1. Welding characteristics of aluminium and aluminium alloys

(1) Aluminum is easily oxidized in the air and during welding, and the generated alumina (Al2O3) has a high melting point, is very stable, and is not easy to remove. Hindering the melting and fusion of the base material, the ratio of oxide film is large, it is not easy to surface, and it is easy to produce slag inclusion, non-fusion, and non-welding penetration. The surface oxide film of aluminum and the absorption of a large amount of water, easy to make the weld porosity. Before welding, the surface should be strictly cleaned by chemical or mechanical methods to remove the oxide film on the surface. Strengthen protection during welding to prevent oxidation. In argon tungsten arc welding, the AC power supply is used to remove the oxide film through the "cathode cleaning" effect. In gas welding, a flux that removes the oxide film is used. When welding thick plates, the welding heat can be increased, for example, the helium arc heat is large, the use of helium or argon-helium mixed gas protection, or the use of large specifications of molten electrode gas protection welding, in the case of direct current connection, there is no need for "cathode cleaning".

(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 more than ten times that of austenitic stainless steel. In the welding process, a large amount of heat can be rapidly conducted to the base metal, so when welding aluminum and aluminum alloy, in addition to the energy consumed in the molten metal pool, there is more heat unnecessary consumption in other parts of the metal, the consumption of this useless energy is more significant than the welding of steel, in order to obtain high-quality welded joints, Energy concentration and power should be used as far as possible, and sometimes process measures such as preheating can be used.

(3) The linear expansion coefficient of aluminum and aluminum alloy is about twice that of carbon steel and low alloy steel. The volume shrinkage rate of aluminum solidification is large, and the deformation and stress of welded parts are large, so it is necessary to take measures to prevent welding deformation. Shrinkage hole, shrinkage porosity, thermal crack and high internal stress are easy to occur during solidification of aluminum welding pool. In production, measures can be adopted to adjust the composition of welding wire and welding process to prevent the occurrence of hot cracks. If the corrosion resistance allows, aluminum alloy other than aluminum magnesium alloy can be welded with aluminum silicon alloy wire. With the increase of silicon content, the crystallization temperature range of the alloy decreases, the fluidity increases significantly, the shrinkage rate decreases, and the thermal cracking tendency also decreases. According to production experience, when the silicon content is 5% ~ 6%, there is no thermal cracking, so the SAlSi bar (silicon content 4.5% ~ 6%) welding wire will have better crack resistance.

(4) Aluminum has a strong ability to reflect light and heat, and there is no obvious color change when solid and liquid are transferred, and it is difficult to judge during welding operation. The strength of high-temperature aluminum is very low, and it is difficult to support the molten pool and easy to weld through.

(5) Aluminum and aluminum alloys can dissolve a large amount of hydrogen in the liquid state, and the solid state almost does not dissolve hydrogen. In the process of solidification and rapid cooling of welding pool, hydrogen can not overflow, and it is easy to form hydrogen pores. The water in the arc column atmosphere, the water adsorbed by the oxide film on the surface of the welding material and the base material are the important sources of hydrogen in the weld. Therefore, the source of hydrogen should be strictly controlled to prevent the formation of pores.

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

(7) When the base metal is deformed or strengthened by solution aging, the welding heat will reduce the strength of the heat affected zone.

(8) Aluminum is a face-centered cubic lattice, there is no isomer, there is no phase change during heating and cooling, the weld grain is easy to coarser, and the grain can not be refined by phase change.

2. Welding method

Almost all kinds of 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 application occasions. Gas welding and electrode arc welding methods, simple equipment, easy to operate. Gas welding can be used for the repair welding of aluminum sheet and castings with low welding quality requirements. Electrode arc welding can be used to repair aluminum alloy castings. The inert gas shielded welding (TIG or MIG) method is the most widely used welding method for aluminum and aluminum alloys. Aluminum and aluminum alloy sheet can be tungsten AC argon arc welding or tungsten pulsed argon arc welding. Aluminum and aluminum alloy thick plates can be tungsten helium arc welding, argon-helium mixed tungsten gas welding, MIG welding, pulsed MIG welding. Mig welding, pulsed MIG welding is more and more widely used (argon or argon/helium mixture).

Several advanced welding processes of aluminum alloy: friction stir welding, laser welding, laser-arc composite welding, electron beam welding. Effective solutions are proposed for the alloys with poor weldability and which were considered unweldable. Several processes have advantages and can be used for welding thick plate aluminum alloys.

3. Difficulty of aluminum alloy welding

Aluminum alloy is widely used in various welding structure products because of its light weight, high specific strength, good corrosion resistance, non-magnetic, good formability and good low temperature performance, etc. The use of aluminum alloy instead of steel plate material welding, structural weight can be reduced by more than 50%.

Aluminum alloy welding has several difficulties:

① Aluminum alloy welded joints are seriously softened and the strength coefficient is low, which is also the biggest obstacle to the application of aluminum alloy;

② Aluminum alloy surface is easy to produce refractory oxide film (Al2O3 melting point is 2060℃), which requires the use of high-power density welding process;

③ Aluminum alloy welding is easy to produce porosity;

④ Aluminum alloy welding is easy to produce hot cracks;

⑤ Large linear expansion coefficient, easy to produce welding deformation;

The thermal conductivity of aluminum alloy is large (about 4 times that of steel), and the heat input is 2 to 4 times larger than that of welded steel at the same welding speed.

