1 welding characteristics of aluminum and its alloys
(1) aluminum is easily oxidized in air and during welding, and the generated alumina (Al2O3) has a high melting point and is very stable and difficult to remove. Hinder the melting and fusion of the substrate and oxidize the film.
Large proportion, difficult surface, easy to produce slag inclusion, incomplete fusion, incomplete penetration and other defects. The oxide film on the aluminum surface and the adsorption of a large amount of water are easy to cause pores in the weld. weld
Before connection, the surface should be cleaned strictly by chemical or mechanical methods to remove the oxide film on its surface. Strengthen protection during welding to prevent oxidation. When TIG welding, choose
The AC power supply removes the oxide film through "cathode cleaning". In gas welding, flux for removing oxide film is used. When welding thick plates, the welding heat can be increased, such as helium arc.
Because of the high heat, it is unnecessary to "clean the cathode" by using helium or argon-helium mixed gas protection or large metal electrode gas shielded welding in the case of direct DC connection.
(2) The thermal conductivity and specific heat capacity of aluminum and its alloys are more than 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 conducted into the base metal, so when welding aluminum and aluminum alloys, energy is not only consumed in the metal bath, but also more heat is consumed.
Other parts of metal consume much more useless energy than steel welding. In order to obtain high quality welded joints, energy concentration and power should be used as much as possible.
For large-scale energy sources, technical measures such as preheating can sometimes be adopted.
(3) The linear expansion coefficient of aluminum and aluminum alloy is about twice that of carbon steel and low alloy steel. When aluminum solidifies, the volume shrinkage rate is large, and the deformation and stress of weldments are large. Therefore, it is necessary
Take measures to prevent welding deformation. Shrinkage, porosity, thermal crack and high internal stress are easy to occur when aluminum welding pool solidifies. The method of adjusting welding wire composition and welding can be adopted in production.
Technological measures to prevent hot cracks. Al-Si alloy welding wire can be used to weld aluminum alloys other than Al-Mg alloy if corrosion resistance allows. In Al-Si alloy
When the silicon content is 0.5%, the tendency of hot cracking is greater. With the increase of silicon content, the crystallization temperature range of the alloy becomes smaller, the fluidity is significantly improved, the shrinkage rate is reduced, and the hot cracking tendency is correspondingly reduced.
According to the production experience, when the silicon content is 5% ~ 6%, there will be no hot cracking, so the welding wire containing SAlSi strip (silicon content is 4.5% ~ 6%) will have better crack resistance.
(4) Aluminum has strong reflection ability to light and heat, and there is no obvious color change when it is in solid-liquid transition state, so it is difficult to judge when welding. The strength of high-temperature aluminum is very low, so it is difficult to support the molten pool.
It is easy to weld through.
(5) Aluminum and aluminum alloys can dissolve a lot of hydrogen in liquid state, but hardly dissolve hydrogen in solid state. In the process of solidification and rapid cooling of welding pool, hydrogen is too late to overflow and is easy to form.
Hydrogen hole. The moisture in the arc column atmosphere, the moisture absorbed by the welding material and the oxide film on the surface of the parent metal are all important sources of hydrogen in the weld. So the source of hydrogen should be strict.
Control to prevent pore formation.
(6) Alloy elements are easy to evaporate and burn, which reduces the welding performance.
(7) When the parent metal of the parent metal is deformed or solution aged, the welding heat will reduce the strength of the heat affected zone.
(8) Aluminum is a face-centered cubic lattice with no isomorphism, and there is no phase change during heating and cooling, so the weld grain is easy to be coarse, and it is impossible to refine the grain through 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 each welding method has its own advantages.
Their respective applications. Gas welding and covered electrode arc welding equipment is simple and easy to operate. Gas welding can be used for repair welding of aluminum plates and castings with low requirements on welding quality. weld
Pole arc welding can be used to repair aluminum alloy castings. Inert gas shielded welding (TIG or MIG) is the most widely used welding method for aluminum and aluminum alloys. Thin aluminum and aluminum alloys
The plate can be welded by tungsten alternating current argon arc welding or tungsten pulse argon arc welding. Thick aluminum and aluminum alloy plates can be welded by tungsten-helium arc welding, argon-helium mixed tungsten gas shielded welding and metal electrode gas shielded welding.
