Welding method and material selection of aluminum alloy

Aluminum alloy welding process Gas welding of aluminum alloy Oxygen-acetylene welding thermal efficiency is low, the welding heat input is not concentrated, welding aluminum and aluminum alloy need to use flux, after welding and need to remove residue, joint quality and performance is not high. Because the gas welding equipment is simple, does not need the power supply, the operation is convenient and flexible, it is often used for welding the aluminum alloy components that do not require high quality, such as thin sheet and small parts, as well as the repair welding of aluminum alloy components and aluminum castings. (1) joint form of gas welding When gas welding aluminum alloy, lap joint and T joint should not be used, because it is difficult to clean up the residual flux and welding slag in the flow into the gap. Butt joint should be used as far as possible. In order to ensure that the welding of welding parts is not only weld penetration and collapse and burn through, can use the grooved pad, pad is generally made of stainless steel or pure copper, with the backing plate welding can get a good back forming, improve welding productivity. (2) the selection of gas welding flux Aluminum alloy gas welding, in order to make the welding process smoothly, to ensure the quality of the weld, the gas welding need to add flux to remove the aluminum surface oxide film and other impurities. Gas welding flux (also known as gas flux) is a flux during gas welding, the main role is to remove the oxide film generated on the surface of aluminum in the process of gas welding, improve the wettability of the base metal, and promote the acquisition of dense weld tissue. Aluminum alloy must be used in gas welding flux, generally before welding flux is directly spread on the groove of the workpiece to be welded, or stick on the welding wire to join the pool. Aluminum alloy flux is potassium, sodium, calcium, lithium and other elements of chlorine salt, is crushed after sifting and according to a certain proportion of powder compounds prepared. For example, aluminum cryolite (Na3AlF6) at 1000℃ can melt alumina, and such as potassium chloride can make refractory alumina into fusible aluminum chloride. This kind of flux has low melting point, good fluidity, and can also improve the fluidity of molten metal, so that the weld is well formed. (3) the selection of welding nozzle and flame Aluminum alloy has strong oxidizing and aspirating properties. During gas welding, in order to prevent the oxidation of aluminum, neutral flame or weak carbonization flame (excess carbonization flame of acetylene) should be used to make the aluminum molten pool under the protection of reducing atmosphere and not be oxidized. The use of oxide flame is strictly prohibited, because with a strong oxide flame will make aluminum strongly oxidized, hinder the welding process; And too much acetylene, free hydrogen may dissolve into the molten pool, will promote the seam to produce pores, so that the weld loose. (4) Positioning weld In order to prevent the size and relative position of the weldments from changing during welding, the weldments need to be fixed spot welded before welding. Because the linear expansion coefficient of aluminum is large, the heat conduction speed is fast, the gas welding heating area is large, therefore, the positioning weld seam should be denser than the steel part. The filler wire used for positioning welding is the same as that used for product welding. A layer of gas agent should be coated in the weld gap before positioning welding. The flame power of position welding is slightly greater than that of gas welding. (5) gas welding operation When welding steel materials, you can judge the temperature of heating from the color change of steel. But when you weld aluminum, you don’t have that convenience. It is difficult for the operator to control the welding temperature because there is no obvious change of color during the process of heating the aluminum alloy from room temperature to melting. But the welding time can be grasped according to the following phenomena: 1) when the surface of the heated workpiece from bright white to dim silver white, the surface oxide film wrinkled, heating metal fluctuation phenomenon, indicating that the melting temperature is about to reach, can be welded; 2) with the flux of the welding wire end and the heating place, the welding wire and the base metal can be fused, that is, to reach the melting temperature, can be welded; 3) when the base metal edge falls down, the base metal reaches the melting temperature and can be welded. The left welding method can be used for the gas welding sheet, and the welding wire is located before the welding flame. This welding method is conducive to preventing the overheating of the welding pool, the growth of metal grains in the heat-affected zone and the burning through of the heat loss due to the flame pointing to the unwelded cold metal. Base material thickness is greater than 5 mm can be used right welding method, this method of welding wire behind the torch, the flame pointing to the weld, small heat loss, deep, high heating efficiency. When the thickness of gas welding is less than 3 mm thin parts, the welding torch inclination is 20 ~ 40°; For gas welding of thick parts, the inclination of welding torch is 40 ~ 80°, and the Angle between welding wire and welding torch is 80 ~ 100°. Aluminum alloy gas welding should be welded into a joint as far as possible, not piling up the second layer, because piling up the second layer will cause slag inclusion in the weld. (6) Post-welding treatment The corrosion of aluminum joints caused by residual flux and slag on the surface of gas welding seam is one of the reasons for the damage of aluminum joints in future use. Within 1 ~ 6h after gas welding, the residual flux and slag should be cleaned away to prevent corrosion of the weldment. The post-welding cleaning procedure is as follows. 1) After welding, put the weldment into a hot water tank at 40 ~ 50℃ for dipping. It is best to brush the weld and the place where the residual flux and slag are near the weld with a stiff bristle brush with flowing hot water until it is cleaned. 