Prealloyability and its role in diamond tools

Zhengzhou Abrasives Grinding Research Institute Wang Guangzu Henan Huajing Superhard Materials Co., Ltd. Lu Huawei Ma Tao Tian Li
1. introduction
Most diamond tools are manufactured using powder metallurgy. Most foreign high-quality diamond tools use ultra-fine cobalt powder as a binder (carcass) for diamond particles. In the mid-1990s, the new concept of ultra-fine pre-alloyed powder in diamond tools was first proposed in foreign countries. In 1998, pre-alloyed powder was used as a substitute for cobalt-based carcass in diamond tools. The prealloyed powder is a homogeneous alloy powder prepared by a hydrometallurgical process from more than one metal element. The powder has a particle size of less than 10 μm and is an ultrafine powder. The aggregate composed of tiny solid particles belongs to the transition region between the microscopic particles and the macroscopic objects, and has a series of special physical and chemical properties.

At present, most diamond saw blades, core drill bits and other diamond tool manufacturers in the world have begun to apply a considerable proportion of prealloyed powders in addition to pure cobalt powder. Pre-alloy powder applications have been growing in recent years, accounting for about 15% of the cobalt powder consumption in the diamond tool industry in 2001. Trends indicate that ultrafine prealloyed powders are increasingly used in the diamond tool industry.

Since the diamonds are all fine particles, it is generally used after the carcass material is used to make a certain shape and has certain mechanical properties. In the product, the diamond as the most basic component is a cutting element, but the carcass is an indispensable component, which plays a decisive role in whether the diamond can fully and effectively function.

2. Five characteristics and analysis of metal carcass
2.1. Wear-resistant matching with the material being processed. The wear resistance of the carcass is too high, the diamond particles are not easy to be exposed, and the processing efficiency is low or even impossible to process; on the contrary, the wear resistance of the carcass is too low, and the service life of the tool is lowered. This characteristic is a complex integrated function of the properties of the processed material and the properties of the material being processed, the wear state, the wear parameters, and the like;
2.2. The holding power of the carcass to the diamond. Refers to the metallurgical bond between the diamond and the carcass and the mechanical inclusion of the carcass to the diamond. The mechanical inclusion force floats from the compressive stress of the cooled shrinkage carcass and the compressive stress of the calcined body that cools the phase change. This enveloping force is related to the elastic modulus of the carcass, and the elastic modulus of the carcass is improved, which is beneficial to enhancing the mechanical inclusion force of the carcass to the diamond;
2.3. Low temperature sintering characteristics of the carcass. The low sintering temperature is greatly beneficial for mitigating the heat loss of the diamond and saving electricity and reducing the loss of the mold;
2.4. The processability of the carcass powder. When the carcass powder is used for cold forming, it should have good compressibility (including compressibility and formability) after adding a small amount of binder. In addition, the carcass powder should also be used as a powder for the transition layer, which should have good weldability;
2.5. The carcass powder is inexpensive, and it is extremely important to prepare the carcass powder without polluting or polluting the environment. It is the advantage of the water mist method to produce the carcass powder.

As a carcass, there are two main functions: one is to "set" the cutting element, and the other is to "match" the wear with the cutting element (diamond). The quality of the product depends to a large extent on the performance of its carcass, while the performance of the carcass depends mainly on the carcass material. Common carcass materials are resin, metal and ceramic. Among them, metal carcass is widely used for good mechanical properties. The diamond product of the metal carcass accounts for about 80% of the total diamond product.

The metal carcass diamond product is obtained by powder metallurgy method, mixing diamond particles with metal powder and pressing and sintering. Due to the presence of diamond: First, the thermal stability is poor; Second, there is a high interfacial energy between diamond and metal, the wettability of metal to diamond is poor, and it is difficult to form a chemical metallurgical bond between diamond and metal, which makes it difficult to obtain high The bonding strength between the diamond and the metal interface; third, the wear resistance of the diamond and the metal carcass is difficult to match. This creates difficulties in the design and selection of metal carcass materials.

There are many researches on metal carcass at home and abroad. In summary, it can be roughly divided into: first, the research on the bonding mechanism between diamond and metal carcass; second, the study on metal carcass system; third, the research on the preparation method of metal carcass material Fourth, the performance of the metal carcass and the evaluation of the evaluation system.

