emergency driver,hot sale,emergency light,exit Jiangmen City Pengjiang District Qihui Lighting Electrical Appliances Co., Ltd , https://www.qihuilights.com
The tool will gradually wear out during the cutting process. When the tool wear reaches a certain level, it can be clearly found that the cutting force is increased, the cutting temperature is increased, the chip color is changed, and even the vibration is generated. At the same time, the workpiece dimensions may also be out of tolerance and the quality of the machined surface is also significantly degraded. Tool wear and durability are related to the efficiency, quality and cost of machining, so it is one of the most important issues in machining.
During the cutting process, the rake face and the flank face are often in contact with the chips and the workpiece, and strong friction occurs in the contact zone, and at the same time, there is a high temperature and pressure in the contact zone. Therefore, the rake face and the flank face gradually wear as the cutting progresses. Tool wear during cutting has the following characteristics: the contact surface between the tool and the chip, the workpiece is often a fresh surface; the contact pressure is very large, sometimes exceeding the yield strength of the material being cut; the temperature of the contact surface is high, for the carbide tool Up to 800 ~ 1000 ° C, for high speed tools up to 300 ~ 600 ° C. Working under the above conditions, tool wear is often the result of a combination of mechanical, thermal, and chemical forms, which can produce the following forms of wear.
First, abrasive wear
Although the hardness of the chips and the workpiece is lower than the hardness of the tool, they often contain some tiny hard spots with extremely high hardness, which can be used to mark the groove on the surface of the tool. This is the wear of the abrasive. Hard spots include carbides (such as Fe3C, TiC, VC), nitrides (such as TiN, Si3N4), oxides (such as SiO2, Al2O3), and intermetallic compounds. The Ti(N, C) particles in the cutting act as a plough on the tool. In addition to abrasive wear on the rake face, grooves on the flank face due to abrasive wear can also be found. Abrasive wear exists at various cutting speeds, but for low speed cutting tools (such as broaches, wrenches, etc.), abrasives are the main cause of wear. This is because the cutting temperature is relatively low at low speed cutting, and the wear caused by other causes is not significant, so it is not essential. The hardness and wear resistance of high-speed steel cutters are lower than that of hard alloys and ceramics, so the proportion of abrasive wear is large.
Second, cold welding wear
During cutting, there is a lot of pressure and strong friction between the chips, the workpiece and the front and back flank, so cold welding occurs between them. Due to the relative movement between the friction surfaces, the cold welded joint will be broken and taken away by one side, resulting in cold welding wear.
Generally, the hardness of the workpiece material or chip is lower than that of the tool material, and the crack of the cold weld often occurs on the workpiece or the chip. However, due to the alternating capacity, contact fatigue, thermal stress, and surface structure defects of the tool, the cracking of the cold welded joint may also occur on the tool side, and the particles of the tool material are carried away by the chips or the workpiece, thereby causing tool wear.
Cold weld wear is generally more severe at moderately low cutting speeds. The research shows that the brittle metal has stronger cold welding resistance than the plastic metal; the same metal or lattice type, lattice spacing, electron density, and electrochemical properties have similar tendency to cold welding; metal compounds are colder than single phase solid solution. The welding tendency is small; the B element of the chemical element periodic table has a smaller tendency to be cold welded than iron.
Under the normal working speed of high-speed steel cutters and the low working speed of cemented carbide tools, the conditions for cold welding can be satisfied, so the proportion of cold welding wear is large. After increasing the cutting speed, the wear resistance of the carbide tool is reduced.
Third, diffusion wear
Diffusion wear occurs at high temperatures. When cutting metal, during the process of contact between the chip and the workpiece, the chemical elements of the two sides diffuse in the solid state, which changes the composition and structure of the original material, making the tool material fragile, thus increasing the wear of the tool. For example, when cutting steel with cemented carbide, the chemical elements in the cemented carbide rapidly diffuse into the chips and the workpiece from 800 ° C, and WC is decomposed into W and C and diffused into the steel. Since the chips and the workpiece are moving at a high speed, the surface of the tool and their surfaces maintain a concentration gradient of the diffusion elements in the contact area, so that the diffusion phenomenon continues. Therefore, the surface of the cemented carbide is depleted in carbon and depleted in tungsten. The binder phase CO is reduced, which in turn reduces the bond strength of the hard phase (WC, TiC) in the cemented carbide. The chips and the Fe in the workpiece diffuse into the cemented carbide, and the Fe diffused into the cemented carbide forms a new hardness and high brittle composite carbide. All of this exacerbates tool wear. In addition to the nature of the tool and the workpiece material itself, temperature is the most important factor affecting diffusion wear. Diffusion wear is often caused by cold weld wear and abrasive wear, and the wear rate is high. The working temperature of high-speed steel cutters is relatively low, and the diffusion between the chips and the workpiece is relatively slow, so the proportion of diffusion wear is much smaller than that of cemented carbide tools.
Fourth, oxidative wear
When the cutting temperature reaches 700-800 °C, the oxygen in the air oxidizes with cobalt and tungsten carbide, titanium carbide, etc. in the cemented carbide to produce soft oxides (such as Co3O4, CoO, WO3, TiO2, etc.). It is rubbed off by the chips or the workpiece to form wear, which is called oxidative wear. Oxidation wear is related to the adhesion strength of the oxide film. The lower the adhesion strength, the faster the wear; otherwise, the wear is alleviated. Generally, air does not easily enter the chip contact area, and oxidative wear is most easily formed at the working boundary of the primary and secondary cutting edges.
Five, thermoelectric wear
Due to the different materials, workpieces, chips and tools will generate a thermoelectric potential in the contact zone during cutting. This thermoelectric potential promotes diffusion and accelerates tool wear. This type of diffusion wear caused by the thermoelectric potential is called "thermoelectric wear". Tests have shown that if the electromotive force opposite to the thermoelectric potential is applied to the workpiece and the tool contact, the thermoelectric wear can be reduced.
In summary, the causes of wear and the wear strength are different under different workpiece materials, tool materials and cutting conditions. For certain tool and workpiece materials, the cutting temperature has a decisive influence on tool wear. The cutting temperature depends on the heat generation and the outflow, which is affected by the amount of cutting, the material of the workpiece, the material of the tool and the beginning of the geometry. Therefore, by reasonably selecting the amount of cutting, tool material and angle, it is possible to reduce the generation of cutting heat and increase the heat transfer. Effectively reducing the temperature in the cutting zone is an important way to reduce tool wear.
As the tool wears to a certain extent, the dimensional accuracy of the workpiece and the quality of the machined surface will be reduced, and the machining cost and tool consumption will also increase. Therefore, it is of great practical significance to reduce tool wear.
Talking about the causes of tool wear
Abstract: Tool wear and durability are related to the efficiency, quality and cost of machining. In this paper, the causes of wear on different materials are analyzed from five aspects: abrasive wear, cold weld wear, diffusion wear, oxidative wear and thermoelectric wear.