Vacuum brazing heat treatment technology of joystick assembly

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Vacuum brazing is a kind of advanced welding in which the workpiece is in a vacuum state, that is, in a negative pressure atmosphere, using a heat source to heat the solder into a liquid, and relying on the action of the capillary to fill the gap between the solid parts and form a metallurgical combination with it. Methods. Its advantages: it can connect parts by metallurgical method without melting the base metal; it has good adaptability to tools with different metals and different qualities.

The joystick assembly consists of a joystick, push rod and clamping bolt, and the 3 parts are welded together with silver copper palladium solder (see picture). The joystick is made of 9Cr18 material, which belongs to semi-martensitic stainless steel; the push rod is made of 3Cr13 spring steel wire; the clamping bolt is made of 1Cr13 material, which belongs to semi-martensitic stainless steel. The oxide film on the surface of stainless steel is more complicated and difficult to remove under normal circumstances, so it is difficult to weld. For martensitic stainless steel, the required mechanical properties can only be obtained by quenching and tempering by heat treatment, so the choice of brazing temperature is very important. The selected temperature should not only ensure the performance requirements of the three parts, but also ensure that the silver-copper-palladium brazing material can be melted and the three parts can be well brazed together and meet their respective performance requirements.

Solder: silver copper palladium brazing alloy, solder melting point: 900~950℃.
Technical requirements: The parts should meet the following technical requirements after vacuum brazing and heat treatment quenching, tempering and cold treatment: (!) Visual inspection. Solder should seep out evenly at the weld. (2) All parts are subjected to load inspection, and the tensile force of the weld should not be less than 455N. (3) Part size. The L length should be within the range of (32.7+0.12) mm. (4) Surface quality of parts. The surface of the parts should be bright and free of oxidation (slight oxidation of the parts is allowed)

Process flow:
For vacuum brazing, the preparation work before brazing has a major impact on the quality of brazing, and in a sense, it is one of the keys to determine the success or failure of brazing.

First, soak the 3 parts, the silver-copper-palladium material and the brazing fixture processed according to the pattern in gasoline, carefully clean the parts and the grease in the parts’ holes with a small brush, and then wash them again with clean gasoline. After volatilization After cleaning with an ultrasonic gas phase cleaner for 20-30 minutes, dry it to fully volatilize the cleaning solution. Otherwise, the oil will not be removed completely, and the brazing filler metal will not be able to wet the splicing surface. After cleaning, put on spun glove and put 3 sections of φ1mm~4mm solder in the welding holes respectively, and then put the combined workpiece hole upward (the solder in the hole should not be too long) and put it in the welding fixture and put it into the furnace. Pay attention to prevent vibration when entering the furnace, so as not to change the position of the parts in the fixture.
We carried out ion nitriding treatment on the fixture, the purpose is to change the surface condition of the fixture and reduce the adhesion between the liquid-solid two phases, so as to achieve the purpose of reducing the adhesion. This method can substantially control the adhesion phenomenon.
The choice of vacuum brazing temperature is very important, so the uniformity of furnace temperature must meet the process requirements. When the vacuum in the furnace is pumped to 0.67Pa (above 5*103Torr), it is allowed to enter the furnace for heating, and the heating rate is controlled at 15℃/min. In the vacuum process, the inert gas should be refilled as much as possible, the main purpose is to clean the oxidizing atmosphere in the furnace. When the temperature continues to rise to the brazing temperature, the protective heating (99.999% high-purity argon) is started with the purification gas, in order to avoid the volatilization of alloy elements at high temperature. After 40 minutes of heat preservation, air-cooled and started the fan to quickly cool to room temperature, and the brazing was completed.

Determination of brazing holding time: The vacuum heating furnace has two characteristics: the heating speed of the furnace is fast; the heating speed of the workpiece is slow. Because the gas in the vacuum heating furnace is extremely thin, the workpiece is mainly heated by radiation, so the temperature of the workpiece is slow, especially below 600 ℃, this feature is more prominent. However, when vacuum brazing is used for heating with purification gas protection, we must take these factors into consideration when formulating the holding time, otherwise it will cause many adverse effects such as the heating of the parts, or the welding time is too long, and the mechanical properties are reduced.

Cold treatment: The parts are cold treated in the fixture, and the temperature is ≤70℃ for 1h; the purpose is that due to the large amount of retained austenite in the 9Cr18 material, it is transformed into martensite by cryogenic treatment, thereby improving the structural stability of the parts.

Tempering: The whole process of vacuum tempering is always protected and heated by a single gas (99.999% high-purity argon gas), so that the surface of the workpiece is not oxidized. The choice of tempering temperature is also crucial, and the technical requirements of the three materials 1Cr13, 9Cr18 and 3CrB must also be taken into account. After testing at different temperatures, we finally chose the best tempering temperature and time, 420 ℃ for 1 hour, after the heat preservation, turn on the fan to quickly cool to room temperature.

After the vacuum brazing of the joystick assembly, all technical indicators have reached the foreign technical standards, and the process is reliable. After the components are brazed, post-weld heat treatment is not required, which simplifies the process and reduces costs, and such processes are now in mass production.

Equipment selection: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.