Mon Jul 04 15:19:24 CST 2022
Cutting tool wear and durability are related to the efficiency, quality and cost of cutting. In this paper, the causes of abrasive wear, cold welding wear, diffusion wear, oxidation wear and thermoelectric wear are analyzed.
Although the hardness of the chip, the workpiece is lower than the hardness of the tool, but they often contain some very high hardness of the tiny hard points, can be carved on the surface of the cutting tool grooves, this is abrasive wear. Hard points include carbide (such as Fe3C, TiC, VC), nitride (such as TiN, Si3N4), oxide (such as SiO2, Al2O3) and intermetallic compounds. Ti(N, C) particles in cutting play a plowing role on the tool. In addition to the front blade surface will have the phenomenon of abrasive wear, after the blade surface, can also be found due to abrasive wear and grooves. Abrasive wear occurs at all cutting speeds, but is the main cause of wear for low-speed cutting tools such as broaches, pullers, etc. This is due to low speed cutting, cutting temperature is relatively low, other causes of wear is not significant, so it is not the main. The hardness and wear resistance of HSS tools are lower than that of cemented carbide and ceramics, so the abrasive wear of HSS tools accounts for a larger proportion.
When cutting, there is a lot of pressure and strong friction between the chip, the workpiece and the front and back of the knife face, so cold welding will occur between them. Due to the relative movement between the friction surfaces, the cold welding junction will produce a rupture to be taken away by one side, resulting in cold welding wear.
In general, the hardness of the workpiece material or chip is lower than the hardness of the cutting tool material, and the crack of the cold welding junction often occurs in the workpiece or chip. However, due to the alternation ability, contact fatigue, thermal stress and tool surface structure defects and other reasons, cold weld joint rupture may also occur in the tool side, the tool material particles are chip or workpiece away, resulting in tool wear.
Cold welding wear is generally more serious at medium and low cutting speeds. The results show that the resistance of brittle metal to cold welding is stronger than that of plastic metal. Metals with the same metal or lattice type, lattice spacing, electron density and similar electrochemical properties are less prone to cold welding. The tendency of metal compound is smaller than that of single phase solid melt cold welding. Group B elements in the periodic table are less prone to cold welding than iron.
Under the normal working speed of high speed steel tool and the low working speed of hard alloy tool, the conditions of cold welding can be met, so the proportion of cold welding wear is larger. With the increase of cutting speed, the wear of carbide tools in cold welding is reduced.
Diffusion wear occurs at high temperatures. When cutting metal, the chemical elements on both sides diffuse in solid state during the contact process of chip, workpiece and tool, changing the composition and structure of the original material, making the tool material become fragile, thus intensifying the wear of the cutting tool. For example, when carbide is cut into steel, starting from 800℃, the chemical elements in carbide are rapidly diffused into chips and workpieces, and WC is decomposed into W and C and diffused into steel. Because the chip and the workpiece are moving at high speed, the tool surface and their surfaces maintain the concentration gradient of the diffusing elements in the contact area, so that the diffusion phenomenon continues. Therefore, the phenomenon of poor carbon and tungsten on the surface of cemented carbide occurs. The bond strength of tungsten carbide (WC, TiC) decreases with the decrease of CO. The Fe in the chip and the workpiece diffuses to the hard alloy, and the Fe in the hard alloy will form a new complex carbide with high hardness and brittleness. All this makes the tool wear worse. In addition to the properties of tool and workpiece material, temperature is the most important factor affecting diffusion wear. Diffusion wear often occurs at the same time with cold welding wear and abrasive wear, and the wear rate is very high. The working temperature of HSS tool is low, and the diffusion between the chip and the workpiece is relatively slow, so the proportion of diffusion wear of HSS tool is much less than that of cemented carbide tool.
When the cutting temperature reaches 700 ~ 800℃, oxygen in the air oxidized with cobalt, tungsten carbide and titanium carbide in cemented carbide, resulting in soft oxides (such as Co3O4, CoO, WO3, TiO2, etc.) erased by chips or workpiece and formed wear, which is called oxidative wear. Oxidation wear is related to adhesion strength of oxide film. The lower the adhesion strength is, the faster the wear is. Conversely, this wear and tear can be reduced. Generally, the air is not easy to enter the chip contact area, and the oxidation wear is most likely to form at the working boundary of the main and secondary knife cutting edges.
Workpiece, chip and cutting tool due to different materials, cutting in the contact area will generate thermoelectric potential, this thermoelectric potential has the effect of promoting diffusion and accelerate tool wear. This diffusion wear under the action of thermoelectric potential is called "thermoelectric wear". It is proved that the thermoelectric wear can be reduced if the electromotive force opposite to the thermoelectric potential is applied at the contact point of workpiece and cutting tool.
In short, the causes of wear and the strength of wear are different for different workpiece materials, tool materials and cutting conditions. For certain tool and workpiece materials, cutting temperature has a decisive effect on tool wear. The height of the cutting temperature depends on the heat generation and out of the situation, it is affected by the cutting amount, workpiece material, tool material and how many open. Therefore, through the reasonable selection of cutting parameters, tool material and Angle, can reduce the generation of cutting heat and increase the heat out. Reducing the cutting zone temperature effectively is an important way to reduce tool wear.
Because the tool wear to a certain extent, will reduce the workpiece size accuracy and machining surface quality, but also will increase the cost of processing and tool consumption, so reducing tool wear has a very important practical significance.