Brazing and Diffusion Welding in Diamond Tool Manufacturing

Preface

Diamond has high hardness, strong wear resistance, high thermal conductivity and low thermal expansion coefficient, and is the best material for making cutting and grinding tools.

Diamond tools are the most commonly used tools for cutting and grinding hard and brittle materials, mainly including sawing tools, drilling bits, finishing tools, wire drawing dies, etc.

They are widely used in stone cutting, 3C processing, geological exploration, automobile parts manufacturing, national defense industry and other fields.

Related reading: Cutting Tools Basics

At present, the common diamond tools are: single crystal diamond tools (SD), polycrystalline diamond tools (PCD), diamond coated tools (CVD).

Brazing and Diffusion Welding in Diamond Tool Manufacturing 1

The diamonds used in single crystal diamond tools are divided into natural single crystal diamond and artificial single crystal diamond;

Polycrystalline diamond tools are usually synthetic diamonds;

Diamond coated tools are divided into diamond thick film tools (diamond film thickness>300 µ m) and diamond film tools (diamond film thickness<20 µ m) according to the coating thickness.

The classification of diamond tools is shown in Fig. 1, and some common diamond tools are shown in Fig. 2.

It is a key technology to convert diamond into diamond tools and achieve the required properties.

This article summarizes the current preparation methods of diamond tools, summarizes the research progress of brazing and diffusion welding of diamond tools at home and abroad, and prospects the development of diamond welding technology in the future.

Brazing and Diffusion Welding in Diamond Tool Manufacturing 2

Fig.1   Classification of diamond tools

Brazing and Diffusion Welding in Diamond Tool Manufacturing 3

Fig.2   Commonly used diamond tools

1. Main preparation methods of diamond tools

Single diamond is difficult to be used as a tool and must be connected with high toughness materials.

The manufacturing of diamond tools generally adopts sintering, electroplating, brazing, diffusion welding and other methods.

Sintering method is a method to prepare diamond tools by pressure sintering in sintering furnace after matrix powder and diamond are uniformly mixed in a certain proportion.

Hot pressing sintering technology is often used to manufacture multilayer diamond tools, such as diamond cutter heads, grinding wheels and wire saws.

The diamond tool matrix made by sintering has low bond strength with diamond and more diamond particles are embedded in the matrix.

It is often used for cutting and grinding materials with high hardness and brittleness.

Electroplated diamond tools can be prepared by using electroplating method to solidify diamond particles in the coated metal deposited on the substrate.

The plating metal of electroplated diamond tools has a small holding force on the diamond, the exposed height of the diamond is low, and the diamond abrasive grains are easy to fall off when the grinding load is heavy.

Brazed diamond tool is a diamond tool made by connecting diamond and matrix by brazing.

The bonding strength between diamond and matrix is high and it is not easy to fall off.

At the same time, it has the advantages of large exposed height of abrasive particles (theoretically more than 70% of the height of abrasive particles), large chip space, good heat dissipation, etc.

It can withstand large loads during use and can be used for high-speed and efficient grinding.

The brazing method of diamond tools has been patented as early as 1975.

Its research and use have attracted the attention of researchers at home and abroad, and has been recognized as a promising technology to improve the connection of synthetic diamond.

Diffusion welding is a solid state connection method.

The atoms on the surface of the contact surface diffuse with each other under high temperature and pressure, resulting in the combination of atoms, so as to obtain a firm connection.

The temperature of vacuum diffusion welding is generally 0.6~0.8Tm (Tm is the melting temperature of the base metal to be welded), which is applicable to the connection between materials with large difference in thermal expansion coefficient, such as the welding of cemented carbide of PDC composite sheet and cutter post matrix.

The advantages and disadvantages of several preparation methods are shown in Table 1.

