Aluminum and aluminum alloys are frequently used in welded constructions. The low density means that the constructions are lightweight. The thin and tenacious layer of aluminum oxide on the surface gives good corrosion
behavior in many environments. The oxide layer is healed in the air if it is damaged. Most aluminum alloys are easily formed by rolling, forging, and extrusion. Extruded profiles of sophisticated shapes can be manufactured and give unique possibilities for the designers. Casting is also often used.
The use of aluminum and its alloys for welded applications is increasing due to their low weight, good corrosion resistance, and high toughness even at low temperatures. However, the special characteristics of welding aluminum have to be taken into account. For instance, one has to avoid hydrogen input, which is introduced during the welding operation through contaminations or moisture. The oxide layer, which usually provides corrosion resistance, can cause problems during welding. Al2O3 has a melting point of over 2000°C, which is much higher than that of aluminum of 660°C. Furthermore, it is porous and can absorb moisture, and it is electrically isolating. Usually, the oxide layer has to be removed before welding, either mechanically or chemically. During MIG welding, the cleaning effect of the arc supports the break up of the oxide layer. For MMAW, flux is added to the covering of stick electrodes.
MIG welding is one of the most important processes for joining aluminum. Filler materials are available as solid wires for example of the types Al, AlSi, AlMg, and AlMn, but no cored wires are available until now.
Some properties are important for the welding of aluminum and its alloys:
- A strong and tough oxide is very easily formed on the surface and can cause weld defects if not properly removed or broken up. The oxide has low electrical conductivity.
- The aluminum oxide has a melting temperature of about 2060 C, which is much higher than the melting temperature of pure aluminum at 660 C. The oxide has a higher density than aluminum and may, therefore, remain in the melted aluminum and form slag inclusions in the solid material.
- Aluminum has a high solubility for hydrogen in the molten state but not in the solid phase. This will very easily lead to porosity since the hydrogen has insufficient time to disappear during solidification.
- The heat source for welding must be very intense depending on the high thermal conductivity of aluminum. MIG welding is therefore suitable.
- Aluminum has a low melting point and it is difficult to see when it melts
since there is no color change.
- There is no arc blow caused by the material itself since aluminum is non- magnetic. However, the arc can deflect considerably from the electrode direction. This may have different reasons. The electric current always chooses the easiest path (shortest or least resistance). The oxide layer also influences this behavior. Magnetic fields generated by the current, the tilting of the welding torch, and the connection of the return cable also have an influence.