Tool edge repair based on vacuum brazing technology

With the continuous development of the industrial machinery field, the consumption of tool heads used in mechanical equipment is becoming increasingly serious. Once the tool is chipped, cracked, or notched, it will seriously affect the use of the equipment and reduce production efficiency. According to current survey data, the workload of various production industries is large, and the output is increasing day by day. The consumption and wear of tools are gradually increasing. In order to ensure production efficiency, tools need to be replaced or repaired in a timely manner.

1. Advantages of vacuum brazing technology
The conventional welding technology currently used for tool edge repair has certain limitations. The welding appearance is not beautiful and the shear strength of the repaired edge is low. Unlike common welding technology, vacuum brazing technology uses a solid-phase connection method to use brazing filler metal as the welding material and a high-melting-point cemented carbide material as the base material. During the welding process, the temperature of the base material exceeds the melting point of the brazing filler metal, but the temperature does not exceed the melting temperature of the base material. At the same time, the base material is wetted by the liquid brazing filler metal, which can ensure that the gap position of the welded joint is filled. During this process, the base material will also diffuse with the liquid brazing filler metal, and finally complete the material welding. This welding method can ensure that the structure and properties of the base material will not be damaged. At the same time, after the welding is completed, the weld will also experience thermal diffusion, which can help the diffusion of elements between the brazing filler metal and the base material, and improve the shear strength of the weld position.

2. Factors affecting vacuum brazing repair technology
Before using carbide vacuum brazing technology to repair the cutting edge of a tool, in order to avoid factors such as brazing filler metal, brazing flux, and welding temperature affecting the repair effect, constraints must be imposed before the repair is implemented, that is, factors that affect the vacuum brazing repair process must be eliminated to a certain extent.
(1) Determine the brazing filler metal
The tool to be repaired is a carbide external cylindrical turning tool on a CNC lathe. It needs to contact a large number of workpieces in daily work, with a high friction temperature. It will also cause a large impact on the tool. Therefore, when determining the brazing filler metal, both temperature and impact strength must be considered. This article uses a brazing filler metal that can withstand a maximum temperature of 450°C. The main component of the brazing filler metal is copper, and it also contains a small amount of Ni, Zn, and Mg.
(2) Determine the brazing flux
Based on brazing experience, a brazing flux is selected to complete the repair of the cutting edge of the tool. The main component is borax, and in some cases, boric acid or borax mixtures are also used. This brazing flux has a high active temperature and viscosity. When used, the welding temperature must exceed 800°C. YJ1 brazing flux is used.
(3) Determine the vacuum brazing temperature
YG8 cemented carbide is used as the base material, and the brazing filler metal and brazing flux are copper-based and borax-based, respectively. Considering the temperature requirements and restrictions of these materials, the brazing temperature cannot be lower than 800℃. Considering that alloy steel can withstand higher temperatures and high temperatures can promote the fusion of alloy steel, the brazing temperature range is determined to be 900℃~1000℃.

3. Effect of heating rate on tool edge repair effect
The heating rate during vacuum brazing is also a key factor affecting the tool edge repair effect. If the heating rate during brazing is too fast, there will be no preheating between the base material and the edge, resulting in a large temperature difference and cracks. If the heating rate is too slow, the flux will be lost and the brazing effect will be reduced. Therefore, the heating rate during brazing should be controlled to obtain a better tool edge repair effect. The wetting angle and shear strength of the tool edge will change at different heating rates.
As the heating rate increases, the wetting angle of the brazing material to the cemented carbide shows a trend of first decreasing and then increasing. When the heating rate is 30℃/s, the wetting angle is 70°, corresponding to the highest shear strength in the experiment, reaching 1636MPa. The shear strength increases first and then decreases with the increase of heating rate. The shear strength is higher when the heating rate is 30℃/s and 40℃/s, exceeding 1600MPa. Combined with the good wetting angle of cemented carbide YG8 at about 70° (based on the range determined by factors such as anti-stripping performance), it is concluded that when the heating rate is 30℃/s, the shear strength of the repaired edge is the best, and this heating rate can effectively improve the edge repair effect.
When the heating rate is low, the brazing material is in a liquefied state for a long time, and the low melting point phase is separated first, so the strength of the welded joint is reduced. After the heating rate exceeds 30~40℃/s, the shear strength of the tool edge shows a downward trend again. This situation may be due to the fact that the brazing agent activity enters the melting stage before reaching the peak due to the excessively fast heating rate, which reduces the shear strength of the repaired edge. Based on the above analysis, it can be seen that when the heating rate is 30℃/s, the tool edge repair effect is better.

Vacuum Brazing Furnace