Table 1   Comparison of several preparation methods

Preparation methodAdvantageDisadvantage
Powder metallurgy sinteringThe preparation method is mature;
Simple process operation;
Low production cost;
Most are multilayer diamond tools.
The diamond particles are irregularly distributed and the exposed height is uneven;
The metal matrix is mechanically wrapped with diamond, so the holding force is weak, and diamond particles are easy to fall off prematurely.
ElectroplatingThe exposed height of arm particles is relatively consistent.The depth of particles embedded in the matrix is up to 2/3, and the service life of the tool is short.
Brazing methodStrong adhesion between diamond and matrix;
The exposed height of particles reaches 2/3, and the service life is long;
The distribution of particles in the matrix is controllable.
The brazing performance of diamond is poor;
High technical requirements for process operation;
Diamond is easily graphitized.
Diffusion weldingThe lower welding temperature is suitable for materials with large difference in thermal expansion coefficient.The equipment cost is high and the scope of application is narrow.

2. Brazing process of diamond tools

At present, the brazing processes available for diamond tools include vacuum brazing, furnace brazing, salt bath brazing, flame brazing, laser brazing, resistance brazing, high-frequency and vacuum induction brazing, etc.

The bonding strength determines the service performance and service life of diamond tools.

In order to prevent solder oxidation, diamond tools are usually brazed in high vacuum or under inert gas protection.

The vacuum and gas shielded welding environment needs a specific furnace to provide;

Resistance radiation heating is generally used for brazing in the furnace.

The heating is uniform, and the diamond tools with complex structure and large size can be welded.

Vacuum brazing single-layer diamond tools are mainly used in the manufacture of dressing tools, grinding wheels, stone tools and glass tools.

Vacuum brazing and furnace brazing are suitable for batch production.

Ammonia decomposition gas is used as reducing gas for continuous atmosphere furnace brazing.

The heating speed is slow and the temperature is uniform.

The brazed joint is beautiful, the quality is reliable, the workpiece surface is bright, the production efficiency is high, and the welding cost is low.

It is commonly used in the production of small saw blades and special-shaped diamond tools.

The salt bath brazing temperature is low, the heating speed is fast and uniform, and the graphitization degree of diamond is low, which is suitable for mass production.

However, the salt bath brazing process is cumbersome, the residual flux of the workpiece is serious after welding, and a large amount of wastewater generated during cleaning pollutes the environment.

In addition, the equipment of salt bath brazing is expensive and the production cycle is long.

The salt bath brazing of diamond tools is contrary to the development concept of green manufacturing.

Flame brazing is an early developed brazing method, which requires simple and light equipment, wide gas sources and low process cost.

However, its heating temperature is difficult to control and has gradually been eliminated by the mainstream market.

During laser brazing, the local heating of laser is fast, the exposure time of diamond in high temperature environment is short, and the graphitization degree of diamond is low.

Laser brazing has the advantages of high heating efficiency, easy temperature control, small heat affected zone, processing of complex surfaces, orderly arrangement of diamond particles, high degree of automation and good working environment.

The proximity effect and skin effect of high-frequency induction brazing can also be avoided by using laser brazing to weld large and complex workpieces.

However, the heating time is too short to ensure the formation of continuous reaction layer or diffusion layer between diamond abrasive particles and filler metal, filler metal and matrix, thereby reducing the holding force of filler metal on diamond particles.

Resistance brazing is often used to braze honing bars, with good weld quality;

However, due to the relatively high temperature of the cutter head in the welding process, which affects the service life of the diamond, a few enterprises currently use this process.

High frequency induction brazing has the advantages of fast heating speed, high brazing efficiency, low production cost, simple operation, low labor intensity, and can be used to braze various complex shapes and multi tooth workpieces.

When automatic feeding, atmosphere protection or vacuum induction brazing are used, the joint has good appearance and brazing quality.

Induction brazing is a promising brazing method for diamond tools.

