Features of welding aluminum and aluminum alloy
Aluminum is easily oxidized in air and during welding, and the aluminum oxide (Al 2 Ó 3 ) generated has a high melting point, is very stable and is not easy to remove.
Prevents melting and melting of the base metal.
The specific gravity of the oxide film is large, so it is not easy to float off the surface, and it is easy to generate defects such as slag inclusion, incomplete melting, incomplete penetration, etc.
The oxide film on the aluminum surface and the absorption of large amounts of water can easily cause porosity in the weld.
Before welding, the surface must be thoroughly cleaned by chemical or mechanical methods to remove the oxide film from the surface.
Strengthen protection during welding to prevent oxidation.
During argon tungsten arc welding, the AC power supply is selected to remove the oxide film through “cathode cleaning”.
During gas welding, use flux to remove the oxide film.
When welding thick plates, the welding heat can be increased, for example, the helium arc heat is high, helium or helium argon mixed gas is used for shielding, or large standard MIG welding is used.
Under the condition of positive DC connection, “cathodic cleaning” is not necessary.
The thermal conductivity and specific heat capacity of aluminum and aluminum alloy are about twice that of carbon steel and low-alloy steel.
The thermal conductivity of aluminum is more than ten times greater than that of austenitic stainless steel.
In the welding process, a large amount of heat can be quickly transmitted into the base metal.
Therefore, when welding aluminum and aluminum alloys, more heat is unnecessarily consumed in other parts of the metal in addition to the molten metal pool.
The consumption of this useless energy is more significant than that of welding steel.
To obtain high-quality welded joints, maximum energy must be used with concentrated energy and large power.
Sometimes technological measures such as preheating can also be adopted.
The coefficient of linear expansion of aluminum and aluminum alloy is about twice that of carbon steel and low-alloy steel.
The volumetric contraction of aluminum during solidification is large, and the deformation and stress of welding are large.
Therefore, measures must be taken to prevent welding deformation.
Shrinkage cavity, shrinkage porosity, hot cracking and high internal stress are easily produced during the solidification of aluminum welding puddle.
In production, measures can be taken to adjust the welding wire composition and welding process to prevent hot cracking.
If corrosion resistance is allowed, aluminum silicon alloy welding wire can be used to weld aluminum alloys except aluminum magnesium alloy.
When the silicon content in aluminum and silicon alloy is 0.5%, the tendency of hot cracking is greater.
With increasing silicon content, the crystallization temperature range of the alloy becomes smaller, the fluidity is significantly improved, the shrinkage is reduced, and the tendency to hot cracking is also reduced.
According to production experience, when the silicon content is 5% ~ 6%, no hot cracking will occur, therefore, the use of SAlSi bars (silicon content 4.5% ~ 6%) of the wire welding will have better crack resistance.
Aluminum has a strong ability to reflect light and heat.
When solid and liquid are transferred, there is no obvious color change.
It is difficult to evaluate during the welding operation.
High temperature aluminum has low strength and is difficult to withstand weld pool and easy to weld.
Aluminum and aluminum alloys can dissolve a large amount of hydrogen in the liquid state, but almost no hydrogen in the solid state.
In the process of solidification and rapid cooling of the welding pool, hydrogen cannot overflow over time and hydrogen pores are easily formed.
Moisture in the arc column atmosphere and moisture absorbed by the oxide film on the surface of welding materials and base metal are important sources of hydrogen in the weld.
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Therefore, the hydrogen source must be strictly controlled to prevent pore formation.
Alloying elements are easy to evaporate and burn, which will reduce welding performance.
If the base metal is strengthened by deformation or solution aging, the heat from welding will reduce the strength of the heat-affected zone.
Aluminum is a face-centered cubic lattice with no isomers.
There is no phase transformation during heating and cooling.
Weld grains are easy to thicken and cannot be refined through phase transformation.
Welding method
Almost all types 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.
Resistance welding
Generally, aluminum alloy resistance butt welding (spot welding) can only be used for overlap welding of plates with a thickness of less than 5 mm or between bars with a thickness of less than 10 mm.
The advantages are low welding cost, high welding efficiency and easy integration into automatic production lines.
For example, automobile manufacturing is widely used.
The limitation is that the welding thickness is limited and different electrodes must be made for different products and structures.
Argon Arc Welding
Manual argon tungsten arc welding is mainly used to weld aluminum alloy sheet structures (thickness <6mm).
Due to the protective effect of argon and the crushing effect of argon ion on the aluminum alloy oxide film, argon arc welding can avoid welding dust, thereby preventing the corrosion of welding residue in the joint.
Therefore, after argon arc welding, cleaning is not necessary and the shape of the joint can also be unrestricted.
In addition, the argon flow that passes through the welding area during welding can significantly cool the welded joint, thereby improving the structure and performance of the joint and reducing welding deformation.
Gas shielded welding
In general, it is difficult to master one-sided welding and two-sided forming of aluminum alloys with gas shielded welding.
Related Reading: Shielding Gas for Laser Welding
If the top plates have gaps, it is easy to weld them, and the further penetration of gap-free welds is not easy to control.
Generally, AC argon arc welding is also used for aluminum alloy welding in China, but for thicker plates, the efficiency of argon arc welding will be very low.
At present, fused electrode pulse gas shielded aluminum alloy welding is only used in a few examination items, and most of the butt plate welding is overhead welding, mainly used for the welding of car body of aluminum alloy and EMUs structure.
Friction welding
The friction stir welding seam of aluminum alloy is formed by plastic deformation and dynamic recrystallization.
The grain in the welding zone is fine, without fusion welding dendrites, and the microstructure is good.
The heat-affected zone is narrower than in fusion welding, and there is no loss from burning alloy elements, cracks, pores and other defects. Comprehensive performance is good.
Compared with the traditional fusion welding method, it has no splash, smoke and dust, does not need to add welding wire and shielding gas, and has good joint performance.
Due to the solid phase welding process, the low heating temperature makes the welding deformation small.
The disadvantage is that the welding speed is slow and the process is not mature enough.
laser welding
Aluminum alloy laser welding technology is a new technology developed in the past ten years.
Related reading: Laser welding: the basic guide
Compared with traditional welding technology, it has the characteristics of strong function, high reliability, no need for vacuum conditions and high efficiency.
It is characterized by high power density, low total heat input, large penetration of the same heat input, small heat affected zone, small welding deformation, high speed, easy industrial automation, etc.
Its disadvantage is that when welding aluminum alloys, the energy cannot be fully absorbed, resulting in large amounts of waste and high equipment acquisition costs.
