PCD tool is made of polycrystalline diamond knife tip and carbide matrix through high temperature and high pressure sintering. It can not only give full play to the advantages of high hardness, high thermal conductivity, low friction coefficient, low thermal expansion coefficient, small affinity with metal and non-metal, high elastic modulus, no cleaving surface, isotropic, but also take into account the high strength of hard alloy.
Thermal stability, impact toughness and wear resistance are the main performance indicators of PCD. Because it is mostly used in high temperature and high stress environment, thermal stability is the most important thing. The study shows that the thermal stability of PCD has a great impact on its wear resistance and impact toughness. The data show that when the temperature is higher than 750℃, the wear resistance and impact toughness of PCD generally decrease by 5% -10%.
The crystal state of the PCD determines its properties. In microstructure, carbon atoms form covalent bonds with four adjacent atoms, obtain the tetrahedral structure, and then form the atomic crystal, which has strong orientation and binding force, and high hardness. The main performance indexes of PCD are as follows: ① hardness can reach 8000 HV, 8-12 times of carbide; ② thermal conductivity is 700W / mK, 1.5-9 times, even higher than PCBN and copper; ③ friction coefficient is generally only 0.1-0.3, much less than 0.4-1 of carbide, significantly reducing the cutting force; ④ thermal expansion coefficient is only 0.9x10-6-1.18x10-6,1 / 5 of carbide, which can reduce thermal deformation and improve processing accuracy; ⑤ and non-metallic materials are less affinity to form nodules.
Cubic boron nitride has strong oxidation resistance and can process iron-containing materials, but the hardness is lower than single crystal diamond, the processing speed is slow and the efficiency is low. The single crystal diamond has high hardness, but the toughness is insufficient. Anisotropy makes it easy to dissociation along (111) surface under the impact of external force, and the processing efficiency is limited. PCD is a polymer synthesized by micron-sized diamond particles by certain means. The chaotic nature of the disordered accumulation of particles leads to its macroscopic isotropic nature, and there is no directional and cleavage surface in the tensile strength. Compared with the single-crystal diamond, the grain boundary of PCD effectively reduces the anisotropy and optimizes the mechanical properties.
1. Design principles of PCD cutting tools
(1) Reasonable selection of PCD particle size
Theoretically, PCD should try to refine the grains, and the distribution of additives between products should be as uniform as possible to overcome the anisotropy. The choice of PCD particle size is also related to the processing conditions. Generally speaking, PCD with high strength, good toughness, good impact resistance and fine grain can be used for finishing or super finishing, and PCD of coarse grain can be used for general rough machining. The PCD particle size can significantly affect the wear performance of the tool. Relevant literature points out that when the raw material grain is large, the wear resistance gradually increases with the decrease of the grain size, but when the grain size is very small, this rule is not applicable.
Related experiments selected four diamond powder with average particle sizes of 10um, 5um, 2um and 1um, and it was concluded that: ① With the decrease of particle size of raw material, Co diffuses more evenly; with the decrease of ②, the wear resistance and heat resistance of PCD gradually decreased.
(2) Reasonable choice of the blade mouth form and blade thickness
The form of blade mouth mainly includes four structures: inverted edge, blunt circle, inverted edge blunt circle composite and sharp angle. The sharp angular structure makes the edge sharp, the cutting speed is fast, can significantly reduce the cutting force and burr, improve the surface quality of the product, is more suitable for low silicon aluminum alloy and other low hardness, uniform non-ferrous metal finishing. The obtuse round structure can passivate the blade mouth, forming R Angle, effectively prevent the blade breaking, suitable for processing medium / high silicon aluminum alloy. In some special cases, such as shallow cutting depth and small knife feeding, the blunt round structure is preferred. The inverted edge structure can increase the edges and corners, stabilize the blade, but at the same time will increase the pressure and cutting resistance, more suitable for heavy load cutting high silicon aluminum alloy.
