Knowledge points of brazing diamond tools(Part 1)

 

Keywords: diamond, diamond tools, brazing materials, brazing technology, Application of Brazing Technology in Diamond Tools

 

 

 

Diamond is a functional material that combines many excellent properties. It is the hardest natural material discovered to date, and its unique optical, thermal, and mechanical properties further enhance its status as a functional material. Diamond is divided into two major categories: natural diamond and synthetic diamond. Synthetic diamond is further divided into single crystal and polycrystalline diamond. All three types of diamond can be used to make diamond tools.

 

In recent years, global synthetic diamond production has reached 15 billion carats, with China consistently ranking first in synthetic diamond production. China, a latecomer to the market, has experienced rapid development in the diamond tool industry over the past two decades, not only achieving the world's highest production but also developing a comprehensive range of new tools.

Diamond tools have a wide range of applications, primarily in stone processing, ceramic modification, geological drilling, oil drilling, and mining. They also play an important role in the construction, building materials, machining, optical glass and jewelry processing, and electronics and electrical industries. Modern manufacturing is increasingly demanding diamond tools, and high-end equipment manufacturing is increasingly reliant on the development of diamond cutting tools. High-speed, ultra-high-speed, high-precision, and ultra-precision cutting and grinding, especially for the processing of hard, brittle, and extremely hard materials, are now inseparable from diamond tools. Diamond tools can be categorized by application, including sawing tools, grinding and polishing tools, cutting tools, drilling tools, and wire drawing dies.

 

 

Diamond sawing tools are categorized by shape, including diamond circular saw blades, diamond gang saws, diamond wire saws, diamond wire saws, and diamond hole saws. Diamond circular saw blades are currently the most common sawing tool in the stone and construction industries, widely used for cutting products such as granite, marble, ceramics, and concrete. In the context of green manufacturing, diamond circular saw blades are developing towards multi-blade combination saws, with combination saws and gang saws being typical examples. Gang saws feature dozens of diamond saw blades mounted side by side on a saw frame, achieving several times higher cutting efficiency than conventional abrasive saws. The resulting cuts create smooth, even cross-sections, significantly reducing grinding and polishing workload. Diamond wire saws are generally used for quarrying granite and marble, cutting reinforced concrete, or cutting metal structures, and are gradually transitioning to processing special-shaped stone and slabs. Diamond wire saws are capable of precise, narrow-kerf cutting of hard and brittle materials, making them widely used in semiconductor and photovoltaic cell slicing. They also demonstrate unique advantages in processing ceramics, quartz, wood, and other materials.

 

Diamond grinding and polishing tools are a general term for geometrically shaped abrasives bonded with diamonds using a binder. Diamond abrasives offer high grinding finish, high efficiency, low processing costs, and a long product life. Common diamond grinding tools include grinding discs, grinding blocks, grinding heads, grinding cups, drums, rollers, squaring wheels, milling wheels, tangential wheels, honing wheels, honing sticks, and honing blocks.

 

Diamond tools are characterized by extremely high hardness and wear resistance, high elastic modulus, low coefficient of friction, low thermal expansion coefficient, excellent heat transfer, and low adhesion to non-ferrous metals. Diamond tools can be used for machining hard and brittle non-metallic materials (such as ceramics, graphite, and composite materials), as well as for precision machining of tough non-ferrous materials (such as copper alloys and aluminum alloys). Diamond tools come in a wide variety of types, each with significantly varying performance, structure, preparation methods, and application areas. Common diamond tools include diamond turning tools, diamond milling cutters, diamond boring tools, diamond broaches, diamond drill bits, and diamond forming tools.

 

Diamond drill bits are advanced drilling tools, offering high drilling efficiency, high hole quality, minimal labor intensity, and low drilling costs. Diamond drill bits are primarily categorized by application, including oil drill bits, coal drill bits, geological exploration drill bits, engineering exploration drill bits, thin-wall engineering drill bits, glass drill bits, and composite material drill bits.

 

 

 

Due to limitations in diamond manufacturing technology, individual diamond particles are relatively small, often supplied as fine particles or even powder. This presents challenges in diamond application. Since diamond can only fully realize its excellent properties when attached to a matrix material, bonding with the matrix is crucial for its application.

 

Diamond is inherently incompatible with most metals. Physically, diamond properties are not compatible with other materials, and chemically, metallurgically, they are incompatible, limiting diamond bonding.

 

In the early days of diamond industrial application, mechanical setting was the primary method, as seen in tools like diamond pens and glass cutters. These tools require large diamond particles.

 

For smaller diamond particles, mechanical setting is nearly impossible, leading to the development of electroplated fixed setting methods. The general process for electroplating diamond tools is as follows: tool body machining, dimensional inspection, mechanical treatment, degreasing, pickling, insulation treatment, rust removal, hot and cold water washing, anodic etching, hot and cold water washing, electroplating, diamond application, thickening the plating layer, tank cleaning, hydrogen removal, inspection, and finished tool.

