Analyze tool wear and tool life-coating technology effectively improves the performance of the blade cutting area
The power consumption in the metal cutting process manifests itself in the form of cutting heat and friction. These factors make the tool under harsh processing conditions, high surface load and high cutting temperature. The high temperature is caused by the high-speed sliding of chips along the rake face of the tool, which generates high pressure and strong friction on the cutting edge.
During the machining process, the tool encounters hard spots in the microstructure of the component or performs intermittent cutting, which can cause fluctuations in the cutting force. Therefore, the cutting tool is required to have the characteristics of high temperature resistance, high toughness, high wear resistance, and high hardness.
In the past half century, in order to continuously improve the performance of cutting tools, people have carried out a lot of research work. A key factor affecting the wear rate of almost all tool materials is the cutting temperature reached during the machining process. Unfortunately, it is difficult to define the relevant parameter values for calculating the cutting temperature, but the experimental determination can provide a basis for the empirical formula.
It is generally assumed that all the energy generated in the cutting process is converted into cutting heat, and 80% of the cutting heat will be taken away by the chips (this
The value will vary with some factors, the cutting speed is the main influencing factor). This allows approximately 20% of the heat to enter the tool. Even when cutting low-carbon steel, the tool temperature can exceed 550℃, and this temperature is the highest temperature that high-speed steel (HSS) can withstand while maintaining its hardness. When using cubic boron nitride (CBN) tools to cut hardened steel, the tool and chip temperature can exceed 1000°C.
The relationship between tool wear and tool life
Tool wear patterns can be divided into the following categories:
· Flank wear
· Groove wear
·Crescent crater wear
·Cutting edge chipping
At present, there is no universally accepted uniform definition of tool life in the industry. People need to specify the tool life for the workpiece material and cutting process. One way to quantify tool life is to define an acceptable maximum flank wear value, namely VB or VBmax.
From a mathematical point of view, the tool life can be expressed by the following formula. Taylor formula provides a good approximate calculation method for tool life prediction.
VcTn = C, this is the general form of Taylor's formula, the relevant parameters are as follows:
Vc = cutting speed
T = tool life
D = depth of cut
F = feed rate
x and y are determined based on experimental conditions, and n and C are constants determined by experimental or empirical values; they differ depending on the tool material, workpiece material, and feed rate.
From a practical point of view, in order to suppress excessive tool wear and overcome high temperatures, three key elements should be noted: substrate, coating, and cutting edge treatment. Each element is related to the success or failure of metal cutting. These three elements, combined with the chip flute shape and the tool tip fillet radius, jointly determine the suitable processed material and application for each tool. All the above related parameters work together to ensure the long life of the tool, which is finally reflected in the economy and reliability of processing.
Tungsten-based cemented carbide tools with both wear resistance and toughness have a wider processing application range. Tool suppliers usually control the WC grain size range: 0.3 microns to 5 microns to grasp the performance of the substrate. The WC grain size has a significant impact on the performance of the cutting tool. The smaller the WC grain size, the more wear-resistant the tool; conversely, the larger the WC grain size, the better the tool toughness. Blades made of ultra-fine grain matrix are mainly used to process processed materials in the aerospace industry, such as titanium alloys, inconel, high temperature alloys, etc.
In addition, adjusting the cobalt content from 6% to 12% can significantly improve the toughness of the matrix. Therefore, it is only necessary to adjust the composition of the matrix material to meet the toughness and wear resistance requirements of tools in metal processing applications.
The performance of the matrix can be enhanced by the "cobalt-rich layer" adjacent to the surface layer, or by selectively adding other types of alloying elements to the cemented carbide, such as titanium carbide (TiC), tantalum carbide (TaC), carbide Vanadium (VC) and Niobium Carbide (NbC). The cobalt-rich layer significantly improves the strength of the cutting edge, which makes the tool have excellent performance in rough machining and interrupted machining applications.
In addition, in order to match the workpiece material and meet specific processing requirements, the following five physical properties should be considered when selecting the appropriate matrix: impact toughness, transverse fracture strength, compressive strength, hardness and thermal shock toughness.
NASCAR R&D engineers improved the matrix composition of the new SUMO TEC blade to further improve performance. The base of the blade grade used for processing steel needs to have stronger resistance to plastic deformation to suppress microcracks that appear due to the brittle coating of the blade. NASCAR has also made similar improvements to a full range of alloy substrates used to process cast iron.
