Knowledge Points Of Brazing Diamond Tools(Part 2)

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

 

In the previous article, we explored the application of brazing technology in diamond tools. Let's continue with the next article.

The previous article covers:

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

 

This article covers:

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

 

 

Diamond tools are often manufactured by brazing diamond tips and their substrates. Brazing is the primary method used in everyday sawing, cutting, grinding, and drilling tools. While these tools come in a variety of forms, the commonality lies in the brazing of porous, complex composites made from powder metallurgy to alloy steel.

 

Diamond tools operate in harsh conditions, often operating at high speeds, under vibration, and at high temperatures. Therefore, brazing diamond tool tips is challenging. To improve the efficiency of diamond tools, the requirements for cutting speed and feed rate are constantly increasing, which in turn requires diamond tools to possess higher braze joint strength. In the technical approach to improving braze joint strength, the brazing material and the brazing process complement and reinforce each other; in the technical approach to improving brazing efficiency, the brazing process and the brazing equipment support and reinforce each other.

 

The melting temperature of the brazing filler metal is closely related to the sintering of the tool tip and should be selected based on the sintering temperature. The brazing filler metal's strength should also be considered in light of the tool's operating conditions. When brazing diamond tool tips to tool substrates, the brazing material should meet the following requirements: the brazing material's melting temperature should be below 850°C, preferably as low as possible; the brazing material should have good wettability and moderate flow properties for both the diamond tool tip matrix and the tool substrate; the brazing material should also exhibit good fatigue resistance and stability, capable of withstanding transient high temperatures of 300-400°C; and the brazing process should be simple to operate, employing either conventional induction brazing or flame brazing.

 

The brazing process primarily influences the mechanical properties of diamond tool brazed joints, including brazing sheet thickness, pre-weld treatment, heating time, heating rate, holding time, and post-weld treatment. These factors determine wettability, porosity, inclusion rate, braze seam thickness, and residual stress in the heat-affected zone (HAZ).

 

The thickness of the brazing sheet influences the braze seam thickness, which in turn affects the shear strength and fatigue strength of the brazed joint. Experimental research has shown that when brazing diamond tools using the same filler metal composition and brazing process, the overall performance of the braze joint is best when the filler metal thickness is between 0.20 and 0.28 mm (the filler metal thickness can be reduced to 0.15 mm during re-welding). A smaller filler metal thickness results in insufficient wetting of the interface between the tool tip and the tool base, resulting in poor brazing efficiency and lower joint strength. Furthermore, during the brazing process, elements such as tin, lead, aluminum, iron, tungsten, and titanium in the tool tip diffuse and dissolve into the weld, embrittlement of the joint structure, and reduced joint strength. Excessive filler metal thickness can easily lead to pores in the joint, reducing the effective brazing area and lowering shear strength.

 

The formation of the filler metal joint is closely related to the entire heating process. Factors such as heating rate, brazing temperature, holding time, and cooling rate can affect wettability and the porosity of the brazed joint, thereby altering the mechanical properties of the joint. During production, higher heating rates are often used to improve efficiency. However, excessively high heating rates can result in larger residual stresses in the joint, affecting the mechanical properties of the weld. Due to the significant differences in the linear expansion coefficients of the various components in diamond tool tips-for example, tungsten carbide, tungsten, and chromium have low linear expansion coefficients, while zinc, lead, tin, manganese, and cobalt have high linear expansion coefficients-the various components experience different deformations upon heating, generating internal stresses. However, excessively slow heating rates not only reduce production efficiency but also exacerbate oxidation of the brazing metal, negatively impacting the diamond tool joint. A two-stage heating process can be used to address this heating rate challenge: first heating the workpiece to 400-500°C, holding the temperature for a specified period, and then continuing to heat the brazing process. Brazing temperature has a significant impact on the mechanical properties of the joint. Excessively high brazing temperatures cause zinc in the brazing filler metal to evaporate, resulting in pores. Furthermore, the filler metal is susceptible to oxidation, producing slag inclusions and weakening the joint strength. Excessively low brazing temperatures reduce the filler metal's fluidity, making slag inclusions more likely, leading to false welds and weakening the joint strength.

 

Using high-frequency vibration during brazing can improve brazing efficiency and reduce residual thermal stress. Magnetic flow control technology can improve energy efficiency, reduce energy consumption, and enhance work efficiency.

