will pollute equipment parts and increase equipment loss.
Application of brass wire;
(1) is not a 24-hour user because of insufficient processing capacity. Because processing efficiency is not the main problem for these users;
(2) Users who have low requirements on machining accuracy, especially surface quality;
(3) Mainly processing users with small size and thin thickness. Due to the high proportion of workpiece clamping adjustment time to the total processing time and less cutting time, the influence on processing efficiency is not obvious.
(4) The material hardness of the workpiece is not high or the thickness is not more than 80- 100 mm. ..
Although the market share of brass wire is declining with the appearance and popularization of various coated electrode wires with better performance, it will continue to be widely used because of its low cost and meeting the general processing needs. At the same time, some new brass wires have appeared in the market, their properties have been improved to varying degrees, and their prices are lower than those of electroplated electrode wires:
(1) Ultra-clean brass wire: it is made by adding a special cleaning process in the subsequent process to overcome the shortcoming of excessive copper on the surface of ordinary brass wire;
(2) Ultra-hard brass wire: By adding other trace elements to brass, the tensile strength of brass wire is as high as 1200 N/mm2. This kind of wire can improve the machining accuracy and speed when machining ultra-thick or ultra-hard workpieces;
(3) High-speed brass wire: increasing the proportion of zinc in brass to the limit of 40% can improve washability and increase cutting speed. But its cutting speed is still slower than that of galvanized electrode wire.
2. Coated electrode wire Because zinc with low melting point can obviously improve the discharge performance of electrode wire, and the proportion of zinc in brass is limited, people thought of adding a layer of zinc outside brass wire, which produced galvanized electrode wire. This method, which was invented by several Swiss engineers in 1979, made the development of electrode wire take a big step forward and led to the emergence of more new coated electrode wires. Main advantages of coated covered electrode:
(1) The cutting speed is fast and the wire is not easy to break. The cutting speed of high-quality galvanized electrode wire is 30-50% faster than that of high-quality brass wire. Many users in Guangdong use 0.25mm galvanized electrode wire, and the average cutting speed is 150- 180mm2/ min.
(2) The surface quality of the machined workpiece is good, there is no copper accumulation, and the metamorphic layer is improved, so the surface hardness of the workpiece is higher and the service life of the die is prolonged.
(3) The machining accuracy is improved, especially the shape error of sharp corners and the straightness error of thick workpieces.
(4) The loss of parts such as the guide wire nozzle is reduced. The hardness of zinc is lower than that of brass, and at the same time, galvanized wire does not have a lot of copper powder like brass wire, so it is not easy to block the guide nozzle and pollute related parts. The production technology of coated electrode wire mainly includes three methods: impregnation, electroplating and diffusion annealing. The core materials of electrode wire are mainly brass, copper and steel. The coating materials are zinc, copper, copper-zinc alloy and silver. At present, there are the following kinds of mature electrode wires in the market according to their uses:
Common galvanized electrode wire:
Because impregnation is a relatively simple process, many wire manufacturers use this method to produce galvanized wire. However, the biggest problem of wire drawing after galvanizing is that the uniformity of coating can not be controlled, so the discharge performance of electrode wire produced by this process is not stable enough, and the speed is only less than 10% higher than that of brass wire. Some poor quality galvanized electrode wires are often not uniform silver gray, and some light yellow can be seen inside, which is the so-called "copper exposure" phenomenon. Although this kind of electrode wire is cheaper, only slightly more expensive than brass wire, it is not used by many people.
Galvanized electrode wire for high precision machining;
This kind of electrode wire is mostly electroplated, which can control the thickness of galvanized layer well, has stable discharge performance and is not easy to break, and is suitable for precision machining with more than four knives. The cutting speed can generally be about 30% faster than that of brass wire.
Coated electrode wire for high-speed machining;
This electrode wire is made by diffusion annealing process, and it is a thick-coated composite electrode wire containing 50% zinc and 50% copper. This coating needs a series of heat treatment processes, and its color changes from bright silver to yellowish brown due to the diffusion of the coating. The core of this welding wire is α structure and the coating is β structure. Finally, the coating is pressed into the core material by stretching process and cold pressing. The surface of the diffusion electrode wire is porous, which helps to improve the cleanability. The cutting speed of this kind of electrode wire is the fastest at present. Common brands include COBRA CUT D, BRONCO CUT X, MEGACUT D, SWX, STAMMCUT Xi, bunol TX, bunol GT, bunol GS, etc. The latest gamma coating technology is adopted to achieve the fastest cutting speed.
Hard-to-machine coated electrode wire;
Steel core welding wire is a kind of composite welding wire. It consists of a steel core with a copper coating in the middle and a brass coating outside. The tensile strength of steel core at room temperature is similar to that of brass wire, but with the increase of temperature, the tensile strength of brass wire decreases rapidly, while the tensile strength of steel is higher than that of brass wire. However, due to the poor conductivity of the port, a layer of red copper is wrapped outside the steel core to improve the conductivity. The external brass layer plays a role in improving the flushing performance. For the more difficult WEDM, although this situation can be improved by using larger diameter (0.30mm) electrode wire or galvanized electrode wire, the best choice is this steel-cored electrode wire to meet higher processing requirements.
1, high-thickness machining: Generally speaking, when the workpiece is thick (usually greater than 100mm), the machining speed is obviously reduced, and the straightness error of the machined surface will be great. At this time, the speed and accuracy can be obviously improved by using steel core wire processing.
2. Processing with poor flushing state: such as large inclination, irregular workpiece thickness, large variation range, multi-workpiece overlapping processing, etc. Poor washing will easily lead to broken wires, and the processing speed will also decrease accordingly. At the same time, it will also lead to the increase of secondary discharge and affect the surface quality.
3. Workpiece materials are difficult to machine: such as graphite, copper, aluminum alloy and other difficult-to-cut materials.
Common brands are: COMPEED steel core welding wire of Fujikura Company of Japan.
Electrode wire for ultra-precision machining;
The diameter of brass wire or electroplated wire is generally between 0.30 mm and 0.07 mm. For some micro parts or ultra-precision machining in electronics, optics and watch industries, the diameter of electrode wire is required to be between 0.10 mm-0.03 mm. Previously, this electrode wire was made of tungsten wire or molybdenum wire, which was very expensive. At present, high tensile strength steel wire (100 carbon piano wire) is generally made of brass plated outside, commonly known as "piano wire". The tensile strength of this kind of electrode wire is twice that of common electrode wire, reaching more than 2000N/mm2. Common brands are Germany's MICRO CUT and Japan's SPWire.