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Welding method and material selection of aluminum alloy

There are many welding methods for aluminum alloy, and each method has its own application. In addition to the traditional fusion welding, resistance welding and gas welding methods, other welding methods (such as plasma arc welding, electron beam welding, vacuum diffusion welding, etc.) can also easily weld aluminum alloys together.

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Common welding methods of aluminum alloy

The welding method of aluminum alloy should be selected according to the grade of aluminum and aluminum alloy, the thickness of weldment, product structure and the requirements for weldability.

Gas welding

The thermal power of oxygen acetylene welding flame is low and the heat is dispersed, so the weldment deformation is large and the productivity is low. Preheating is needed when gas welding thick aluminum weldment. The weld metal after welding is not only coarse grain and loose structure, but also easy to produce defects such as alumina inclusion, porosity and cracks. This method is only used for the welding repair of unimportant aluminum structural parts and castings with thickness ranging from 0.5 to 10 mm.

TIG welding

This method is welding under the protection of argon, the heat is relatively concentrated, the arc combustion is stable, the weld metal is dense, and the strength and plasticity of the welded joint are high, so it is widely used in industry. TIG welding is a perfect welding method for aluminum alloy, but the equipment of TIG welding is complex, so it is not suitable for outdoor operation.
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MIG welding

Automatic and semi-automatic TIG arc welding has the advantages of high arc power, concentrated heat and small heat affected zone. The production efficiency is 2-3 times higher than manual TIG arc welding. It can weld pure aluminum and aluminum alloy plates with thickness less than 50 mm. For example, it is not necessary to preheat the aluminum plate with welding thickness of 30mm, only welding the positive and negative layers can obtain smooth surface and excellent quality weld. Semi automatic TIG welding is suitable for positioning weld, intermittent short weld and irregular structure and shape of weldment. Semi automatic TIG welding torch can be used for welding conveniently and flexibly, but the diameter of semi-automatic welding wire is smaller and the porosity sensitivity of weld is larger.

Pulsed argon arc welding

Pulsed TIG welding

This method can obviously improve the stability of low current welding process, and is convenient to control arc power and weld formation by adjusting various process parameters. The weldment has small deformation and heat affected zone, which is especially suitable for thin plate, all position welding and welding of forged aluminum, hard aluminum and super hard aluminum with strong heat sensitivity.

MIG pulse argon arc welding

It is suitable for all position welding of aluminum alloy sheet with thickness of 2-10 mm.

Resistance spot welding and seam welding

It can be used to weld aluminum alloy sheet with thickness less than 4mm. For products with high quality requirements, DC shock wave spot welding and seam welder can be used. Welding requires more complex equipment, high welding current, high productivity, especially suitable for mass production of parts and components.

Friction stir welding

Friction stir welding (FSW) is a kind of solid state joining technology which can be used for welding various alloy plates. Compared with the traditional fusion welding method, friction stir welding has no spatter, no dust, no need to add welding wire and shielding gas, and no pores and cracks in the joint. Compared with ordinary friction, it is not limited by shaft parts and can weld straight welds. This welding method also has a series of other advantages, such as good mechanical properties, energy saving, no pollution, low preparation requirements before welding. Because of the low melting point of aluminum and aluminum alloy, friction stir welding is more suitable.
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Welding materials for aluminum