3. Brazing equipment for diamond tools

3.1 Vacuum brazing furnace equipment

Vacuum brazing furnace is the most commonly used equipment in diamond tool brazing manufacturing, which can realize batch production, stable operation process and strong repeatability.

However, the volume of traditional vacuum furnace cavity is limited, and most domestic furnaces are single chamber furnaces, with low capacity of each furnace, which is only suitable for small batch production.

With the development of technology, multi-function vacuum furnaces have been produced, such as continuous vacuum brazing furnace, vacuum induction brazing furnace, hot wall vacuum brazing furnace, etc.

3.2 Complete set of equipment for continuous brazing under controlled atmosphere

Taking the mesh belt continuous tunnel furnace as the representative, this kind of complete set of equipment has been widely used in some annealing and bright heat treatment occasions.

It consists of feeding area, heating area, cooling area and discharging area.

The workpiece completes the brazing process under the protection of ammonia decomposition atmosphere.

The equipment has the advantages of continuous heating, adjustable heating time of workpiece, high production efficiency, convenient for mass production, low labor intensity of workers, automation, etc.

The complete set of controllable atmosphere continuous brazing equipment integrates various professional technologies such as chemical industry, machinery, control, electric furnace brazing, etc., providing technical guarantee for automatic continuous brazing production with high efficiency, high quality and low cost in large quantities.

3.3 Laser brazing equipment

Laser brazing refers to the brazing technology with laser as the heat source, and its core component is the laser.

According to the type of working materials, lasers are mainly divided into gas lasers (CO2 gas lasers), liquid lasers, semiconductor lasers, solid state lasers (Nd: YAG lasers), fiber laser free electron lasers, etc.

Among them, fiber lasers have developed rapidly and been widely used in communication, sensing, laser material processing and other fields.

High conversion, high quality, high stability and small volume will be the focus of future fiber laser research.

Ultra short pulse mode-locked fiber laser with smaller duty cycle, high power femtosecond pulse fiber laser, broadband output and tunable fiber laser will become the research focus.

Laser beam for welding has developed from pulse waveform to continuous waveform.

Most of the first lasers used for welding are solid state lasers with pulse output.

In recent years, high-power continuous CO2 gas lasers and solid Nd: YAG lasers are widely used as welding heat sources.

The absorptivity of diamond to laser beam is lower than that of alloy filler metal, so high energy density laser should be selected for brazing diamond tools.

When the pulsed laser is used as the welding heat source, the process parameters are more complex.

At present, the research on laser brazing diamond mainly uses CO2 continuous laser, but the research on laser brazing diamond is very little.

3.4 High frequency induction brazing equipment

The core component of high-frequency induction brazing equipment is induction heating power supply.

Modern induction heating power supply is developing towards high-power, high-frequency and intelligent direction.

The miniaturized induction heating power supply system, which is suitable for field operation, high efficiency and energy saving, is becoming the development goal in the future.

With the increase of manpower cost for diamond tool manufacturing and the improvement of requirements for the stability of brazing quality of diamond tools, automatic induction brazing process has gradually become the main method for welding diamond sawing tools.

Automatic induction brazing of diamond tools can realize automatic tooth splitting, automatic identification and picking of cutter heads, and automatic sending of welding pads.

It has the advantages of high welding accuracy, firm brazing joints, and long service life. One person can operate multiple machines, Greatly reduce labor costs.

4. Brazing mechanism of diamond tools

The principle of brazing diamond tools is to use elements with high affinity to C (such as Ti, Cr, Zr, etc.) to generate carbides through chemical reaction during brazing, so as to realize the metallurgical combination of diamond, filler metal and matrix.

For brazing of diamond tools, brazing technology and brazing materials are the most important factors.

The brazability of diamond is very poor because it is difficult for the commonly used metal filler metals to wet the diamond surface.

At present, active metal brazing or diamond surface metallization are mainly used to improve the brazing performance between diamond and metal.