 

Electroplated diamonds have low holding strength and low diamond exposure, which affect cutting force and sharpness. Brazed diamond tools address these shortcomings. Brazed diamond tools are categorized as surface-set and impregnated. Surface-set tools involve brazing diamond directly onto the substrate surface, while impregnated tools involve sintering diamond with other materials to form a composite. The sintering process of the diamond composite is essentially a diffusion brazing process between the diamond and the substrate. By hot pressing the diamond and carbide into a compact (PDC), the diamond bond is transformed into a carbide bond.

 

 

The sintering process of the diamond matrix (commonly known as the blade) is also a diffusion brazing process. Diamond tool brazing can be categorized into three main types: single-layer diamond tool brazing, diamond blade brazing, and PDC composite sheet brazing.

 

The hot or cold pressing sintering of diamond blades is a typical diffusion brazing process. In the early days of diamond blade manufacturing, low-temperature metal powder was typically melt-diffusion brazed with high-temperature metal powder to further secure the diamond. With technological advancements, active pre-alloyed powders containing carbide-forming elements (such as chromium, titanium, vanadium, and molybdenum) have emerged. Diffusion brazing or brazing is achieved by reacting the active pre-alloyed powder with diamond to form carbides. Carbide formation is slow, resulting from the mutual diffusion and migration of the active elements in the pre-alloyed powder and the carbon in the diamond. This process requires a long period of time at a certain temperature for atomic diffusion and reaction to occur, making hot pressing sintering more easily achieve this diffusion process.

 

Brazing single-layer diamond tools typically uses an active or nickel-based brazing filler metal. This process, through the presence of strong carbide-forming elements or alloys, creates a chemical metallurgical bond between the matrix material and the diamond, enhancing the brazing layer's grip on the diamond. Single-layer brazed tools feature a high percentage of exposed diamonds that resist shedding, resulting in sharp cutting and excellent chip removal. The orderly arrangement of diamonds not only ensures a rational distribution of the diamond layer on the tool surface, maximizing the diamond's cutting power, but also significantly reduces diamond usage, lowering tool costs and improving cutting efficiency.

 

Brazing diamond tool tips present unique challenges. First, tool tips are powder metallurgy products with numerous capillaries. Second, tool tips are sintered, resulting in the presence of oxides on both the surface and within. Third, tool tip compositions vary, placing significant demands on brazing materials and processes.

 

Because PCD's permissible heating temperature is limited (generally heat-resistant temperatures not exceeding 720-780°C), silver-based brazing materials are primarily used. However, common silver-based brazing materials have low heat resistance and poor wettability for diamond and carbide. Currently, specialized PDC tool brazing materials are available that can meet the requirements of PDC brazing by improving the brazing material's high-temperature strength and fatigue properties and optimizing the brazing process.

 

 

 

After the 1930s, powder metallurgy principles began to be used to manufacture diamond circular saw blades. Early tools were primarily sintered from diamond, high-melting-point skeleton metal powder, low-melting-point elemental metal powder, and filler materials, with the low-melting-point metal powder serving as the brazing filler. After the 1960s, diamond tools rapidly developed, and a growing number of institutions began systematically researching matrix powders. Pre-alloyed powdered brazing fillers, in particular, garnered increasing attention for their diffusion brazing properties within diamond tools.

 

During the use of diamond tools, a significant amount of diamond is lost due to matrix wear, rather than wear and failure of the diamonds themselves. This results in a relatively low diamond utilization rate. This is because traditional diamond segment manufacturing relies on the mechanical encapsulation of the diamonds within the matrix. Since diamonds do not wet the metal matrix material, once the matrix wears to a certain height, the diamonds easily separate from the metal matrix, significantly reducing the performance and life of the diamond tool. Improving the diamond encapsulation capacity of the matrix is the most effective technical measure to prevent premature diamond loss. Initially, this approach primarily focused on improving grip through mechanical extrusion and encapsulation, resulting from phase transformations in the matrix powder. Over the past two decades, research on the chemical metallurgical bonding of active pre-alloyed powders with diamond has intensified.

 

Adding strong carbide-forming elements such as nickel, titanium, zirconium, vanadium, chromium, molybdenum, and tungsten to pre-alloyed powders improves the alloy's wettability to diamond and, through diffusion brazing during the sintering process, strengthens the chemical bond between the matrix and diamond. This increases the diamond's edge height during grinding, improving cutting efficiency and diamond utilization. Furthermore, the pre-alloyed powder, acting as a brazing filler metal, enhances the stability and consistency of diamond cutting bits. The State Key Laboratory of New Brazing Materials and Technologies has systematically studied powdered brazing filler metals based on the powder diffusion brazing mechanism, developing a series of silver- and copper-based powdered brazing filler metals, including FBAg625, FBAg737, FBCu14, FBCu17, FBCu18, FBCu70, and FBCu423.