 

Pre- and post-brazing treatments significantly impact brazing quality. Before welding, the brazing surface must be ground to remove the oxide layer. The gap between the tool tip and the base must be uniform after grinding. Common methods for removing surface oxide layers include grinding with a grinder, filing, and sandblasting. After welding, diamond tools should be cooled slowly or insulated to minimize the cooling rate, which helps reduce post-weld thermal stress. In production, a more economical method is to use asbestos for insulation. After welding, the workpiece surface often retains a large amount of oxide slag and brazing flux. These should be cleaned using sandblasting or other methods to prevent corrosion. After cleaning, the brazing seam should be inspected for quality. A good brazing seam exhibits a smooth, bright, curved surface without noticeable pores or cracks.

 

 

 

Polycrystalline diamond compact (PDC) is a composite crystal formed by sintering diamond powder and a binder with cemented carbide under high temperature and high pressure. PDC boasts advantages such as high wear resistance, high elastic modulus, high hardness, isotropy, and relatively high thermal conductivity. PDC tools have become the material of choice for high-end tools such as oil drilling tools, coal mining drill bits, and high-speed, high-precision cutting tools. Brazing is a key process in PDC tool manufacturing, and the widespread use of PDC has placed higher technical requirements on brazing technology.

 

PDC tool brazing essentially involves the brazing of carbide to the tool steel body. Because both carbide and tool steel have excellent brazing properties, PDC tools offer a wide range of brazing filler metals and brazing processes. Silver-based or copper-based filler metals can be used, and induction heating and flame heating methods can be employed. High-frequency induction brazing, with its low environmental pollution and high production efficiency, has become the predominant brazing method for PDC tools worldwide. Due to the high precision and high strength of PDC tools, brazing accuracy, braze seam strength, and braze seam quality are critical technical issues. PDC tools can fail in three main ways: PDC sheet detachment from the tool base; sheet fracture; and damage to the tool base. To address these three failure modes, PDC tool quality is ensured through optimized brazing materials and appropriate brazing processes.

 

Currently, commercially available PDC-specific brazing filler metals feature high silver content, along with elements such as nickel, manganese, and cobalt. These fillers exhibit excellent wettability for cemented carbide, high brazing strength, and a solid-liquid phase temperature difference generally controlled between 50°C and 80°C.

 

To address the issue of sheet detachment, engineers are committed to improving brazing material performance, such as replacing higher-strength filler metals like BAg40CuZnNi, BAg45CuZn, and BAg50CuZn. However, the brazing process has been overlooked. Due to the brittle nature of PDC composite sheets, the use of brazing materials and processes with excessively high melting temperatures can lead to cracks, chipping, disintegration, and chipping in the composite sheets. Research has shown that the allowable brazing temperature for high-quality PDC sheets should not exceed 780-800°C, while that for general-quality PDC sheets should not exceed 750°C.

 

 

 

Due to the wide variety and variety of diamond tools, the resulting brazing processes and equipment are numerous. Flame brazing, salt bath brazing, induction brazing, resistance brazing, furnace brazing, vacuum brazing, and laser brazing all compete with each other. Currently, induction brazing and vacuum brazing are the dominant processes .

 

Flame brazing is an early developed brazing method. The equipment required is simple and lightweight, the gas source is widely available, and the process cost is low. However, its heating temperature is difficult to control and it has gradually been eliminated from the mainstream market.

 

Salt bath brazing offers fast and uniform heating, high brazing efficiency, and less deformation of the base metal during brazing, making it suitable for mass production. However, salt bath brazing leaves a large amount of flux on the workpiece after welding, resulting in a large amount of wastewater and environmental pollution from post-weld cleaning. Furthermore, salt bath brazing equipment is expensive, the process is complex, and the production cycle is long. More importantly, the trend of green manufacturing is phasing out dip brazing for diamond tools.

 

High-frequency induction brazing offers fast heating and high brazing efficiency; it is easy to operate and labor-intensive; and it can braze various complex shapes and multi-tooth workpieces. When using automatic feeding, atmosphere protection, or vacuum induction brazing, the joints exhibit excellent appearance and brazed quality. This method not only offers low production costs for a single weld, but also allows the substrate to be re-welded and reused. With increasing labor costs in diamond tool manufacturing and increasing demands for consistent brazing quality, automatic induction brazing has become the primary method for brazing diamond sawing tools. Automatic induction brazing of diamond tools enables automatic tooth separation, automatic identification and pick-up of blades, and automatic feeding of brazing pads. It offers advantages such as high welding precision, strong brazed joints, and long service life. One person can operate multiple machines, significantly reducing labor costs.

