Aluminum alloying element binding affinity
Describes the chemical tendency of Aluminum to form stable nitrides or oxides with interstitial trace elements, thereby strengthening the matrix or impeding dislocation movement.
Aluminum has a high affinity for oxygen and immediately forms a stable oxide layer. When welding, you have to break it up, otherwise you won't get a clean fusion, but slag and lack of fusion defects. The alloying elements like silicon or magnesium determine how weldable the material is and how the structure reacts. With common wrought alloys, you have to be careful not to put in too much heat, or the weld will crack or the strength in the heat-affected zone will drop. Basically: clean it, remove the oxide layer mechanically, prepare properly, and work with the right filler material. Then you'll get a proper weld that holds.