There are many welding methods for aluminum alloy, and each method has its own application. In addition to the traditional fusion, resistance and gas welding methods, other welding methods (such as Plasma arc welding, electron beam welding, vacuum diffusion welding, etc.) can also be easily welded together. 1. The characteristics and application scope of commonly used welding methods for aluminium alloys are shown in Table 1. Should be based on aluminum and aluminum alloy brand, welding piece thickness, product structure and weldability requirements and so on.

(1) gas welding oxygen-acetylene gas welding flame low thermal power, heat is more dispersed, so welding deformation, low productivity. It is necessary to preheat welding when welding thick aluminum weldment by gas welding. After Welding, the weld metal is not only coarse in grain and loose in structure, but also easy to produce defects such as alumina inclusion, porosity and crack. This method is only used for welding of non-important aluminum structures and castings with thickness ranging from 0.5 mm to 10 mm. 

(2) Argon Tungsten Arc welding is a kind of welding method under the protection of Argon Gas. The heat is concentrated, the ARC combustion is stable, the weld metal is compact, the strength and plasticity of the welded joint are high, and it is widely used in industry. Tig Welding of aluminum alloy is a perfect welding method, but the equipment of Tig welding is complex, so it is not suitable to operate in the open air. 

(3) the automatic and semi-automatic mig welding has the advantages of high arc power, concentrated heat and small heat-affected zone. THE PRODUCTION EFFICIENCY IS 2 ~ 3 times higher than that of manual mig welding. Can Be welded thickness of 50 mm below the pure aluminum and aluminum alloy plate. For example, the aluminum plate with thickness of 30 mm need not be preheated, only the positive and negative layers can be welded to obtain smooth surface and good quality weld. Semi-automatic mig welding is suitable for locating weld, intermittent short weld and welding parts with irregular structure. semi-automatic tig welding torch can be used for easy and flexible welding, but the wire diameter of semi-automatic welding is small, the porosity sensitivity of the weld is high.

 (4) PULSED ARGON ARC welding

 (1) tig welding with this method can obviously improve the stability of low current welding process, and is convenient to control the arc power and weld shape by adjusting various process parameters. It is especially suitable for welding thin plate, all-position welding and welding of wrought aluminum, Duralumin and super-duralumin, which are sensitive to heat. 

2) the average welding current is small, the parameter adjustment range is large, the deformation and heat-affected zone of the welding piece is small, the productivity is high, the resistance to porosity and crack is good, suitable for all-position welding of aluminum alloy sheet with thickness of 2 ~ 10 mm. 

(5) resistance spot welding and seam welding can be used to weld aluminum alloy sheets with thickness less than 4 mm. For high quality products can be used DC shock wave spot welding, seam welding machine welding. Welding needs to use more complex equipment, welding current, high productivity, especially suitable for mass production parts. (6) friction stir welding friction stir welding is a solid state bonding technique used in the welding of various alloy plates. Compared with the traditional fusion welding method, the friction stir welding has no spatter, no smoke and dust, no need to add welding wire and protective gas, and the joint has no blowhole and crack. Compared with ordinary friction, it is not limited by shaft parts, can be welded directly. This welding method has a series of other advantages, such as good mechanical properties of joints, energy-saving, pollution-free, pre-welding preparation requirements are low. Aluminum and its alloys are more suitable for friction stir welding because of their low melting point.

