12-23

How to choose the welding process of aluminum and aluminum alloy

2022-12-23

Welding process 4.1 Selection of Welding materials In principle, aluminum and aluminum alloy welding wires or strips with the same composition as the base metal should be selected. Argon purity> 99.95%, try to use large diameter welding wire. Aluminum-magnesium-aluminum alloy arc welding usually recommend the use of CB-AMr2, CB-AMr3, CB-AMr6, CB- AMr61, CB-AMr63, 1557 and 1577 electrodes, and aluminum-copper-aluminum alloy recommend use 01201 and 01217 electrodes. 4.2 group butt welding and spot welding Due to the fast heat conduction of aluminum and aluminum alloy pipes, the crystallization is fast in the molten pool, the weld length of 10-15mm is easy to locate, and there is no gap and blunt edge, so the large residual stress caused by strong exercise after welding should be avoided. The welding positions are at 7 o'clock, 9 o'clock and 12 o'clock of the pipeline. Positioning welds are often reserved as formal welds, so problems should be dealt with in time. Before welding, remove the black powder and oxide film on the tack welding surface, and make both ends smooth. Welding parts do not require preheating. Before welding, try welding on the test plate to confirm that there is no air hole before formal welding. Using high-frequency arc, the starting point should be about 20mm above the center line, and it should be stationary for about 2-3 seconds. Then, under the condition of ensuring the penetration depth, the high current rapid welding is adopted. The welding wire shall not swing and the end of the welding wire shall not leave the argon protection zone. If leaving the argon protection zone, the end of the wire should be cut off. The angle between the wire and the weld surface shall be 15 degrees right angle. The angle between the torch and the weld surface should be maintained between 80 degrees and 90 degrees. In order to increase the argon protection area and enhance the protection effect, the large-diameter ceramic nozzle of the welding gun can be used to increase the argon flow of the welding gun. When there is spatter on the nozzle that significantly impedes the flow of argon. The spatter must be removed or the nozzle replaced. When the tungsten electrode is contaminated and irregularly shaped, it must be trimmed or replaced. The tungsten should not protrude from the nozzle. The control of welding temperature is mainly the control of welding speed and welding current. The test results show that high current rapid welding can effectively prevent the generation of pores. This is mainly due to the fast penetration of the weld during the welding process, the short heating time of the molten metal, and the less chance of absorbing gas. When closing the arc, pay attention to fill the arc pit, reduce the molten pool, and avoid shrinkage. The connection of the end point shall be welded for 20~30mm, and the air shall be stopped for 6 seconds after the arc is stopped. When rotating aluminum and aluminum alloy pipe butt welding, the welding torch should be in a slightly uphill welding position. This facilitates penetration. Thick wall tube bottom welded. Wire cannot be added. However, the welding wire shall be added in the subsequent welding layer. 5 Welding inspection All welds shall be subjected to surface and radiographic examination in accordance with the HGJ222-92 Technical Specification for Welding of Aluminium and Aluminium Alloys. 6 Effect of implementation Using the above welding process, the problem of air holes and burn-through in the actual welding construction is effectively solved. The qualified rate of welding flaw detection reached 97%. Of course, there is also the problem of back molding, which mainly depends on the hand feeling of the operator, and the technical requirements for the operator are relatively high.

