Vacuum electron beam brazing process of capillary plate joint

When a 5mm, tube-thickness 0.1mm capillary tube and 6mm thick circular plate tube joint member are used in a conventional vacuum furnace radiant heating brazing process, it is difficult to accurately control the brazing temperature and cooling rate due to the long residence time at high temperatures. , easily lead to loss of volatilization of the alloy components of the solder, and more serious is the problem of thin-wall capillary penetration, plugging and leakage. In addition, some brittle phases are formed during the brazing process, which reduces the plasticity and toughness of the joint. Therefore, it is necessary to find a brazing method in which energy and welding process parameters can be precisely controlled, so as to ensure that the welded structure obtains a good joint during brazing.

Instructor: Prof. Yao Yao uses high-energy beam as brazing heat source to realize brazing of precision components. It is a new research direction at home and abroad in recent years. Vacuum electron beam brazing has a series of advantages such as local heating, rapid temperature rise, and precise controllable high-temperature residence time. This method can not only reduce and avoid defects such as blockage and dissolution of thin-walled components, but also can greatly reduce energy consumption and improve processing efficiency. . In recent years, vacuum electron beams of carbon-carbon composite materials and superalloys m, A123 and cermets, cubic boron nitride (CBN) cutter heads and tungsten carbide matrix m, and heat exchanger tube plate structure M have been successively realized at home and abroad. Xuan welding connection, but the capillary beam tube joint electron beam brazing technology is currently not reported.

The basic research on the influence of electron beam process parameters and workpiece preparation status on the quality of capillary tube plate joints was carried out, and the process specification parameters were optimized, and a high quality capillary tube plate solder joint was obtained.

2 Test materials and methods 2.1 Test materials Capillary and tube plate materials are stainless steel lCrl8Ni9Ti, its 08C, the balance of Ni. After the brazing material with 2.5% polyvinyl alcohol aqueous solution, pre-positioned in the tube on the back of the capillary around the tube.

2.2 Test method The test adopts the independently developed electron beam brazing flexible processing system, which consists of electron beam welding machine, industrial computer, PLC, power amplifier, external deflection coil, AD/DA converter, infrared thermometer and related control software. constitute. Through computer virtual instrument technology, a series of functions such as editing of electron beam scanning trajectory, automatic control of electron beam brazing process, temperature signal acquisition and processing, and automatic adjustment of electron beam process parameters, etc., are completed, and the vacuum electron beam of the capillary tube plate joint is finally completed. Automatic closed-loop control of the brazing process. An electron beam scan pattern edited by this system and the actual electron beam scanning path can be seen. In addition, special heat-insulating brackets have also been designed and manufactured to reduce heat loss due to heat conduction in the electron beam brazing process and to ensure reliable grounding of the welded test panels.

The effect of brazing process parameters on the diffusion depth of brazing material to the capillary wall and the influence of the height of the brazing feet were studied using a group comparison test method. The macroscopic metallographic analysis was performed on the grouped specimens using a LEICA-GZ6 type optical microscope. The height of the brazing feet and the diffusion depth of the brazing filler metal were measured using a JGX-1 tool microscope. Finally, the electron beam brazing process was optimized. The parameters mainly include: electron beam current, focusing current, heating time, and the amount of solder added. This is an illustration of the joint of the test piece, where D is the solder diffusion depth and H is the solder foot twist.

3 Test results 3.1 Electron beam brazing process parameters on the impact of the height of the brazing feet and the depth of diffusion Electron brazing process parameters only change one of the factors, the other parameters remain unchanged to observe the impact. The test results show that under the same conditions, with the increase of the beam, the diffusion depth of the solder to the capillary wall increases slightly, while the height of the solder joint on the joint surface increases significantly (as shown in the figure); when the heating time At shorter times, the diffusion depth of the solder does not change with time, but as the heating time continues to increase, its value begins to increase. The height of the foot height increases significantly with the increase of the heating time (b). Under the conditions of this test, the height of the foot increases significantly with the increase of the focusing current, and the diffusion depth of the solder increases slightly with the increase of the focusing current. (c) The height of the braze foot increases significantly with the increase in the amount of brazing material. When the brazing material content is greater than 75 mg, the diffusion depth of the brazing material to the capillary increases with the addition of the brazing material (d).

3.2 The impact of the preparation process on the solder diffusion and the height of the legs The preparation process of the test piece includes the assembly gap and the processing state of the inner surface of the capillary pores of the tube plate. It can be seen that when the joint clearance is increased from 0.015mm to 0.047mm, the height of the brazing foot increases significantly with the increase of the clearance; when the clearance is equal to *57mm, the height of the brazing foot does not change much. The diffusion depth of the solder increases as the gap increases.

The surface state of the joint is different, and the brazing quality is also very different.

