1. introduction
Conductive heating cutting is a non-traditional cutting method with a unique machining mechanism. The tool wear has completely different wear patterns and wear patterns from the traditional cutting methods. This is determined by the machining mechanism and the friction mechanism of conductive heating cutting. Studying the tool wear characteristics of the conductive heating cutting and the use performance of the tool have certain practical significance for accelerating the practical process of the conductive heating cutting technology.
2. Cutting test
The test conditions are as follows:
Machine Tools: CM6140 Precision Lathe Material: 45 steel, quenched and tempered; 38CrNi3MoVA, tempered cutting tools: YG8, YT5, YT15, YW1, CN25 Carbide Gripper Clamps, indexable turning tools and welding integral turning tools Power Supply: High power IGBTs High-frequency inverter switching power supply measuring instrument: scanning electron microscope (S-550); reading microscope (40×)
The test adopts a conventional standard test method, that is, a round turning test on a lathe. The cutting speed is selected to be about 0.3 m/s, and on the premise of obtaining the surface of the workpiece (strip, blue) and similar surface roughness (Ra1.25 to 3.20 μm) of the same shape as the high-speed non-conductive cutting, each is used. This type of tool is cut by conductive heating under different cutting conditions and compared with conventional cutting.
3. Tool wear characteristics
As we all know, in the traditional cutting process, the tool wear is generally based on mechanical wear, with diffusion, oxidation wear, etc. The wear pattern of the tool is “Crescent†wear. Cutting speed is the most important factor affecting the tool's durability. Usually the cutting speed is increased, the durability is reduced, and the amount of wear increases [1].
Conductive heating cutting is usually performed at a low speed, and should have a small amount of wear. However, when the conductive heating is cut, a large low-voltage current must be applied to heat the cutting zone to soften the workpiece metal material. And, conductive heating cutting is relying on the knife-worker in close contact with the conduction current. When the current enters the cutting zone, especially the knife-tool and knife-chip contact areas, it is like entering a “narrow bottleneck†where both the charge density and the current density increase rapidly. Not only the contact point burns red heat, but also The air gap space at the contact point may also produce sparks or even arcs [2]. This is the most essential factor that influences tool wear during cutting by conductive heating. In the conductive heating cutting, the tool wear will be based on thermoelectric wear and electric erosion, while the mechanical wear will be weakened due to the softening of the material.
Due to the combined effect of thermal effects and electrical corrosion, the wear patterns of the tools are also different. The results of the study indicate that the typical wear pattern of the electrically conductive heated cutting tool is the "edge wear" of the main cutting edge [3].
4. Tool durability analysis
Domestic and foreign scholars who conduct conductive heating cutting research generally have the following conclusions: under certain conditions, conductive heating cutting can improve the tool durability. For example, when processing titanium alloys, it can be increased by 1 to 4 times[4], and when processing nickel-chromium alloy chilled cast irons, it can be increased by nearly 14 times[5].
In fact, only when heating can significantly reduce the strength properties of the part material, but soften the tool material to a lesser extent, can significantly improve the durability of the tool.
Some scholars have pointed out that, under the condition of maintaining the optimum cutting temperature (using a method of reducing cutting speed), heating the heat-strength alloy parts may increase the durability of the tool and may also reduce the tool durability. When processing hot-strength alloy parts, only with a relatively low cutting speed, and when the cutting temperature is close to the optimum temperature, the purpose of improving the durability of the tool can be achieved by the heating method. When machining at a cutting speed equal to or higher than the optimum speed, the use of a heating method will deviate the cutting temperature from the optimum temperature, thus reducing the tool life [6]. For example, conductive heating cutting 38CrNi3MoVA steel, can improve the tool durability 4 to 5 times at a lower cutting speed, but when the cutting speed is greater than 1.8m/s, there is no advantage of conductive heating cutting, and even the durability of the tool will decrease. the trend of. Therefore, the cutting tool durability of conductive heating cutting is not always higher than that of conventional cutting.
Comprehensive analysis of the relevant research data shows that only in the cutting of high-strength, heat-resistant alloys, quenching hardware and other difficult-to-machine materials, tool-to-workpiece material hardness ratio is relatively small, under certain conditions, the conductive heat cutting tool life expectancy is expected to be large Increased in magnitude. In the case of better machinability of the workpiece material, higher ratio of tool-to-workpiece hardness, or higher cutting speed, conductive heating cutting does not produce significant effects. Due to the electrothermal effect, the tool life may be longer than conventional cutting. Even lower.
5. Tool material selection
As long as the strength and hardness ratio of the knife-tool material is greater than 1.4-1.5, the tool can be cut. However, if the cutting tool can continuously cut the machining material, it must keep the shape of the cutting edge unchanged. This is mainly related to the strength ratio and hardness ratio between the tool material and the workpiece material. Obviously, the greater the ratio between the two, the more favorable the cutting.
Tests have shown that the thermal conductivity of the tool material and the workpiece material has a great influence on the thermal cutting effect. In the test, it was found that the use of a better thermal conductivity of K-type carbide tools for conductive heating cutting effect is better, especially the P type of improved YW1 tool, has a higher tool durability, YW2-based coating blades CN25 also With a very high tool life. Workpiece materials are better with smaller thermal conductivity, which can increase the heating efficiency and increase the durability of the tool (usually the order of thermal conductivity of metal materials is: carbon steel> alloy structure> tool steel> heat-resistant steel> stainless steel) .
6. Selection of tool structure and geometric parameters
After high-temperature welding and sharp grinding, carbide inserts have decreased in hardness. Micro-cracking occurs due to multiple thermal stresses, which reduces the tool's durability. Tests have shown that the conductive heating cutting tool structure should use the machine tool is not re-grinding tool as well, when the cutting conditions are the same, usually welding the integrated tool than the machine tool does not re-grinding tool wear greater.
In order to ensure the close contact between the knife and the work during the cutting process, the current conduction is smooth, the arc is avoided, and the electric erosion is reduced. When considering the geometric parameters of the tool: (1) The tool should be subjected to a certain radial component, the main declination angle Should be less than 90 °, generally 45 ° ~ 75 ° is appropriate; (2) As the workpiece material heat softened, cutting force decreases, the tool strength has not been the most prominent, in order to prevent the tool from rebounding vibration caused by rebound, The angle should be increased as much as possible to maintain a certain degree of sharpness of the blade; (3) In order to ensure that the knife has sufficient strength and heat transfer volume, increase the wear resistance of the cutting edge, improve tool life, usually based on The amount of feed is to ensure that the cutting edge has edges of 0.2-0.5 mm.
7. in conclusion
(1) Conductive heating cutting As the knife-working and knife-chip contact areas are strongly affected by low-voltage high-current and high-temperature, the wear patterns and friction mechanisms of conductive heating cutting tools are unique.
(2) The degree of tool wear is not only related to the contact stress, cutting temperature, and continuous action time, but also related to the adaptation of the cutting amount and cutting conditions to the heating current.
(3) Conductive heating cutting can improve the tool durability. This can only be achieved when the cutting speed is low, the strength of the tool-workpiece material and the hardness ratio are relatively small, and the heating energy can significantly reduce the strength properties of the part material and soften the tool material to a lesser extent. Produce obvious results.
(4) When the conductive heating is cut, the thermal conductivity of the tool material has a great influence on the durability of the tool. The test shows that YW tools are more suitable for conductive heating cutting; in order to avoid arcing, when selecting tool geometry parameters, it is necessary to consider not only maintaining the sharpness of the cutting edge as much as possible, but also taking into consideration that the tool is subjected to a certain radial pressure during the cutting process.
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