12-22

Preparation before welding for welding of aluminium and aluminium alloy tubes

2022-12-22

3.1 groove processing adopts mechanical processing method. The surface of the groove after processing shall be smooth without burrs and flashes. Groove form is generally V-shaped, no blunt edge, groove angle should be 70~75 ℃. Butt welds of different wall thicknesses shall have a transition section of 14O. Preparation before 3.2 welding Before welding, remove the welding wire, welded pipe groove and the oil stain and oxide film within 30~50mm inside and outside the groove. The removal sequence and method are as follows: use acetone or carbon tetrachloride and other organic solvents to remove oil stains on the surface, and the removal range inside and outside the groove should not be less than 50mm. After oil stains are removed, the welding wire adopts chemical method, the groove is easy to use mechanical method, and the test tube also uses chemical method to remove the surface oxide film. On the mechanical side, the surface of the groove and its vicinity can be cleaned by filing, scraping, grinding or brushing with a stainless steel wire of about 0.2mm until the metallic luster is exposed. The cleaning range on both sides shall not be less than 30mm from the groove edge, and the tools used shall be degreased regularly. Chemical methods. Soak in 5-10% NaOH solution at 70°C for 30-60 seconds, or soak in 5-10% NaOH solution at normal temperature for 3 minutes. Then, it was soaked in about 15% HNO3 (room temperature) for about 2 minutes, and then washed with warm water. Or rinse it with cold water and let it dry completely. The welding wire that has undergone a reliable surface treatment and is not oxidized or contaminated can be directly used without the above treatment. The cleaned groove and welding wire shall not be contaminated before welding. If there is no effective protective measures, welding should be carried out within 8 hours. Otherwise, it should be cleaned again. When assembling the pipeline, the inner wall should be flush, without burrs and chips, and the misalignment should conform to B ≤ 0.5mm. The internal welded joints are not lined, and the clearance should be equal to zero as much as possible, especially at the overhead welding position. The inner wall of the pipeline should be rounded by 1~1.5mm. 3.3 welding machine and other matters needing attention The welder must be an AC TIG welder with steep external characteristics and sufficient capacitance. Moreover, the parameters are stable, the adjustment is flexible, and the performance is safe and reliable. It should also have a device for arc starting, arc stabilizing and eliminating DC components. The current and voltage meter on the welding machine shall be verified by the metering department. Before using the welding machine, check whether the grounding is good, whether the cooling water and gas circuit are unblocked, and its various functions must be guaranteed to work normally. The welding place shall be kept clean. In addition to windproof, rainproof and snowproof facilities, relative humidity ≤ 80% and ambient temperature> 5 ℃ shall be ensured during welding.

12-21

Welding characteristics and methods of aluminum alloy tube

2022-12-21

铝合金因其重量轻、强度高、耐腐蚀性好、无磁性、成型性好、低温性能好等优点，被广泛应用于各种焊接结构产品中。 用铝合金代替钢板进行焊接，可使结构重量减轻50%以上。因此，铝及铝合金广泛应用于航空、航天、电气等领域，越来越多也用于石油化工。 使用铝镁合金(主要是:5083、5183、5A02相当于老品牌的LF2、LF4)。但铝及铝合金在焊接过程中容易氧化、气孔、热裂、烧穿和塌陷。这类材料是公认的难焊焊接材料，尤其是小直径薄壁管的焊接更难掌握。因此，解决铝及铝合金的这些焊接缺陷是施工过程中必须解决的问题。 铝及铝合金的理化性能及焊接特点      2.1 易氧化         铝和氧的亲和力很强。在常温下，铝表面就能被氧化成厚度约0.1～0.2 m致密的AL2O3薄膜。虽然这层氧化铝薄膜比较致密，能防止金属的继续氧化，对自然防腐有利，但它给焊接带来了困难，这是由于氧化铝的熔点(2050℃)远远超过了铝的熔点(600℃左右)，比重约为铝的1.4倍。在焊接过程中，会阻碍金属之间的熔合，易形成夹渣，而且氧化铝薄膜还吸附了较多的水份，焊接时会促使焊缝生成气孔。      2.2 较大的导热系数和比热容         铝的导热系数约为钢的四倍，因此，焊接铝材管时，比钢管焊接要消耗更多的热量，为得到高质量的焊接接头，必需采用能量集中，功率大的热源。     2.3 易形成氢气孔         铝及铝合金的焊接气孔主要氢气孔。铝在液态时能大量吸收和溶解氢，在熔融状态下溶解度为0.0069ml/g，而在高温凝固状态下为0.00036 ml/g，前后相差近20倍。铝的导热系数很大，在相同的焊接工艺条件下，其冷却速度为钢的4～7倍，使金属结晶加快，焊接熔池在快速冷却过程中，氢的溶解度急剧下降，此时析出大量过饱和气体，氢气来不及析出在焊缝金属中形成气孔。因此，在焊接铝材时，焊缝产生气孔的倾向很大。      2.4 易形成热裂纹      铝的线膨胀系数和结晶收缩率比钢大约一倍，易产生较大的焊接变形和应力，加上某些杂质或合金元素的不利影响，在刚性较大的接头中将导致产生裂纹。     2.5 烧穿和塌陷     铝及铝合金由固态转变为液态时.由于没有明显的颜色变化，所以，不易判断熔池的温度。焊接时，常因温度过高不易被察觉而导致烧穿或严重塌陷。