4.1 Active metal brazing

For the connection of diamond, the active metal brazing method refers to adding active elements (usually strong carbide forming elements) such as Ti, Zr, Cr, V, etc. into the filler metal to generate carbide through chemical reaction, so as to achieve the metallurgical connection between diamond and filler metal, and then connecting the diamond to the metal substrate through the metal filler metal.

See Fig. 3 for the brazed diamond joint interface.

Brazing and Diffusion Welding in Diamond Tool Manufacturing 4

Fig.3   Interface brazed diamond joint

Ag Cu Ti and Cu Sn Ti alloys containing Ti are commonly used as active solders;

Ag Cu Cr and Ni Cr alloys containing Cr are brazed in inert gas or vacuum.

The chemical affinity of Ti, Cr and C is relatively high.

The formation of carbides such as TiC and Cr-C at the interface is the main reason for realizing the metallurgical connection between diamond, filler metal and matrix.

4.2 Metallization treatment of diamond surface

Metal activation treatment on the surface of diamond makes the surface have metal like properties, which is more conducive to the combination between diamond and metal filler metal.

The surface metallization mainly includes chemical plating plus electroplating, vacuum evaporation, plasma sputtering, magnetron sputtering, chemical vapor deposition, physical vapor deposition, mechanical coating, etc.

The chemical plating plus electroplating and vacuum evaporation are widely used.

Metals with strong activity are usually selected for metallization, such as Ti, Cr, W, Mo, etc.

The metal coating is beneficial to the metallurgical bonding between diamond and matrix, and can improve the holding power of diamond tool matrix to diamond particles.

5. Solder for brazing diamond tools

The strength of brazed joint or interface mainly depends on two factors: brazing materials and brazing process.

Common metal solders are difficult to be wetted and spread on the diamond surface, and the wetting angle of metal Ni on the diamond surface is small at high temperature;

Cu does not wet diamond surface at high temperature, but the wettability of Cu based alloy on diamond surface is greatly improved after adding Ti to Cu.

At present, the brazing filler metals used for diamond tool brazing can be basically divided into: Ni based high melting point alloy brazing filler metals (Ni Cr, etc.) and Ag based and Cu based low melting point alloy brazing filler metals (Cu Sn Ti, Ag Cu Ti and Ag Cu Cr, etc.).

5.1 Nickel base solder

The brazing layer of nickel base filler metal has high hardness, good corrosion resistance, good wear resistance and high temperature performance, and has been widely used in the aerospace field.

Since its birth, there have been a lot of basic and application research on nickel based solders, and its commercial development has also been paid much attention.

Among them, Ni Cr alloy is the commonly used filler metal for brazing diamond tools.

The diamond tools are brazed with nickel based filler metals, which complement each other’s advantages.

The tools can be used for high-strength cutting, grinding and drilling.

In 1975, J T. Lowder et al. initiated the research on brazing diamond tools with Ni Cr alloy filler metals.

Wiand et al. studied the influence of metal powders with different compositions, such as Ni, Cr, B, Si, Fe, Mo, etc., and brazing process (temperature and holding time, etc.) on strength by vacuum brazing coated diamond.

Switzerland A K. Chatopadhyay et al. first sprayed Ni Cr alloy flame onto the steel substrate, then used induction brazing to connect diamond and steel substrate under argon protection, and studied the relationship between filler metal amount and wettability and strength.

Lin Zengdong and others in China used Ni-Cr-P filler metals to braze diamond tools, and introduced the brazing process of diamond tools.

The results show that the brazing temperature should be 50~100 ℃ higher than the melting temperature of the solder alloy, the holding time should be short, and the exposed height of diamond is inversely proportional to the amount of solder.

Xiao Bing, Feng Bojiang and other scholars found that when brazing diamond with Ni Cr filler metal, the interface presents a double-layer structure of inner layer Cr3C2 and outer layer Cr7C3.