Welding wire

When gas welding and TIG welding are used to weld aluminum alloy, filler wire should be added. Aluminum and aluminum alloy welding wires are divided into homogeneous welding wire and heterogeneous welding wire. In order to get a good welding joint, the welding wire suitable for the base metal should be selected as the filler material.
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The selection of welding wire should first consider the requirements of weld composition, as well as the mechanical properties, corrosion resistance, structural rigidity, color and crack resistance of the product. Choosing the filler metal whose melting temperature is lower than the base metal can greatly reduce the tendency of intergranular cracks in the heat affected zone. For non heat treated alloy, the strength of welded joint increases in the order of 1000 series, 4000 series and 5000 series.
5000 series welding wires containing more than 3% magnesium should not be used in structures with service temperature above 65 ℃, because these alloys are very sensitive to stress corrosion cracking, and stress corrosion cracking will occur in the above temperature and corrosion environment. Welding wire with higher alloy content than base metal is usually used as filler metal to prevent crack tendency of weld metal.
At present, most of the commonly used welding wires for aluminum alloy are standard grade welding wires with similar composition to the base metal. In the absence of standard grade welding wire, the narrow strip can be cut from the base metal instead. The common welding wire is hs311, which has good liquid metal fluidity, small shrinkage during solidification and excellent crack resistance. In order to refine the grain size and improve the crack resistance and mechanical properties of the weld, a small amount of Ti, V, Zr and other alloy elements are usually added into the wire as modifier.
Attention should be paid to the following problems in the selection of aluminum alloy welding wire:
1) Crack sensitivity of welded joints
The direct factor affecting the crack sensitivity is the matching of base metal and welding wire. The weld metal with melting temperature lower than base metal can reduce the crack sensitivity of weld metal and heat affected zone. For example, when welding 6061 alloy with 0.6% silicon content, choosing the same alloy as the weld, the crack sensitivity is very high. However, er4043 welding wire with 5% silicon content has good crack resistance because its melting temperature is lower than 6061 alloy and it has higher plasticity in the cooling process. In addition, the weld metal avoids the combination of magnesium and copper, because al mg Cu has high crack sensitivity.
2) Mechanical properties of welded joints
The strength of industrial pure aluminum is the lowest, 4000 series aluminum alloy is in the middle, and 5000 series aluminum alloy is the highest. Although Al Si welding wire has high crack resistance, the plasticity of Si containing welding wire is poor. Therefore, for the joints that need plastic deformation processing after welding, Si containing welding wire should be avoided.
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3) Performance of welded joint
The choice of filler metal depends not only on the composition of base metal, but also on the geometry of joint, the requirements of corrosion resistance in operation and the appearance requirements of weldment. For example, in order to make the container have good corrosion resistance or prevent the pollution of the stored products, the welding container storing hydrogen peroxide requires high purity aluminum alloy. In this case, the purity of the filler metal should be at least equal to that of the base metal.

Welding rod

The model, specification and application of aluminum alloy welding rod are shown in Table 2. The chemical composition and mechanical properties of aluminum alloy electrode are shown in Table 3.
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Shielding gas

Inert gases for welding aluminum alloy are argon and helium. The technical requirements of argon are ar > 99.9%, oxygen < 0.005%, hydrogen < 0.005%, moisture < 0.02mg/l and nitrogen < 0.015%. The increase of oxygen and nitrogen worsens the cathodic atomization. When the oxygen content is more than 0.3%, the burning loss of tungsten electrode will be intensified, and when the oxygen content is more than 0.1%, the weld surface will be dull or blackened.
For TIG welding, pure argon is selected for AC plus HF welding, which is suitable for thick plate; Ar + he or pure AR is selected for DC positive electrode welding.
When the thickness of the plate is less than 25 mm, pure AR is used. When the thickness of the plate is 25 ~ 50 mm, Ar + he mixed gas with 10% ~ 35% AR is used. When the plate thickness is 50-75mm, Ar + he mixture with 10% – 35% or 50% he should be used. When the plate thickness is greater than 75 mm, it is recommended to add Ar + he mixture of 50% – 75% he.

Welding process of aluminum alloy

Gas welding of aluminum alloy

The thermal efficiency of oxygen acetylene gas welding is low, the heat input is not concentrated, the flux is needed when welding aluminum and aluminum alloy, and the residue needs to be removed after welding, so the quality and performance of the joint is not high. Because gas welding equipment is simple, no power supply, convenient and flexible, it is often used to weld aluminum alloy components with low quality requirements, such as thin sheet and small parts, as well as repair welding of aluminum alloy components and aluminum castings.

Joint form of gas welding

Lap joint and T-joint are not suitable for gas welding of aluminum alloy. It is difficult to clean up the residual flux and welding slag in the gap, so butt joint should be used as far as possible. In order to ensure that the weldment is fully welded without collapse and burn through, the backing plate with groove can be used. The backing plate is generally made of stainless steel or pure copper. The backing plate welding can obtain good reverse forming and improve the welding productivity.