Because the melting temperature range of the pre-alloyed powders is compatible with the sintering temperature of diamond tool bits, hot pressing and sintering are diffusion brazing processes, while cold pressing and sintering are transient liquid-phase diffusion brazing processes. Both hot and cold pressing processes enable wetting reactions between high-melting-point elements and diamond at moderate temperatures.

 

Reactive pre-alloyed powders, replacing elemental metal powders, are used in the brazing sintering of impregnated diamond composites, addressing technical bottlenecks such as uneven mixing of diamond tool matrix metal powders, easy loss of low-melting-point elements, weak interaction of high-melting-point active elements, volatilization of high-vapor-pressure elements, structural segregation of components, and unstable bonding strength. The mechanical holding effect of the matrix on the diamond inlay is optimized to an inlay/brazing composite connection, which improves the diamond blade height and sharpness. When cutting granite with a blade prepared with pre-alloyed powder developed by the State Key Laboratory of New Brazing Materials and Technologies, the cutting speed can be increased by 1.5 to 2 times and the saw blade life can be extended by 1.2 to 1.6 times.

 

 

Direct brazing of single-layer diamond tools is primarily used in the manufacture of grinding wheel dressing pens, measuring tools, stone tools, gemstone or glass engraving tools, and diamond grinding wheels, diamond grinding wheels, and diamond files. The brazing process achieves a metallurgical bond between the diamond, brazing filler metal, and the metal substrate, resulting in a high bond strength. The diamond's edge height is significantly greater than that achieved with electroplating. As a result, brazed single-layer diamond tools are sharp, have a large chip clearance, are less susceptible to clogging during use, and achieve high diamond grit utilization.

 

Most brazing filler metal alloys have difficulty wetting diamond, and diamond is susceptible to graphitization and oxidation at high temperatures. Due to the graphitization transition temperature of diamond, the brazing temperature should not exceed 1050°C, even in a vacuum environment. Diamond brazing has a limited selection of brazing filler metals. The selected filler metal must ensure good wetting with diamond and form a chemical and metallurgical bond with it. It must also maintain sharpness and avoid excessive corrosion. Furthermore, the filler metal's wear properties must be compatible with the material being worked to ensure optimal diamond exposure and a long service life.

 

Carbide-forming elements such as titanium, zirconium, chromium, and vanadium wet diamond well, but their melting temperatures are too high, causing the diamond to become severely graphitized at high temperatures. Cobalt, iron, and aluminum effectively wet diamond in their liquid state, but they are severely corrosive within their wettable temperature range. Currently, two common processes are used to improve diamond wettability and reduce thermal damage. One involves adding active elements to conventional brazing alloys to improve their wettability and affinity for diamond. The other involves coating the diamond surface with metal. When brazing with high-melting-point brazing alloys, the surface metal effectively protects the diamond, minimizing thermal damage and improving its wettability.

 

Two major types of new brazing alloys are created by adding active elements to conventional brazing alloys: copper-based and silver-based brazing alloys such as Ag-Cu, Cu-Sn, and Ni-Cr. Ag-Cu and Cu-Sn low-melting-point brazing alloys are primarily designed to minimize thermal damage to the diamond, but the resulting brazed tool is weak, making it difficult to achieve aggressive grinding. Diamond tools brazed with Ni-Cr alloys exhibit excellent wear resistance and high-temperature resistance.

Processes for brazing single-crystal or single-layer diamond tools include vacuum furnace brazing, laser brazing, and shielded gas high-frequency induction brazing. Brazing in a vacuum furnace has high production efficiency and uniform heating, and can weld workpieces with complex structures and large sizes. Laser brazing uses a laser beam as a welding heat source. During welding, the workpiece is locally heated quickly, and the diamond stays in a high-temperature state for a short time, which can effectively prevent the diamond from undergoing graphitization. Gas-shielded high-frequency induction brazing uses high-frequency induction to heat the substrate and brazing alloy at the same time. It has a high heating temperature, a fast heating speed, and is easy to control the temperature. It can also heat locally and is easy to achieve automatic control.

 

This article discusses the principles of brazing technology used in brazed diamond tools, the development of brazing technology, and information on brazing materials, processes, and equipment. It also proposes the future direction of brazing technology in the diamond tool industry.

 

(This brazed brazed diamond backed velvet polishing pad effectively combines the brazing process with the technology of flexible diamond polishing pad. Makes grinding sharp and more cost-effective.)

 

 

Contents of this article:

• 1. Diamond Tools and Their Classifications
• 2. Forms of Diamond Connections
• 3. Applications of Brazing Technology in Diamond Tools
• 4. Diffusion Brazing During Diamond Matrix Sintering
• 5. Brazing of Single-Layer Diamond Tools
• 6. Brazing of Diamond Segments to Matrix
• 7. Brazing of Diamond Composites
• 8. Diamond Tool Brazing Equipment and Processes
• 9. Development Trends in Diamond Tool Brazing
• 10. Conclusion

Due to limited space, this website is divided into two sections for your reference.

 

Knowledge points of brazing diamond tools(Part 2)