Welding characteristics of aluminum and aluminum alloy

(1) Aluminum is very easy to oxidize in the air and during welding. The resulting aluminum oxide (Al2O3) has a high melting point, is very stable, and is not easy to remove. It hinders the melting and fusion of the base material. The oxide film has a high specificity and is not easy to float on the surface. It is easy to generate defects such as slag inclusion, lack of fusion, and incomplete penetration. The oxide film on the surface of aluminum and the absorption of a large amount of moisture can easily cause pores in the weld. Before welding, chemical or mechanical methods should be used for strict surface cleaning to remove the surface oxide film. Strengthen the protection during the welding process to prevent its oxidation. In tungsten argon arc welding, an AC power source is used to remove the oxide film through the "cathodic cleaning" function. For gas welding, use flux that removes the oxide film. When welding thick plates, the welding heat can be increased. For example, the helium arc has a large heat, using helium or argon-helium mixed gas for protection, or adopting large-scale molten electrode gas shielded welding. In the case of direct current connection, the "cathode" is not required Clean up".

(2) The thermal conductivity and specific heat capacity of aluminum and aluminum alloys are about twice that of carbon steel and low-alloy steel. The thermal conductivity of aluminum is ten times that of austenitic stainless steel. During the welding process, a large amount of heat can be quickly transferred to the inside of the base metal. Therefore, when welding aluminum and aluminum alloys, in addition to the energy consumed in the molten metal pool, more heat is needlessly consumed in other parts of the metal. This kind of useless energy consumption is more significant than steel welding. In order to obtain high-quality welded joints, energy concentrated and high-power energy should be used as much as possible, and sometimes preheating and other technological measures can be used.

(3) The coefficient of linear expansion of aluminum and aluminum alloys is about twice that of carbon steel and low-alloy steel. The volumetric shrinkage rate of aluminum during solidification is large, and the deformation and stress of the weldment are large. Therefore, measures to prevent welding deformation must be taken. When the aluminum welding molten pool is solidified, shrinkage cavities, shrinkage porosity, thermal cracks and high internal stress are prone to occur. In production, measures to adjust the composition of the welding wire and the welding process can be used to prevent the occurrence of hot cracks. Where the corrosion resistance permits, aluminum-silicon alloy welding wire can be used to weld aluminum alloys other than aluminum-magnesium alloys. When the silicon content of aluminum-silicon alloy is 0.5%, the hot cracking tendency is greater. As the silicon content increases, the alloy crystallization temperature range becomes smaller, the fluidity is significantly improved, the shrinkage rate decreases, and the hot cracking tendency decreases accordingly. According to production experience, when the silicon content is 5% to 6%, thermal cracking will not occur, so the use of SAlSi (silicon content 4.5% to 6%) welding wire will have better crack resistance.

(4) Aluminum has a strong ability to reflect light and heat. There is no obvious color change when the solid and liquid transition states, and it is difficult to judge the welding operation. High temperature aluminum has very low strength, it is difficult to support the molten pool, and it is easy to weld through.

(5) Aluminum and aluminum alloys can dissolve a large amount of hydrogen in the liquid state, but hardly dissolve hydrogen in the solid state. During the process of solidification and rapid cooling of the weld pool, hydrogen is too late to overflow, and hydrogen pores are easily formed. The moisture in the arc column atmosphere, the moisture absorbed by the welding material and the oxide film on the surface of the base metal are all important sources of hydrogen in the weld. Therefore, the source of hydrogen must be strictly controlled to prevent the formation of pores.

(6) Alloy elements are easy to evaporate and burn, which reduces the weld performance.

(7) If the base metal is deformation strengthening or solid solution aging strengthening, welding heat will reduce the strength of the heat-affected zone.

(8) Aluminum is a face-centered cubic lattice, without allotropes, and there is no phase change during heating and cooling. The weld grains are easy to be coarse and cannot be refined by phase change.