In order to facilitate EDM, usually choose a thin PDC sheet layer (0.3-1.0mm), plus the carbide layer, the total thickness of the tool is about 28mm. The carbide layer should not be too thick to avoid stratification caused by the stress difference between the bonding surfaces
2, PCD tool manufacturing process
The manufacturing process of PCD tool directly determines the cutting performance and service life of the tool, which is the key to its application and development. The manufacturing process of the PCD tool is shown in Figure 5.
(1) Manufacturing of PCD composite tablets (PDC)
① Manufacturing process of the PDC
PDC is generally composed of natural or synthetic diamond powder and binding agent at high temperature (1000-2000℃) and high pressure (5-10 atm). The binding agent forms the binding bridge with TiC, Sic, Fe, Co, Ni, etc. as the main components, and the diamond crystal is embedded in the skeleton of the binding bridge in the form of covalent bond. PDC is generally made into disks with fixed diameter and thickness, and grinding and polished and other corresponding physical and chemical treatments. In essence, the ideal form of PDC should retain the excellent physical characteristics of single crystal diamond as much as possible, therefore, the additives in the sintering body should be as little as possible, at the same time, the particle D-D bond combination as much as possible,
② Classification and selection of binders
The binder is the most important factor affecting the thermal stability of the PCD tool, which directly affects its hardness, wear resistance and thermal stability. Common PCD bonding methods are: iron, cobalt, nickel and other transition metals. Co and W mixed powder was used as the bonding agent, and the comprehensive performance of the sintering PCD was best when the synthesis pressure was 5.5 GPa, the sintering temperature was 1450℃ and the insulation for 4min. SiC, TiC, WC, TiB2, and other ceramic materials. SiC The thermal stability of SiC is better than that of Co, but the hardness and fracture toughness are relatively low. Appropriate reduction of raw material size can improve the hardness and toughness of PCD. No adhesive, with graphite or other carbon sources in the ultra-high temperature and high pressure burned into a nanoscale polymer diamond (NPD). Using graphite as the precursor to prepare NPD is the most demanding conditions, but the synthetic NPD has the highest hardness and the best mechanical properties.
Selection and control of ③ grains
The raw material diamond powder is a key factor affecting the performance of PCD. Pretreating diamond micropowder, adding a small amount of substances hindering abnormal diamond particles growth and reasonable selection of sintering additives can inhibit the growth of abnormal diamond particles.
High pure NPD with a uniform structure can effectively eliminate the anisotropy and further improve the mechanical properties. The nanographite precursor powder prepared by high-energy ball grinding method was used to regulate oxygen content at high temperature pre-sintering, transforming graphite into diamond under 18 GPa and 2100-2300℃, generating lamella and granular NPD, and the hardness increased with the decrease of lamella thickness.
④ Late chemical treatment
At the same temperature (200 °℃) and time (20h), the cobalt removal effect of Lewis acid-FeCl3 was significantly better than that of water, and the optimal ratio of HCl was 10-15g / 100ml. The thermal stability of PCD improves as the cobalt removal depth increases. For coarse-grained growth PCD, strong acid treatment can completely remove Co, but has great influence on the polymer performance; adding TiC and WC to change the synthetic polycrystal structure and combining with strong acid treatment to improve the stability of PCD. At present, the preparation process of PCD materials is improving, the product toughness is good, the anisotropy has been greatly improved, has realized commercial production, related industries are developing rapidly.
(2) Processing of the PCD blade
① cutting process
PCD has high hardness, good wear resistance and high difficult cutting process.
② welding procedure
PDC and the knife body by mechanical clamp, bonding and brazing. Brazing is to press PDC on the carbide matrix, including vacuum brazing, vacuum diffusion welding, high frequency induction heating brazing, laser welding, etc. High frequency induction heating brazing has low cost and high return, and has been widely used. The welding quality is related to the flux, welding alloy and welding temperature. Welding temperature (generally lower than 700 °℃) has the greatest impact, the temperature is too high, easy to cause PCD graphitization, or even "over-burning", which directly affects the welding effect, and too low temperature will lead to insufficient welding strength. The welding temperature can be controlled by the insulation time and the depth of PCD redness.