 2. Welding materials for aluminum

 (1) when welding aluminum alloy with gas welding and tig welding, it is necessary to add filler wire. Aluminum and aluminum alloy welding wire is divided into homogeneous welding wire and heterogeneous welding wire. In order to get a good welding joint, the suitable welding wire for base metal should be chosen as filling material according to the requirement of welding components. The choice of welding wire should first consider the composition of the weld, but also consider the mechanical properties of products, corrosion resistance, structural rigidity, color and crack resistance. The tendency of intergranular crack in Haz can be greatly reduced by selecting filler metal whose melting temperature is lower than that of base metal. For the welded joints of non-heat-treated alloys, the strength increases in the order of 1000 series, 4000 series and 5000 series. 5000 series welding wires containing more than 3% magnesium should be avoided in structures with service temperatures above 65 °C, as these alloys are very sensitive to stress corrosion cracking, which can occur at the above temperatures and corrosive environments. The tendency to crack in the weld metal is usually prevented by using a filler metal with a higher alloy content than the base metal. At present, most of the commonly used welding wires for aluminum alloys are standard grade welding wires with similar composition to the base metal. In the absence of standard grade welding wire, can be cut from the base metal strip instead. The more general welding wire is HS311, this welding wire liquid metal fluidity, solidification shrinkage rate is small, specific excellent crack resistance. In order to refine the weld grain and improve the crack resistance and Mechanical Properties of the Weld, a small amount of alloy elements such as TI, V and Zr are usually added to the wire as modifiers.

3. The following problems should be paid attention to when choosing aluminum alloy welding wire: 

1) the direct factor that influences the crack sensitivity of welded joint is the match of base metal and welding wire. Using the weld metal whose melting temperature is lower than the base metal can reduce the crack sensitivity of the weld metal and heat affected zone. For example, when the 6061 alloy with 0.6% SI content is welded, the same alloy is chosen as the weld and the crack sensitivity is very high, but the ER4043 welding wire with 5% SI content has good crack resistance because its melting temperature is lower than that of 6061 alloy and it has higher plasticity in cooling process. In addition, weld metals avoid the combination of magnesium and copper because Al-Mg-Cu has a high crack sensitivity. 

2) the Mechanical Properties of welded joints are the lowest in strength of industrial pure aluminum, the middle of 4000 series and the highest in strength of 5000 series. Although al-si welding wire has higher crack resistance, but the plasticity of si-containing welding wire is poor, so we should avoid using si-containing welding wire for the joints that need plastic deformation after welding. 

3) the selection of filler metal for welded joints depends not only on the composition of the base metal, but also on the geometry of the joints, the corrosion resistance in operation and the appearance of the welded parts. For example, in order to provide good corrosion resistance or to prevent contamination from stored products, welded containers containing hydrogen peroxide require high-purity aluminum alloys. In this case, the filler metal must be at least as pure as the parent material. 

(2) type, specification and application of electrode for aluminium alloy are shown in Table 2. The chemical composition and mechanical properties of aluminum alloy electrodes are shown in table 3.

(3) the inert gases for protecting gas welding aluminum alloys are Argon and helium. Ar > 99.9% , oxygen < 0.005% , hydrogen < 0.005% , moisture < 0.02 mg/l, nitrogen < 0.015% . The increase of oxygen and nitrogen aggravates the CATHODIC atomization. When oxygen is more than 0.3% , the tungsten electrode will burn more severely, and when oxygen is more than 0.1% , the weld surface will be dull or black. For Tig welding, pure argon gas is used for AC and high frequency welding, while Ar + He or pure Ar is used for DC positive polarity welding. Pure Ar is used when the thickness of the plate is less than 25 mm. When the plate thickness is 25 ~ 50 mm, AR + He gas mixture with 10% ~ 35% Ar is used. When the plate thickness is 50 ~ 75 mm, AR + He mixture gas with 10% ~ 35% or 50% He should be used. When the plate thickness is more than 75 mm, it is recommended to add 50% ~ 75% He Ar + He gas mixture.

