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Stainless steel and titanium alloy vacuum brazing process

Titanium alloy has the advantages of high specific strength, low density, high temperature resistance, good toughness, good thermal conductivity and good fatigue resistance, especially good corrosion resistance, and can not corrode in most acids, alkalis, salts and seawater . Therefore, titanium has been widely used in aerospace, national defense, nuclear energy, shipbuilding, electronics, petroleum, chemical and other industrial sectors. Stainless steel is one of the most commonly used structural materials, with a series of excellent properties, such as mechanical properties, weldability, thermal stability, etc., and relatively low cost. However, the corrosion resistance of steel is far inferior to that of titanium alloy, and the density of steel is relatively high. Therefore, in some cases, it is necessary to connect stainless steel and titanium alloys to make full use of their respective strengths.

However, because intermetallic compounds (Ti2Fe, TiFe, TiFe2) are easily formed at the interface when titanium alloy and steel are connected, the internal stress of the joint after welding is large, which reduces the performance of the joint. In order to obtain a joint with ideal performance, it is necessary to solve the above two main welding problems from the aspects of welding method and welding process.

1 Experimental materials and techniques

The sample size of the base material 1Cr18Ni9Ti stainless steel and TC4 titanium alloy used in the experiment is 10 mm × 10 mm × 2 mm, and the brazing filler metal is Ag72Cu28)Ti3. Lap joints are used, the lap length is 10mm, and the gap between brazing joints is 0.05~0.5mm. The vacuum brazing parameters are vacuum degree of 3.0×10-3Pa, the brazing temperature is 790, 830, 850 and 870°C, and the holding time is 1 and 3min, respectively. When the temperature is lower than 700°C, the heating rate is 20°C/s; in order to ensure the high vacuum degree at high temperature, stay at 700°C for 1 hour, then rise to the brazing temperature, keep warm for a period of time, and cool to room temperature with the furnace.

After the vacuum brazing test, the cross-section of the sample was polished and corroded for microscopic analysis. The interface structure and products were observed by SEM (JXA840A), and the composition of the interface products between the solder and the base metal was determined by energy spectroscopy.

2 Experimental results and analysis

2.1 Influence of vacuum brazing temperature

The picture below shows the structure photos of the vacuum brazing interface when the holding time is 3min and the brazing temperature is 790, 830 and 850°C (the left side of the photo is TC4, and the right side is 1Cr18Ni9Ti). It can be seen from the figure that when the holding time is constant, the width of the brazing seam decreases with the increase of the brazing temperature. This is because the higher the temperature, the stronger the dissolution of the base metal into the solder, and the greater the dissolution rate. Before the components of the base metal have time to react with the elements diffused from the solder, they will directly continue to diffuse and dissolve to the center of the weld. The decomposition and loss of the material itself leads to a reduction in the width of the weld. In addition, it can be seen that when the vacuum brazing temperature is 790 ° C, no cracks appear at the interface of 1Cr18Ni9Ti/brazing seam, and when the brazing temperature is 830 and 850 ° C, there are cracks at the interfaces of Ti6A14V/brazing seam and 1Cr18Ni9Ti/brazing seam Cracks appear. The reason for this phenomenon is: on the one hand, there is a large difference between the physical properties such as the linear expansion coefficient between the stainless steel and the titanium alloy, and a large stress concentration will be generated at the interface after vacuum brazing; on the other hand On the one hand, during the vacuum brazing process, due to the diffusion of elements and the chemical reaction at high temperature, Ti-Fe intermetallic compounds will grow at the interface of 1Cr18Ni9T/brazing seam, and Ti-Fe intermetallic compounds are generally brittle. In this way, under the action of stress, Ti-Fe intermetallic compounds form cracks on the side. On the TC4/brazing seam side, in addition to the large stress concentration, the Cu element in the brazing filler metal diffuses to the TC4 side of the base metal when the temperature rises, enriches on the titanium alloy side and forms a Ti2Cu compound at high temperature, under the action of residual stress Cracks form. At 790°C, the reason why there is no crack at the base metal/brazing seam interface is that: on the one hand, the vacuum brazing temperature is relatively low at this time, and the Ti and Cu elements in the brazing seam have not been well diffused. Only a small amount of Cu and Ti elements appear at the interface between the /brazing seam and 1Cr18Ni9Ti/brazing seam; The probability of compound reaction is small, and there are no Ti-Cu and Ti-Fe brittle compounds at the interface, resulting in no cracks at the interface.

3 Conclusion

Vacuum brazing experiments were carried out on 1Cr18Ni9Ti/TC4 by selecting different holding times at 790, 830 and 850 ° C, and the results of the experiments were analyzed, and the following conclusions were drawn:

(1) Different vacuum brazing temperatures have a great influence on the weld structure. If the vacuum brazing temperature is too low, the action of the base metal of the brazing filler metal will not be sufficient, and a good brazed joint cannot be formed; increase, increasing the probability of crack formation. Increase the holding time, the role of the base metal and the solder is sufficient, the solder and the base metal diffuse and dissolve, but at the same time it leads to an increase in the amount of intermetallic compounds at the interface, which increases the brittleness of the joint.

(2) The weld width decreases with the increase of temperature and increases with the increase of holding time; the thickness of the diffusion layer increases with the increase of holding time. The active element Ti is easy to react with other elements, and is one of the main elements forming intermetallic compounds in the brazing joint. Therefore, it is necessary to control the dissolution of the base metal TC4 into the brazing joint.

(3) The vacuum brazing temperature was 790°C and the temperature was kept for 3 minutes. No cracks and large intermetallic compound structures appeared in the joints, and a good brazed joint structure was obtained.