3. Study on the mechanism of carcass binding
The bonding agent has three kinds of holding power for diamond: mechanical setting force, physical adsorption force and chemical bonding force. Among these three forces, the physical adsorption force is small and negligible; the mechanical mosaic force depends on the degree of sintering alloying of the carcass, the porosity and the strength and hardness of the carcass; the chemical bonding is the strongest, but the premise It is a chemical metallurgical bond between the metal carcass and the diamond interface.

The key to improving the holding force of the carcass on the diamond is to improve the interface state of the carcass and the diamond, so that a strong bond is formed between the carcass and the diamond. The measures to improve the interface bonding state are summarized as the addition of strong carbide forming elements and diamond surface metallization. Metallization of diamond surface is by physical vapor deposition, chemical vapor deposition, or even electroless plating or electroplating. A metal film or a carbide film is formed on the surface of the diamond in advance, and then mixed with the carcass material to make a product.

At present, there are still some difficulties in the diamond surface metallization method to be solved; how to minimize the diamond strength loss during the surface metallization process; how to prevent the transition layer formed after the surface metallization from appearing cracks or even delamination during the production process.

4. Research on metal carcass system
The metal carcass comprises a binder, an additive and a trace amount of active elements, wherein the binder mainly refers to a metal having a low melting point and a low hardness such as Cu, Sn or Zn. The additive generally refers to an additive component having a high hardness and a high melting point in the carcass. It can improve the strength, hardness, wear resistance of the carcass and adjust the toughness of the carcass. The trace active element is mainly to improve the bonding state between the diamond and the carcass interface, thereby improving the interfacial bonding strength of the two.

The carcass systems that have been used until now have bronze, cobalt, iron and WC-Co groups. The bronze base has a lower liquidus temperature and can achieve liquid phase sintering at a lower temperature, which can better protect the diamond and reduce the heat loss of the diamond. Mainly used as diamond grinding wheel and marble saw blade.

Cobalt is the most widely used system for high quality diamond products. Cobalt is a relatively good element for wettability of diamonds among all metal elements, and has a lower temperature viscosity than other metals. At a temperature much lower than its melting point, a high cobalt matrix can be obtained very well. The degree of alloying makes it a high mechanical inlay for diamond, while cobalt has little effect on diamond erosion and can effectively protect diamond. Due to the high price and lack of resources of cobalt, it has seriously hindered its large-scale popularization and application, and is currently mainly used in high-grade diamond products. Therefore, many researchers are actively studying the replacement of cobalt in the carcass.

Cobalt has always been an important material for the tool holder in the traditional diamond saw blade manufacturing industry due to its excellent intercalation properties for diamond. Depending on the different uses of the diamond saw blade, ie sawing different materials, other metals are added to the carcass to adjust its hardness, or to adjust its wear properties. For example, diamond saw blades for cutting marble and limestone often have a high ratio of bronze in the carcass of the cutter head; diamond saw blades for harder stones such as granite are sawed, and low-volume bronze is added to the carcass of the cutter head. . Sometimes in order to increase the service life of the diamond saw blade, the cobalt content is increased in the bit body, which serves to increase the hardness of the carcass.
As we all know, Portugal is the world's top nine decorative stone producers, and is also the third largest stone producer in Europe, second only to Italy and Spain, with rich stone resources, granite, marble, limestone and many other varieties. In order to reduce the cost of stone production, the use of low cobalt content of the carcass material, the use of low cobalt content material is Co-Cu-Fe, can reduce the cutting force of diamond saw blade sawing, and the carcass has better Hardness and yield strength, hard stone such as sawn granite and quartzite have good effects.

Germany's Dr.Fritsch is a well-known manufacturer of diamond saw blades and cutter heads. In order to meet the international demand for cobalt-free or low-cobalt matrix materials, a new carcass material was developed in early 2005 for the manufacture of saws. Cut the granite saw blade and get good results. In the new carcass material, bronze is added, and since it has a small tin content, it is malleable, thereby increasing the toughness of the carcass. This is important for the manufacture of cutter heads for large diameter diamond saw blades.

Iron and cobalt are in the same sub-group VIII. Many properties are similar to cobalt, iron resources are abundant and cheap, and it is a very economical carcass system. Therefore, iron-based carcass is a hot spot in carcass research in recent years. It has been studied that iron-based carcasses have been used in low-quality diamond products [4~6], especially in China's stone cutting diamond tools, which have achieved good economic benefits.


