The development and application of surface coating technology have played a key role in improving the performance of cutting tools and the progress of cutting machining technology. Coated tools have become an important symbol of modern tools.
Commonly used tool coating materials mainly include TiC, TiN, Al2O3, TiCN, TiAlN, CBN and so on. As a new type of coating material, TiAlN has excellent characteristics such as high hardness, high oxidation temperature, good thermal rigidity, strong adhesion, low friction coefficient, and low thermal conductivity, especially for high-speed cutting. In Japan and Taiwan, the application of TiAlN coating tools has been quite extensive. The new TiAlN coated milling cutter introduced in this article uses ultra-fine particles (about 1 μm in diameter) tungsten carbide-cobalt cemented carbide substrate, and the surface is coated with a high-hardness TiAlN single-layer coating using a special low temperature physical vapor deposition (PVD) method Thickness about 6μm). High-strength oxide (corundum) is formed on the surface of the TiAlN coating. The Ti content in the coating is controlled at about 37% and the Al content is controlled at 10% to 13% to ensure the sharpness of the cutting edge. The main performance comparison between the new TiAlN coating and the TiN coating is shown in Table 1.
Table 1 Comparison of main properties of new TiAlN coating and TiN coating
|Coating performance||TiAlN coating||TiN coating|
|Vickers hardness Hv (kgf / m2)||2720||1930|
|Oxidation temperature (℃)||840||620|
|Scratch test critical load adhesion (N)||80.3||60.3|
|Structural characteristics||thin columnar face-centered cubic structure||face-centered cubic structure|
|Friction performance||0.30 with steel friction coefficient||0.41 with steel friction coefficient|
Application of high-speed milling technology in mold manufacturing
In recent years, high-speed cutting technology has developed rapidly, such as the milling machine speed has reached as high as 30,000 ~ 50000r / min. High-speed milling has good application prospects in precision and ultra-precision machining such as mold manufacturing. Because the transition edges of some molds are sharp and weak, and the corners are prone to collapse, it is difficult to ensure the machining accuracy at the junction of the mold contour by ordinary milling. However, high-speed milling can cut sharp contour sharp corners and realize precision molds. High precision and high efficiency machining.
In recent years, foreign mold manufacturers have developed a number of advanced mold high-speed machining technologies, such as: ① shape pre-recognition control technology: this technology can achieve high-precision machining control functions during high-speed machining of freeform surfaces of molds, to avoid sharp corners due to cutting Impact, mechanical lag, etc. cause path errors. ②SF technology: When the free-form surface of the mold is processed with a spherical milling cutter, the cutting speed of the actual cutting point continuously changes due to the constantly changing cutting points of the workpiece and the tool, which affects the surface machining quality. Using SF technology, the cutting point information that changes at any time can be read in advance from the NC data, and the cutting speed of the cutting point can be kept constant by controlling the rotation speed of the spindle; at the same time, the equivalent feed per revolution is controlled to keep the feed speed stable. ③Regional machining technology: a certain area is preset on the mold machining surface, and the application of regional machining technology can be used without changing the original
In the case of a machining program, it is possible to change the cutting depth and other cutting conditions within or outside the set area. ④Right-angle adjacent edge arc internal angle milling technology: the three-sided milling shank is embedded with a triangular blade for machining, and the internal angle of the arc can be directly milled to a right angle, and the machining efficiency can be increased by about 20 times compared with EDM wire cutting. ⑤The use of special CNC code G260 can realize the machining of inclined holes on the plane. During machining, the milling cutter bar utilizes the angle required by the universal joint deflection, which avoids the trouble of re-loading the workpiece and using special fixtures during conventional machining. ⑥ The control system adopts NURBS (Non-uniformRationalBSpline) compensation, which solves the problem of waiting for data transmission in the control system, and the machine tool cannot be moved during small feed (1μm) machining.
The high-speed milling technology is used to directly mold the mold, which can reduce electrical machining and related process flows, significantly improve the machining efficiency, and shorten the machining time by 1/3 to 1/4. In order to achieve high-speed milling of molds, it is particularly important to develop and apply advanced high-speed milling tools. TiAlN coated milling cutters are currently the most commonly used ideal tools for high-speed milling of hardened die steel.
TiAlN coated milling cutter high-speed milling die steel wear and tear performance
When high-speed milling AlSiH13 / JISSKD61 hardened die steel (52HRC) with VC-MD type six teeth TiAlN coated hard milling cutter (φ10mm) (milling speed: 628m / min; milling length: 50m; milling depth: axial cutting depth 10mm , Radial cutting depth 0.5mm), the effect of different cooling methods on tool wear morphology shows that the tool wear is the smallest when using air-cooled cutting; the tool wear is the second when using dry cutting; the tool wear is used when cutting with emulsion cooling maximum. Because the cutting edge of the milling cutter is in an intermittent cutting state, for example, the coolant is directly sprayed on the cutting tool during cutting, and the temperature change of the cutting edge when it is cold can easily cause thermal cracking, which causes the cutting edge to break and the blade to be damaged. Therefore, coolant should not be used when high-speed milling mold steel, otherwise it will shorten the tool life.
