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Influence of nickel and tin elements on vacuum brazing of titanium materials

In the welding of titanium and titanium alloys, vacuum brazing is suitable for welding joints that are not heavily loaded or work at room temperature, especially for precision, micro-complex and multi-brazed weldments. The brazing filler metals commonly used for titanium and titanium alloys in engineering applications are silver (Ag-Cu, Ag-Cu-Li, Ag-Cu-Ni-Li), aluminum-based (Al(3003), Al(1100), Al-Cu -Sn) and titanium-based (Ti-Ni-Cu, Ti-Zr-Cu, Ti-Zr-Cu-Ni) 3 types, and copper-based and nickel-based solders are generally not suitable for use due to the formation of brittle intermetallic compounds. Among these brazing materials, its wettability and joint mechanical properties are good, but its vacuum brazing temperature is generally high, which cannot meet some special engineering requirements (vacuum brazing temperature below 600°C). Titanium-based brazing filler metal is the material of choice to ensure that vacuum brazing welds have properties similar to those of the base metal, such as joints with superior high-temperature strength and corrosion resistance. However, the melting point of conventional titanium-based brazing filler metals is generally above 900C~950°C. Brazing at such a temperature will deteriorate the performance of the base metal, and it is difficult for titanium-based brazing filler metals to achieve low-temperature vacuum brazing of titanium parts. weld. The brazing temperature of silver-based and aluminum-based solders is relatively low, but the vacuum brazing temperature is generally above 600C. In addition, the corrosion resistance and other properties of the joint cannot fully meet the requirements. Therefore, it is necessary to develop a low-temperature brazing filler metal with low melting point and good corrosion resistance.

1.Materials

Base material: Industrial pure titanium (TA1) is used. Brazing filler metal: Ag-Cu-Sn-Ni silver-based filler metal is used, and the raw materials used are: first-grade pure silver chips, red copper rods, pure tin ingots and nickel powder.

2.Method

Brazing filler metal preparation test: Ag-based filler metal was used to change the content of tin and copper elements and add a small amount of nickel element. Determined by differential thermal analysis (DTA). Tensile strength test of lap joint: The size of the brazed test plate is 30 mmx70 mmx1.0 mm. Due to the strong chemical activity of titanium, it is easy to absorb oxygen, nitrogen, hydrogen and other impurity elements during the heating process, which drastically reduces the plasticity and toughness of the welded joint. Therefore, the welding of titanium should be carried out in a vacuum furnace or under the protection of inert gas. In this paper, 8 kinds of brazing materials were used to braze TA1 pure titanium base material in a vertical vacuum furnace (vacuum 1.0 x103 Pa) to prepare brazing tensile specimens, metallographic specimens and corrosion tensile specimens.

Corrosion test: The corrosion conditions are 80°C, 5% NaCl aqueous solution, and the coupons are corroded in a self-made constant temperature water bath, and the solution is replaced regularly.

3.Results and Analysis

3.1 Influence of tin and nickel on the melting point of solder

Figure 1 shows the effect of tin on the solidus temperature of the solder, and Table 1 lists the effect of nickel on the solidus temperature of the solder. The test is to add 10%, 15%, 25%, 30%, 35% tin element to the Ag-Cu solder to make 5 kinds of solder, and analyze the influence on the melting point of the solder. In Ag-Cu-Sn Add 0.5%, 1.0%, and 1.5% of nickel on the basis of the solder, and observe the change of its melting point. It can be seen from Figure 1 that when the content of tin is changed, the solidus temperature of the solder changes. As the tin content increases, the solder solidus temperature decreases. When the tin content increases from 10% to 25%, the curve is relatively flat, and the temperature of the solidus line of the solder does not decrease much. When the tin content continues to increase, the slope of the curve increases, the curve becomes very steep, and the solidus temperature drops significantly. When the tin content is 35%, the solidus temperature of the solder decreases from the original 6209 ° C to 360 ° C C. It can be calculated from the figure that for every 5% increase in tin content, the solidus temperature of the solder decreases by about 65°C on average. The tin element can greatly reduce the melting point of the solder, the main reason is that Cu-Sn-Ag, Ag-Cu-Sn is a ternary low-melting eutectic system, so when the heating process is close to the melting point of the solder, Cu-Sn-Ag, The Ag-Cu-Sn ternary eutectic system melts first, which leads to the decrease of the solidus temperature, and the tin element acts as a flux reducing agent.

Table 1 shows that, on the basis of the constant content of Ag, Cu, and Sn, increasing the content of nickel element, the solidus temperature and liquidus temperature of the solder are increased to varying degrees, while the melting temperature range is reduced. The nickel element has a great influence on the melting point of the solder. The analysis shows that Sn-Ni is a high-melting eutectic system, and the formation of this system increases the melting point of the solder. Although the addition of nickel element increases the solidus temperature and liquidus temperature of the solder, it can narrow the melting range, which is conducive to the preparation of welded joints, and can also avoid the occurrence of excessive temperature differences between the solidus and liquidus. However, the amount of nickel added should be limited.