2) Immerse the weldment in the nitric acid solution. When the room temperature is above 25°, the solution concentration is 15%-25%, and the impregnation time is 10-15min. At room temperature of 10-15 ℃, the concentration of the solution was 20%-25%, and the impregnation time was 15min. 3) Place the welds in a tank of running hot water (40 ~ 50℃) and impregnate them for 5 ~ 10min. 4) Wash the weldment with cold water for 5min. 5) The welding parts can be naturally dried, or dried in a drying oven or blown dry with hot air. Argon tungsten arc welding of 2 aluminum alloys (TIG welding) Also known as tungsten inert gas shielded arc welding, the use of tungsten electrode and the workpiece between the formation of a large amount of heat generated by the arc to melt the place to be welded, and then filled with welding wire to obtain a solid welding joint. The argon arc welding of aluminum can remove the oxide film by itself by using its “cathodic atomization” characteristic. The tungsten electrode and the seam area are shielded by inert gas from the nozzle to prevent reaction between the weld area and the surrounding air. TIG welding process is most suitable for welding thickness of less than 3 mm sheet, workpiece deformation is significantly less than gas welding and hand arc welding. The cathode of ac TIG welding has the cleaning effect of removing oxide film, and can avoid the corrosion of joint by residual flux and slag after welding. The joint form can be unrestricted, and the weld is well formed and the surface is bright. The erosion of the welding zone by argon gas flow accelerates the cooling of the joint, improves the structure and properties of the joint, and is suitable for all-position welding. Because no flux is used, the requirements for pre-weld cleaning are more stringent than those for other welding methods. The suitable welding process of aluminum alloy is AC TIG welding and AC pulse TIG welding, followed by DC reverse TIG welding. In general, AC welding of aluminum alloys can achieve the best fit in terms of current carrying capacity, arc control and arc cleaning effect, so most of the TIG welding of aluminum alloys using AC power. When the direct current connection is used (the electrode is connected to the negative electrode), the heat is generated on the surface of the workpiece to form a deep penetration. A larger welding current can be used for the electrode of a certain size. Even thick sections do not require preheating and the base metal is almost deformable. Although rarely used DC reverse connection (electrode positive) TIG welding method to weld aluminum, but this method in continuous welding or repair welding thin-wall heat exchanger, pipe thickness of 2.4 mm below similar components have the advantages of melting depth, arc easy to control, arc has good purification effect. (1) tungsten electrode Tungsten has the highest melting point of any metal, with a melting point of 3,410 ℃. Tungsten has a strong electron emission ability at high temperature. When the tungsten electrode is added with trace oxides of rare earth elements such as thorium, cerium, zirconium, etc., the electron emission work is significantly reduced and the current carrying capacity is significantly improved. During TIG welding of aluminum alloy, tungsten electrode as electrode mainly plays the role of conducting current, igniting arc and maintaining the normal combustion of arc. Commonly used tungsten electrode materials are pure tungsten, thorium tungsten and cerium tungsten, etc. (2) Welding process parameters In order to obtain excellent weld forming and welding quality, welding process parameters should be reasonably selected according to the technical requirements of welding parts. The main technological parameters of manual TIG welding of aluminum alloy include current type, polarity and current size, shielding gas flow rate, tungsten electrode extension length, the distance between nozzle and workpiece, etc. Automatic TIG welding process parameters also include arc voltage (arc length), welding speed and wire feeding speed, etc. Process parameters are based on the welding material and thickness, first determine the tungsten electrode diameter and shape, welding wire diameter, protective gas and flow, nozzle aperture, welding current, arc voltage and welding speed, and then according to the actual welding effect to adjust the relevant parameters, until meet the requirements of use. The key points of selection of TIG welding process parameters of aluminum alloy are as follows. 1) Nozzle aperture and protective gas flow The nozzle aperture of aluminum alloy TIG is 5 ~ 22㎜; The flow rate of protective gas is generally 5 ~ 15L/min. 2) tungsten electrode extension length and the distance between nozzle and workpiece Tungsten extension length: butt weld is generally 5 ~ 6㎜, fillet weld is generally 7 ~ 8㎜. The distance from the nozzle to the workpiece is generally about 10㎜ appropriate. 3) Welding current and welding voltage are related to plate thickness, joint type, welding position and welder’s technical level. Manual TIG welding, the use of AC power supply, welding thickness of less than 6㎜ aluminum alloy, the maximum welding current can be determined according to the electrode diameter D according to the formula I= (60 ~ 65) D. The arc voltage is mainly determined by the arc length and it is usually reasonable to make the arc length approximately equal to the tungsten diameter. 4) Welding speed In order to reduce deformation, faster welding speed should be adopted during TIG welding of aluminum alloy. Manual TIG welding is generally according to the welding pool size, pool shape and fusion on both sides of the welding speed at any time to adjust, the general welding speed is 8 ~ 12m/h; In automatic TIG welding, after the process parameters are set, the welding speed is generally unchanged during the welding process. 