Quality process of aluminum alloy vacuum brazing
The influence of brazing agent, solder, vacuum degree, fixture and brazing technology on the quality of aluminum alloy vacuum brazing is experimentally studied, the influence mechanism of these key factors is clarified, and the key points of brazing technology design are summarized. The working pressure is less than or equal to 1 * 10-3 Pa, which is a necessary condition for aluminum vacuum brazing.
The complex cavity structure of a part requires brazing. This part needs multi-layer lap welding, forming brazing weld on multiple edges and rings. It is required that the welding round Angle R of the welding seam should be no more than 0.2mm, and there should be no filler metal diffusion. The other part is to weld 5 components together, which requires two rectangles facing welding to form vertical two profile cavities. The brazing weld shall be flat on the inner surface of the cavity, not convex or concave, and the two cavities shall be even, and there shall be no dislocation. The brazing of these parts is characterized by the formation of weld on narrow and long edges, which requires high welding strength.
This kind of hard brazing of aluminum alloy, which requires extremely strict roughness of welding round Angle and profile tolerance, welding seam and cavity, affects the quality of brazing with many technological factors and complicated process design.
For precision parts brazing, vacuum brazing is a more suitable process, it does not need to remove the brazing agent after brazing. Vacuum brazing is a welding method to realize brazing by placing the solder and brazing assembly clamping device into the vacuum brazing furnace for overall heating, heat preservation and cooling. Vacuum brazing has the advantages of small size deformation, no need to be cleaned after welding, multiple welds can be realized, multiple parts can be welded simultaneously, green and environment-friendly. In order to formulate the vacuum brazing technology and inspection rules of this kind of complex precision aluminum alloy parts, we have conducted systematic experimental research on the factors influencing the quality of aluminum alloy vacuum brazing.
1. Brazing material
1.1 and soldering flux
Aluminum has strong chemical activity and usually has a very dense oxide film on its surface. Its properties are very stable. When the old film is damaged, new film can be formed quickly. It is necessary for the soldering agent to break the film, otherwise the molten solder cannot be wetted with the base material. The wettability of the filler metal is improved by using a certain amount of magnesium powder as the soldering agent for aluminum vacuum brazing. The infiltration of magnesium vapor will also reduce the interfacial tension between the filler metal and the base metal, thus enhancing the wetting ability of the filler metal.
The experimental results show that the oxide film on the surface of the base material and the solder can not be welded if it is not completely removed before entering the furnace or if certain oxidation occurs during heating. If more magnesium powder is used, it will lead to the diffusion of solder and damage the roughness of the parts. The radius of the welding round Angle of the weld section is not easy to control. Using Al - Si - Mg solder, if before charging can thoroughly remove parent metal and oxide film on the surface of the solder, appropriate control of heating rate, above 450 ℃ for 30 min, eliminate air as much as possible and to maintain the pressure in the whole process of heating up 1 x 10 or less - 3 pa, even without the use of solder can be a very good welding, the welding strength and presence of adding magnesium powder, no difference. Al-Si-Mg series solder is adopted, no more brazing agent is added, and the brazing effect is good.
The material of parts is 3 a21, 6063, solid - liquid phase line temperature is 643 ℃ ~ 654 ℃ and 643 ℃ ~ 655 ℃, the selection and parent metal have the same major components of aluminum and silicon solder, add a certain amount of magnesium can reduce the surface tension of liquid solder, experiments show that the amount of magnesium is not too high, otherwise it will produce the floodplains, even welds not form liquid solder capillarity, not form fillet weld section. The rapid evaporation of magnesium in the vacuum environment also changes the composition of solder and makes the welding strength difficult to control. The closer the liquid - phase line of the solder is to the solid - phase line, the better. Based on the three components (Al - Si - Mg) of experimental screening of solder, the solder melting point range in 555 ℃ ~ 580 ℃ Al80Si14Mg6 solder, welding strength is good, weld cross section radius is also easy to control. Compared with the imported solder, the results show that the brazing weld formed by the solder is better than that of the imported solder.
2. Vacuum brazing furnace
Aluminum has high chemical activity and is easy to be oxidized. When the pressure is greater than 6 * 10-3 Pa, during the heating process, the parent material and the filler metal are oxidized to grey, and the filler metal cannot be wetted. Only when the pressure 5 x 10 or less - 3 Pa, and slow to heat up before 500 ℃, 450 ℃ for 30 min, in order to make sure the pressure in the process of heating up in 5 x 10 or less - 3 Pa, to realize the brazing.