4. Welding process of aluminum alloy 01. The oxygen-acetylene gas welding of aluminum alloy has low thermal efficiency, and the heat input is not concentrated. Flux should be used to weld aluminum and aluminum alloy, and the residue should be removed after welding. Because gas welding equipment is simple, no power source, easy to operate, often used for welding on the quality of aluminum alloy components, such as thin plate and small parts, as well as repair welding aluminum alloy components and aluminum castings. (1) it is not suitable to use lap joint and t-joint when welding aluminum alloy with gas welding joint. It is difficult to clean up the residual flux and slag flowing into the seam. In order to ensure that the weldment can be welded through without collapse and burn-through, the backing plate with groove can be used. The backing plate is usually made of stainless steel or pure copper. (2) in order to make the welding process smoothly and ensure the quality of welding seam, it is necessary to add flux to remove the oxide film and other impurity on the aluminum surface. Gas Welding flux (also known as gas flux) is the flux during gas welding, its main role is to remove the oxide film formed on the surface of aluminum during gas welding, improve the wettability of base metal, and promote the formation of dense weld microstructure. Gas Welding aluminum alloy must use flux, usually in the flux directly before welding on the workpiece bevelled, or stick on the wire to join the pool. Aluminum Alloy flux is a kind of chloride salt of potassium, sodium, calcium, lithium and so on. For example, aluminium cryolite (Na3AlF6) can melt alumina at 1000 °C, and refractory alumina can be transformed into fusible aluminium chloride such as potassium chloride. The flux has the advantages of low melting point, good fluidity, and can improve the fluidity of the molten metal, so that the weld is well formed. (3) the choice of nozzle and flame aluminum alloy has strong oxidation and gettering. During gas welding, in order to avoid oxidation of aluminum, neutral flame or weak carbonization flame should be used to protect the aluminum molten pool from oxidation under reducing atmosphere. Do not use oxidation flame, because with strong oxidation flame will make aluminum strong oxidation, hinder the welding process; and excessive acetylene, free hydrogen may dissolve into the pool, will promote the formation of porosity, make the weld loose. (4) to prevent the size and relative position of weldment from changing during welding, spot welding is needed before welding. Because of the large coefficient of linear expansion of aluminum, high heat conduction speed, gas welding heating area, therefore, the positioning weld should be closer than steel. The filler wire used for positioning welding is the same as that used for product welding. Before positioning welding, a layer of gas flux should be applied in the gap of welding seam. The flame power of positioning welding is slightly larger than that of gas welding. (5) when gas welding is used to weld iron and steel materials, the heating temperature can be judged by the color change of the steel. But when it comes to welding aluminum, there is no such convenience. Because the aluminum alloy from room temperature heating to the melting process of no significant change in color, to the operator to control the temperature of the welding difficult. However, the welding time can be controlled according to the following phenomena: 1) when the surface of the work piece being heated changes from bright white to dull silvery white, the surface oxide film is wrinkled and the metal at the heating place fluctuates, indicating that the melting temperature is about to be reached, the welding can be carried out; 2) when the end of the welding wire with the flux and the heated place can be dipped, the welding wire and the base metal can be fused, and the melting temperature can be reached, and the welding can be carried out; 3) when the edges of the base metal edge fall down, the base metal can reach the melting temperature and can be welded. The left welding method can be used for gas welding thin plate, and the welding wire is located before the welding flame. This welding method can point the flame at the unwelded cold metal and lose part of the heat, which is helpful to prevent the overheating of the Molten Pool and the metal grain growth and burn-through in the heat affected zone. The right welding method can be used when the thickness of base metal is more than 5 mm. The welding wire is behind the welding torch, the flame points to the weld, the heat loss is small, the penetration is deep and the heating efficiency is high. When gas welding thickness is less than 3 mm, the welding torch inclination angle is 20 ~ 40 ° , when gas welding thick piece, the welding torch inclination angle is 40 ~ 80 ° , the welding wire and welding torch angle is 80 ~ 100 ° . Aluminum alloy gas welding joints should be welded once, not deposited on the second layer, because the second layer will cause weld slag, etc. .