 

Resistance brazing is commonly used for brazing honing rods, and it produces excellent weld quality. However, due to the high overall temperature of the blade, this affects the life of the diamond. Currently, only a few companies use this process.

 

Furnace brazing (continuous atmosphere brazing) is primarily used in the production of small saw blades and special-shaped diamond tools. It generally uses hydrogen and nitrogen, generated by the decomposition of ammonia, as reducing gases. Heating is slow and uniform, resulting in aesthetically pleasing brazed joints, reliable quality, and a bright workpiece surface. It also boasts high production efficiency and low welding costs, making it suitable for mass production.

 

Vacuum brazing, which generally uses radiation heating from a resistance furnace, offers high production efficiency and uniform heating, making it suitable for welding complex and large diamond tools. Currently, vacuum brazing of single-layer diamond tools is widely used in the manufacture of restoring tools, grinding wheels, stone tools, and glass tools. Vacuum brazing of multi-layer diamond tools also holds promising industrial application prospects.

 

In recent years, laser brazing has gradually gained application in the production of single-layer diamond tools. Laser brazing heats the workpiece rapidly, minimizing the time the diamond remains at high temperature during the welding process, effectively preventing graphitization. Furthermore, the laser brazing process achieves an orderly arrangement of diamond particles, resulting in high heating efficiency and precise temperature control. It is capable of machining complex curved surfaces, boasts a high degree of automation, and offers a favorable working environment. Laser brazing can be used to weld large, complex, and irregularly shaped workpieces while avoiding the proximity and skin effects associated with high-frequency induction brazing.

 

 

 

Diamond tool brazing has become a specialized occupation, consuming nearly 1,000 tons of brazing materials annually and employing tens of thousands of people domestically. The industry is flourishing, and there are five consensus trends: automation, green manufacturing, integrated processes, high-quality manufacturing, and cost-effective manufacturing.

 

Automation and green manufacturing are fundamental concepts and key projects of Made in China 2025. Labor costs in the diamond tool industry are rising, even threatening the industry's development. Automated brazing is a common industry need. All products with large production volumes are moving towards automated brazing. Currently, automated brazing of saw blades is becoming increasingly mature, and automated brazing equipment for other products is imminent.

 

Green manufacturing encompasses reducing emissions, conserving materials, improving energy efficiency, and lowering energy consumption. Diamond tool brazing has made rapid progress in reducing material consumption and energy conservation, but progress has been slower in adopting cadmium-free and flux-cored brazing filler metals and reducing flux usage. Diamond tools (such as drill bits, picks, saw blades, and cutting tools) are dissipative products. The brazing filler metal within them depletes during normal operation. Elements such as lead and cadmium in the filler metal can be released into the air, soil, and water, polluting the environment. Some elements can be ingested by humans, potentially harming human health.

 

Combined brazing processes, including flame-induction, induction-furnace, and furnace-induction-flame brazing, enable rapid, all-position brazing of complex, large-scale tools. This shortens brazing time, improves tool durability and reliability, and extends tool life.

 

Quality improvement demands longer fatigue life for brazed joints, and defects in the braze seam are a major factor affecting fatigue life. Therefore, as the life of superhard tools increases, there is a growing demand to continuously reduce defects in the braze seam and improve the quality of brazed joints. When the brazing materials and processes are appropriate, filler metal cleanliness is a key factor influencing defect formation.

 

Economy is an eternal demand of the manufacturing industry. Low-silver solder, copper-based solder, composite brazing and prefabricated solder are the main directions for saving solder costs; efficient brazing process and high-reliability brazing technology are another way to reduce brazing costs.

 

 

(1) Diamond tools are of various types, and brazing technology is a key technology in the manufacture of various tools.

(2) Brazing technology has four main applications in diamond tool manufacturing: diffusion brazing in cutter head sintering, cutter head brazing, single-layer diamond tool brazing, and PDC tool brazing.

(3) Pre-alloyed powder is an effective way to improve the comprehensive performance of diamond cutter heads. Active pre-alloyed powder can synergistically improve the sharpness and service life of diamond tools.

(4) The key technology for brazing diamond cutter heads is personalized brazing filler metal and efficient brazing process. The key technology for brazing PDC tools is high-quality brazing filler metal. The key technology for brazing single-layer diamond tools is equipment and brazing filler metal.

(5) The brazing of diamond tools is developing towards automation, greening, process integration, quality improvement, and economy.