Welding, pulsed metal gas shielded welding. Metal arc welding and pulsed metal arc welding are widely used (argon or argon/helium mixture).
3. Welding materials
(1) welding wire The selection of aluminum and aluminum alloy welding wire should not only consider the good welding process performance, but also make the tensile strength and plasticity of butt joint meet the requirements of the container (through bending test).
In order to meet the specified requirements, the impact toughness of aluminum-magnesium alloy with magnesium content exceeding 3% should be met, and the corrosion resistance of welded joints should reach or approach the level of parent metal for containers with corrosion resistance requirements. Therefore, the selection of welding wire mainly follows the following principles:
1) The purity of pure aluminum welding wire is generally not lower than that of base metal;
2) The chemical composition of aluminum alloy welding wire generally corresponds to or is similar to the parent metal;
3) Content of corrosion-resistant elements (magnesium, manganese, silicon, etc.). ) in aluminum alloy welding wire is generally not lower than the parent metal;
4) When welding dissimilar aluminum materials, the welding wire should be selected according to the parent material with high corrosion resistance and high strength;
5) High-strength aluminum alloy (heat treatment strengthened aluminum alloy) that does not require corrosion resistance can use welding wires with different compositions, such as aluminum-silicon alloy welding wire SAlSi- 1 with good crack resistance (note).
Degree may be lower than the parent metal).
(2) Protective gas The protective gas is argon, helium or their mixture. In AC plus high frequency TIG welding, more than 99.9% pure argon gas is used, and helium gas is suitable for DC positive polarity welding.
Gas. When MIG welding, the plate thickness
It is appropriate to add 10% ~ 35% or 50% helium to the gas; When the plate thickness >; At 75 mm, argon containing 50% ~ 75% helium is recommended. Argon gas shall conform to GB/T 4842? 995
Requirements for pure argon. When the pressure of argon cylinder is lower than 0.5 MPa, the pressure is insufficient and it cannot be used.
(Tungsten electrode materials used in TIG welding include pure tungsten, thorium tungsten, cerium tungsten and zirconium tungsten. Pure tungsten electrode has high melting point and boiling point, which is not easy to melt and volatilize, and the electrode is burned and the tip is polluted.
Less, but the electron emission ability is poor. The electrode containing 1% ~ 2% thorium oxide in pure tungsten is thorium tungsten electrode, which has strong electron emission ability, high allowable current density and arc combustion.
It is stable, but thorium has certain radioactivity, so appropriate protective measures should be taken when using it. Electrode with 1.8% ~ 2.2% cerium oxide (impurity ≤ 0. 1%) added to pure tungsten.
This is a cerium tungsten electrode. Cerium-tungsten electrode is widely used because of its low electron work function, high chemical stability, high allowable current density and no radioactivity. Zirconium-tungsten electrode can prevent electrode pollution.
Dyed base material, the tip is easy to keep hemispherical, suitable for AC welding.
(4) The flux of gas welding is chloride and fluoride of potassium, sodium, lithium, calcium and other elements, which can remove the oxide film.
4. Preparation before welding
(1) When cleaning aluminum and aluminum alloy before welding, the oxide film and oil stain on the surface of workpiece welding joint and welding wire should be strictly removed before welding, and the cleaning quality directly affects the welding process and joint quality.
Such as porosity and mechanical properties of welds. Chemical cleaning and mechanical cleaning are often used.
1) chemical cleaning chemical cleaning has high efficiency and stable quality, and is suitable for cleaning welding wires and workpieces with small size and mass production. There are two methods available: soaking and scrubbing. Available c
Surface degreasing of organic solvents such as ketone, gasoline and kerosene. Alkaline washing with 5% ~ 10% NaOH solution at 40℃ ~ 70℃ for 3 min~7 min (the time for pure aluminum is slightly longer but not more than 20 min),
Rinse with flowing clean water, then pickle with 30% HNO3 solution at room temperature to 60℃ for 65438±0min ~ 3min, rinse with flowing clean water, and air dry or dry at low temperature.