The main failure mechanisms of cutters include crescent crater wear, thermal deformation, and rupture. Cutting heat and mechanical vibration are important factors affecting tool failure. Crescent crater wear usually occurs on the rake face of the blade. When high-speed machining of steel parts and other hard materials, the chips are fused to the surface of the tool under the action of high temperature, and the tool material particles are peeled off, forming crescent crater wear. Excessive crater wear will weaken the cutting edge strength, hinder the flow of chips, increase the temperature and pressure of the tool, and eventually cause the tool to break. Coating the tool can add a layer of inert hard medium between the tool and the workpiece, significantly reducing crescent wear. Through reasonable application of coating technology, the blade can have both high hardness and high toughness. When coating, the element distribution of the substrate and the coating can be adjusted according to the needs, so that the cutting edge area has a higher cobalt content, thereby combining the impact resistance of the cobalt-containing substrate with the wear resistance of the coating, so that the cutting edge has a good Toughness while the rest of the tool maintains high hardness.
When high-speed milling AlSiH13 die steel (50HRC) with VC-MD TiAlN coated milling cutter (feed rate: 0.10mm / tooth; axial cutting depth 10mm, radial cutting depth 0.5mm), the relationship between milling status and tool damage It shows that when the cutting speed V = 157m / min, the cutting length can reach 300m when using dry cutting and air-cooled cutting; the cutting edge cracking occurs at 200m when cutting with emulsion cooling. When the cutting speed V = 314m / min, the tool breaks at 150m when dry cutting is used; normal milling is possible at 300m when using air-cooled cutting; the tool cracks at 50m when cutting with emulsion cooling. When the cutting speed V = 471m / min, the tool wears at 200m when dry cutting is used; the tool wears at 300m when using air-cooled cutting; the tool cracks at 50m when cutting with emulsion cooling . When the cutting speed V = 628m / min, when dry cutting is used, the tool will wear more at 100m, and the tool will break and break at 120m; when using air-cooled cutting, the tool will break and break at 150m; when using emulsion cooling for cutting The tool wears and breaks quickly.
Air-cooled cutting can generally use liquid nitrogen cooling and compressed air cooling, and can be supplemented with oil mist lubrication in the cutting area to improve the surface finish. Air-cooled cutting can obtain better machining results, but the machining cost is higher. Dry cutting eliminates the need for cooling and lubrication devices, which can reduce machining costs and reduce environmental pollution.
In order to achieve dry cutting, the tool coating must have two important functions: ① It can act as a thermal barrier between the tool and the workpiece to reduce the thermal stress acting on the tool base; ② It can act as a solid lubricant Role to reduce cutting friction and the adhesion of chips to the tool. TiAlN coating is a high-performance coating that can better meet the above requirements.
Many new carbide tool grades (especially coated grades) can achieve higher cutting efficiency by using dry cutting during high-speed cutting. In fact, for intermittent cutting, the higher the temperature in the cutting zone, the less suitable it is to use cutting fluid. Adding cutting fluid during milling can make the tool withstand severe temperature changes (the milling blade is cooled when it is cut out from the workpiece, and the temperature rises again when it is cut into the workpiece). Although there is a similar heating-cooling cycle during dry cutting, the temperature change is much gentler than when cutting fluid is added. Dramatic changes in temperature can cause stress in the blade, which can cause cracks.
The thickness of the coating (generally 2 ~ 18μm) has an important influence on the performance of the tool. For intermittent cutting with large impact force, rapid tool cooling and heating, the performance of thin coatings withstanding temperature changes is better than that of thick coatings, which has less stress and is less prone to cracking. Therefore, the life of thin coating blades during dry cutting can Extension by 40%. Generally speaking, the PVD process can obtain a thinner coating than the CVD process, and it is more firmly bonded to the substrate, so circular tools and milling blades are often used PVD coating. In addition, because the deposition temperature of the PVD coating is low, it is often used in tools with sharp edges, large positive rake angle milling cutters, turning tools, etc.
TiAlN coating is currently the best performance PVD coating for high-speed dry cutting. Its high-temperature continuous cutting performance index is 4 times higher than that of titanium nitride (TiN) coating. This is mainly due to the coating surface during high-temperature cutting Alumina film formed on the chip / tool interface after oxidation of the aluminum.