3.2 Microstructure of brazing seam and interface

The microstructure of the brazed joint interface is shown in Figure 2, in which Figure 2a shows a tin content of 15%, Figure 2b shows a 25% tin content, Figure 2c shows a 35% tin content, and Figure 2d shows the joint interface microstructure with nickel added. The microstructure analysis of the interface in Figure 2 shows that the composition of the black area in the solder in Figure 2a is Cu: 80.09% (atomic fraction, the same below), Ag: 9.32%, Sn: 5.59%. The composition of the white tissue area is Ag: 87.88%, Cu: 7.39%, Sn: .4.12%. The interface structure consists of 2 layers, namely the light gray area near the brazing seam and the dark gray area near the base metal. The composition of the light gray area is Ti: 45.17%, Sn: 29.78%, Cu: 24.05%. The composition of the dark gray area is Ti: 61.80%, Cu: 36.54%, and a small amount of Ag, Sn. The microstructure of the brazing joint and interface in Fig. 2b is similar to that in Fig. 2a. It can be seen from the above data that the brazing joint is mainly composed of two phases, one is Cu (AgSn) phase based on Cu, and the other is Ag (CuSn) phase based on Ag. From the composition of the interface structure, it can be concluded that the Ti element in the matrix metal diffuses along the matrix to the interface, forming a concentration gradient. Sn is mainly concentrated in the light gray area near the brazing seam in the interface layer. Ag element is mainly concentrated in the solder and does not penetrate into the matrix. The Cu element has better wettability. As can be seen from Figure 2c, when the tin content increases to 35%, the brazing performance of the solder becomes brittle, and there are obvious cracks between the brazing seam and the base metal, and cracks also appear in the solder itself. There is basically no diffusion during the time, the reason is that when the content of Sn element is high, the solder is easy to become brittle.

After adding a small amount of nickel element to the Ag-Cu-Sn solder, its microstructure is shown in Fig. 2d. The composition of the dark gray area in the solder is Ti: 33.37%, Sn: 33.48%, Cu: 27.59%, Ni: 4.63%. The composition of the light gray area is Ag: 76.97%, Sn: 15.29%, Cu: 7.74%. The composition of the interface layer between the matrix and the solder is Ti: 47.02%, Cu: 25.09%, Ag; 4.33%, Ni: 3.69%, Sn: 2.66%. After the addition of nickel element, Ti in the matrix diffuses into the solder along the interface, forming a new phase Ti Sn (Cu Ni) phase. The melted brazing filler metal flows fully on the metal surface, resulting in wetting, and at the same time, a diffusion layer is formed between the metal base metal.

3.3 Influence of tin and nickel on the strength and corrosion resistance of lap joints

Figure 3 shows the effect of changing the tin content on the shear strength of lap joints. Figure 4 shows the effect of tin on the corrosion resistance of lap joints. Table 2 shows the effect of adding a small amount of nickel on the basis of Ag-Cu-Sn on the strength of the lap joint and the effect on the corrosion resistance. It can be seen from Figure 3 that with the increase of tin element, the shear strength of the lap joint decreases. When the tin content is less than 25%, its strength does not decrease significantly. When the tin content continues to increase to 35%, the brazing of titanium base metal cannot be achieved by increasing the brazing temperature and prolonging the brazing time. The reason It is the increase of tin element to form a brittle phase. It can be seen from Figure 4 that with the increase of tin content, the corrosion resistance of the solder initially increases and then decreases sharply. When the tin content is 15%, the corrosion resistance reaches the highest peak, and the corrosion time is about 450h . Table 2 shows that the shear strength and corrosion resistance of lap joints are greatly improved after adding a small amount of nickel. The shear strength of the lap joint of Ag-Cu-Sn solder without nickel element is 8MPa, and the strength can be increased to about 30MPa after adding a small amount of nickel element. The corrosion resistance is also improved with the different amount of nickel added. When no Ni is added, the joints fall off after being corroded in 5% NaCl aqueous solution at 80 °C for 52 h, while a small amount of Ni is added. After that, its corrosiveness can reach more than 1000 hours, and the lap joint does not separate. When the addition of nickel is 1.0% and 1.5%, the joints after corrosion can also maintain high strength.

4. Conclusion

1) The addition of tin element can reduce the melting point of the solder to a certain extent. For every 5% increase in the tin content, the solidus temperature of the solder decreases by about 65°C on average. The addition of nickel element increases the melting point of the solder to a certain extent.

2) With the increase of tin element content, the strength of vacuum brazed joint decreases. When the content of tin element is 15%, the corrosion performance of the vacuum brazed joint is the best. The addition of nickel element greatly improves the strength and corrosion resistance of Ag-Cu-Sn solder brazing joints. The nickel in the solder improves the wetting ability of the base metal.