Aluminum and aluminum alloy welding process

Welding characteristics of aluminum and aluminum alloys

(1) aluminum in the air and welding is easy to oxidize, the alumina (Al2O3) generated high melting point, very stable, not easy to remove. Hinder the melting and fusion of the base metal, the ratio of the oxide film is significant, it is not easy to surface, easy to produce slag inclusion, not fusion, not penetration and other defects. The surface of aluminum oxide film and adsorption of a large amount of water, easy to make the weld porosity. Before welding, the surface shall be strictly cleaned by chemical or mechanical methods to remove the oxide film on the surface. Strengthen protection during welding to prevent oxidation. In GTAW, ac power supply is used to remove the oxide film through "cathode cleaning". In gas welding, flux to remove oxide film is used. In thick plate welding, welding heat can be increased, for example, helium arc heat, using helium or argon-helium mixture protection, or the use of large specification of fusion gas welding, in the case of direct DC connection, "cathode cleaning" is not required.

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(2) The thermal conductivity and specific heat capacity of aluminum and its alloy 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 welding process, a lot of heat transmission can be rapidly into the base metal, so when the welding aluminium and aluminium alloy, in addition to consume energy in the molten metal molten pool, have more useless heat consumption in other parts of the metal, the useless energy consumption is more significant than steel welding, in order to obtain high quality welded joint, Energy concentration and high power should be used as far as possible, and sometimes preheating and other technological measures can also be used.

(3) The linear expansion coefficient of aluminum and its alloy is about twice that of carbon steel and low alloy steel. The volume shrinkage rate of aluminum during solidification is large, and the deformation and stress of welding parts are large. Therefore, measures should be taken to prevent welding deformation. It is easy to produce shrinkage cavity, shrinkage porosity, thermal crack and high internal stress 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. Aluminum alloys other than al-mg alloys can be welded with al-si alloy wires if corrosion resistance is allowed. With the increase of silicon content, the crystallization temperature range of the alloy decreases, the fluidity increases significantly, the shrinkage rate decreases, and the hot cracking tendency decreases correspondingly. According to the production experience, when the silicon content is 5% ~ 6%, there is no hot cracking, so the use of SAlSi strip (silicon content is 4.5% ~ 6%) wire will have better cracking resistance.

(4) Aluminum has strong reflection ability of light and heat. When solid and liquid turn into state, there is no obvious color change, and it is difficult to judge during welding operation. High temperature aluminum strength is very low, support molten pool difficult, easy to weld through.

(5) Aluminum and its alloys can dissolve a lot of hydrogen in liquid state, but hardly in solid state. In the process of solidification and rapid cooling of welding pool, hydrogen has no time to overflow and hydrogen pores are easily formed. The moisture in the arc column atmosphere, the moisture absorbed by the oxide film on the surface of welding material and base metal 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) Alloy elements are easy to evaporate and burn, so that the performance of the weld decreases.

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

(8) Aluminum is a face-centered cubic lattice with no isomers. There is no phase change during heating and cooling. The weld grain is easy to be coarse and cannot be refined by phase change.

2. Welding method

Almost all welding methods can be used to weld aluminum and aluminum alloys, but the adaptability of aluminum and aluminum alloys to various welding methods is different, and each welding method has its own application. Gas welding and electrode arc welding method, simple equipment, easy to operate. Gas welding can be used for repair welding of aluminum sheet and casting with low welding quality requirements. Electrode arc welding can be used for repair welding of aluminum alloy castings. Inert gas welding (TIG or MIG) 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 pulse argon arc welding. The thick aluminum and aluminum alloy plate can be used in the tungsten he arc welding, argon-he mixed tungsten gas welding, fusion gas welding, pulse fusion gas welding. Gmaw and PULSE GMAW are more and more widely used (argon or argon/helium mixture)

3. Welding materials

(1) welding wire

The selection of aluminium and aluminium alloy wire besides considering the good welding performance, in accordance with the requirements of the container should keep butt joint tensile strength, plasticity (by bending test) to achieve specified requirements, the magnesium content more than 3% of the aluminum magnesium alloy should meet the requirements of impact toughness, there is a requirement for corrosion of container, the corrosion resistance of welding joint should be at or near the level of the parent 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 metal;

2) the chemical composition of aluminum alloy welding wire is generally corresponding to or close 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 metal;

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

5) does not require corrosion resistance of high strength aluminum alloy (heat treatment reinforced aluminum alloy) can use dissimilar composition of the wire, such as good crack resistance of aluminum silicon alloy wire SAlSi 1, etc. (note that the strength may be lower than the base material).

(2) Protective gas

The shielding gas is argon, helium, or a mixture thereof. For AC and HF TIG welding, more than 99.9% pure argon is used, while helium is suitable for DC positive polarity welding. When MIG welding, plate thickness < Argon gas should be used for 25 mm; When the plate thickness is 25 mm ~ 50 mm, 10% ~ 35% helium should be added in argon. L0% ~ 35% or 50% helium should be added in argon when the plate thickness is 50mm-75mm. When the thickness & gt; Argon with 50% to 75% helium is recommended for 75 mm. Argon shall conform to GB/T 4842? 995 "Pure Argon" requirements. Argon cylinder can not be used because of insufficient pressure after being lowered to 0.5mpa.