Vacuum brazing furnace pressure rise rate is an important index to ensure the quality of brazing, pressure rise rate is the vacuum furnace pressure to work the pressure, close vacuum system to stop after extraction, the recovery rate of the furnace per unit time pressure, and it reflects the brazing furnace sealing leakage performance of a technical indicators, also known as the leakage rate, vacuum brazing furnace pressure rise rate < 0.3 Pa • h - 1 can welding aluminum alloy parts of high quality. If the pressure rise rate is large, the brazing quality of aluminum alloy parts will not be guaranteed even if the working pressure is maintained at less than or less than 1 * 10-3pa. Because the high power pumping system can cover the leakage index of the seal, the leakage gas into the furnace is quickly drawn out, but the brazing parts are oxidized.
In 500 ℃ ~ 600 ℃ temperature stage heating rate has important influence to the welding quality, low heating rate, low melting point elements in vacuum filler metals rapid evaporation, keep high melting point group yuan investment, even form the floodplains. The cooling rate at the end of heat preservation is also important. Too slow cooling is equivalent to prolonged heat preservation time, resulting in diffusivity, leakage welding, corrosion and other defects.
The ultimate pressure of the vacuum brazing furnace should be no more than 10-4 Pa, the working pressure should be no more than 1 x 10-3 Pa, and the pressure rise rate should be less than 0.3Pa• h-1. Should have enough heating power, can rapid heating, heating rate, 12 ℃ or higher, min - 1. The vacuum brazing furnace should be equipped with air cooling system, which can be cooled quickly. The fastest speed cooling is applied in the condition of ensuring the shape tolerance of parts. Aluminum alloy brazing quality of brazing furnace temperature control precision, higher requirements on furnace temperature uniformity, + / - 5 ℃ can satisfy the vacuum brazing of small parts, plus or minus 1 ℃ in the market now, plus or minus 3 ℃ temperature control precision on furnace temperature uniformity can aluminum vacuum brazing furnace brazing of high quality parts.
3. The brazing welder shall install clamps
The fixture should be designed with high precision of assembly position and adjustable clamping force. It is better to use 1Cr18Ni9Ti for the fixture. The clamp has a certain elasticity at the brazing temperature, so that there is a suitable gap at the joint of the welding seam.
The volatile solder will form capillary adsorption between the threaded teeth and finally solidify in the teeth. Bolts should be threaded with coarse teeth. The fixture should be heated and oxidized in the air furnace before it is used for the first time, so as to reduce the wettability of the solder and the reflectance of the radiant heat.
The heat capacity of the fixture in the brazing experiment we used in the early stage is large, and the defects such as non-melting of solder, diffuse flow, leakage welding, corrosion and so on keep appearing, even if other technological parameters are adjusted, they cannot be avoided. According to the observation and analysis, the reason is that the vacuum brazing is radiative heat transfer. The heating and cooling rates of the parts are greatly affected by the thermal capacity of the fixture, and the heating and cooling rates are small, which is equivalent to slow heating rate and long heat preservation time. After hollowing out the workpiece and opening the weight reduction hole, the weight of the workpiece is reduced by more than 50%. The thermal capacity of the tooling should be minimized without damaging the rigidity. This can be achieved by reducing the weight of the tooling and using materials with less density and smaller heat capacity than graphite, for example.
4. Brazing technology
4.1 process flow of aluminum alloy vacuum brazing
Process flow of aluminum alloy vacuum brazing:
Solder cutting - removal of oxide film on the base material and solder surface of brazing parts - solder fixing - assembly - vacuum brazing - inspection.
4.2 removal of surface oxidation film
Corrosion solder and oxide film of the base material with sodium hydroxide solution are slow in alkali corrosion reaction at room temperature, high concentration is required, corrosion speed is fast and difficult to control, and hemp spots are easy to appear. The corrosion time is long and the solder is thinner. Through contrast experiment select easy to control the corrosion process: temperature 80 ℃ ~ 85 ℃, and the corrosion time 45 s and 50 s, caustic wash rinse with clear water after the lye and debris, with dilute nitric acid to remove corrosion products, acid washed with water, alcohol or acetone dehydration. The key parameters are composition, concentration, corrosion temperature, corrosion time, concentration and corrosion time of nitric acid solution. If the composition and concentration of the alkali solution are not properly selected, it is easy to cause incomplete removal or over-corrosion of the oxide film. If the oxide film is not completely removed, the liquid solder will not be able to wet the base material well. Excessive corrosion of parts and solders may cause a certain degree of change in their composition ratio, or even a certain degree of change in the composition of solders, which affects their wettability and weld strength. In addition to the composition and concentration of alkali liquor, corrosion temperature and corrosion time also have an important influence on the removal effect, and the proper removal process parameters of oxide film need to be optimized through experiments. The corrosion rate of solder is different from that of base metal.