5) Wire diameter It is generally determined by plate thickness and welding current, and the diameter of the welding wire is proportional to the two. 1) Causes of stomata Argon gas is of low purity or there is water or air leakage in the argon gas pipeline; The welding wire or the base metal groove is not cleaned up before welding or is stained with dirt, water and so on after cleaning; Welding current and welding speed is too large or too small; Poor pool protection, arc instability, arc too long, tungsten electrode extended too long, etc. Prevention measures Ensure the argon gas pipeline, select carefully clean the welding wire, welding parts, after cleaning timely, and prevent re-pollution. Update the air delivery pipeline, select the appropriate gas flow rate, adjust the tungsten extension length; Correct selection of welding parameters. If necessary, preheating process can be adopted and windshield device can be installed at the welding site to prevent the wind from flowing at the site. 2) Causes of crack Improper selection of welding wire alloy composition; When the content of magnesium in the weld is less than 3%, or the content of iron and silicon impurities exceeds the requirements, the crack tendency increases. When the melting temperature of the wire is too high, the liquefaction crack in the heat-affected zone will be caused. The structure design is unreasonable, the welding seam is too concentrated or the temperature of the hot zone is too high, resulting in the joint binding stress is too large; High turbidity long residence time, tissue overheating; The crater is not filled, and there are cracks in the crater. Prevention measures The composition of the selected welding wire should match with the base metal; Add an arc plate or fill the arc pit with a current attenuation device; Correct design of welding structure, reasonable layout of welding seam, so that the welding seam as far as possible to avoid the stress concentration, choose the appropriate welding sequence; Reduce welding current or increase welding speed appropriately. 3) Causes of penetration failure Welding speed is too fast, arc length is too large, welding gap, groove Angle, welding current are too small, blunt edge is too large; The burr on the edge of the workpiece groove and the dirt on the bottom edge are not removed before welding; Incorrect Angle between welding torch and welding wire. Prevention measures Correct selection of clearance, blunt edge, groove Angle and welding process parameters; Strengthen the cleaning of oxide film, flux, slag and oil; Improve operation skills and so on. 4) Causes of tungsten in welds Contact arc caused by; The shape of the end of the tungsten electrode and the welding current are not selected properly, which makes the tip fall off. Wire filling touches the hot tungsten tip and misuse of oxidizing gas. Prevention measures High frequency and high voltage pulse arc initiating; According to the selected current, the reasonable shape of the tungsten tip is adopted. Reduce welding current, increase tungsten electrode diameter, shorten tungsten electrode extension length; Update inert gas; Improve operation skills, do not make wire and tungsten electrode contact. 5) Causes of underbite The welding current is too large, the arc voltage is too high, the welding torch swing is not uniform, the wire filling is too little, and the welding speed is too fast. Prevention measures Reduce the welding current and arc voltage, keep the welding torch swing uniform, appropriately increase the wire feeding speed or reduce the welding speed. Routine repair welding process of casting The defects of aluminum alloy castings can be remedied by argon arc welding, and the effect of AC TIG welding is better. Repair welding technology on casting defects repair welding, in addition to the above mentioned general practices such as pay attention to clean up before welding wire and workpiece to be welded parts, choose reasonable welding wire material, choose a short arc welding wire and small Angle to join way for welding points, such as, according to different defect types in practice there are many successful experience is worth reference, such as selection of small current welds as far as possible; The composition of welding wire alloy during repair welding is higher than that of base metal, so as to supplement the burned alloy during repair welding and keep the composition of welding seam consistent with that of base metal. For castings with crack defects before repair welding at both ends of the hole; During welding, the welding part should be heated first, and the wire should be filled by the left welding method to facilitate the observation of the melting situation of the weld. After the melting of the welding place, the wire should be filled to form a fully wetting pool. When defect size is larger in order to improve the efficiency of repair welding, can be in front of the conventional TIG welding will be a very thin layer of surfactant (hereinafter referred to as ATIG surfactant) coating in the surface of welding position, welding when the surfactant cause welding arc contraction or molten pool inside the metal flow pattern change, make the weld penetration increases, when the ac TIG welding for aluminum alloy, is a layer of Si in the weld surface coating O2 active agent to change the weld depth, reduce preheating procedures and reduce the difficulty of welding. conclusion The convenient and low-cost TIG and MIG argon arc welding methods are usually used for welding and repair of aluminum alloys. When new welding processes such as high-energy beam welding and friction stir welding are adopted, the burning of alloy elements, joint softening and welding deformation can be effectively avoided, especially the friction stir welding has the characteristics of green environmental protection as solid phase connection. When the conventional repair welding method is used for the defect repair welding of aluminum alloy castings, in order to avoid the welding defects, attention should be paid to the cleaning before welding, the selection of reasonable welding wire filler and the correct welding process specification, and the AC TIG repair welding is usually appropriate. In order to improve the repair welding quality of aluminum alloy castings, special repair welding method can be used in combination with practice when casting defects are special and conditions are available.铜焊丝,Copper Welding Rods,铝焊丝,aluminium welding wire,镍焊条,Nickel electrode,药皮焊条,Flux Coated Brazing Welding Rods