Lu Jinbin et al. showed that when diamond was brazed with Ni Cr filler metal, the Cr7C3 generated at the interface took on a shoot like morphology and the Cr3C2 took on a sheet like morphology.

5.2 Copper based solder

The high melting temperature of nickel based filler metal and the high brazing temperature required will lead to graphitization of diamond and damage the thermal stability of diamond.

In the field of good working environment and low wear resistance requirements, copper based or silver based filler metals can be selected to reduce the graphitization degree of diamond.

  • of Pakistan A. Khalid et al. used Cu-14.3Sn-10.2Ti-1.5Zr, a copper based active filler metal, to realize vacuum brazing of diamonds.
  • The research found that the interface between diamond and filler metal produced a 200 nm thick double-layer TiC structure: 50~70 nm thick TiC particles near diamond, and 100~200 nm thick columnar TiC near filler metal.

With the increase of temperature or reaction time, columnar TiC grows on the filler metal side, and the TiC particle layer on the diamond side remains unchanged.

S. Buhl et al. used Cu-14.4Sn-10.2Ti-1.5Zr filler metal to realize brazing of diamond single crystals, explored the relationship between brazing parameters and interface residual stress and joint strength.

When the brazing temperature increased from 880 ℃ to 930 ℃, the shear strength of diamond joints decreased from 321 MPa to 78 MPa.

W. C. Li et al. used Cu-10Sn-15Ti alloy solder to braze diamonds at 925 ℃/5 min and 1050 ℃/5 min, forming a continuous TiC layer about 500 nm thick at the interface. S. F. Huang et al. used Cu-10Sn-15Ti filler metal to compare the effects of vacuum brazing and laser brazing on the microstructure of diamond interface.

Under vacuum conditions, TiC transition layer is continuous;

Due to the short reaction time under the laser condition, diamond shows that fewer reaction products are produced.

Meng Weiru, et al. of Xi’an Jiaotong University, used BNi-2, BNi-7, CuSnNiTi and other filler metals to realize vacuum brazing of diamond tools.

Through comparison, it is found that CuSnNiTi filler metal has low melting temperature, good wettability on diamond surface, small thermal damage to diamond, high holding power, and good usability of diamond tools.

5.3 Silver based solder

Silver based brazing filler metal for diamond tools is usually Ag Cu eutectic added with active elements such as Ti, Zr, etc.

Ag based alloy solder has low melting point and good wettability to diamond.

Some scholars from Harbin University of Science and Technology, Yanshan University, Harbin University of Technology, and Nanjing University of Aeronautics and Astronautics have carried out research on brazing diamonds with Ag based brazing filler metals, and have come to a similar conclusion: that is, under brazing conditions, chemical reaction occurs between Ti and C, and the formation of TiC compounds enables the metallurgical connection between brazing filler metals and diamonds.

The team of the State Key Laboratory of Brazing of Zhengzhou Machinery Research Institute Co., Ltd. analyzed the commonly used Ag based solders.

According to the role and characteristics of Ag, Cu, Zn, Cd, Ni, Co, Mn and other elements in the solders, more than 10 kinds of solders were prepared, and their melting temperature, strength and brazing process were studied.

A series of solders suitable for brazing PCD tools under different working environments were developed.

However, due to the high price of Ag metal, Ti element has high requirements for vacuum degree and high brazing cost;

At the same time, the holding force of the filler metal on diamond is small, and it is easy to fall off when working, so its application in diamond tool brazing is not very extensive.

Silver based solders are often used in special cases with low wear resistance.

The comparison of advantages and disadvantages of several different solders is shown in Table 2.

