The hottest needle is the new material grade desig

2022-08-16
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New material grade design for high-speed machining

processing trend

the metal processing industry is characterized by rapid technological development. This is the result of factors such as globalization, intensified market competition, larger and more stable machine tools that allow higher cutting speeds, the use of difficult to machine materials, and the growing awareness of environmental problems. As a result, the end user of the tool puts forward continuous improvement requirements for the tool manufacturer on the performance of the tool. The general trend of metal processing industry is to develop more cost-effective processing technologies

the new laws and regulations will increase the use cost of coolant and lubricant. This promotes the use of dry processing (Figure 1). Then, this will increase the demand for more heat-resistant coated cemented carbide and force the metal processing industry to consider new alternatives

Figure 1 high speed cutting with cemented carbide inserts

especially in the automotive industry, the market demand for reducing energy consumption will emphasize the importance of material selection. For example, light alloys are preferred. In addition to aluminum and other light alloys, specially designed high-strength steel will be used more in the future

the following are the most important trends in the market:

higher cutting speeds aimed at improving productivity

dry processing and/or minimum lubrication (MQL) for cost and environmental reasons

difficult to machine materials, such as high-strength materials for lighter parts

all these trends put forward more requirements for the wear resistance, plastic deformation resistance and toughness of the same tool material grade. If toughness is the only problem, PVD (physical vapor deposition) coating can be regarded as an option. However, PVD coatings cannot compete with modern CVD (chemical vapor deposition) coatings in terms of wear resistance. PVD coating deposition thickness is usually less than 5 m, while CVD coating may be 20 m thick

traditional design

a sintered cemented carbide cutting blade is an optimized combination of matrix and coating customized for a specific application field, which is usually regarded as a material grade. To design a new material grade is to find the best combination of physical properties of substrate and coating. The most important properties are hardness and toughness, which in turn are related to material properties. As a result, a compromise between wear resistance and toughness is usually inevitable in tool development. In this paper, we will deal with some new ways to overcome this dilemma, such as the combination of wear resistance and toughness. The research and development of a wear-resistant material grade intended for high-speed cutting will be discussed

the properties of WC Co based matrix can be controlled by the changes of CO content, WC particle size and the number of cubic carbide phases. McEuen is building complex stretchable and folded graphene structures. The application of functional gradients is also feasible: the matrix immediately below the coating creates a surface layer rich in CO and exhausting hard cubic carbides at the same time. In this way, a hard matrix core can be obtained without sacrificing the toughness of the cutting edge. Wear resistant applications usually require plastic deformation resistant substrates and traditional hard substrates. The matrix with functionally graded distribution is usually not used. The disadvantage is to reduce toughness in many difficult to machine materials

in traditional coatings, different coating materials are combined and optimized according to the multilayer structure derived from the generally accepted wear model. The internal (middle) coating is usually based on the tic Ti (C, n) -tin combination that ensures good resistance to flank wear and good adhesion between the coating and the cemented carbide matrix. The most commonly used intermediate layer is Ti (C, n) and today it is almost exclusively deposited using medium temperature chemical vapor deposition (mtcvd). Al2O3 is traditionally used on the top of the intermediate layer to reduce the crater wear of the rake face, and it is also used as a thermal barrier. Finally, a thin layer of tin is usually deposited on the top of the composite coating in order to make the tool golden and easy to observe 5 Wear occurs every time you enter the program. Most coatings on the market today are composed of Ti (C, n), Al2O3 and tin. Usually, Al2O3 is only covered with a thin layer of tin

oxide dilemma

al2o3 has many unstable allotropes, such as G, h, D, Q, C and K, plus stable a-Al2O3 phase. All these changes will then be transformed into stable a-Al2O3 phase, such as heat treatment during deposition, heat treatment after deposition and metal cutting. The three Al2O3 phases a-Al2O3, k-al2o3 and g-Al2O3 can be deposited by CVD in a controlled manner. The characteristics of these phases are given in Table 1

table 1: characteristics of stable and unstable CVD Al2O3 phases

the only CVD phases applied to industrial scale are a-Al2O3 and k-al2o3. G-Al2O3 has not been commercially used as a wear reduction or friction coating, even though it is most commonly obtained by PVD Al2O3 phase

surprisingly, stable a-Al2O3 has been found to be the most difficult CVD deposition in industrial applications compared with unstable k-al2o3. This is why about 80% of all CVD alumina coatings are composed of k-al2o3. However, k-al2o3 is an unstable phase, which may be transformed into stable a-Al2O3 phase during deposition and metal cutting (especially at high speed). The volume shrinkage encountered in the phase transition will reduce and eventually destroy the adhesion of k-al2o3 layer

