Silver-based Solder Vacuum Brazing

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Vacuum brazing is a process technology developed in the 1950s, mainly used in high-tech products, such as aviation, aerospace, electronics and medical supplies. The brazing filler metal used for vacuum brazing is different from other brazing filler metals. It is unnecessary and unfavorable to use the filler in the vacuum protection state. Therefore, the volatile elements in the vacuum state are removed from the vacuum brazing filler metal, and it contains relatively chemical properties. Active elements to play the role of flux. Therefore, the brazing filler metal used for vacuum brazing is also called self-brazing filler metal, and it can also be used for inert gas shielded brazing. Such solders mainly include nickel-based solders, copper-based solders, silver-based solders, aluminum-based solders, cobalt-based solders, and so on.

With the development of industry, the application range of vacuum brazing has been continuously expanded, and the variety of its solders has also increased. Aiming at the problems of silver-based solders in vacuum brazing, the reasons for their formation have been analyzed and studied with examples. The solution is described.

1. Vacuum BrazingComponents

1.1 Component Structure

The component structure is shown in Figure 1. The material of the two parts 207 and 208 of the assembly is AMS5643, namely 0Cr17Ni4Cu4Nb, and the brazing filler metal is AMS4765 (domestic grade is material 317).

Fig 1 Component structure diagram

1.2 Technical Conditions of Component Vacuum Brazing

Technical conditions are the quality requirements that must be met clearly specified in the design drawings and technical documents, mainly including the following:

(1) The brazing gap is 0.025~0.102 mm;

(2) The vacuum degree does not exceed 2.67Pa;

(3) 100% of the solder round roots should be visible on the front side of the brazed joint (that is, the side where the solder is filled) and the back;

(4) The brazing area should not be less than 80%;

(5) The brazing joints should be clean, free of foreign impurities, no excess brazing material, etc.

After several years of welding, the welding quality cannot fully meet the technical requirements. To this end, the reasons for this state are comprehensively analyzed.

2. Improvement of Vacuum Brazing Measures

2.1 Improve the Heating System

Improved the heating system to solve the problem of uneven temperature on the outside and inside of the parts during vacuum brazing. During heating, because the radiant heating effect inside the part is affected by its own structure, to a greater extent, the heating heat is conducted from the outside to the inside through the part itself to achieve the purpose of uniform temperature. Because of the relatively low thermal conductivity of the material, a longer heating time and holding time are required.

The improved heating system is shown in Figure 2. 940 °C is the uniform temperature before welding, and the holding time is increased from 12 min to 15 min to improve the uniform temperature effect; from 940 °C. The time for the brazing temperature of C to rise to 1020 °C is increased to 50 minutes to maintain the uniform temperature effect as much as possible. The holding time of brazing is increased from 15 min to 20 min to ensure that the liquid solder on the front side of the weld fully penetrates to the back side and the brazing area is guaranteed.

Fig 2 Brazing temperature control diagram

In Figure 2, 450. C heat preservation and 760 °C heat preservation mainly consider the factor of vacuum degree. In order to keep the vacuum degree not exceeding 2.67 Pa, the proper setting of the heat preservation time also has a certain effect on the uniform temperature of the parts.

2.2 Anti-oxidation and delayed melting measures of silver-based solder

Anti-oxidation in the vacuum brazing process usually involves placing some chemically active metal materials in the furnace, such as titanium alloys, magnesium alloys, etc., as getters. When the furnace temperature rises, the getters are first absorbed in the vacuum. The remaining oxygen, hydrogen, and nitrogen combine with them and play a role of inhalation to further protect the parts. When silver-based solder is used for vacuum brazing, due to its lower melting point and the more active chemical properties of silver, it tends to combine with gases such as oxygen before the getter, making it difficult for conventional getter methods to function. For this reason, this research broke the traditional getter method, combined with physical factors and adopted new measures to add a protective ring cover for the solder, as shown in Figure 3.

Fig 3 Schematic diagram of solder protection measures

Its protection principle is as follows: When heating, because the protective ring covers the solder and shields the radiation, the solder is not directly exposed to heat radiation and slows down its rapid temperature rise. The protection ring and the parts are heated by heat radiation at the same time. Because of the small size of the protection ring, the temperature rises before the parts and the solder, and the heat penetrates faster to reach the uniform temperature. When the protective ring is red-heated, the brazing filler metal will be heated again by radiation, so that the brazing filler metal will be heated and melted by the second radiation. According to the principle of physics and chemistry, the higher the temperature of the metal material, the easier it is to oxidize. At this time, the red-hot guard ring has a higher ability to absorb residual oxygen and other gases in the vacuum than the silver-based solder, thereby absorbing the release from the surface of the solder and its surroundings. Oxygen and other gases play a role in protecting the solder. At the same time, the protective ring also makes the brazing material lag behind the parts reaching the brazing temperature. After the brazing material is melted, it can quickly penetrate and fill the brazing gap and reach the back of the welding seam, so that the front and back round roots are better without flowing to the surface of the part. .

Using this method effectively solves the problem of unsuccessful vacuum brazing of the component for many years.

The design of the protection ring generally does not require sophisticated structure and calculations, but the following principles and precautions should be mastered. The material should be made of materials with the same or similar chemical composition content as the parts to avoid contamination of the parts at high temperatures and adverse effects. Regarding the size of the structure, it is necessary to know that it can not only achieve the purpose of protecting the solder, but also affect the radiant heating of the parts as little as possible. It should be appropriately designed according to the specific characteristics of the part structure.

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