Welding method

Almost various welding methods can be used to weld aluminum and aluminum alloys, but aluminum and aluminum alloys have different adaptability to various welding methods, and various welding methods have their own applications. The gas welding and electrode arc welding methods have simple equipment and convenient operation. Gas welding can be used for repair welding of aluminum sheets and castings that do not require high welding quality. Electrode arc welding can be used for repair welding of aluminum alloy castings. Inert gas shielded welding (TIG or MIG) method is the most widely used aluminum and aluminum alloy welding method. Aluminum and aluminum alloy thin plates can be welded by AC argon tungsten arc welding or pulsed argon tungsten arc welding. Aluminum and aluminum alloy thick plates can be welded with tungsten helium arc welding, argon-helium mixed tungsten gas shielded welding, MIG welding, pulse MIG welding. MIG welding and pulse MIG welding are more and more widely used (argon or argon/helium mixture)

Welding materials

(1) Welding wire

The selection of aluminum and aluminum alloy welding wire, in addition to considering good welding process performance, according to the requirements of the container, the tensile strength and plasticity (through bending test) of the butt joint should meet the specified requirements. For aluminum with a magnesium content of more than 3% Magnesium alloys should meet the requirements of impact toughness. For vessels with corrosion resistance requirements, the corrosion resistance of welded joints should also reach or approach the level of the base material. Therefore, the selection of welding wire is mainly based on?strong>aluminum ying?br />

1) The purity of pure aluminum welding wire is generally not lower than the base material;

2) The chemical composition of aluminum alloy welding wire is generally corresponding or similar to that of the base metal;

3) The content of corrosion-resistant elements (magnesium, manganese, silicon, etc.) in the aluminum alloy welding wire is generally not lower than the base material;

4) When welding dissimilar aluminum materials, the welding wire should be selected according to the base material with higher corrosion resistance and high strength;

5) High-strength aluminum alloys (heat-treated reinforced aluminum alloys) that do not require corrosion resistance can use welding wires of different compositions, such as aluminum-silicon alloy welding wire SAlSi-1 with good crack resistance (note that the strength may be lower than the base material ).

(2) Shielding gas

The protective gas is argon, helium or their mixture. When AC and high frequency TIG welding, use more than 99.9% pure argon gas, and helium gas is suitable for DC positive polarity welding. For MIG welding, it is recommended to use argon with 50% to 75% helium when the plate thickness is 75 mm. Argon should meet the requirements of GB/T 4842-995 "Pure Argon". If the pressure of the argon cylinder is lower than 0.5 MPa, the pressure is insufficient and cannot be used.

(3) Tungsten electrode

There are four types of tungsten electrode materials for argon arc welding: pure tungsten, thorium tungsten, cerium tungsten, and zirconium tungsten. The pure tungsten electrode has a high melting point and boiling point, which is not easy to melt and volatilize, and the electrode burns less and the tip pollution is less, but the electron emission ability is poor. The electrode with 1% to 2% thorium oxide added to pure tungsten is thorium tungsten electrode, which has strong electron emission ability, high allowable current density and stable arc combustion, but thorium element has certain radioactivity, so proper protection should be taken when using it. Measures. In pure tungsten, 1.8%-2.2% of cerium oxide (impurity & le; 0.1%) is added to the electrode of cerium tungsten electrode. Cerium tungsten electrode has low electron work function, high chemical stability, high allowable current density, and no radioactivity. It is currently a commonly used electrode. The zirconium tungsten electrode can prevent the electrode from contaminating the base metal, and the tip is easy to maintain a hemispherical shape, which is suitable for AC welding.

(4) Flux The flux for gas welding is chloride and fluoride of potassium, sodium, lithium, calcium and other elements, which can remove oxide film.

Pre-welding preparation

(1) Cleaning before welding

When welding aluminum and aluminum alloys, the oxide film and grease on the surface of the weld and wire of the workpiece should be strictly removed before welding. The removal quality directly affects the welding process and joint quality, such as the tendency of weld pores and mechanical properties. Two methods of chemical cleaning and mechanical cleaning are often used.

1) Chemical cleaning

The chemical cleaning efficiency is high, and the quality is stable. It is suitable for cleaning welding wire and small-size, batch-produced workpieces. Two kinds of dipping method and scrubbing method are available. Use acetone, gasoline, kerosene and other organic solvents to degrease the surface. Use 5%～10% NaOH solution at 40℃～70℃ for alkaline washing for 3 min～7 min (the pure aluminum time is slightly longer but not more than 20 min), and rinse with flowing water , Followed by pickling with 30% HNO3 solution at room temperature to 60°C for 1 min to 3 min, rinse with running water, and air-dry or dry at low temperature.

