Research status and development of the hottest PCD

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Research status and development of PCD tools

1 Introduction

polycrystalline diamond composite is composed of PCD (polycrystalline diamond) layer and cemented carbide substrate. PCD layer has high hardness, while cemented carbide substrate has good toughness. The combination of the two makes PCD composite sheet widely used in cutting, wood processing, drilling and other industries. There are many ways to make PCD tools, which are generally divided according to the heating method and the use of solder. In the manufacturing process, the heating method plays a decisive role, the selection of solder will have a significant impact on the welding quality, and the structural design of welded joints is also an important means to improve the quality. Therefore, welding has become the key technology in the manufacturing process of PCD tools

the welding of PCD composite sheet is essentially the welding of cemented carbide substrate and cutter bar and other supports. For the welding of cemented carbide, Mn based solder is often used, and the brazing temperature is about 1000 ℃. However, the heat-resistant temperature of the PCD layer generally does not exceed 700 ℃, otherwise it will cause thermal damage to the PCD layer and reduce the service performance of the tool after welding. Therefore, it is necessary to find a welding method that can not only reduce the brazing temperature, but also ensure sufficient welding strength. At present, the welding methods of PCD composites include laser welding, vacuum diffusion welding, vacuum brazing, high-frequency induction brazing, water-cooling brazing, inert gas brazing, etc. Although the welding strength is sufficient, some methods use equipment with high cost, large maintenance cost, and its process is complex, which is not convenient for production and operation, which greatly increases the production cost of PCD composite cutting tools and is not conducive to the promotion and application of PCD cutting tools. This paper compares and analyzes the advantages and disadvantages of various welding methods for PCD laminations

2 PCD welding method

1) laser welding

laser beam welding is an efficient and precise welding method using high-energy density laser beam as heat source. Laser welding has the advantages of high energy density, focusing, deep penetration, high efficiency and strong adaptability. Laser welding belongs to conduction welding, that is, the heat generated by laser irradiating the surface of the workpiece is transferred internally through heat transfer guidance. By controlling the parameters of laser pulse width, energy, peak power and repetition frequency, the workpiece can be welded when it reaches a certain depth of molten pool without obvious vaporization on the surface. Due to the high power density (up to 109w/cm2), small holes are generated on the metal material during the laser welding process, and the laser energy is transmitted to the deep part of the workpiece through the small holes to reduce the horizontal diffusion. The fusion depth of the material is large, the welding speed is fast, and the welding area per unit time is large. In addition, the weld formed by laser welding is deep and narrow, the depth width ratio is large (up to 2 ~ 10), the energy required for welding per unit area is small, the heat affected zone is small, and the welding deformation is small. Generally, no filler metal is added, and the fusion depends on the weldment itself. Laser welding system is highly flexible and easy to realize automation. However, when using laser welding, the weldment is required to have high assembly accuracy. The original assembly accuracy cannot be changed due to thermal deformation in the welding process, and the light spot should be scanned strictly along the weld to be welded without obvious deviation, which overcomes the difficult deviation that the old digital tube display is prone to failure, otherwise serious welding defects will be caused. In addition, the wide application of laser welding is restricted by the large one-time investment of laser and its welding system, high welding cost, high requirements for base metal, many parameters, and high requirements for operating skills. Using laser to weld PCD composite sheets, the strength of the welded joint can be as high as 1800mpa, and there will be no thermal damage to the diamond layer. It is an ideal PCD welding method. At present, it is mostly used for the welding of diamond circular saw blades

2) vacuum diffusion bonding

vacuum diffusion bonding refers to a welding method that makes the surfaces of parts cleaned in vacuum close to each other under high temperature and pressure, and the atoms diffuse each other within a relatively small distance, so as to connect the two parts together. Vacuum diffusion welding is generally carried out at the temperature of 60% - 80% of the melting point temperature (absolute temperature) of the material to be welded. Therefore, this method is very effective for materials with great differences in expansion coefficient (such as cemented carbide substrate of PCD composite sheet and 45# steel knife bar). During diffusion welding, the heating of parts in the vacuum chamber is carried out under the condition of constantly pumping air out, so the adsorbed gas and oxide film on the surface of parts can be removed. In addition, vacuum diffusion welding can maintain the geometric size and shape accuracy of the workpiece, and obtain a vacuum sealed, heat stable and earthquake resistant joint. Therefore, vacuum diffusion welding has been widely used in the welding of PCD geological bits. Its application can ensure the quality of the drill bit, improve the welding strength and increase the footage depth of the drill bit. Sandia Laboratory of the United States conducted nickel plating on the welding surface with a coating thickness of 25 ~ 50, and then subjected to a pressure of 214.62mpa at 650 ℃ for 4 hours to conduct vacuum diffusion welding, with a shear strength of 413.36 ~ 551.2mpa

when using vacuum diffusion welding to weld PCD composite sheet, its welding process is complex, the welding time is long, the cost is high, special equipment is required, and the one-time investment is large. At present, vacuum diffusion welding is generally only used for the welding of geological drill bits with high welding strength requirements and high vibration when used, and has not been used in the production of mass manufacturing general tools