Selection of flux for gas welding

During gas welding of aluminum alloy, in order to make the welding process smooth and ensure the weld quality, flux is needed to remove the oxide film and other impurities on the surface of aluminum alloy.
Gas welding flux (also known as gas flux) is a flux used in gas welding. Its main function is to remove the oxide film formed on the surface of aluminum during gas welding, improve the wettability of base metal, and promote the obtaining of dense weld microstructure. Flux must be used for gas welding of aluminum alloy. Generally, the flux is directly sprinkled on the groove of the workpiece to be welded before welding, or added into the molten pool on the welding wire.
Aluminum alloy fluxes are chlorides of potassium, sodium, calcium, lithium and other elements. They are powdery compounds that are crushed, sieved and prepared in a certain proportion. For example, aluminum cryolite (Na3AlF6) can melt alumina at 1000 ℃, and potassium chloride can transform refractory alumina into fusible aluminum chloride. The flux has low melting point and good fluidity. It can also improve the fluidity of molten metal and make the weld well formed.

Selection of welding nozzle and flame

Aluminum alloy has strong oxidation and air absorption. In gas welding, in order to prevent aluminum from being oxidized, neutral flame or weak carbonization flame (acetylene is excess carbonization flame) should be used to make aluminum molten pool under the protection of reducing atmosphere without being oxidized. It is strictly forbidden to use oxidation flame, because using oxidation flame with strong oxidizability will make aluminum oxidize strongly and hinder the welding process; However, if there is too much acetylene, the free hydrogen may dissolve into the molten pool, which will cause pores in the weld and make the weld loose.

Tack weld

In order to prevent the change of size and relative position of weldment in welding, spot welding is needed before welding. Due to the high coefficient of linear expansion, fast heat conduction speed and large heating area of gas welding, the positioning weld should be denser than steel parts.
The filler wire for positioning welding is the same as that for product welding. Before positioning welding, a layer of gas flux should be applied in the weld gap. The flame power of positioning welding is slightly higher than that of gas welding.

Gas welding operation

When welding steel materials, the heating temperature can be judged from the color change of steel. But when welding aluminum, there is no such convenient condition. Because there is no obvious color change in the process of aluminum alloy heating from room temperature to melting, it is difficult for the operator to control the welding temperature. However, the welding time can be controlled according to the following phenomena:

  • 1) When the surface of the heated workpiece changes from bright white to dull silver white, the surface oxide film is wrinkled, and the metal at the heating place fluctuates, it indicates that the melting temperature is about to reach and welding can be carried out;
  • 2) When the end of the welding wire dipped with flux and the heated part can be fused with the parent material, the melting temperature can be reached and the welding can be carried out;
  • 3) When the edge of the base metal falls down, the base metal reaches the melting temperature and can be welded.

The left welding method can be used for gas welding thin plate, and the welding wire is in front of the welding flame. Because the flame points to the cold metal that has not been welded, part of the heat is lost in this welding method, which is conducive to preventing the overheating of the molten pool, the growth of metal grains in the heat affected zone and burning through. The right welding method can be used when the thickness of base metal is greater than 5mm. The welding wire is behind the torch, and the flame points to the weld, so the heat loss is small, the melting depth is large, and the heating efficiency is high.
When the thickness of gas welding is less than 3mm, the inclination angle of torch is 20 ~ 40 degrees; When gas welding thick parts, the inclination angle of welding torch is 40-80 ° and the angle between welding wire and welding torch is 80-100 °. For gas welding of aluminum alloy, the joint shall be welded at one time as far as possible, and the second layer shall not be deposited, because the slag inclusion in the weld will be caused when the second layer is deposited.
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Post weld treatment

The corrosion of aluminum joint caused by residual flux and slag on the weld surface of gas welding is one of the reasons for the damage of aluminum joint in the future. In 1 ~ 6h after gas welding, the residual flux and slag should be cleaned to prevent the weldment from corrosion. The cleaning process after welding is as follows:

  • 1) After welding, the weldment shall be immersed in a hot water bath at 40-50 ℃. It is better to use flowing hot water and brush the weld and the residual flux and slag near the weld with a bristle brush until it is cleaned.
  • 2) Immerse the weldment in nitric acid solution. When the room temperature is above 25 ℃, the solution concentration is 15% ~ 25%, and the soaking time is 10 ~ 15min; When the room temperature is 10 ~ 15 ℃, the solution concentration is 20% ~ 25%, and the soaking time is 15 min.
  • 3) Immerse the weldment in hot water (40-50 ℃) for 5-10 minutes.
  • 4) Rinse the weldment with cold water for 5min.
  • 5) The weldment can be dried naturally, or dried in a drying oven or blown dry with hot air.