(6) the corrosion of aluminum joint caused by residual flux and slag on the surface of gas welding seam after welding is one of the reasons for the damage of aluminum joint. Within 1 ~ 6 hours after gas welding, residual flux and slag should be cleaned to prevent corrosion of welding parts. The cleaning procedure after welding is as follows. 1) after welding, soak the welding parts in the hot water tank at 40 ~ 50 °C. it is better to use flowing hot water, brush the weld seam with a hard brush and the place where the flux and slag remain near the weld, until it is clean. 2) immerse the solder in a nitric acid solution. When the room temperature is above 25 ° , the concentration of the solution is 15% ~ 25% and the immersion time is 10 ~ 15 min. When the room temperature is 10 ~ 15 °C, the concentration of the solution is 20% ~ 25% and the immersion time is 15 min. 3) soak the weldment in a bath of flowing hot water (temperature 40 ~ 50 °C) for 5 ~ 10 Min. 4) rinse the weldment with cold water for 5 min. 5) let the weldment dry naturally, or put it in a drying box to dry or blow dry with hot air. 2 Tig welding of aluminum alloy, also called Tungsten Inert Gas Metal Arc welding, uses the heat generated by the ARC formed between the tungsten electrode and the workpiece to melt the place to be welded. Argon arc welding of aluminum takes advantage of its "cathode atomization" characteristic to remove oxide film by itself. The Tungsten electrode and the seam area are shielded by the inert gas ejected from the nozzle to prevent the reaction between the seam area and the surrounding air.

The TIG welding process is most suitable for welding thin plate with thickness less than 3 mm, and the deformation of workpiece is obviously smaller than that of gas welding and manual arc welding. The Cathode of AC TIG welding can remove oxide film without flux and avoid the corrosion of residual flux and slag after welding. The joint type can be unrestricted, the weld is well formed and the surface is bright. The flushing of Argon flow on the welding zone 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 requirement of cleaning before welding is stricter than other welding methods. AC Tig welding and AC pulse TIG welding are the most suitable methods for aluminum alloy welding, followed by DC REVERSE TIG welding. In general, AC welding of aluminum alloy can achieve the best coordination in current carrying capacity, ARC controllability and arc cleaning function, so most of aluminum alloy TIG welding uses AC power source. When DC positive connection (electrode connected with negative electrode) is used, heat is generated on the surface of the workpiece and deep penetration is formed. Even the thick section does not need to be preheated, and the base metal hardly deforms. Although it is seldom used to weld aluminum by DC REVERSE TIG welding (electrode with positive electrode) , however, this method has many advantages such as low penetration, easy control of ARC and good cleaning effect when welding thin-wall heat exchanger or similar parts with pipe thickness less than 2.4 mm. (1) tungsten has a melting point of 3410 °C and is the metal with the highest melting point. Tungsten has a strong electron emission ability at high temperature. When the tungsten electrode is doped with trace rare earth elements such as thorium, cerium, zirconium, etc. . Tungsten electrode plays the role of conducting current, igniting arc and keeping arc burning normally in TIG welding of aluminum alloy. The commonly used tungsten electrode materials include pure tungsten, thorium tungsten and cerium tungsten. (2) welding process parameters in order to obtain excellent weld shape and welding quality, welding process parameters should be selected reasonably according to the technical requirements of welding parts. The main technological parameters of aluminum alloy manual TIG welding are current type, polarity and current, protective gas flux, tungsten pole length, distance from nozzle to workpiece, etc. . The technological parameters of automatic TIG welding also include arc voltage (arc length) , welding speed and wire feeding speed, etc. . According to the material and thickness to be welded, the diameter and shape of tungsten electrode, diameter of welding wire, shielding gas and flux, diameter of nozzle, welding current, arc voltage and welding speed are determined, then adjust the parameters according to the actual welding effect until they meet the use requirements. 5. The main points of selecting technological parameters for TIG welding of aluminum alloy are as follows: 1) The nozzle aperture and the nozzle aperture of TIG welding of aluminum alloy are 5ー22 mm, and the shielding gas flux is 5ー15 L/min. 2) the extension length of Tungsten electrode and the distance from nozzle to workpiece: 5 ~ 6 mm for Butt Weld and 7 ~ 8 mm for fillet weld. The distance from the nozzle to the workpiece should be about 10 mm. 3) welding current and welding voltage are related to plate thickness, joint type, welding position and welder's technical level. For Manual Tig welding, the maximum welding current can be determined according to the Formula I = (60 ~ 65) D when the welding thickness is less than 6 mm. The ARC voltage is mainly determined by the ARC length. It is reasonable to make the arc length approximately equal to the diameter of Tungsten electrode. 4) the welding speed of aluminum alloy TIG welding should be faster in order to reduce the deformation. Manual Tig welding is usually used to adjust the welding speed according to the size of pool, the shape of pool and the fusion of two sides. The general welding speed is 8 ~ 12m/h. 5) the diameter of welding wire is determined by the thickness of plate and welding current, and the diameter of welding wire is proportional to them.