many commercial a-Al2O3 coatings on the market are k? The result of phase a conversion. Therefore, this coating exhibits thermal cracks and is fragile (Fig. 2a). Only recently has it been found that the nucleation of a-Al2O3 phase can be completely controlled and controlled by adjusting the chemical action on the surface of the nucleus. The resulting coating is composed of fine particles a-Al2O3, avoiding transformation cracks (Fig. 2b). As a result, the results shown in Fig. 3B show excellent toughness compared with the a-Al2O3 obtained by the previous technology and the current k-al2o3 layer

a) microstructure of a-Al2O3 in previous technology B) modern a-Al2O3 microstructure with controlled nucleation and fine particle size

Figure 2

classical wear model and traditional coating design are questionable. For example, due to the high chemical stability of Al2O3 to the melting point of steel, the crescent wear of Al2O3 is most unlikely to be a constrained diffusion process. The crater wear of Al2O3 is actually the result of plastic deformation. Therefore, when processing many steel parts, Ti (C, n) shows better resistance to crater wear than Al2O3, and it can be expected that the resistance to flank wear is always better than Al2O3. Therefore, it is necessary to discuss the new coating design according to the following examples

Fig. 3 shows two experimental coatings with equal total thickness on the same substrate. According to the traditional method, Al2O3 is deposited on the top of Ti (C, n) layer (Fig. 3a) and Al2O3 is deposited between two Ti (C, n) layers (Fig. 3b). In the later example, the Ti (C, n) layer protects the Al2O3 layer from craters and flank wear and allows full use of the thermal barrier properties of Al2O3. As shown in Figure 4, this new design (Figure 4b) is superior to the traditional approach. In particular, the ability to resist flank wear is improved, and compared with the traditional coating design, the ability to resist deformation is increased. According to the performance test of the newly designed coating tape, the layer life is about 100% longer than the traditional coating

Figure 3

a) traditional coating design: alumina is placed on the Ti (C, n) layer as a diffusion barrier

b) a thick layer of Ti (C, n) is deposited on alumina to reduce plastic deformation

figure 4 Figure 3 shows the cutting performance of the hypothetical coating

a) traditional design B) according to new design

new coating concept

coating structure is shown in Figure 5. The fine Al2O3 layer is sandwiched between two layers of mtcvd Ti (C, n). The most striking feature is that a thick layer of Ti (C, n) is coated on the Al2O3 layer. Since such a thin layer of tin is applied to the Ti (C, n) coating to detect wear. The total thickness of the coating is about 20 m and has a functionally graded matrix designed for high-speed cutting

Figure 5 The cemented carbide matrix with precise functional gradient distribution and enhanced structural control allows the application of thicker,

more wear-resistant CVD coatings that maintain toughness. The thickness of CVD coating is about 20 m

as shown in Figure 5, the cobalt rich area immediately below the coating is quite thick, but its thickness is negligible compared with the size of perceptible wear. When combined with a hard matrix, the plastic deformation resistance will not decrease

proof of concept

some completed field tests have verified the laboratory test results. These experiments focused on comparing the new material grade design with the traditional coatings on the market. Here are two representative examples. They include two different steels and two different processes. These examples clearly show that the wear resistance of products with traditional coating design is improved

example 1

the first example is about the cylindrical dry rough machining of the shaft. The workpiece material is 16MnCr5 with hardness of hb180. The cutting conditions are:

cutting speed VC = 430 m/min

feed FN = 0.3 mm/R

cutting depth AP = 1.5 mm

new design traditional coating a traditional coating B

Figure 6 Optical micrographs show that the cutting edge wear based on the newly designed blade after 6.5 minutes of turning is compared with the material grade of traditional coating design and similar substrate of the two main competitors

example 2

the second example is about turning the outer diameter and end face of small bearing ring. Add coolant to the test. The workpiece material is 100Cr6 with hb=190. The cutting condition is:

cutting speed VC = 480 m/min

feed FN = 0 73 mm/R

cutting depth AP = 1.0 mm

new design traditional design

figure 8 Optical micrographs show the cutting edge wear based on the newly designed blade and the material grade of competitors based on the traditional coating design with the best performance

Figure 9 The chart shows the comparison of expected tool life

and flank wear based on the number of processed parts of the newly designed product with the best performance of the material grade of competitors based on traditional coating design

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