3) vacuum brazing

vacuum brazing refers to the brazing and welding of parts in a vacuum state. Because the diameter of this method is similar to that of grade 2 reinforcement, and it is carried out in an atmosphere without oxidizing gas, it can obtain excellent joints with high strength, toughness and uniformity, which is a new welding method. Special equipment must be used for vacuum brazing. During the welding process, the vacuum degree and welding temperature must be controlled at the same time, so the process is complex and the operation is difficult. At present, the vacuum brazing method is used to weld PCD oilfield bits, and the shear strength of the brazing joint can reach 451.9mpa

4) high frequency induction brazing

high frequency induction heating technology is a heating technology developed in the early 20th century. Because it has a series of advantages, such as fast heating speed, high internal heating and thermal efficiency of materials, uniform and selective heating, good product quality, almost no environmental pollution if special accessories are needed, and easy to realize production automation, it has been rapidly promoted. At present, this heating technology has been widely used in machine tool manufacturing, automobile, tractor manufacturing, bearing manufacturing, measuring tools and cutting tools manufacturing and general mechanical parts manufacturing, and its application scope is expanding day by day. High frequency induction brazing is one of the main application directions

hi frequency induction brazing is a welding method that uses the principle of electromagnetic induction to convert electromagnetic energy into heat energy in solder and parts, heat the solder to the molten state, and then weld the parts together. Using this method, the brazing heating speed is fast, and the power density can reach 10 ~ 100kw/cm2. Generally, the product quality is directly related to the overall safety of the car and the life safety of passengers. The heating process can be completed in a few seconds, and the dimensional accuracy of parts can be guaranteed. Its shear strength can reach 300 ~ 400MPa

compared with laser welding, vacuum diffusion welding, vacuum brazing and other welding methods, the biggest advantage of high-frequency induction brazing is that it has less equipment investment and easy to master the welding process, and its disadvantage is that the temperature of high-frequency induction heating is difficult to control. At present, the application of high-frequency induction brazing PCD composite sheet is relatively extensive, but its process needs to be further improved

3 design of welded joint structure

the special joint structure design of diamond circular saw blade enhances the service performance of the saw blade and reduces the turning rate. Its structure is shown in Figure 1. British De Beers company launched a new product of fried dough twist drill embedded with PCD. Its novel joint structure design reduces the waste of PCD materials, saves expenses, and can realize the change of geometric parameters of the drill (see Figure 2)

Figure 1 design of circular saw blade joint to reduce the turning rate Figure 2 structural design scheme of fried dough twist drill joint embedded with PCD

4 research on welding filler metal and flux

in addition to the research on welding methods and processes, researchers also developed special filler metal and flux according to the characteristics of PCD composite sheet to improve the wettability of filler metal to welding metal. For example, a new solder developed by De Beers laboratory, which contains 53% steel, 14.5% gold, 29.0% manganese and 3.5% nickel, has improved the welding quality; The brazing flux containing cobalt chloride can make the silver based or copper based brazing filler metal well wet the surface of PCD composite and cemented carbide, and prevent them from cobalt removal, so as to improve the welding strength and reduce the chip removal rate of drill bit; The special pre welding treatment method can burn a layer of Ni Co Pd alloy layer on the surface of PCD or cemented carbide, fill its surface cracks and other defects, and prevent the cemented carbide from cobalt removal. It has good and consistent wettability, and the shear strength of brazed joints can reach 320MPa. The research of low silver solder has always been an important direction. A low silver cadmium free solder combined with the matching flux makes the shear strength of diamond tools up to 179.5mpa; The silver solder paste composed of potassium fluoride, sodium chloride, potassium fluoroborate, boric acid, distilled water, etc. can be used with various silver solder at 500 ~ 700 ℃ to increase the wetting ability. At the same time, there are many researches on low silver and silver free solders to reduce costs and replace silver based solders, and the results are very good

5 conclusion

in view of the poor heat resistance of PCD, the improvement methods such as water-cooling brazing and inert gas shielded brazing are used to reduce the thermal damage to the PCD layer during the heating process, so as to correspondingly increase the welding temperature and obtain higher welding strength

while improving the existing technology, new manufacturing methods are also emerging. NASA Jet Propulsion Laboratory has developed a microwave heating process for welding tungsten carbide and diamond brazing joints. This kind of contact can withstand the drilling temperature of hard rock up to 900 ℃, and can be used to make diamond coated bits for geothermal wells. The principle is that microwave has the characteristics of selective heating, which can not overheat the diamond and meet the requirements of brazing temperature. In order to connect the materials, place the diamond disc with a thickness of 2 ~ 3mm on the top of the brazing layer with a thickness of 0.08 ~ 0.8mm (the brazing layer is located on the tungsten carbide substrate). Put the device in a strong electric field in the microwave room and heat it until the solder melts. The solder and its size are determined according to the different thermal expansion coefficients of the weldment, which makes the contact strength reach the maximum and the residual stress is the minimum

to sum up, a reasonable heating method is revolutionary, which can greatly improve the welding quality. Joint structure design, reasonable selection of solder and flux, and auxiliary measures such as water cooling and shielding gas can improve the shortcomings of the existing technology. (end)

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