Brazing of Diamond Segment and Substrate

The diamond cutter head and the substrate are often connected by brazing to make diamond tools. All kinds of sawing, cutting, grinding and drilling tools seen in daily life are mainly connected by brazing. There are various forms of such tools, but the joints have their commonality, that is, the brazing of porous complex composites manufactured by powder metallurgy and alloy steel.

 

The working conditions of diamond tools are relatively harsh, and they are generally used in high-speed, vibration, and high-temperature environments, so brazing of diamond cutter heads is difficult. In order to improve the working efficiency of diamond tools, the requirements for cutting speed and feed rate are constantly increasing, which requires diamond tools to have higher brazing seam strength. In the technical way to improve the strength of the brazing seam, the brazing material and the brazing process rely on and promote each other; in the technical way to improve the brazing efficiency, the brazing process and the brazing equipment support and promote each other.

 

The melting temperature of the brazing filler metal is closely related to the sintering of the cutter head. The melting temperature of the brazing filler metal should be selected according to the sintering temperature; the strength selection of the brazing filler metal should refer to the use conditions of the tool. The brazing material should meet the following conditions when the diamond cutter head is brazed with the substrate: the melting temperature of the solder should be below 850 °C, and the lower the better; the brazing filler metal has good wettability to the diamond cutter head matrix material and the tool substrate and moderate flow spreading; the solder should also have good fatigue performance and stability, and can withstand instantaneous high temperatures of 300-400 °C; the brazing process is easy to operate, and general-purpose induction brazing or flame brazing can be used.

 

The main influencing factors of the brazing process on the mechanical properties of the diamond tool brazed joints are the thickness of the welding sheet, pre-welding treatment, heating time, heating rate, holding time, post-welding treatment, etc. These factors determine the wetting condition, porosity, inclusion rate, brazing seam thickness and residual stress in the brazing heat-affected zone, etc.

 

The thickness of the solder sheet affects the thickness of the brazing seam of the joint, thereby affecting the shear strength and fatigue strength of the brazed joint. Experimental research shows that when using the same solder composition and brazing process to braze diamond tools, when the thickness of the brazing seam is between 0.20 and 0.28 mm, the comprehensive performance of the brazing seam is better (the thickness of the brazing filler metal can be reduced to 0.15 mm when re-welding ). When the thickness of the brazing seam is small, the wetting of the solder to the joint surface of the cutter head and the tool substrate is insufficient, resulting in a low brazing rate and a low strength of the brazing seam; at the same time, the tin, lead, and Aluminum, iron, tungsten, titanium and other elements diffuse and dissolve into the weld, which promotes the embrittlement of the braze joint and reduces the strength of the joint. When the thickness of the brazing seam is too large, it is easy to generate pores in the brazing seam, which reduces the effective brazing area and reduces the shear strength.

 

The formation of the brazing seam is closely related to the whole heating process. Factors such as heating rate, brazing temperature, holding time, and cooling rate will affect the wettability and porosity of the brazed joint, thereby changing the mechanical properties of the joint. In the production process, a higher heating rate is often used to improve work efficiency. If the heating rate is too high, the joint will have a large residual stress and affect the mechanical properties of the weld. Due to the large difference in the linear expansion coefficient of each component in the diamond segment, for example, the linear expansion coefficient of tungsten carbide, tungsten, and chromium is small, while the linear expansion coefficient of zinc, lead, tin, manganese, cobalt and other elements is large. The deformation of the element after heating is different to generate internal stress. But too low heating rate not only leads to low production efficiency, but also aggravates the oxidation of brazing seam metal, which has adverse effects on diamond tool joints. To solve the problem of heating speed, a two-stage heating process can be used. First, the workpiece is heated to 400-500°C, and the temperature is kept for a certain period of time before continuing to heat up and weld. The brazing temperature has a certain influence on the mechanical properties of the joint. When the brazing temperature is too high, the zinc element in the brazing material evaporates to cause pores, and at the same time, the brazing material is easily oxidized to produce slag inclusions, resulting in a decrease in the strength of the joint; when the brazing temperature is too low, The fluidity of the brazing material is poor, it is easy to produce slag inclusions, cause false welding, and also reduce the strength of the joint.

 

High-frequency vibration in the brazing process can improve the brazing rate and reduce residual thermal stress. Magnet drive technology can improve energy efficiency, reduce energy consumption, and improve work efficiency.

The pre- and post-processing of brazing has an important influence on the quality of brazing. Before welding, the brazed surface needs to be ground to remove the oxide layer. It is required that the gap between the grinding tool head and the mating part of the substrate is uniform. The commonly used methods for removing the surface oxide layer include grinding machine grinding, filing and sandblasting. The diamond tools after welding should take slow cooling or heat preservation measures to reduce the cooling rate after welding as much as possible, which is beneficial to reduce the thermal stress after welding. The more economical way in production is to use asbestos for heat preservation. After welding, there are usually a large amount of oxide slag and flux remaining on the surface of the workpiece, which should be cleaned by sandblasting to avoid corrosion of the workpiece by the flux. After the workpiece is cleaned, it is necessary to check the welding quality of the brazing seam. The surface of the brazing seam with good welding quality is smooth, smooth, arc-shaped, and has no obvious pores and cracks.