TIG welding of aluminum alloy

Also known as tungsten inert gas arc welding, is the use of tungsten and workpiece between the formation of arc generated a lot of heat to melt the place to be welded, plus filler wire to obtain a solid welding joint. Argon arc welding aluminum is to use its “cathode atomization” characteristics to remove the oxide film. The tungsten electrode and the seam area are shielded by the inert gas emitted from the nozzle to prevent the reaction between the weld area and the surrounding air.
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TIG welding process is most suitable for welding thin plates with thickness less than 3mm, and the workpiece deformation is obviously less than that of gas welding and manual arc welding. The cathode of AC TIG welding can remove the oxide film and avoid the corrosion of residual flux and slag after welding. The joint form can be unrestricted, and the weld has good shape and bright surface.
The erosion of argon flow on the welding zone accelerates the cooling of the joint, improves the microstructure and properties of the joint, and is suitable for all position welding. Because of no flux, the requirement of cleaning before welding is more strict than other welding methods.
AC TIG welding and AC pulse TIG welding are more suitable for welding aluminum alloy, followed by DC reverse TIG welding. In general, AC welding aluminum alloy can achieve the best cooperation in current carrying capacity, arc controllability and arc cleaning function, so AC power is used in most aluminum alloy TIG welding.
When DC positive connection (electrode connected to negative electrode) is adopted, heat is generated on the surface of the workpiece to form deep penetration, and larger welding current can be used for a certain size of electrode. Even the thick section does not need preheating, and the base metal hardly deforms. Although the direct current reverse connection (electrode to positive electrode) TIG welding method is rarely used to weld aluminum, this method has the advantages of shallow melting depth, easy control of arc and good purification effect in continuous welding or repair welding of thin-walled heat exchanger and similar components with pipe thickness less than 2.4mm.

Tungsten electrode

The melting point of tungsten is 3410 ℃. Tungsten has strong electron emission ability at high temperature. After adding trace rare earth elements such as thorium, cerium and zirconium, the work of electron emission is significantly reduced and the current carrying capacity is significantly improved. In TIG welding of aluminum alloy, tungsten electrode is mainly used to conduct current, ignite arc and maintain normal combustion of arc. Commonly used tungsten electrode materials are pure tungsten, thorium tungsten and cerium tungsten.

Welding process parameters

In order to obtain excellent weld forming and welding quality, the welding process parameters should be reasonably selected according to the technical requirements of the weldment. The main process parameters of manual TIG welding of aluminum alloy include current type, polarity and current size, shielding gas flow rate, tungsten electrode extension length, distance between nozzle and workpiece, etc. The process parameters of automatic TIG welding also include arc voltage (arc length), welding speed and wire feeding speed.
According to the material and thickness to be welded, the process parameters include the diameter and shape of tungsten electrode, the diameter of welding wire, protective gas and flow rate, nozzle diameter, welding current, arc voltage and welding speed, and then the relevant parameters are adjusted according to the actual welding effect until they meet the use requirements.
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The key points of selecting TIG welding parameters for aluminum alloy are as follows:
1) Nozzle diameter and shielding gas flow
The nozzle diameter of aluminum alloy TIG is 5 ~ 22mm; The flow rate of protective gas is generally 5 ~ 15L / min.
2) The length of tungsten electrode and the distance between nozzle and workpiece
The extension length of tungsten electrode is generally 5-6 mm for butt weld and 7-8 mm for fillet weld. The distance between nozzle and workpiece is generally about 10 mm.
3) Welding current and welding voltage
It is related to plate thickness, joint form, welding position and welder’s technical level. In manual TIG welding, AC power supply is used. When the thickness of aluminum alloy is less than 6 mm, the maximum welding current can be determined according to the formula I = (60-65) d according to the electrode diameter D. 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) Welding speed
In order to reduce the deformation of TIG welding of aluminum alloy, a faster welding speed should be adopted. In manual TIG welding, the welder adjusts the welding speed at any time according to the size and shape of the weld pool and the fusion condition on both sides. The general welding speed is 8 ~ 12m / h; In automatic TIG welding, after the process parameters are set, the welding speed is generally unchanged in the welding process.
5) Wire diameter
Generally determined by plate thickness and welding current, the diameter of welding wire is proportional to them.
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Common problems of aluminum alloy welding

Stoma

The causes are as follows:
The purity of argon is low or there is moisture or 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 cleaning; The welding current and welding speed are too large or too small; Poor protection of molten pool, unstable arc, too long arc and too long extension of tungsten electrode, etc.
Preventive measures:
Ensure the argon pipeline, carefully clean the welding wire and weldment, timely weld after cleaning, and prevent re pollution. Update the gas supply pipeline, select the appropriate gas flow, and adjust the tungsten electrode extension length; Correct selection of welding process parameters. If necessary, preheating process can be adopted and wind proof device can be installed on welding site to prevent wind flow.