③ blade grinding process
PCD tool grinding process is the key to manufacturing process. Generally, the peak value of the blade and the blade is within 5um, and the arc radius is within 4um; the front and back cutting surface ensure certain surface finish, and even reduce the front cutting surface Ra to 0.01 μ m to meet the mirror requirements, make the chips flow along the front knife surface and prevent sticking knife.
Blade grinding process includes diamond grinding wheel mechanical blade grinding, electric spark blade grinding (EDG), metal binder super hard abrasive grinding wheel online electrolytic finishing blade grinding (ELID), composite blade grinding machining. Among them, the diamond grinding wheel mechanical blade grinding is the most mature, the most widely used.
Related experiments: ① the coarse particle grinding wheel will lead to serious blade collapse, and the particle size of the grinding wheel decreases, and the quality of the blade becomes better; the particle size of ② grinding wheel is closely related to the blade quality of fine particle or ultrafine particle PCD tools, but has limited effect on coarse particle PCD tools.
Related research at home and abroad mainly focuses on the mechanism and process of blade grinding. In the blade grinding mechanism, thermochemical removal and mechanical removal are the dominant, and brittle removal and fatigue removal are relatively small. When grinding, according to the strength and heat resistance of different binding agent diamond grinding wheels, improve the speed and swing frequency of the grinding wheel as far as possible, avoid brittleness and fatigue removal, improve the proportion of thermochemical removal, and reduce the surface roughness. The surface roughness of dry grinding is low, but easily due to high processing temperature, burn tool surface,
Blade grinding process needs to pay attention to: ① choose reasonable blade grinding process parameters, can make the edge mouth quality more excellent, front and back blade surface finish higher. However, also consider high grinding force, large loss, low grinding efficiency, high cost; ② select reasonable grinding wheel quality, including binder type, particle size, concentration, binder, grinding wheel dressing, with reasonable dry and wet blade grinding conditions, can optimize the tool front and rear corner, knife tip passivation value and other parameters, while improve the surface quality of the tool.
Different binding diamond grinding wheel have different characteristics, and different grinding mechanism and effect. Resin binder diamond sand wheel is soft, Grinding particles are easy to fall off prematurely, Not having a heat resistance, The surface is easily deformed by the heat, Blade grinding surface is prone to wear marks, Large roughness; Metal binder diamond grinding wheel is kept sharp by grinding crushing, Good formability, surfacing, Low surface roughness of the blade grinding, Higher efficiency, However, the binding ability of grinding particles makes the self-sharpening poor, And the cutting edge is easy to leave an impact gap, Causing serious marginal damage; Ceramic binder diamond grinding wheel has a moderate strength, Good self-excitation performance, More internal pores, Favfor dust removal and heat dissipation, Can adapt to a variety of coolant, The low grinding temperature, The grinding wheel is less worn, Good shape retention, The accuracy of the highest efficiency, However, the body of diamond grinding and binder leads to the formation of pits on the tool surface. Use according to the processing materials, comprehensive grinding efficiency, abrasive durability and surface quality of the workpiece.
The research on grinding efficiency mainly focuses on improving productivity and control cost. Generally, grinding rate Q (PCD removal per unit time) and wear ratio G (ratio of PCD removal to grinding wheel loss) are used as evaluation criteria.
German scholar KENTER grinding PCD tool with constant pressure, test: ① increases the grinding wheel speed, PDC particle size and coolant concentration, the grinding rate and wear ratio are reduced; ② increases the grinding particle size, increases the constant pressure, increases the concentration of diamond in the grinding wheel, grinding rate and wear ratio increase; ③ binder type is different, the grinding rate and wear ratio is different. KENTER The blade grinding process of PCD tool was studied systematically, but the influence of the blade grinding process was not analyzed systematically.
