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Pre-brazing Preparations and Post-brazing Operations


Joint and Jig Design

  • Usually simple aluminum brazing is done by lap, socket or T-joint. For many brazed seams with special requirements, such as sealing, special stress or brazing without deformation, it is necessary to seriously consider the brazing seam. the design of.
  • There are several basic situations to design for brazing seams:
  • When brazing, the change of the width of the brazing joint affects the accuracy of the tooling, especially when using the sheet-like brazing material (clamping the brazing seam) and pressing the brazing material, the solder should be melted, and the overall size of the workpiece should be observed. The coefficient of linear expansion of the aluminum alloy is about 1/3 larger than that of the usual metal, so the brazing fixture should be flexible.
  • Aluminum alloys soften when brazing is heated, and slender and vertical t-pieces must be supported?
  • For some aluminum alloy parts that need to be heat treated, it is inevitable that there will be deformation when quenching to restore the original strength after brazing, and parts should be considered with machining allowance.
  • The brazed joint I’ul of aluminum brazing affects the quality of the brazing process and the brazing joint. The narrower the gap, the stronger the capillary action of the molten flux and the brazing filler metal in the brazing joint, but the slag is easy to be slag; the gap is too wide, the brazing material is difficult to flow to the end, and the stress of the brazing seam is not uniform.
  • The molten flux and solder automatically maintain a reasonable gap between the brazes when placed naturally. How do the Vacuum brazing furnace seams with special clearance requirements remain? A small tapered cone can be used to gently puncture the small holes on the surface of the base material to be brazed. The curling of the edge of the small hole helps to maintain the width of the gap and is measured after clamping to maintain the required gap.
  • For flanges or T-joints, it is also possible to use needle-punching holes to maintain clearance.
  • The blind holes and closed air M at the brazed joint are not allowed to exist and must have a passage for venting or discharging residual flux. Figure 242 shows the brazing of a solid rod inserted into a hole in the plate, forming a closed space at the joint. The connector on the left is incorrect and the heart is correct. The venting and draining holes are not fixed on the bottom side, but can be placed on the other side. If the brazed center element is not a solid rod but a n-open tube, there is no problem with a dedicated vent.
  • In the practice of aluminum brazing, various forms of brazed joints are encountered, and the butt joint method is rarely used. This is because the butt welded surface is small, the sides of the nail are not rounded, and the strength of the brazing material It is often lower than the base metal. Figure 243 shows various typical brazed joint designs.
  • In the case of a sheet, a straight joint is used, and a lap joint is often used instead of a simple butt to enhance the strength of the joint. When brazing thinner sheets (<3mm) and the thickness and strength of the two parts are different, the appropriate length of the overlap should be 2 to 3 times the thickness of the thin side (or weak side). If the strength of the sheet is higher or the shear strength of the solder is lower, the length of the overlap should be increased.
  • In the case of large-area overlap, in order to prevent the core trap and slag inclusion, if the design allows, a small hole can be drilled on the upper piece of material to facilitate the discharge of gas and residual flux.
  • There are three ways to supply solder during brazing:
  • Place the brazing material next to the gap of the brazing joint = use the plated material.
  • Temporary supply by hand during brazing.
  • The first type of feedstock can be placed using a shaped braze: plasticized commodity brazing filler metal, various forms such as wire, rod, sheet/ring, gasket, tube, etc., can be used as needed. For example, the tube can be connected to the flange by a ring-shaped material, and the large-area overlap can be used to sandwich the nail foil in the gap. The most commonly used is the filament and rod-shaped solder. The solder wire can be cut into small sections. Carefully consider the walking route of the molten solder on the workpiece. Place a number of points to place a number of brazing bars, so that long and complicated intersections can be completed at one time. Multiple brazing joints of :

Pay attention to the following points when placing solder:

  • Do not allow the squeezing brazing material to flow too far in the brazing joint to avoid uneven formation of the agglomerated base metal and the brazing joint.
  • If the quality of the brazing filler metal is too small compared to the base metal, it must be placed in a stable place, such as a ditch or a trough, so as not to be melted away due to the small heat content and the melting of the parent metal. If the quality of the parts of the base material differs greatly, the solder will rest on the part of the large mass a, so that it can be consistent with the temperature of the large mass parts when heated. If the heating during brazing is mainly based on the radiation of the heat source. Heat, such as in automatic flame soldering lines and furnaces, prevents the solder from over-melting and rolling away before the base material has not warmed up to the predetermined brazing temperature.
  • Use anhydrous propylene to adjust the chloride flux into a paste, stick the solder to the desired position, and cover it with a small amount of flux on the upper M. This can reduce the problems caused by items 2) and 3) above.
  • Clamping and fixing of the workpiece: When the part is welded, the + is specially fixed, and the weight of the part itself is enough to maintain the original position. Salt bath brazing must be fixed with a clamp. The design of the firearm can be determined according to the specific conditions, but the volume and quality of the fixture itself should be reduced, and flexible and elastic materials should be used. It is best to use blue-treated steel or oxidized stainless steel to avoid the fixture and aluminum mother. The materials are also brazed together, but fixtures of this material cannot be used for salt bath brazing. The better fixture material in the salt bath is the imonel X750, which is twice as resistant to corrosion as stainless steel. Figure 244 shows a spring-loaded clamp.
  • The use of jigs is always a very inconvenient car, whether it is occupying space, or heating, after cleaning, etc., it is always troublesome. Therefore, self-clamping joints are often used in factories, especially in mass-produced factories. That is, it is fixed by rivets, mechanical expansion pipes, convex line pressing, lock seams and even tack welding without using clamps. Figure 245 shows indium-like self-clamping joints.