Crackle

The causes are as follows:
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 specification, the crack tendency increases; When the melting temperature of the welding wire is too high, it will cause liquefying cracks in the heat affected zone; Unreasonable structure design, too concentrated weld or too high temperature in heating zone result in excessive restraint stress of joint; High turbidity, long residence time, tissue overheating; The crater is not filled and cracks appear.
Preventive measures:
The composition of welding wire should match with base metal; Adding arc striking plate or using current attenuation device to fill the arc pit; The welding structure should be designed correctly, the welding seam should be arranged reasonably, the stress concentration should be avoided, and the proper welding sequence should be selected; Reduce welding current or increase welding speed appropriately.
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Incomplete penetration

The causes are as follows:
Welding speed is too fast, arc length is too long, welding gap, groove angle, welding current are too small, blunt edge is too large; The burr on the edge of the groove and the dirt on the bottom edge of the workpiece are not removed before welding; The inclination angle between welding torch and welding wire is incorrect.
Preventive measures:
Correct selection of gap, blunt edge, groove angle and welding process parameters; Strengthen the cleaning of oxide film, flux, slag and oil; Improve operation skills, etc.

Tungsten inclusion in weld

The causes are as follows:
It is caused by contact arcing; The shape of the end of the tungsten electrode and the welding current are not reasonable, which makes the tip fall off; The fillings touch the tip of the hot tungsten electrode and misuse the oxidizing gas.
Preventive measures:
High frequency and high voltage pulse arc ignition is adopted; According to the selected current, the reasonable shape of tungsten electrode tip is adopted; The welding current is reduced, the diameter of tungsten electrode is increased, and the length of tungsten electrode is shortened; Renew inert gas; Improve the operation skills, do not make the filler wire contact with tungsten electrode, etc.

Undercut

The causes are as follows:
The welding current is too large, the arc voltage is too high, the swing of the torch is not uniform, the wire filling is too little, and the welding speed is too fast.
Preventive measures:
Reduce the welding current and arc voltage, keep the torch swing uniform, increase the wire feeding speed or reduce the welding speed appropriately.
Conventional repair welding process of castings
Generally, the defects of aluminum alloy castings can be repaired by argon arc welding, and the effect of AC TIG welding is better.
When using repair welding process to repair casting defects, in addition to the above-mentioned general practices, such as cleaning the welding wire and the parts to be welded before welding, selecting reasonable welding wire materials, selecting short arc and small angle welding wire for welding, there are many successful experiences in practice for different types of defects, such as choosing small current welding as far as possible.
The alloy composition of the welding wire is higher than that of the base metal, so as to supplement the burnt alloy in the process of repair welding and keep the weld composition consistent with that of the base metal; For the casting with crack defect, crack stop holes shall be made at both ends before repair welding; When welding, the part to be welded should be heated first, and the left welding method should be used to fill the wire, so as to observe the melting of the weld. After the welding part is melted, the wire should be filled to form a fully wetted molten pool.
When the defect size is large, in order to improve the repair welding efficiency, a thin layer of surfactant (referred to as atig surfactant) can be coated on the surface of the welding position before the traditional TIG welding. During welding, the surfactant causes the welding arc to shrink or the metal flow state in the weld pool to change, which increases the weld penetration, It is to coat a layer of SiO2 active agent on the weld surface to change the weld penetration, reduce the preheating procedure and reduce the welding difficulty.
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Summary

TIG and MIG arc welding, which are convenient and low-cost, can be used for welding and repair of aluminum alloy. When high energy beam welding and friction stir welding are used in aluminum alloy welding, the problems of alloy element burning, joint softening and welding deformation can be effectively avoided, especially the friction stir welding is a solid-state connection, which has the characteristics of green and environmental protection.
When the conventional repair welding method is used to repair the defects 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 preferred.
In order to improve the repair welding quality of aluminum alloy castings, special repair welding method can be used in combination with the actual situation when the casting defects are special and conditions are available.

Source: Network Arrangement – China Aluminum Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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