3. Use and failure of PCD cutting tools
(1) Selection of tool cutting parameters
During the initial period of PCD tool, the sharp edge mouth gradually passited, and the machining surface quality became better. Passivation can effectively remove the micro gap and small burrs brought by the blade grinding, improve the surface quality of the cutting edge, and at the same time, form a circular edge radius to squeeze and repair the processed surface, thus improving the surface quality of the workpiece.
PCD tool surface milling aluminum alloy, cutting speed is generally in 4000m / min, hole processing is generally in 800m / min, processing of high elastic-plastic non-ferrous metal should take a higher turning speed (300-1000m / min). Feed volume is generally recommended between 0.08-0.15mm/r. Too large feed volume, increased cutting force, increased residual geometric area of the workpiece surface; too small feed volume, increased cutting heat, and increased wear. The cutting depth increases, the cutting force increases, the cutting heat increases, the life decreases, excessive cutting depth can easily cause blade collapse; small cutting depth will lead to machining hardening, wear and even blade collapse.
(2) Wear form
Tool processing workpiece, due to friction, high temperature and other reasons, wear is inevitable. The wear of the diamond tool consists of three stages: the initial rapid wear phase (also known as the transition phase), the stable wear phase with a constant wear rate, and the subsequent rapid wear phase. The rapid wear phase indicates that the tool is not working and requires regrinding. The wear forms of cutting tools include adhesive wear (cold welding wear), diffusion wear, abrasive wear, oxidation wear, etc.
Different from traditional tools, the wear form of PCD tools is adhesive wear, diffusion wear and polycrystalline layer damage. Among them, the damage of polycrystal layer is the main reason, which is manifested as the subtle blade collapse caused by external impact or the loss of adhesive in PDC, forming a gap, which belongs to physical mechanical damage, which can lead to the reduction of processing precision and the scrap of workpieces. PCD particle size, blade form, blade Angle, workpiece material and processing parameters will affect the blade blade strength and cutting force, and then cause the damage of the polycrystal layer. In engineering practice, the appropriate raw material particle size, tool parameters and processing parameters should be selected according to the processing conditions.
4. Development trend of PCD cutting tools
At present, the application range of PCD tool has been expanded from traditional turning to drilling, milling, high-speed cutting, and has been widely used at home and abroad. The rapid development of electric vehicles has not only brought impact to the traditional automobile industry, but also brought unprecedented challenges to the tool industry, urging the tool industry to accelerate the optimization and innovation.
The wide application of PCD cutting tools has deepened and promoted the research and development of cutting tools. With the deepening of research, PDC specifications are getting smaller and smaller, grain refinement quality optimization, performance uniformity, grinding rate and wear ratio is higher and higher, shape and structure diversification. The research directions of PCD tools include: ① research and develop thin PCD layer; ② research and develops new PCD tool materials; ③ research to better welding PCD tools and further reduce cost; ④ research improves PCD tool blade grinding process to improve efficiency; ⑤ research optimizes PCD tool parameters and uses tools according to local conditions; ⑥ research rationally selects cutting parameters according to the processed materials.
brief summary
(1) PCD tool cutting performance, make up for the shortage of many carbide tools; at the same time, the price is far lower than single crystal diamond tool, in modern cutting, is a promising tool;
(2) According to the type and performance of the processed materials, a reasonable selection of the particle size and parameters of PCD tools, which is the premise of tool manufacturing and use,
(3) PCD material has a high hardness, which is the ideal material for cutting knife county, but it also brings the difficulty for cutting tool manufacturing. When manufacturing, to comprehensively consider the process difficulty and processing needs, in order to achieve the best cost performance;
(4) PCD processing materials in knife county, we should reasonably select cutting parameters, on the basis of meeting the product performance, as far as possible to extend the service life of the tool in order to achieve the balance of tool life, production efficiency and product quality;
(5) Research and develop new PCD tool materials to overcome its inherent drawbacks
This article is sourced from the "superhard material network"
Post time: Mar-25-2025