The mainstream coating materials currently on the market include:
·Titanium Nitride (TiN)-PVD coating is usually used, which has the characteristics of high hardness and high oxidation temperature.
·Titanium Carbide Nitrogen (TiCN)-Adding carbon helps to improve the hardness of the coating and the self-lubricity of the coating surface.
·Titanium Aluminum Nitride (TiAlN or AlTiN)-includes a layer of alumina, which can extend tool life in high cutting temperature applications, especially suitable for quasi-dry cutting/dry cutting. Compared with TiAlN coating, the surface hardness of AlTiN coating is higher due to the difference of aluminum/titanium ratio. This coating scheme is very suitable for high-speed processing applications.
· Chromium Nitride (CrN)-With the advantages of high hardness and high wear resistance, it is the preferred solution for anti-chip buildup.
·Diamond (PCD)-has the best processing performance of non-ferrous alloy materials, especially for processing graphite, metal matrix composite materials, high silicon aluminum alloy and other abrasive materials. It is completely unsuitable for processing steel, because chemical reactions will destroy the bond between the coating and the substrate.
Crescent crater wear
By analyzing the development of coating materials in recent years and the growth of market demand, we can see that PVD coated tools are more popular than CVD coated tools. The thickness of the CVD coating is generally between 5-15 microns, and the thickness of the PVD coating is generally between 2-6 microns. When the CVD coating is coated on the upper surface of the substrate, the CVD coating will generate tensile stress, while the PVD coating will generate compressive stress on the contrary. These two factors have a significant impact on the cutting edge, especially the performance of the tool during interrupted cutting or continuous machining. Adding new alloying elements in the coating process is not only beneficial to improve the bonding force of the coating, but also can improve the characteristics of the coating.
NASCAR recently announced a unique 3P beam magic coating (SUMO TEC) post-treatment process, which improves the toughness, surface finish and chipping resistance of PVD and CVD coatings respectively. The beam magic coating technology also helps to reduce the friction in the process, and therefore improves the resistance to buildup and ultimately reduces power consumption.
This unique process is embodied in the special process control of the blade cooling process after the CVD coating, which effectively reduces the micro-cracks on the blade coating surface. Similarly, this process can remove the undesirable droplets left on the surface during the PVD coating process. Therefore, whether it is a CVD coating or a PVD coating, a smoother coating surface can be obtained in the end, so that the blade has lower cutting heat, longer life, smoother chip removal, and faster cutting speeds.
Another innovation of NASCAR is the DO-TEC dual coating technology, which is based on the Al2O3 medium temperature chemical coating (MTCVD) surface with TiAlN (PVD) coating. This combination brings multiple benefits to end users, such as cutting cast iron with different composition content at medium and high speeds, with high wear resistance and high chipping resistance.
Insert cutting edge edge treatment
In many cases, cutting edge treatment (passivation) determines the success or failure of machining. The passivation parameters are determined by the preset application. For example, the cutting edge edge treatment required for high-speed finishing of steel is completely different from the cutting edge edge treatment applied to rough machining.
Generally speaking, continuous turning requires passivation of the cutting edge, as does most steel and cast iron milling. For harsh intermittent machining, it is necessary to increase the passivation parameters or carry out T-LAND negative chamfering treatment on the cutting edge.
In contrast, when processing stainless steel or high-temperature alloys, the blade needs to be passivated to obtain a small passivation radius, and a sharp cutting edge is adopted. This is because it is easy to produce built-up edge when processing such materials. Characteristics. Similarly, sharp cutting edges are required when machining aluminum.
In terms of geometry, NASCAR offers many inserts with spiral cutting edges. The contour of the cutting edge is uniformly encircled on a cylindrical surface along the axis. The direction of the spiral blade is similar to a spiral. One of the benefits of the spiral blade design is to make the cutting process smooth and excessive, reduce the chatter, so as to obtain a higher surface finish. In addition, the spiral cutting edge can withstand greater cutting load, so that while reducing the cutting force, more metal can be removed. Another advantage of the spiral cutting edge tool is that the tool life is longer, because the tool cutting force and cutting heat are lower.