12-20

Introduction of several new and commonly used aluminum alloy brazing filler metals

2022-12-20

In the brazing process, the welds are joined together by solidification of the molten solder. Therefore, the quality of the weld depends to a large extent on the solder. Aluminum brazing filler metal is mainly aluminum silicon alloy. Sometimes copper, zinc, germanium and other elements are added to meet the requirements of process performance. After years of practice and exploration, several series of aluminum brazing filler metals have been formed, many of which have achieved satisfactory brazing results under proper process conditions. The following mainly introduces several new, commonly used aluminum alloy brazing filler metal. 3.1 aluminum silicon solder Aluminum-silicon brazing filler metal is based on aluminum-silicon eutectic composition, and also includes sub-eutectic, over-eutectic and aluminum-silicon alloy with added elements not higher than 5%. This series of brazing filler metal has good brazing strength, base metal color, plating and corrosion resistance, and is an excellent brazing filler metal. Moreover, this series of solders can be modified, which can greatly increase the toughness and bending performance of solders and solder joints. At present, a new type of aluminum-silicon alloy solder has been developed by using rapid solidification technology. Compared with the ordinary crystalline solder, the liquid phase point of the solder is about 3-5 ℃ lower than that of the ordinary crystalline solder of the same composition, the wetting coefficient is increased by 18%, the strength is increased by 28.4, and the fluctuation is small, and it has certain processing flexibility. 3.2 copper solder The copper is welded by the contact reaction brazing principle. At present, aluminum contact reaction brazing is considered as an ideal method to solve the problem of aluminum brazing. The invention has the following advantages:(1) no flux is added, no pollution to the environment, the brazing product does not need to be cleaned, and the brazing seam has no chemical corrosion; ② selecting a suitable eutectic reaction alloy layer can reduce the brazing temperature, which not only reduces The energy consumption makes the brazing process easy to control, and the equipment requirements are low. The contact reaction of Cu on the aluminum substrate has obvious preferential spreading on the surface, which can destroy the oxide film and facilitate the formation of a uniform liquid filling layer between the joint interface during the contact reaction brazing process. On the other hand, the contact reaction preferentially penetrates the grain boundary in the depth direction of the aluminum matrix, and the bonding strength of the brazed joint can be ensured. According to the data, the suitable process parameters for aluminum contact reaction brazing with Cu as the intermediate layer material are: brazing temperature 570-580 ℃, holding time 15-20min. However, the electrochemical corrosion resistance of the copper welded joint is poor, and the eutectic reaction layer is brittle. In order to improve the performance of single copper as a solder, some other elements can be added to it. Solder metals for contact reaction brazing with aluminum alloys include silver, nickel, silicon, zinc, titanium, and the like. 3.3 a composite layer of copper and zinc as a reactive solder In order to make up for the deficiency of zinc and copper alone as solder, a composite layer of them may be used. Copper and zinc composite layers are used for contact eutectic brazing. Peritectic reaction occurs at the Cu/Zn interface, and eutectic reaction occurs at the Cu/Al interface, forming a eutectic liquid phase that destroys the oxide film on the aluminum surface. As a reactive brazing filler metal for aluminum, the copper-zinc composite layer must have a suitable copper and zinc content. Some data show that when δ(Zn)= 0.2mm and δ (Cu) < 0.1mm, the brazing effect is ideal. At this time, the reaction layer can not only destroy the oxide film, but also the joint has strong electrochemical corrosion resistance and high shear strength. 3.4 aluminum silicon copper zinc solder The liquidus temperature of the solder is in the range of 500-577°C. After adding copper to the aluminum-silicon solder, the fluidity of the solder is obviously enhanced. Due to the high content of CuAl2 intermetallic compounds, this ternary eutectic solder is very brittle and is only suitable for casting into strips, but it is difficult to process into wires or foils. The wettability and fluidity of the solder are enhanced when zinc is added to the aluminum-silicon solder. The solubility of silicon decreases rapidly with increasing zinc concentration. Since this solder does not contain compounds, its thermal workability is better than that of the aluminum-silicon-copper system. 3.5 aluminum-copper-silver-zinc solder The liquidus temperature range of the solder is 400-500 ° C., which is close to the liquidus temperature range of the aluminum alloy solder. Al-Cu-Ag ternary eutectic composition makes its color close to that of Al base metal, and it has good brazing fluidity but is fragile. Another ternary system is aluminum-copper-zinc. The color of this solder is also close to the base metal, and the workpiece can be better. The corrosion resistance of the solder can be improved by adding 0.05-0.08%(mass fraction, the same below) of Mg and 0.05% of Ni or Cr into the solder. There are many ideal aluminum solders. In general, the melting point of most existing solders for aluminum is close to that of aluminum alloys. Therefore, the search for lower melting point, better process performance of the solder is the task faced by the welding workers.