Therefore, aluminum alloy welding requires the use of high energy density, small welding heat input, high welding speed and efficient welding methods.

4. Welding materials

(1) The selection of welding wire aluminum and aluminum alloy welding wire in addition to considering good welding process performance, according to the requirements of the container should make the tensile strength of the butt joint, plasticity (through bending test) to meet the requirements of the impact toughness of the aluminum magnesium alloy containing more than 3% magnesium should meet the requirements of the corrosion resistance of the container, the corrosion resistance of the welded joint should also reach or close to the level of the base metal. Therefore, the selection of welding wire is mainly in accordance with the following principles:

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 to or similar to the base metal;

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

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

5) High-strength aluminum alloy (heat-treated strengthened aluminum alloy) that does not require corrosion resistance can use welding wires with heterogeneous components, such as aluminum-silicon alloy welding wire with good crack resistance SAlSi 1, etc. (note that the strength may be lower than the base metal).

(2) Protective gas The protective gas is argon, helium or its mixture. When AC plus high-frequency TIG welding, use more than 99.9% pure argon, DC positive polarity welding should use helium. When MIG welding, plate thickness <25mm when using argon; When the plate thickness is 25mm ~ 50mm, 10% ~ 35% helium should be added to argon. When the plate thickness is 50mm-75mm, l0 ~ 35% or 50% helium should be added to argon; Argon with 50% ~ 75% helium is recommended when the plate thickness is >75mm. Should Argon conform to GB/T4842? 995 "Pure Argon" requirements. The argon gas cylinder can not be used because the pressure is insufficient after the pressure is lower than 0.5MPa.

(3) Tungsten electrode materials for argon tungsten arc welding are pure tungsten, thorium tungsten, cerium tungsten and zirconium tungsten. Pure tungsten electrode has high melting point and boiling point, is not easy to melt and volatilize, electrode burning and tip pollution is less, but the electron emission ability is poor. In pure tungsten adding 1% to 2% of thorium oxide electrode thorium tungsten electrode, electron emission ability is strong, allowed current density is high, arc combustion is more stable, but thorium element has a certain radioactivity, should take appropriate protective measures when used. A cerium tungsten electrode with 1.8% ~ 2.2% cerium oxide (impurity ≤ 0.1%) is added to pure tungsten. Cerium tungsten electrode has low electron work, high chemical stability, large allowable current density and no radioactivity, so it is widely used at present. Zirconium tungsten electrode can prevent the electrode from polluting the base metal, the tip is easy to keep hemispherical, suitable for AC welding.

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

5. Preparation before welding

(1) When cleaning aluminum and aluminum alloy welding before welding, the oxide film and oil on the surface of the workpiece welding joint and wire should be strictly removed before welding, and the removal quality directly affects the welding process and joint quality, such as the tendency and mechanical properties of the weld porosity. Chemical cleaning and mechanical cleaning are often used.

1) Chemical cleaning Chemical cleaning efficiency is high, stable quality, suitable for cleaning welding wire and small size, mass production of the workpiece. It can be used in two ways: soaking and scrubbing. Use acetone, gasoline, kerosene and other organic solvents to remove oil from the surface, wash with 5% ~ 10% NaOH solution at 40℃ ~ 70℃ for 3min ~ 7min(pure aluminum time is slightly longer but not more than 20min), rinse with running water, then pickling with 30% HNO3 solution at room temperature to 60℃ for 1min ~ 3min, and rinse with running water. Air dry or dry at low temperature.

2) Mechanical cleaning When the workpiece size is large, the production cycle is long, and the multi-layer welding or chemical cleaning is contaminated, mechanical cleaning is often used. First wipe the surface with organic solvents such as acetone and gasoline to remove oil, and then brush directly with a diameter of 0.15mm ~ 0.2mm copper wire brush or stainless steel wire brush until the metallic luster is exposed. Generally, it is not appropriate to use grinding wheel or ordinary sandpaper to polish, so as to avoid sand remaining on the metal surface and entering the welding pool to produce defects such as slag inclusion. In addition, scraper, file can also be used to clean the surface to be welded.

After the workpiece and the welding wire are cleaned and cleaned, the oxide film will be re-generated during the storage process, especially in the humid environment, in the environment polluted by acid, alkali and other vapors, the oxide film will grow faster. Therefore, the storage time of the workpiece and welding wire after cleaning and cleaning should be shortened as far as possible, and in the case of humid climate, welding should generally be performed within 4h after cleaning. After cleaning, if the storage time is too long (such as more than 24h), it should be processed again.