Pre-cleaning of Workpieces to be Brazed

  • The brazed workpiece must be carefully removed from various dirt on the surface, excessively thick oxide film, and oil from processing, otherwise a good braze seam cannot be obtained. The most important way to remove the workpiece’s surface oil is to flush it with a vapor of organic solvent in a closed compartment. Rinse with steam of tri-ethane, tetra-ethylene or other commercial solvents. It can also be scrubbed with petroleum solvents or chlorinated hydrocarbon solvents.
  • The aqueous deoiling formulation is preferably brushed with an alkaline aqueous solution of trisodium phosphate plus a little detergent (sodium alkylbenzene sulfonate) and finally rinsed with water.
  • After the oil is removed, there should be a method of determining whether the thickness of the oxide film on the surface of the aluminum alloy affects the function of the flux. According to the visual surface, the state of oxidation is very unreliable, and sometimes it seems that oxidation is very strong. «The real oxide film is very #. On the contrary, a very smooth and flat surface sometimes has a very thin oxide film. Whether the thickness of the oxide film is suitable for soldering can be judged simply by measuring the surface resistance of the alloy by micro ohms. The table resistance of common aluminum alloys is shown in Table 2-17. In general, this measurement is not very accurate, and the values ​​of each point and each surface are different, and some points can be measured. If the surface resistance is within the range of the table or low, this value is the surface and the film is thin enough for direct brazing.
  • The excessively thick oxide film can be partially removed by a stainless steel wire or a copper wire brush or a grinding method. Never use abrasive cloth or sandpaper, beware of sand particles embedded in the aluminum surface. A common chemical method for removing oxide film over a large area. Usually use “KNa0H” 5% sodium hydroxide solution to clean, the temperature is maintained at about 60T, when the cleaning release large children’s hydrogen, entrainment of large alkali mist is very irritating to the respiratory tract, and easy to fire and explosion, should be in a good ventilation get on. As the scrubbing reaction progresses, the temperature will rise rapidly and attention should be paid to cooling to avoid excessive reaction.
  • In the above hot solution, the aluminum base material was roughly thinned by about 25 pm per minute. At the same time as the oxide film is removed, the alkali solution mainly dissolves the pure aluminum and the alloying elements form black scum or sediment deposited on the surface of the base material. The larger the amount of alloying elements, the more black scum is. Therefore, when cleaning aluminum alloys, care should be taken to control the temperature of the solution and the time of ff washing. Sometimes a more dilute cleaning solution can be used.
    Table 2-17 The surface resistance of commonly alloy alloy resistance
    used aluminum alloys GB AA
    3004 500-3000
    alloy alloy
    5A02(LF2) 5052 1000-10000
    GB AA
    8A06(L6) 1100 100-500 6A02(LD2) 6061 1000-20000
    3A21(LF21) 3003 300-1000 7039 2000-20000
  • The time of alkaline washing is preferably controlled within 10 ~ 15s. When the liquid is removed, the layer is evenly etched away, and the base material is often etched out of the uneven pit and leaves some debris. Figure 2>46 shows the SEM image of the 20丨4 alloy after cleaning with NaOH solution. It can be seen that the cleaning of the alkaline solution should be carried out under strict control conditions, and should not be excessive.
  • After alkaline washing, the lye should be washed carefully with water. It is very difficult to completely regenerate the trace amount of lye. The black sediment left by the alloy cannot be washed away by washing with water. It is easy to remove it with acid soaking. To prevent acid corrosion of aluminum, oxidizing acid should be used. It is usually washed at room temperature with dilute nitric acid or chromic acid (CrO, aqueous solution plus some potassium dichromate). It is reported that a foreign factory uses dilute nitric acid plus 0.5% hydrofluoric acid, and some use Up to 20% hydrogen acid has a better light-emitting effect. The most comprehensive introduction to the formulation and operation of aluminum and aluminum alloy rinses can be found in the specials of Wemick and Pinner.
  • After soaking with acid, it should be rinsed with water. The last one or two times should be rinsed with deionized water or distilled water to avoid leaving scale; then air dry or warm wind. In this process, you can’t touch the touch by hand, otherwise the clean watch is easy to leave sweat marks and fingerprints.
  • If the surface of the alloy is relatively clean, the oxide film is not thick, so that it is not washed with alkali and only with pickling. Pickling nf uses a mixture of nitric acid and a hydrofluoric acid, which not only removes the oxide film but also thins off a layer of metal, but the rate of cutting is much slower than that of the alkali solution, and uniform parts are also preferred. It should be pointed out that hydrofluoric acid is a very dangerous acid. Do not touch the skin when using it. If it is in contact (including steaming), it should be washed immediately with water and thoroughly washed with soap or alkaline solution. The danger is that it is not related to sulfuric acid, nitric acid, etc. at the initial contact, and it is especially difficult to cause convulsions. It is too late when the skin is itchy and soaking, which will cause skin tissue to be deeply necrotic, which is extremely painful. endure.
  • After cleaning the workpiece after the drying, the welding work should be completed in time. If the storage period needs to exceed 48 hours, the plastic bag should be packed first and sealed.

