Oxide Layer in Aluminum Welding

Aluminum is essentially coated with an amorphous Al₂O₃ structure consisting of two superimposed sublayers. When exposed to the atmosphere, an oxide layer forms rapidly.

These two layers are as follows:

• Amorphous Aluminum Oxide: A nearly non-porous layer that acts as a base or barrier.
• Porous Layer: A hydrated surface layer containing Al-hydroxides and low-crystalline bayerite.

The thickness of the oxide layer increases depending on the oxygen content in the environment and the temperature. Although the oxide layer has a dense structure and provides resistance to corrosive effects with a high melting point of 2300°C, it can absorb moisture due to its porous structure.

The surface quality of aluminum directly affects MIG and TIG welding methods and depends on the following:

• Arc stability: (The presence of aluminum oxide is necessary for a stable arc)
• Geometry of the arc focal point
• Arc voltage drop and distance from the weld spot
• Weld geometry
• Weld quality
• Repeatability, especially in automated welding processes

Since the oxide layer is only a few nanometers thick, it is difficult to measure under practical conditions. To obtain a defined oxide layer, the old layer must be completely removed using chemical methods (chemical cleaner) under controlled conditions (environment and duration). Since the density of aluminum oxide is higher than that of the metal itself, the oxide sinks into the molten metal and can cause inclusions. In contrast, in metals such as steel, the oxide rises to the surface of the molten pool and floats.