1) the cause of gas hole is low purity of Argon gas or water and gas leakage in the Argon Pipeline; the welding wire or base metal groove is not cleaned before welding or is contaminated by dirt and water after being cleaned; the welding current and welding speed are too large or too small; the Weld Pool Protection is not good, the ARC is unstable, the ARC is too long, the tungsten pole extends too long, etc. . Preventive measures to ensure the Argon pipeline, the choice of careful cleaning welding wire, welding parts, cleaning in time after welding, and to prevent re-pollution. Update the gas supply pipeline, select the appropriate gas flow, adjust the tungsten extension length, correct selection of welding process parameters. When necessary, can take the preheating process, welding the field installation of wind-proof device to prevent the field wind flow. 2) improper selection of Alloy composition of welding wire; when the content of Magnesium in the weld is less than 3% , or the content of iron and silicon impurities exceeds the regulation, the crack tendency increases; when the melting temperature of welding wire is on the high side, it will cause liquefaction crack in heat-affected zone;. The composition of the welding wire should match the base metal, the ARC plate should be added or the current attenuator should be used to fill the ARC crater, the welding structure should be designed correctly, the welding seam should be arranged reasonably, the welding seam should avoid the stress concentration as far as possible, the suitable welding sequence should be selected, the welding current should be decreased or the welding speed should be increased properly.

3) The reason of incomplete penetration is that the welding speed is too fast, the arc length is too long, the gap, groove angle, welding current are too small, the obtuse edge is too large, the burr on the edge of groove and the dirt on the bottom edge of workpiece are not removed before welding, and the inclination angle of Welding Torch and welding wire is not correct. Preventive measures include proper selection of clearance, Blunt Edge, groove angle and welding process parameters, strengthening the cleaning of oxide film, flux, slag and oil, and improving operating skills. 4) the reason of Tungsten inclusion in welding seam is caused by contact arc starting, the unreasonable shape of Tungsten electrode end and welding current make the tip fall off, the filling wire touches the tip of hot tungsten electrode and the wrong use of oxidizing gas. Measures to prevent arc ignition by high-frequency and high-voltage pulse; reasonable tungsten tip shape according to the selected current; reducing welding current, increasing tungsten diameter, shortening tungsten extension length; renewing inert gas; improving operating skills, do not make wire filling contact with tungsten electrode, etc. . 5) the causes of undercut are too large welding current, too high arc voltage, uneven welding torch swing, too little wire filling and too fast welding speed. Preventive measures to reduce welding current and ARC voltage, keep uniform swing of welding torch, increase wire feeding speed or reduce welding speed. 6. The defects of Aluminum Alloy Castings can be repaired by Argon arc welding, but the effect of AC TIG welding is better. Sentence is too long, please supply a shorter sentence.

7. In conclusion, easy and low-cost TIG and MIG welding methods are usually used for welding and repair welding of aluminum alloys. When adopting new welding technologies such as high energy beam welding and friction stir welding welding, the problems such as alloy element burning loss, joint softening and welding deformation can be effectively avoided, in particular, friction stir welding solid phase connections are environmentally friendly. In order to avoid welding defects when conventional repair welding is used to repair defects in aluminum alloy castings, attention should be paid to cleaning up before welding, selecting proper wire filler and correct welding procedure specification, and AC TIG repair welding is usually used. In order to improve the repair welding quality of Aluminum Alloy Castings, special repair welding method can be used when casting defects are special and conditions are available. 

Source: Ablution Machinery

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