Post-braze Cleaning of Workpieces

  • The workpiece with the flux adhered after soldering must be thoroughly cleaned to prevent corrosion. The most effective rinsing is to hot dip into the boiling water after welding and continue to boil. The water in the washing tank should be agitated by the flow or blown air and need to be replaced periodically. Manual or mechanical brushing of the workpiece is also required if necessary. Ultrasonic vibration scrubbing is extremely effective – a method. Complex, slit or small deep-hole workpieces often need to be soaked in flowing, hot water from time to time. In this case, the aluminum workpiece only needs to be washed and re-updated, and generally does not cause pitting corrosion.
  • The final residue of the flux is often removed by chemical means. The cleaning solution in Changzhou is described as follows:
  • The nitric acid cleaning solution is prepared by dissolving nitric acid and water in a volume ratio of i:i and washing at room temperature. This lotion works by washing for 10 to 20 seconds and then washed with water. Nitric acid is quickly consumed by residual flux, so this lotion is only used to wash small workpieces and where there is not much solder.
  • When the gasification in the nitric acid washing solution contains pen: more than 5R/L, the thin-walled or slender workpiece may be etched. At this time, the thiourea with the highest fraction of 1% can be added as a corrosion inhibitor to prevent vaporization. corrosion.
  • Nitric acid and hydrofluoric acid mixed cleaning solution Nitric acid: Hydrofluoric acid: Water (volume ratio) = 1: 0.06: 9 Kisei cleaning solution, used at room temperature. After the workpiece is immersed in this liquid, not only the residual flux is quickly removed, but also the base metal is etched. The depth to which the metal is etched depends on the soaking time, and it is usually sufficient to clean it for 10 to 15 minutes. Rinse with cold water and rinse with hot water of about 75 ,. Do not rinse for more than 3 minutes, otherwise rust will appear. The cleaning solution should be analyzed weekly, and if the vapor (eg HCI) reaches 3 g/L, it needs to be replaced.
  • Hydrofluoric acid cleaning solution Hydrofluoric acid: water (volume ratio) = 0.3: 10 is formulated into a cleaning solution and used at room temperature. This cleaning fluid removes residual flux and is quite effective and fast. Since this liquid can dissolve aluminum, the dipping time should not exceed lOmin. It is consumed by residual flux at a lower rate than nitric acid, so it is used more, but hydrogen is produced when it is not washed and must be ventilated. When washing, if the workpiece is found to be discolored and dark, the nitric acid can be used to restore the gloss.
  • Nitric acid-sodium dichromate cleaning solution 5L nitric acid +3. 5kg sodium dichromate (Na2Cr207) +40L water is used as a rinsing liquid, which is most suitable for washing fine 丨’. Pieces. The cleaning temperature is maintained at about 65 弋, 5 ~ 10 minutes after the completion of washing, and then washed with hot water u chromic anhydride-phosphoric acid cleaning solution 1L aqueous solution containing chromic anhydride w (CrO,) = 2% and “; (ratio 1 > 04) = 5% heating 80t:. This solution is suitable for delicate workpieces. A new fluid is required when the concentration of the gasified substance washed in the rinsing liquid exceeds 100 &/L.
  • Sodium Phosphate-Sodium Dichromate-Sodium Fluoride Washing Solution: 135] 120_W This rinsing solution is suitable for extremely fine workpieces, removing residual flux and surface and etching, and the corrosion is extremely slight.
  • After cleaning with the above various methods, the cleaning solution should be thoroughly rinsed with clean water, otherwise the cleaning liquid itself will cause perforation corrosion to the weak parts of the workpiece. Ding
  • Factory reported that adding 0.5% sodium dichromate or potassium dichromate to the final washing liquid can make the workpiece more resistant to corrosion.
  • For highly demanding workpieces, it is necessary to wash with i-ion water or distilled water until the concentration of vapor on the surface of the washing liquid and the workpiece does not exceed 5×10-. The simple test method is to take some of the last cleaning water and put it in the test tube. A few drops of human silver nitrate solution, let stand for 1 min, if it is not turbid, it is qualified. The preparation method of the silver nitrate solution is as follows: take AgNO^g, add 100 g of secondary deionized water, and then drop HN0 with a mass fraction of 20% to 50%, shake a few drops, and shake for use.
  • When using nitric acid as the rinsing liquid, the rinsing tip can be made of stainless steel; when using the HN03-HF mixed washing liquid or HF rinsing liquid, a glass fiber reinforced plastic tank is required, and the groove can also be used for HN0. , -Na2Cr207 cleaning solution hot rinsing solution can also be used.