4.3 vacuum brazing heating
The heating rate of vacuum brazing has an important influence on the quality of brazing. The heating rate is fast, the surface color of the parts is dark, and the flatness is poor. When heated, the activity of aluminum is enhanced, and a very small amount of air can also make it oxidized. At the same time, the gas is heated to expand, reducing the heating rate, so as to extract as much hot air as possible. The heating rate is fast. Another adverse effect is that the parts are easy to be deformed. The parts are heated by radiation. When approaching the insulation temperature of brazing welding, rapid heating is required. The heating rate is slow, and the temperature range of the solid - liquid phase line of the solder stays for a long time. In the vacuum environment, the composition of the solder with low melting point and the composition with high steam pressure has serious volatilization, and the melting point of the remaining filler metal components is increased to maintain solid non-melting and brazing is not possible. This is the same situation that occurs when the brazing welder has a large heat capacity. For the flatness of the parts, rapid heating is also allowed at this stage. In 500 ℃ when stress is released, the recrystallization temperature of aluminum alloy is lower than 500 ℃, 500 ℃ above aluminum alloy plasticity is good, easy to release heat stress. In a simulated specimen brazing experiments, 30 min at 400 ℃ heat preservation and fast heat up to 600 ℃ heat preservation of 25 min, than directly to 600 ℃ soldering effect is good. Further experiments result is only at 450 ℃ for 30 min, and then 10 ℃, min - 1 to 620 ℃ soldering some parts of the fully qualified.
4.4 aluminum alloy vacuum brazing insulation
Generally, the temperature of thermal insulation is lower than the solid phase line temperature of the base material and higher than the liquid phase line temperature of the solder. The temperature is too high, which is easy to cause the corrosion defect of the base material. The thermal insulation time of brazing is about 2min after the workpiece reaches the liquid phase line temperature of the solder. If the thermal insulation time is too long, the filler metal will overflow or leak welding occurs. The length of thermal insulation time is affected by the thermal capacity of parts and tooling. Different parts and the amount of vacuum brazing need to be experimentally screened out the brazing thermal insulation time.
4.5 aluminum alloy vacuum brazing cooling
The thermal insulation time of brazing welding should not be too long. Rapid cooling is required after the end of the thermal insulation. The protective gas is used to force uniform cooling. The slow cooling is the same as the extended heat preservation time, which will lead to the overflow of solder or leakage welding defects. For larger parts, rapid cooling is very important.
(1) selection of solder
Note: solder the main group of RMB should be the same parent metal, the liquidus temperature of solder is lower than the base solidus temperature for at least 30 ℃, the smaller the difference in temperature of brazing furnace temperature control precision, the higher the temperature uniformity requirements; The temperature difference between solid phase line and liquid phase line of solder should be as small as possible. At the brazing temperature, the main components of solder should have high chemical stability, that is, low oxidation and low steam pressure. The position of the main component elements of the filler metal and the main component elements of the base metal should be as close as possible in the periodic table, so that the corrosion resistance of the brazing joint is good.
(2) brazing technology
Because of aluminum alloy and solder type, shape and size of part, different requirements for brazing quality differences, such as surface oxidation film to clear the composition and concentration of alkali, corrosion temperature and corrosion time, vacuum degree of vacuum brazing, heating rate, holding time, cooling rate and other parameters will be slightly different, should be screening optimized through experiments according to different situations, ultimately determine the best process parameter combination.
(3) aluminum alloy vacuum brazing furnace
Performance indexes of vacuum brazing furnace:
1 x 10-3 Pa or less work pressure, and pressure rise rate < 0.3 Pa • h - 1, + / - 1 ℃ accuracy of temperature control, furnace temperature uniformity of plus or minus 3 ℃, with gas cooling function.
(4) tooling fixtures
The fixture should be as simple and light as possible on the premise of ensuring the rigidity meets the requirement of brazing.