Note: The standard* is the cited literature data, and the rest is the data measured by Antai Technology Co., Ltd.
It can be seen from Table 1 that the foreign A and B powders have smaller particle size, lower pine ratio and higher oxygen content, and the C powder has the same particle size, pine ratio and oxygen content as the 04, 05 and 06 powders prepared herein. The biggest advantage of water atomization is that the powder has a high degree of alloying, uniform composition, high productivity and low cost.



As can be seen from the table, the hardness HRB of the 04, 05, and 06 powders formed exactly three steps from 109 to 97. It has good adaptability for cutting reinforced concrete and granite as well as sandstone cutter heads. If necessary, add other powders of hard or soft phase to prepare tools with high processing sharpness and long life.

In recent years, high-temperature brazing technology has been introduced into the diamond products industry, although the time is not long, but it has shown the feasibility of greatly improving the performance of diamond products. Compared with the conventional method (electroplating, sintering), the maximum allowable blade height of the diamond can be increased from 0.3D to 0.7D, which indicates that the holding force of the carcass to the diamond is greatly improved. The role of brazing is to weld the diamond (base metal) plus the carcass material (base metal) and the brazing material (also known as the carcass material) to achieve the purpose of metallurgical chemical bonding. For diamonds, the prerequisite for good cladding is also the key to the technology. The binder is in the liquid phase and wets the diamond well. However, the current brazing materials (also called carcass materials) do not wet the diamond well, or the liquidus temperature is too high. In order to be promoted and applied, it is necessary to conduct in-depth research to develop brazing materials suitable for diamond properties.

5. Rare earth in metal carcass
The application experience of rare earth in cemented carbide was transplanted into the diamond tool (carcass) material, and the Co-based in the diamond tool (carcass) material formulation was replaced by Fe base, and the rare earth Fe-based diamond was successfully produced. The tool material, through its actual performance test, has a significant increase in flexural strength, hardness and impact toughness, and the porosity has been significantly reduced. Several sets of data are cited below to support.



It can be seen from the data in Table 4 that the average bending strength of the 35% Fe-based carcass material with rare earth added is at least 10% higher than that of the non-added rare earth carcass material (comparison group Nos. 13 and 14), and the highest is improved. 62% (comparison group number 13 and 17), the hardness also has a certain increase. Comparing the group numbers 17 and 18, it is known that the bending strength and hardness of the 35% Fe-based carcass with rare earth added are close to the bending strength and hardness of the 35% Co-based carcass without the rare earth.



It can be seen from Table 5 that the flexural strength of the coated diamond material is generally higher than that of the uncoated diamond material, which is mainly due to the partial formation of metallurgical inlays between the coated metal layer and the carcass material, and the uncoated diamond and the carcass. The material is completely a mechanical inlay.

It can also be seen from Table 5 that the flexural strength of the rare earth-added diamond material is increased by more than 10% and the hardness is slightly improved compared with the un-raised rare earth diamond material. And the performance of rare earth Fe-based diamond materials is very close, even exceeding the performance of similar Co-based (without rare earth) diamond materials.

6. Study on the preparation method of metal matrix material
In the 1980s, some researchers proposed the use of pre-alloyed methods for the preparation of carcass materials. The pre-alloyed powder is made of more than one metal element, which is made into a very uniform alloy powder by hydrometallurgy. It has: easy to adjust the tire Body properties, can reduce sintering temperature, improve compactness and homogenization, improve the wear state of diamond matrix, improve the carcass to diamond encapsulation ability, and good impact resistance, can prevent burning or segregation of low melting point metal, powder It is not easy to oxidize, suitable for large-scale automated production, simplifies the manufacturing process, and can achieve better economic benefits. Therefore, the pre-alloyed powder is quickly popularized and applied in the metal matrix. The prealloyed powder of this period was mainly produced by atomization. The pre-alloyed powder produced by the atomization method has high alloying degree and good fluidity, can effectively adjust the wear resistance of the carcass, and has high production efficiency and low cost compared with the production mode of electrolysis.

In the mid-1990s, the new concept of using ultra-fine pre-alloyed powder in diamond products was first proposed in foreign countries. In 1998, pre-alloyed powder was used as a substitute for cobalt-based carcass in diamond tools. The ultrafine prealloyed powder has a particle size of less than 10 μm, and the aggregate composed of these fine particles belongs to a transition region between the microscopic particles and the macroscopic object, and has a series of special physical and chemical properties. The ultrafine prealloyed powder has excellent sintering properties and is not commonly used in elemental mixed powders or even atomized prealloyed powders, and can achieve high density and sintered hardness at lower sintering temperatures. Ultrafine prealloy production methods generally have a coprecipitation method and a metal vapor synthesis method.