2) Mechanical cleaning
Mechanical cleaning is often used when the workpiece is large in size, long in production cycle and polluted after multi-layer welding or chemical cleaning.
(1) First, wipe the test surface with organic solvents such as acetone and gasoline to remove oil stains, and then directly brush it with a copper wire brush or a stainless steel wire brush with a diameter of 0. 15 mm ~ 0.2 mm until the metallic luster is exposed. Generally, it is not suitable to use grinding wheel or ordinary sandpaper to polish, so as to avoid defects such as slag inclusion caused by sand particles left on the metal surface and entering the molten pool during welding. In addition, scrapers and files can also be used to clean the surface to be welded. After cleaning, the workpiece and welding wire will produce oxide film again during storage, especially in humid environment, polluted by steam such as acid and alkali, and the oxide film will grow faster. Therefore, the storage time of workpieces and welding wires from cleaning to welding should be shortened as much as possible. In humid climate, welding should generally be carried out within 4 h after cleaning. After cleaning, if the storage time is too long (such as more than 24 h), it should be reprocessed.
(2) The strength of back aluminum and aluminum alloy is very low at high temperature, and the liquid aluminum has good fluidity, so the weld metal is easy to collapse during welding. In order to ensure penetration without collapse, backing plates are usually used to support the molten pool and nearby metals. The backing plate can be made of graphite plate, stainless steel plate, carbon steel plate, copper plate or copper rod. Form a circle on the surface of the pad.
Arc groove to ensure that the weld is formed on the opposite side. It can also be formed by single-sided welding and double-sided welding without backing plate, but it requires skilled welding operation or strict automatic feedback control of arc welding energy.
System and other advanced technical measures.
(3) Preheating thin and small aluminum parts before welding is generally unnecessary. When the thickness is 10 mm ~ 15 mm, preheating can be carried out before welding, and the preheating temperature can be 100℃ according to different aluminum alloy types.
200℃ can be heated by oxygen-acetylene flame, electric furnace or blowtorch. Preheating can reduce the deformation and porosity of weldments.
5. Post-welding treatment
(1) After welding, cleaning the residual flux and slag in the weld and its vicinity will destroy the passive film on the aluminum surface and sometimes corrode the aluminum parts, so it should be cleaned up. The shape is simple and important.
Generally, workpieces can be cleaned by simple methods such as hot water washing or steam blowing. Aluminum parts with high requirements and complex shapes are brushed in hot water with a hard brush and then heated at 60℃
Soak in 2% ~ 3% aqueous chromic anhydride solution or potassium dichromate solution at about 80℃ for 5 min ~ 10 min, clean with a hard brush and then clean with hot water.
Dry in an oven, or dry with hot air, or naturally dry.
(2) Post-weld heat treatment of aluminum containers generally does not require post-weld heat treatment. If the aluminum material used does have obvious stress corrosion sensitivity under the medium conditions of container contact, it needs to be welded.
Post-heat treatment to eliminate high welding stress and reduce the stress on the container below the critical stress of stress corrosion cracking should be proposed by the container design document.
No need, and then carry out post-welding stress relief heat treatment. If welding annealing is needed, for pure aluminum, 5052, 5086, 5 154, 5454, 5A02, 5A03, 5A06, etc.
The recommended temperature is 345℃; For 20 14, 2024, 3003, 3004, 5056, 5083, 5456, 606 1, 6063, 2A 12, 2A24, 3A2 1 etc. , the recommended temperature is.
4 15℃; For 20 17, 2A 1 1, 6A02, etc. The recommended temperature is 360℃. According to the size and requirements of the workpiece, the annealing temperature can be adjusted by 20℃ ~ 30℃ positively or negatively.
The heat preservation time can be between 0.5 hours and 2 hours.