12-19

Properties of flux for aluminium

2022-12-19

Aluminum is active, and its surface is easy to form a layer of dense and chemically stable oxides, which is one of the main obstacles to brazing aluminum and aluminum alloys. In order to obtain a high quality joint, the surface oxide must be removed. When brazing aluminum and its alloys, the use of brazing filler metal can remove the oxide film on the aluminum surface and reduce the interfacial tension between the brazing filler metal and the base metal. The brazing agent for aluminum is divided into soft brazing agent and hard brazing agent. Usually, the brazing flux used when the brazing temperature is higher than 450 ℃ is a brazing flux, and the brazing flux used when the brazing temperature is lower than 450 ℃ is a brazing flux. The following describes the rapid development of aluminum flux, especially Nocolok flux. The traditional aluminum flux is mainly chloride flux, generally based on lic 1-KCl or lic 1-KCl-na C1 system. This kind of flux has the advantages of high activity, stable heating and not easy to fail. Moreover, various heat sources can be used, which is convenient and inexpensive. The disadvantage of this kind of flux is that due to the presence of Cl ions, it has strong electrochemical corrosion to the base material, strong hygroscopicity, and the flux is not easy to store. Therefore, it is important to clean the residue when brazing is performed using such a flux. In the late 1970 s, a non-corrosive, insoluble flux developed rapidly. The flux is composed of α-KF eutectic and has little solubility in water. It avoids the shortcomings of chloride flux easy to absorb moisture, corrosion is also very small, so it is also called Nocolok flux. Characteristics of 4.1 fluxes Nokolok solder is a fine white powder whose main component is a mixture of potassium fluoroaluminate, which may contain crystal water. The molten solder will dissolve the oxide on the aluminum surface, preventing it from being oxidized again. Under the action of the flux, the solder can freely penetrate into the bonding surface by capillary action. After cooling, the flux forms a paste-like film with strong adhesion on the surface of the element. The flux residue layer is non-hygroscopic, non-corrosive, and insoluble in water-based solvents. Although the solubility of potassium fluoroaluminate flux in water is very small, but the thermal stability is not good, heating in the air will occur chemical reaction. Improvement and new progress of 4.2 noco lok solders In recent years, a large number of literatures have reported the improved Nocolok method, mainly including two aspects: one is to add a third or more salts to potassium fluoroaluminate solder to increase the activity of the solder and other properties; the other is to develop a new application method of potassium fluoroaluminate solder. Si can improve the activity of potassium fluoroaluminate solder, ideally in the form of K2SiF6, but to calculate the excess KF. When W (Si) > 2%, self-brazing is possible. K2GeF6, SnF2, znF2, and the like are added. In particular, K2GeF6 is preferred for improving the activity of the flux. In a Nocolok improvement, solder powder was mixed with this solder. There is also a method of brazing KA1F4 as a gas phase: one is to directly mix KA1F4 vapor brazing aluminum alloy under a low pressure oxygen-free atmosphere; the other is to vacuum deposit a layer of KA1F4 on the outside of the aluminum piece, and then assemble and braze as needed. The composite solder formed by depositing a layer of KA1F4 solder on the surface of the Al-Si eutectic solder powder can be mixed with an organic solvent to form a solder paste.

12-18

There are many welding methods used in brazing of aluminum and aluminum alloys.