(2) The strength of aluminum plate and aluminum alloy is very low at high temperature, the flow performance of liquid aluminum is good, and the weld metal is prone to collapse during welding. In order to ensure welding penetration without collapse, welding often uses a backing plate to support the molten pool and nearby metals. The gasket can be made of graphite plate, stainless steel plate, carbon steel plate, copper plate or copper rod. A circular arc groove is opened on the surface of the backing plate to ensure that the opposite side of the weld is formed. It can also be formed by single-side welding without backing plate, but it requires skilled welding operation or advanced technological measures such as strict automatic feedback control of arc welding energy.

(3) Pre-welding preheating thin, small aluminum parts generally do not need to preheat, the thickness of 10mm ~ 15mm can be preheated before welding, according to different types of aluminum alloy preheating temperature can be 100℃ ~ 200℃, available oxygen acetylene flame, electric furnace or blowtorch heating. Preheating can reduce the deformation and porosity of weldment.

6. Post-welding treatment

(1) Cleaning after welding the residual flux and welding slag left in the weld and nearby will destroy the passivation film on the surface of aluminum, and sometimes corrosion aluminum parts, should be cleaned up. The workpiece with simple shape and general requirements can be cleaned by simple methods such as hot water washing or steam blowing brush. The aluminum parts with high requirements and complex shapes can be washed with a hard brush in hot water, and then soaked in an aqueous solution of chromic anhydride or potassium bichromate solution with a concentration of 2% to 3% at about 60 ° C ~ 80 ° C for 5min ~ 10min, and washed with a hard brush, and then washed in hot water, dried in an oven, or dried with hot air, and can also be naturally dried.

(2) post-welding heat treatment Aluminum containers generally do not require heat treatment after welding. If the aluminum used does have significant stress corrosion sensitivity under the medium conditions of the container contact, post-welding heat treatment is needed to eliminate the high welding stress, so that the stress on the container is reduced to below the critical stress of stress corrosion cracking, then the special requirements should be made by the container design document before the post-welding stress relief heat treatment. If you need to backfire welding 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℃; 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 in a positive or negative direction of 20 ° C ~ 30 ° C, and the holding time can be between 0.5h ~ 2h.

7.MIG welding aluminum and aluminum alloy process characteristics

Aluminum and aluminum alloy are relatively active, and the affinity with oxygen is very large, it is easy to combine with oxygen to produce Al2O3, and its melting point is 2050 ° C, about 3 times the melting point of aluminum. In addition, a solid and dense oxide film is formed on the surface of aluminum at room temperature. This layer of oxide film is not conducive to welding, preventing the joint. To this end, the influence of oxygen must be excluded, first of all, the protective gas of MIG welding must be an inert gas, and pure argon or Ar+He gas can be used, and oxidizing gas (O2 or CO2) must not be mixed. Secondly, direct current reverse polarity (DCRP) should be used, so that the workpiece is the cathode, and the welding seam and the metal oxide film (Ar2O3 film) near it are removed during the welding process by relying on the crushing effect of the cathode, while ensuring the transition stability of the droplet. Third, when MIG welding aluminum, the arc temperature is high (especially in the high current), the arc is filled with metal vapor, when the vapor loses gas protection, and the oxygen phase in the air to generate Ar2O3 and other oxides, in the near seam zone, and even on the weld surface will form black powder. The test shows that the black powder can be greatly reduced by pulsed MIG welding.

The droplet and pool in liquid state easily absorb moisture and form pores. Therefore, the welding wire and material surface should be carefully cleaned before welding, and the purity of the gas should be paid attention to. Due to the fast thermal conductivity and large thermal expansion coefficient of aluminum and aluminum alloy, the welding deformation is large, and it is easy to produce non-fusion and non-penetration. When MIG welding, the heat is more concentrated, so it is more suitable for welding aluminum. However, when welding large thickness workpieces, in order to reduce deformation, preheating measures should be taken, and generally welding should be done in the fixture.

8. How to clean the base metal and welding wire before MIG welding aluminum?

The surface of aluminum and aluminum alloy weldments should be cleaned before welding. The aim is to remove oxide film and oil to prevent porosity and slag inclusion in the weld.

The commonly used cleaning methods in production are cleaning oil and removing oxidation film two processes.

1) Cleaning of oil stains on the surface of the workpiece, you can use gasoline, carbon tetrachloride, trichloroethylene and acetone to wipe, wipe when the use of clean white cloth dipped in solvent cleaning, pay attention not to use cotton yarn.

2) Cleaning of oxide film Surface oxide film using the above solvent cleaning is ineffective, can only use chemical cleaning and mechanical cleaning.

a. Chemical cleaning is the use of alkali and acid to clean the surface of the workpiece, the method can remove the oxide film, but also can remove the oil, the specific process is as follows: the volume fraction of 6% to 10% of sodium hydroxide solution, at about 70℃ soaking 0.5min→ water → volume fraction of 15% of nitric acid at room temperature soaking for 1min for neutralization → water washing → warm water washing → drying. The finished surface is a dull silver white.

b. Mechanical cleaning can use pneumatic or electric milling cutter, can also use scrapers, files and other tools. For thinner oxide film can also be used stainless steel wire brush or fine wire brush brush until exposed metallic luster.

It is best to weld immediately after cleaning, if the parking time exceeds