See b, the capillary hole in the tube plate is directly brazed without any treatment after electric spark machining. The brazed leg height is the lowest; after the pickling, the reamer reaming test piece is used, and the height of the brazing pin is the highest. Pickled specimens are centered on the brazed feet. The surface state of the joint has little effect on the diffusion depth of the solder. 3.3 Optimization of process parameters Based on the above process tests, the optimized vacuum electron beam brazing process parameters are: acceleration voltage 60 kV, beam current 6.5 mA, heating time 37 s, solder mass 25 mg, focusing current 654 mA, assembly clearance 0.027 mm, surface state : Pickling. The results of the brazing test of the capillary plate joints with this parameter show that the brazing joints at the joints have smooth surfaces and the penetration rate of the appearance inspection reaches 100%. After the macroscopic gold phase inspection, there are no dissolution defects on the capillary surface, and no melt is found. Wear, block, leak, etc. Its appearance and profile macroscopic appearance are seen.

4 Analyze and discuss that when the temperature reaches the melting point of the solder in the electron beam brazing process, the liquid solder begins to wet, spread and fill the base metal. Due to the concentration difference between the alloying elements of the brazing material and the base metal, the alloy elements will produce mutual diffusion. The diffusion rate of solder in the capillary wall is closely related to its diffusion coefficient, and the diffusion coefficient of the material is not only related to the type of material and the type of crystal lattice, but also a function of temperature. As the temperature increases, the diffusion coefficient of the element increases, that is, the diffusion distance and the diffusion amount increase in a unit time.

It shows that with the increase of the beam current, the heating time and the increase of the focusing current, the solder diffusion depth increases. The reason is that due to the increase of the above parameters, the electron beam power and power density of the input to the workpiece are increased, so that the maximum temperature value of the brazing temperature field is increased, and the high-temperature residence time is prolonged. This not only increases the diffusion coefficient of the element, but also prolongs the high-temperature diffusion time of the element, so that the diffusion amount and the diffusion depth of the alloying element in the solder alloy into the stainless steel capillary wall are increased.

As the temperature increases, the viscosity of the liquid solder decreases and the fluidity increases. In addition, the increase in temperature also results in a decrease in the interfacial energy between the liquid solder and the solid metal, thereby increasing the interstitial capacity of the liquid solder and decreasing the wetting angle. Therefore, as the electron beam current increases, the heating time increases. The elongation and the focusing current increase, and the height of the brazing feet increases significantly.

As the brazing process progresses, the alloying elements in the brazing alloy rapidly diffuse into the base metal, and the concentration gradually decreases with time. Therefore, increasing the amount of solder added will increase the content of the elements involved in diffusion, which is conducive to the continued diffusion of these elements to the capillary walls, thereby increasing their diffusion depth. Similarly, as the assembly gap increases, the solder diffusion depth will gradually increase, also due to the increase in the amount of solder in the gap.

In the case of constant temperature, solder, and surface conditions, the wetting angle of the liquid solder and the solid metal is constant, so that the gap is reduced and the height of the solder leg becomes smaller. As the gap increases, the brazed feet gradually increase, but when the gap is too large, the capillary action weakens, which will restrict the filling and climbing ability of the liquid brazing filler metal, so that the brazed foot size will no longer increase.

The surface state of the sample determines the size of the wetting angle of the brazing material to the base metal. Samples directly brazed after spark drilling do not impede the contact between the liquid solder and the base metal due to failure to remove impurities such as oil and oxides, weakening the adhesion therebetween, and therefore the capillary action of the liquid solder is very small. The lowest degree of athlete's foot; After pickling the sample, although the debris attached to the hole was removed, a passive film was formed on the surface of the workpiece after pickling, which also hindered the brazing material spreading to a certain extent. The height is thus not high; after the acid-washed specimen is scraped and retorted by a reamer, the oxide film on the inner surface of the pores is scraped off, exposing the fresh metal surface, so that the solder has the smallest wetting angle and the strongest capillary effect. With the same process parameters, the height of the brazing feet is the highest.

5 Conclusion Vacuum electron beam brazing of various capillary plates can be successfully realized by using a fully automatic closed-loop vacuum electron beam brazing system. This not only effectively avoids the occurrence of defects such as dissolution, burn-through and plugging of capillary tubes, but also can greatly Improve brazing work efficiency.

For the capillary plate connectors studied, the optimized parameters of the electron beam brazing were: acceleration voltage 6 kV, beam current 6.5 mA, heating time 37 s, solder mass 25 mg, focusing current 654 mA, and assembly clearance. 27mm. With the increase of the beam current, the increase of the focusing current and the extension of the heating time, the diffusion depth of the solder to the capillary wall is gradually increased, and the increasing trend of the height of the solder leg is more obvious.

The depth of solder diffusion and the height of the legs increase with the increase in the amount of solder and the assembly gap. When the gap is larger than *47mm, the height of the legs almost does not change.

The acid-washed and hinged test plates with capillary pores are most conducive to the spreading of the solder, the highest height of the brazing feet, followed by pickling, and the lowest height of the brazed feet without any treatment.

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