Finishing

  • After the brazed joint is cleaned, it is generally not plated. The color of the brazing seam is always in contrast to the base metal, depending on the composition, type and relationship of the brazing filler metal. The difference in color from the base metal is Al-Si solder. The biggest difference is zinc-based and heavy metal-containing solder. After the brazing, the brazing seam is most likely to be washed with alkali. Even if the Al-Si brazing filler metal is used (such as NaOH), the alkali-soluble AI is faster than the dissolving Si, so that the brazing seam I exposes the Si crystallites and becomes black. In the case of pickling, if HF is contained, it should be carefully controlled in concentration and pickling time. In this sense, if the brazing joint after washing is slightly unclean, it is better to use a softer gold brush, such as copper wire. Brush the whole.
  • 1. Anodizing of the workpiece
  • When brazed parts require surface protection, anodizing is often performed. Although aluminum without brazing is often anodized in a strong acid medium solution, strong acid anodizing on the brazed parts often results in brazing The color becomes darker and the contrast is larger. The phenomenon of zinc-containing solder is more serious, and even the nail seam is severely corroded. Therefore, strong acid is usually not used as the electrolyte, and one is more suitable for the chromic acid method; moreover, it is more suitable for the brazing joint. The anodizing of aluminum parts is the oxalic acid (H2C204) method. Table 2-18 lists the formulation and operating conditions of the anode gasification electrolyte of aluminum brazed parts. The cathode of the electrolysis can be made of stainless steel, lead or graphite. : Cathode (area ratio) = 5: 1-10: 1. After oxidation, rinse with water, and then immerse in a mass fraction of 1% ammonia or N^HCO to remove traces of acid.
  • Table 2-18 Some anodizing processes in use for aluminum brazed parts
  • Serial number Electrolyte + water Electrolyte concentration / (g / L) power supply Current density /(A/dm)2 Low voltage / V Temperature / X. Oxidation time /min Cladding color Special feature
    1 Cr03 90 DC 0.3 40 30 30 Micro green One
    2 Cr03

    K2Cr207

    12.5

    37.0

    DC 0.1 -0.6 40 20-60 ? Micro green Purer Cr03 has low corrosion resistance
    3 h2c2o4 30-50 DC I ~2 40 〜60 18-20 40-60 yellow One
    4 II2 C2 O4 30 〜50 DC 1〜2 30-35 35 20-30 colorless Porous for dyeing
    5 h2c2o4 30-50 AC 2-3 20-60 25-35 40-60 yellow Suitable for thin parts and with village
    h2c2o4 30-50 AC

    DC

    1 -2

    2〜3

    40-60

    30-60

    20-30 15-20 ? Anti-wear layer suitable for Al-Mg alloys
    7 H2L204 1 50 -100 DC 1 -1.5 50-65 30 10-30 translucent ?
  • The flj contains brazing joints of heavy metal brazing filler metal (such as Zn-Al eutectic solder), which tends to have a considerable color difference with the parent metal after anodizing. There is a factory in Japan, a moving wire, a flame brazing household aluminum pot, using Zn-Al solder. After the anodization, the overall dyeing is appropriate, which significantly reduces the chromatic aberration of the brazing seam.

2. Pre-plating treatment of the workpiece

  • In many cases, if the surface of the aluminum workpiece is required to have a high electrical conductivity (such as a microwave device), it is necessary to plate a layer of metal with a more stable conductivity and more stable performance on the aluminum surface, but the metal is plated on the aluminum surface. A simple process is often to first deposit a layer of zinc on the aluminum chimney by chemical precipitation, and then plate other metals by conventional methods to: list several formulations of chemically deposited zinc: ZeUcy formula
  • A small amount of FeCl in the formulation, combined with tartrate, improves the adhesion of zinc to aluminum, especially to magnesium-containing alloys. Formulations 2 and 3 are especially recommended for depositing zinc on complex workpieces. Formulation 3 is suitable for magnesium-containing alloys such as Al-Zn-Cu-Mg and Al-Cu-Mg-Mn. In the case of high magnesium-containing alloy 1: deposition of 7, n, sodium potassium tartrate can be increased to IOOr/L. The operating temperature of this formula is 21 -24^, and the person’s time should not exceed 30s.