2022-12-18

Induction brazing, vacuum brazing and protective atmosphere furnace brazing are widely used in the brazing of aluminum and aluminum alloys due to their unique and excellent processes, and their development is relatively fast. Focus on them below. 2.1 induction brazing Compared with flame brazing, induction brazing has obvious advantages, such as small deformation of base metal, environmental protection, easy operation, automation, energy saving and emission reduction, low investment cost, high cost performance, and low requirements for employee operation skills. In recent years, some domestic induction brazing equipment manufacturers (such as Bairui Ship Welding Technology Co., Ltd.) have fully applied induction brazing to the brazing of automotive air-conditioning accessories, such as aluminum pipe joints, aluminum pressure plates, aluminum water cooling belts, and aluminum water Cooling plate, condenser and evaporator inlet and outlet pipes, etc. Here are some aluminum alloy brazing samples using induction brazing: 2.2 vacuum brazing Aluminum is more active, easy to form a dense oxide film on its surface. When brazing, it is difficult to remove oxides only by vacuum conditions, and some metal activators, such as Mg and Bi, must be used at the same time. Generally speaking, the mechanism of activator stripping is: on the one hand, the activator reacts with residual O and HO in vacuum to eliminate their harmful effects on aluminum brazing; on the other hand, magnesium vapor penetrates into the lower surface layer of the film and forms a low melting point aluminum-silicon-magnesium alloy together with the diffused silicon. During brazing, the alloy melts and destroys the combination of the oxide film and the base metal, so that the molten solder wets the base metal and spreads on the base metal under the film, so that the surface film is floated and removed. When vacuum brazing aluminum alloy, the vacuum furnace should be selected according to productivity, cost, weldment size and structure. Before brazing, weldments shall be carefully cleaned. The surface oxides can be washed off with acid or alkali. If there is oil on the surface, it can be wiped with alcohol. Generally, the solder is polished with sandpaper to remove the oxide film on the surface, and then the oil is wiped off with alcohol. For larger workpieces, preheating is performed before welding to ensure that all parts of the weldment are evenly heated before reaching the brazing temperature. As the aluminum alloy vacuum brazing mainly depends on Mg activator stripping, for the complex structure of the weldment, in order to ensure that the base metal is fully affected by the Mg vapor, some domestic units have taken the supplementary process measures of local shielding, and achieved good results. The common method is to put the workpiece into a stainless steel cover, put the Mg chip, and then put it into a vacuum brazing furnace for brazing, which can greatly improve the brazing quality. Vacuum degree is the most important and the most difficult to control process parameters in vacuum brazing. To obtain a high-quality joint, it depends to a large extent on the degree of vacuum. According to the experience of some workers for many years, if the brazing equipment is not used for a long time, the vacuum furnace will run for several hours before it can be used. When using, especially in mass production, the time interval between the two uses should be shortened as much as possible, so that the vacuum degree of the vacuum furnace can meet the requirements easily and quickly. Vacuum brazing is an excellent brazing method, but it also has some shortcomings, such as complex and expensive equipment, difficult maintenance technology of vacuum system, etc. The development of aluminum alloy vacuum brazing in China has begun to take shape. The difficulty and key of future development is to develop low melting point solder with low melting point, high mechanical properties and corrosion resistance. Brazing in 2.3 protective atmosphere Due to the expensive equipment and complicated process, the vacuum brazing application of aluminum is limited. To solve this problem, we can replace the vacuum with a neutral atmosphere. This reduces the requirements on the leakage rate of the system, the equipment is simpler, the equipment maintenance problems caused by the deposition of volatile elements are reduced, and the production cost is lower. The heating method mainly relies on electric current, and the heating speed is fast and uniform, which can not only ensure product quality but also improve productivity. In recent years, neutral gas shielded aluminum brazing has been paid attention to and developed rapidly, which is a promising aluminum brazing method. The stripping mechanism of gas shielded brazing of aluminum alloy is the same as that of vacuum brazing of aluminum alloy, mainly by Mg activator. I won't repeat it here. The quality of brazing can be improved by adding bismuth to the brazing filler metal. The gas shielded brazing of aluminum alloy generally uses argon and pure nitrogen, and the purity of nitrogen must be greater than 99.99%. It is reported that for aluminum/aluminum, aluminum/copper joints, there is a connection method using the principle of diffusion brazing. The mixed powder composed of Si and aluminum potassium fluoride flux was sprayed on the aluminum surface for brazing under nitrogen atmosphere near atmospheric pressure, and the effect was satisfactory. The Si may be replaced by a metal such as Cu, Ge, Zn, etc. It forms a eutectic with the low melting point al.

