Summary of Welding Knowledge of Copper-based Solder and Vacuum Brazed Diamond, Welding Expert Fujing

The base material is 45 steel, and the brazing filler metal is Cu2Sn2Ti alloy. Two series of A and B series are used in the test solder ratio. The experiment was carried out at five points by changing the proportion of titanium from 5% to 15% based on A and B alloys.

The diamond in brazing is uncoated, the particle size is 45/50 mesh, and is evenly arranged on the alloy brazing filler metal. The surface of the steel substrate and diamond is degreined before brazing. The brazing test was carried out under vacuum conditions, and the brazing temperature was about 30 ℃ higher than the melting point. After holding for a certain period of time, it was cooled to room temperature with the furnace.

Test analysis method:SEM and energy spectrum were carried out on the tungsten filament electron microscope of Japan Electronic JSM25610LV. Strength test was carried out on the WDW-E100 microcomputer controlled electronic universal testing machine of Jinan test gold group co., ltd.; Solder melting is carried out by vacuum non-consumable electric arc furnace.

Analysis of interface between 3.1 diamond and Cu2Sn2Ti alloy

Figure 1 shows the SEM and BSE images of the combination of Cu2Sn2Ti alloy and diamond after vacuum brazing.

It can be seen from Figure 1 that the edge of the diamond is covered by a lot of white crescent-shaped alloy. It is obvious that the Cu2Sn2Ti alloy shows good wettability to the diamond abrasive particles, which indicates that the diamond abrasive has a wetting reaction with the molten solder. It can be seen from the figure that the diamond exposed height effect is ideal.

If the solder layer reaches a certain thickness, the entire diamond grain will be wrapped by the solder due to capillary action. However, whether there is a chemical metallurgical reaction between diamond and Cu2Sn2Ti alloy can not be determined by morphological observation alone.

In addition, it can be seen that the crystal form of the diamond grit remains intact after brazing. No crack additional thermal damage phenomenon.

Fig.1 SEM and BSE images of Cu2Sn2Ti welding

Table 1 shows the results of the energy spectrum analysis of the solder composition on the diamond surface.

The concentration of Ti element in the metal on the diamond surface is significantly higher than that of the solder itself, which indicates that the Ti element in the solder diffuses from the solder to the diamond surface under the effect of the brazing temperature, and combines with the elements in the diamond to form carbide.

Table 1 Distribution of diamond surface composition of Cu2Sn2Ti brazing filler metal (wt%)

In order to further prove the existence of carbides, X-ray diffraction analysis of brazed diamond grains was used to determine the existence of new compounds at the welding interface between diamond and brazing filler metal.

2 shows the X-ray diffraction results of the brazing interface.

Among them, Ti compounds are TiC, SnTi2C, TiCu and Cu2SnTi, and the main reason for the formation of metallurgical connection between solder and diamond is the formation of new compounds TiC and SnTi2C, which is formed by the combination of Ti in solder and C in diamond. Because of the formation of TiC and SnTi2C, the interfacial tension between diamond and solder is reduced, and the wettability of solder is relatively improved.

Fig.2 Results of X-ray diffraction analysis

The Role of 3.2 Solder and Steel Matrix

In order to understand the microstructure of the brazing material and the combination of the brazing material and the steel substrate in the process of brazing diamond, the energy spectrum scanning analysis of the brazed diamond sample is shown in Figure 3. Ti element is enriched at the interface of the two, and the concentration gradient of other elements Cu and Sn at the interface shows a slow transition trend.

During the brazing process, the brazing filler metal and the steel substrate undergo inter-diffusion at the brazing temperature to form a solid phase diffusion zone. When the composition of the solid phase diffusion zone is close to the eutectic composition, a liquid phase will appear. At this time, the brazing filler metal and the steel substrate melt together at the interface.

Fig.3 Line scanning of brazing filler metal and steel substrate after brazing

3.3 brazing process

The test method of compressive shear strength: under vacuum condition, the heating temperature in the brazing process is 10 ℃/min, the holding time is 20min, and then it is cooled with the furnace. After brazing, the test was carried out in the manner shown in FIG. 4, and the lowering speed of the indenter was: 1 mm/min. The results of the strength test are shown in Table 2.

Table 2 Solder strength relationship

Fig. 4 Strength test method

The strength of the solder alloy increases with the increase of titanium content. In the test process, the fracture occurs on the junction surface of the polycrystalline wafer and the solder. After partial fracture, a layer of fine diamond is distributed on the solder surface. The diamond here is the diamond powder in the polycrystalline wafer, that is to say, the fracture occurs between the diamond powder in the polycrystalline wafer and the filler. In the actual production and use process, the strength of the solder may be greater than the current test results.

Cu-Sn-Ti alloy is an ideal binder, high temperature liquid phase alloy on diamond completely wet; diamond has sufficient bonding force; and Ti can react with C to generate TiC, solder and diamond to form a metallurgical bond.

(1) The alloy brazing filler metal containing strong carbide forming element Ti is used to braze single-layer diamond. The brazing filler metal has good wettability to diamond, and the carbide forming elements are segregated on the surface of diamond, which makes the chemical metallurgical bonding between diamond and brazing filler metal and the substrate, and improves the holding force of the substrate to diamond.

(2) The copper-based brazing filler metal is brazed to achieve high-strength bonding with the steel substrate.

(3) The best brazing process of B5 is 880 ℃ ~ 900 ℃,10min ~ 15min can meet the process requirements.