The use of prealloyed powder as a binder for the diamond tool carcass has the following advantages:

(1). The performance of the root sawing work object is selected by using different adhesives to improve the pertinence of the use of the tool;
(2). The diamond tool made of pre-alloyed powder improves the compactness and uniformity of the carcass, thereby improving the wear state of the carcass;
(3). The diamond tool made of pre-alloyed powder can improve the ability of the carcass to be inlaid, and the impact resistance is better;
(4). The pre-alloyed powder is not easy to be oxidized, convenient to store, and can simplify the manufacturing process of the tool and achieve better economic benefits.

The prealloyed powder acts as a binder in the carcass. When designing the alloy composition of the prealloyed powder, the following points should be considered:

(1). The temperature of the sintering temperature depends mainly on the melting point of the bonding metal. In order to avoid the thermal damage caused by high temperature to the diamond, the composition should be selected as a low melting point metal;
(2). The pre-alloyed binder can wet the skeleton component and diamond in the carcass at a certain sintering temperature, and can generate flow under the action of external pressure;
(3). During the sintering process, if the prealloyed binder reacts with the skeleton component, it can only be beneficial to the mechanical properties of the carcass, and can only be advantageous for lowering the sintering temperature, and does not allow the formation of inferior alloys, and The disappearance of the liquid phase does not completely wet the skeleton component and the diamond, and it does not cause damage to the diamond;
(4). The cooling process after sintering can only be beneficial to the mechanical properties of the carcass to ensure that the quality of the diamond tool is not affected;
(5). At normal working temperature, the pre-alloyed bonding metal shall ensure that the bonding material layer can withstand the stress transmitted to it by the hard particles (skeletal component and diamond) in the carcass without deformation or displacement.

7. Carcass evaluation system
There are two core requirements for the performance of diamond products on the carcass: one is that the carcass can hold the diamond firmly, and it will not fall off prematurely during use; the other is that the carcass wear and the wear of the diamond match during use, keeping The product should have a sharpness and at the same time ensure the design performance of the product. At present, the performance of metal carcass is still characterized by flexural strength, hardness and impact toughness. These indicators do not reflect the bearing strength of the carcass and the matching of the wear of the two, and can not directly establish a direct correspondence with the final performance of the diamond products.

Therefore, researchers are actively exploring the relationship between mechanical properties and application properties of the carcass. The wear of the carcass is mainly the mutual grinding effect of the rock powder and the carcass. In the actual sawing process, the rock powder is ground with the carcass under high temperature conditions, so the high temperature performance of the carcass is good and bad, and the carcass The wear resistance is closely related. The higher the high temperature strength of the carcass, the better the high temperature wear resistance of the carcass, and the better the wear resistance of the corresponding carcass. It can be seen that the high temperature softening point of the carcass can be used to indicate the wear resistance of the carcass.

Up to today, the characterization of gold powder carcass performance is still an international difficulty and hotspot. The performance characterization method has not been implemented, and it is impossible to establish a carcass performance evaluation system. This is the research focus in the field of diamond products in the present and subsequent years.

8. Deficiencies in carcass research
Through the analysis of the research status of metal matrix at home and abroad, it is known that there are still obvious deficiencies from the combination mechanism of diamond and metal matrix to the performance of carcass. The specific performance is:

8.1 The research on the bonding mechanism between diamond and metal matrix is not deep enough. The interface reaction between the two and the formation mechanism of the transition layer are unclear, and the required conditions are not clear;
8.2 The design of metal carcass material lacks theoretical calculations. There is no theoretical basis for the selection of additive components, active elements and surface metallization elements. It is not clear which factors lead to the difference in metallization effect of diamond surface;
8.3 Lack of research on pre-alloying methods, powder pretreatment and process requirements, resulting in the inability of the pre-alloyed powder to be fully utilized;
8.4 No accurate and direct characterization methods for carcass performance were found, nor did a scientific performance evaluation system be established. A large number of studies have shown that mechanical properties such as carcass strength, hardness, impact toughness and wear resistance can only be used as an auxiliary characterization method for carcass performance.
8.5 So far, the cobalt-based carcass is still the best comprehensive carcass system, and no complete replacement materials have been found, resulting in high cost of high-quality diamond products. It can be seen from a large number of studies that it is very difficult to replace cobalt with a single element, but it is highly likely that a multi-alloy powder is used instead of cobalt.