12-17

Brazing of aluminium and aluminium alloys

2022-12-17

Aluminum and aluminum alloys can be brazed by induction brazing, flame brazing, furnace brazing, and salt bath brazing. Flame brazing has the advantages of simple equipment, wide gas source, great flexibility and wide application. Mainly used for brazing small weldments and single piece production. There are many types of flame that can be used. It is reported that China has cooperated with other countries to produce a sharp gas between liquefied gas and oxyacetylene. This gas has a soft flame and an intensity between liquefied gas and oxyacetylene. It is a good heat source for aluminum brazing. However, compared with other connection methods, the heating temperature of aluminum and aluminum alloy flame brazing is difficult to master, and the experience of the operator is required. Salt brazing has the advantages of rapid and uniform heating, difficult deformation of weldments, sufficient film removal, etc., so the weldments are of good quality and high production efficiency. Especially suitable for mass production, especially suitable for welding structure dense brazing seam. Typically, a paste, foil-like solder or solder coating is used for aluminum salt bath brazing, and the solder coating is a Al-Si eutectic composition or a Al-Si hypoeutectic composition. At present, brazing coating is mostly used in brazing production, which can not only improve production efficiency but also ensure brazing quality. Its disadvantages: First, because the heating of the workpiece and the removal of the oxide film are carried out by molten salt, it is difficult to enter and discharge salt for the workpiece with complex structure, which limits the structural design and process, and complicates it. The welding quality is also difficult to guarantee. Secondly, due to the specific use environment and service life requirements, some products have higher requirements for corrosion resistance, and a large amount of flux remains in the workpiece after salt bath brazing, which requires a longer cleaning time. In addition, salt bath brazing equipment investment, complex process, long production cycle. Air brazing has the advantages of less equipment investment, simple brazing process and convenient operation. However, this method has a slow heating speed, heating in the air, the surface of the workpiece is easy to oxidize, especially when the temperature is high, which is not conducive to the removal of the solder film. Moreover, during the heating process, the solder will fail due to moisture in the air. In view of this situation, dry air furnace brazing was developed.

12-16

Research Status of Aluminum and Aluminum Alloy Brazing

2022-12-16

Aluminum alloy has the advantages of low density, high strength and corrosion resistance, so it is widely used in automobile, high-speed railway vehicle, aerospace and military industry. Due to its unique physical and chemical properties, a series of difficulties will be encountered in the welding process, such as oxidation, thermal cracks and pores in the weld. For aluminum alloy welding, the traditional method is mainly fusion welding, the equipment is complex, and the technical requirements for welders are strict. As an important method of aluminum alloy connection, aluminum brazing has the characteristics of small deformation of brazed parts. The advantages of high dimensional accuracy have been widely used in China in recent years. The brazing technology of aluminum and aluminum alloys is a hot topic in recent years. With the continuous emergence of new materials and new methods, the brazing process of aluminum and aluminum alloys has developed rapidly, and the brazing methods, brazing materials and fluxes have made great progress. Aluminum and aluminum alloy brazing is one of the fields that have been studied and developed rapidly in recent years. This is mainly because it has a series of excellent properties, such as high strength, good corrosion resistance, high electrical conductivity and thermal conductivity, so it is widely used in aerospace, aviation, electronics, metallurgy, machinery manufacturing and light industry and other fields. Especially with the sharp rise in the price of copper, in order to reduce the quality, improve the efficacy and enhance the appearance, the technology of replacing copper with aluminum and steel with aluminum has been successfully applied in some fields. The most typical example is that the copper water tank of the car has been replaced by an aluminum water tank. There are few domestic manufacturers of large-scale production of aluminum flux, and most of the aluminum flux currently used is imported from abroad. Due to the low melting point of aluminum and aluminum alloy, strong chemical activity, high melting point, stable oxide film, can be firmly and densely attached to the surface of aluminum or aluminum alloy, the general brazing agent can not meet the requirements of brazing aluminum and aluminum alloy, must use special brazing agent-aluminum and aluminum alloy brazing agent. In addition, the corrosion resistance of aluminum and its alloy brazing joints is easily affected by the brazing filler metal and flux, which is mainly due to the large difference between the electrode potential of the brazing filler metal and the base metal, which reduces the corrosion resistance of the joint, especially the effect on the brazing joint is more obvious. In general, in order to remove the oxide film on the surface of aluminum and its alloys, most of the solder is filled with highly corrosive materials. Even if these materials are cleaned after brazing, it is difficult to completely remove the influence on the corrosion resistance of the joint.

