Why is Aluminum Alloy Welding So Difficult?

The welding of aluminum alloys is still a difficult problem that troubles manufacturers to this day. What is the reason why there are so many difficulties in welding aluminum alloys?

(1) Aluminum oxidizes very easily in air and during welding, and the generated aluminum oxide (Al2O3 ) has a high melting point,is very stable and not easy to remove. Impede the melting and fusion of the base metal, the specific gravity of the oxide film, not easy to float to the surface, easy to produce defects such as slag inclusion, lack of fusion, and lack of penetration.

Aluminum surface oxide film and adsorption of a large amount of water, easy to make the weld produce porosity. Before welding, chemical or mechanical methods should be used for strict surface cleaning to remove the surface oxide film. Strengthen protection during welding to prevent its oxidation. When tungsten argon arc welding, using AC power supply, through the "cathodic cleaning" effect, removes the oxide film. When gas welding, use flux that removes the oxide film. In thick plate welding, can increase the welding heat, for example, if the helium arc has high heat, use helium or argon-helium mixed gas for protection, or use large-scale gas metal arc welding. In the case of DC positive connection, there is no need for "cathode cleaning".

(2) Thermal conductivity and specific heat capacity of aluminum and aluminum alloys are more than twice that of carbon steel and low alloy steel.  The thermal conductivity of aluminum is more than ten times that of austenitic stainless steel. During the welding process, a large amount of heat can be quickly applied to the base metal. Therefore, when welding aluminum and aluminum alloy, in addition to the energy consumed in the molten metal pool, more heat is also consumed unnecessarily in other parts of the metal,the useless energy consumption is more significant than the welding of steel, in order to obtain high quality welded joints, should try to use energy concentration, power energy, and sometimes can be used to preheat and other process measures.

(3) Aluminum and aluminum alloy coefficient of linear expansion is about twice that of carbon steel and low alloy steel. The volume shrinkage of aluminum solidification is larger, and the deformation and stress of the weldment is larger, therefore, it is need to take measures to prevent the welding deformation. When the aluminum welding molten pool solidifies, it is easy to produce shrinkage cavities, shrinkage porosity, hot cracks, and high internal stress. Production can be used to adjust the composition of the wire and welding process measures to prevent hot cracks. In the case of corrosion resistance permits, aluminum-silicon alloy welding wire can be used to weld aluminum alloys other than aluminum-magnesium alloys. When the aluminum-silicon alloy contains 0.5% silicon, the tendency of thermal cracking is greater, with the increase in silicon content, the alloy crystallization temperature range becomes smaller, mobility significantly improves the shrinkage rate decreases, and the tendency of thermal cracking is also correspondingly reduced. According to the production experience, when the silicon content of 5% to 6% can not produce thermal cracking, so using a SAlSi strip (Silicon content of 4.5% ~ 6%) wire welding will have better crack resistance.

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

(5) Aluminum and aluminum alloy in the liquid state can dissolve a large amount of hydrogen, the solid-state is almost insoluble hydrogen. During welding molten pool solidification and rapid cooling process, hydrogen is too late to overflow, very easy to form hydrogen pores. Moisture in the arc column atmosphere, welding material and the moisture adsorbed by the oxide film on the surface of the base material are 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, which reduces the performance of the weld.

(7) If the base metal is deformation strengthened or the solid solution aging strengthened, the welding heat will cause the strength of the heat-affected zone to decrease.

(8) Aluminum is a face-centered cubic lattice and has no allotropes, there is no phase change during heating and cooling, the weld grain is easy to be coarsened, and the grain cannot be refined by phase change.