2) Mechanical cleaning

When the size of the workpiece is large, the production cycle is long, multi-layer welding or chemical cleaning is contaminated, mechanical cleaning is often used. First wipe the surface with acetone, gasoline and other organic solvents to remove oil, and then directly use a copper wire brush or stainless steel wire brush with a diameter of 0.15mm to 0.2mm until the metallic luster is exposed. Generally, it is not advisable to use a grinding wheel or ordinary sandpaper for sanding, so as to prevent the sand particles from staying on the metal surface and entering the molten pool during welding to cause defects such as slag inclusion. In addition, scrapers, files, etc. can also be used to clean the surface to be welded.

After the workpiece and welding wire are cleaned and cleaned, an oxide film will regenerate during storage. Especially in a humid environment, the oxide film grows faster in an environment contaminated by vapors such as acids and alkalis. Therefore, the storage time of the workpiece and welding wire after cleaning and cleaning up to before welding should be shortened as much as possible. Generally, welding should be performed within 4 hours after cleaning in a humid climate. After cleaning, if the storage time is too long (such as more than 24 hours), it should be processed again.

(2) backing plate

Aluminum and aluminum alloys have very low strength at high temperatures, and liquid aluminum has good flow properties, and the weld metal is prone to collapse during welding. In order to ensure the penetration of the weld without collapsing, a backing plate is often used to support the molten pool and nearby metal during welding. The backing plate can be graphite plate, stainless steel plate, carbon steel plate, copper plate or copper rod, etc. An arc-shaped groove is opened on the surface of the backing plate to ensure the formation of the reverse side of the weld. It is also possible to weld on both sides without backing plate, but it requires skillful welding operation or adopts advanced technological measures such as strict automatic feedback control of arc welding energy.

(3) Pre-heating thin and small aluminum parts before welding generally does not require pre-heating. When the thickness is 10 mm～15 mm, it can be pre-heated before welding. According to different types of aluminum alloy, the preheating temperature can be 100℃～200℃ , It can be heated by oxygen-acetylene flame, electric furnace or torch. Preheating can reduce the deformation of the weldment and reduce the defects such as pores.

Post-welding treatment

(1) Cleaning up after welding

The residual flux and welding slag left on the weld and nearby after welding will destroy the passivation film on the aluminum surface, and sometimes corrode the aluminum parts, so it should be cleaned up. Workpieces with simple shapes and general requirements can be cleaned by simple methods such as hot water washing or steam blowing. The aluminum parts with high requirements and complex shapes are washed with a hard brush in hot water, and then immersed in a chromic anhydride aqueous solution or potassium dichromate solution at a concentration of 2% to 3% at a concentration of 60℃～80℃5 Min to 10 min, and scrub with a hard brush, then rinse in hot water, dry in an oven, or blow dry with hot air, or dry naturally.

(2) Post-weld heat treatment

Aluminum containers generally do not require heat treatment after welding. If the aluminum material used does have obvious stress corrosion sensitivity under the medium conditions of the container contact, post-weld heat treatment is needed to eliminate the higher welding stress, so that the stress on the container can be reduced to below the critical stress for stress corrosion cracking. At this time, special requirements should be put forward by the vessel design documents before the post-weld stress relief heat treatment is carried out. For post-weld annealing heat treatment, for pure aluminum, 5052, 5086, 5154, 5454, 5A02, 5A03, 5A06, etc., the recommended temperature is 345℃; for 2014, 2024, 3003, 3004, 5056, 5083, 5456, 6061, 6063 , 2A12, 2A24, 3A21, etc., the recommended temperature is 415°C; for 2017, 2A11, 6A02, etc., the recommended temperature is 360°C. According to the size and requirements of the workpiece, the annealing temperature can be adjusted to 20°C to 30°C in both positive and negative directions. The holding time can be between 0.5 h and 2 h.