12-15

SMD components repair process

2022-12-15

The repair process can be divided into flux coating method, solder bridging method and solder surrounding method according to the material; according to the size of the component removal force, it can be divided into surface tension method, tweezers method and vacuum method. The first step in the repair process is non-destructive welding. Tables 2 to 6 show the Welding tools and processes for repairing SMD components, QFP, BGA, LCCC and other major surface mount components, respectively. After successful removal, the liner needs to be cleaned. The preparation of the substrate pads is very important before reassembling the components on the old pads. During this process, care was taken to avoid thermal and mechanical damage to the liner. Pad preparation includes: (1) Removal of old solder. Can use soldering iron, tin absorption soldering iron, copper braid, special soldering iron, etc. (2) Clean the pad surface. It is mainly to remove flux residue, as well as alcohol, brushes, etc. can be used; (3) Add new solder. This step can also be regarded as the step of mounting components, mainly pre-plating solder on the pads. The method may use a soldering iron, a welding wire, and a solder paste. 37 shows a process comparison for removing old solder when repaired with a copper braid. Table 7 shows several common pad cleaning methods. Table 8 lists several common methods of soldering and surface tinning. After the surface of the backing plate is cleaned and leveled, it can be repaired and assembled according to the component assembly process described herein. It is worth noting that when QFP is repaired and disassembled, if there is no special repair table, you can also refer to the soldering iron repair process shown in Figure 38, but it will cause great damage to the pins of the components and is generally not recommended. When repairing BGA, it is necessary to simulate the BGA welding process, dry the PCBA, remove the moisture that may invade, prevent pitting or delamination, and set the preheating melting curve. Notably, some fine-pitch elements are easily bridged during the repair process. Soldering iron common removal methods are drag and diffusion method. The mechanism is that the attraction of the solder head to the solder must be greater than the attraction of the pin to the solder. The temperature of the drag soldering iron is generally 280°C, and the temperature of the diffusion soldering iron is generally 315°C. Table 9 shows the method and procedure for bridging the airfoil pin and the J-pin.

12-14

Desoldering process of in-line components

2022-12-14

The desoldering process for the flat/cylindrical in-line assembly is shown in Figure 35. First, clean the pad of contamination, such as oil, oxide, flux residue, etc., Then adjust the temperature of the soldering iron to about 315 ℃, install the heat-driven soldering head with a special tool, wipe the soldering iron mouth with a sponge, and apply solder. Then, gently touch the pad to be cleaned with a soldering iron. After the solder melts, use a soldering iron to suck off the excess solder. Leave the soldering iron pad and work in a vacuum for 2 seconds to ensure that all molten solder is sucked into the soldering iron cavity. Repeat the tin suction operation to thoroughly clean all gaskets. The above method is applicable to vertical pins, partially bent pins, and fully bent pins. For bent pins, a straightening process should be added. In addition to tin absorption, tin absorption can also be removed with tin absorption tape. It is necessary to maintain and clean the welding head. Careful maintenance of the welding head can ensure good heat conduction at the lowest temperature and complete the welding. This not only reduces the damage to the surrounding materials, but also greatly extends the service life of the soldering iron. After removing the soldering iron, re-tin the soldering tip and put it back in place, and clean the soldering pad. The soldering iron needs to be cleaned regularly, as shown in Figure 36. It is worth noting that after cleaning the "S" plug, brush a layer of mineral oil on the surface to prevent tin.