(3) the tungsten electrode

Argon arc welding tungsten electrode materials are pure tungsten, thorium tungsten, cerium tungsten, zirconium tungsten. Pure tungsten electrode has high melting point and boiling point, which is not easy to melt and volatilize, less electrode burning loss and tip pollution, but poor electron emission ability. When 1% ~ 2% thorium oxide electrode is added to pure tungsten, thorium tungsten electrode has strong electron emission ability, high allowable current density and relatively stable arc combustion. However, thorium element has certain radioactivity, and appropriate protective measures should be taken when using. In pure tungsten, 1.8% ~ 2.2% cerium oxide (impurity ≤ 0.1%) electrode cerium tungsten is added. Cerium-tungsten electrode is widely used at present because of its low electron emission, high chemical stability, high permissible current density and no radioactivity. Zirconium tungsten can prevent the electrode from contaminating the base metal, the tip is easy to maintain hemispherical, suitable for AC welding.

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

4. Preparation before welding

(1) Clean before welding

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

1) Chemical cleaning

High chemical cleaning efficiency, stable quality, suitable for cleaning welding wire and small size, batch production of the workpiece. Available dip method and scrub method two kinds. Acetone, gasoline, kerosene and other organic solvents can be used to remove oil on the surface, using 5%-10% NaOH solution of 40℃ ~ 70℃ alkaline washing for 3 min ~ 7 min(pure aluminum time is a little longer but not more than 20 min), rinse with flowing water, and then pickling with 30% HNO3 solution of room temperature to 60℃ for 1 min ~ 3 min, rinse with flowing water. Air drying or low temperature drying.

2) Mechanical cleaning

Mechanical cleaning is often used when the workpiece size is larger, the production cycle is longer, and the multilayer welding or chemical cleaning is contaminated. First use acetone, gasoline and other organic solvents to wipe the test surface to remove oil, then directly use 0.15mm ~ 0.2mm diameter copper wire brush or stainless steel wire brush, brush until the metal luster is exposed. Generally, it is not suitable to use grinding wheel or ordinary sandpaper to avoid sand particles left on the metal surface and slag inclusion and other defects when entering the molten pool during welding. In addition, scraper and file can be used to clean the welding surface.

Workpiece and welding wire after cleaning and cleaning, in the storage process will re-produce oxide film, especially in the humid environment, in the environment of acid, alkali and other vapor pollution, oxide film growth faster. Therefore, the workpiece and welding wire cleaning and cleaning before the storage time should be as short as possible, in the humid climate, generally should be cleaned within 4 h after welding. After cleaning, if the storage time is too long (such as more than 24 h), it should be reprocessed.

(2) the plate

Aluminum and aluminum alloy at high temperature strength is very low, liquid aluminum flow performance is good, welding seam metal is prone to collapse phenomenon. In order to ensure the penetration of welding without collapse, welding is often used to support the molten pool and the nearby metal. The backing plate can be made of graphite plate, stainless steel plate, carbon steel plate, copper plate or copper bar, etc. A circular arc groove is opened on the surface of the plate to ensure that the reverse side of the weld is formed. It can also be single-sided welding and double-sided forming without backing plate, but it requires skilled welding operation or advanced technological measures such as strict automatic feedback control of arc welding energy.

(3) preheating before welding thin, small aluminum parts generally do not preheat, thickness of 10 mm ~ 15 mm can be preheating 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 deformation, porosity and other defects.

5. Post-welding treatment

(1) Clean up after welding

Residual flux and welding slag left on and near the weld after welding will destroy the passivation film on the aluminum surface and sometimes corrode the aluminum parts, which should be cleaned. Simple shape, general requirements of the workpiece can be washed with hot water or steam blowing brush and other simple methods to clean. Demand is high and complex shape aluminum, with bristle brush to brush after washing in hot water, then at around 60 ℃ ~ 80 ℃, the concentration of 2% ~ 3% of chromic anhydride in aqueous solution or potassium dichromate solution immersion 5 min ~ 10 min, and hard brush wash, and then wash in hot water washing, use oven to dry, or use the hot air blow dry, also can be natural drying.

(2) post-welding heat treatment

Aluminum containers generally do not require heat treatment after welding. If the aluminum used under the condition of the medium in contact with the container, there is obvious stress corrosion sensitivity through heat treatment after welding are needed to eliminate the welding stress is higher, to stress on the container down to produce stress corrosion cracking under the critical stress, at this time should be put forward special requirements by the container design documents, to eliminate stress after welding heat treatment. If you need to reverse heat treatment, for pure aluminum, 5052, 5086, 5154, 5454, 5A02, 5A03, 5A06, the recommended temperature is 345℃; For 2014, 2024, 3003, 3004, 5056, 5083, 5456, 6061, 6063, 2A12, 2A24, 3A21, the recommended temperature is 415℃; For 2017, 2A11, 6A02, the recommended temperature is 360℃, according to the size and requirements of the workpiece, the annealing temperature can be adjusted forward or negative 20℃ ~ 30℃, the holding time can be between 0.5 h ~ 2 h.

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