Table 2   Advantages and disadvantages of several different brazing filler metals

TypeAdvantageDisadvantage
Ni Cr alloyExcellent corrosion resistance and oxidation resistance. Cr reacts with C in diamond to form Cr3C2 and Cr7C3, with high bonding strength and high holding power on diamond.When brazing temperature is higher than 1000 ℃, diamond is easy to be graphitized and thermal damage occurs.
C-Sn Ti alloy solderWhen TiC is generated, the brazing filler metal can realize high strength connection with diamond, the brazing temperature is low, and the thermal damage to diamond is small. The activity of Ti element is high, and brazing requires high atmosphere.
Ag Cu Cr alloy solderCr and C in diamond form Cr-C compound, and the brazing filler metal has a high holding power on diamond.  The brazing temperature must be lower than 800 ℃ to avoid graphitization and oxidation of diamond;Too low temperature leads to too long brazing time and energy waste.
Ag Cu Ti alloy solderTiC is generated, and the brazing filler metal is metallurgically connected with the diamond. The low brazing temperature has no thermal damage to the diamond. The content of Ag is high and the cost of solder is high; The activity of Ti element is high, and brazing requires high atmosphere.
CuMn based Ti containing prealloyed solderTiC is generated to achieve gold bonding, and the holding power of diamond is high. The thickness of TiC layer is large or TiC is continuously distributed, which is easy to produce cracks and make diamond particles fall off.

6. Diffusion welding connection of diamond tools

The joint properties of diffusion welding can be equivalent to that of base metal, which is suitable for welding dissimilar materials.

The vacuum diffusion welding joint has high strength, good thermal stability and good seismic resistance.

In the manufacture of diamond tools, it is mostly used for the connection of PDC geological bits.

While ensuring the quality of the bit and improving the welding strength, it increases the footage depth of the bit.

Diffusion welding is an effective method to improve the performance and service life of PDC bits.

Varnado et al. plated 25~50 μm thick metal nickel on the surface of PDC, and then conducted vacuum diffusion welding at 650 ℃ and 214 MPa for 4 hours, obtaining a joint shear strength of 413~551 MPa.

Qiu Huizhong et al. used unidirectional pressure diffusion welding and hot isostatic pressure diffusion welding to realize the high-strength connection between PDC and tungsten carbide pillar, meeting the requirements of PDC bit.

With unidirectional pressure diffusion welding, the connection strength between PDC and tungsten carbide reaches about 500 MPa at 680 ℃;

By using hot isostatic pressure diffusion welding, the cutting teeth are stable and reliable, and the abrasion resistance of the finished drill bit is not significantly reduced.

The field test also verifies the excellent quality of the product.

Liu Jie et al. conducted a vacuum diffusion welding test on PDC bits by analyzing the problems and causes in the use of PDC bits, and verified the feasibility of using vacuum diffusion welding to weld polycrystalline diamond composites to improve the service performance and life of PDC bits.

Sun Fenglian et al. adopted the method of diffusion welding and brazing, using Ti foil and Ag Cu foil as the interlayer material, and realized the solid connection between the thick film of CVD diamond and the cemented carbide on the vacuum radiation heating diffusion welding machine.

In the near area of the interface between diamond and intermediate layer, there is mutual diffusion of C and Ti elements, and compound TiC is formed.

Qiao Peixin et al. realized the diffusion brazing connection between diamond and matrix alloy by adding prealloyed powder by vacuum hot pressing sintering, and clarified that the diffusion brazing connection of diamond using prealloyed powder is an effective way to improve diamond tools.

Diffusion welding process is complex, welding time is long, equipment is expensive, and welding cost is high.

At present, vacuum diffusion welding is generally only used for welding geological drill bits with high strength and large vibration requirements, and has not been used for mass production of general diamond tools.

Brazing and diffusion welding have unique advantages in the connection of heterogeneous materials, such as carbon materials and metals, due to their excellent process characteristics, and will become a research hotspot in the preparation of diamond tools in the future.

The brazing research of diamond tools has blossomed in a hundred flowers.

At present, it is developing towards the direction of automation, greening, process compounding, quality and economy.

Diffusion welding of diamond tools is still limited to the connection of special tools, which needs to be further developed.

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