All porosity in Aluminum welding is caused by hydrogen. Aluminum in its liquid state has a high affinity for hydrogen, as it solidifies however, it loses this affinity and consequently tends to create porosity as the hydrogen is removed. There can be many, many potential sources of hydrogen in a weld, here are a few.

1. Poorly cleaned base metal.
   
2. Residual drawing lubricants on the filler metal.



3. Condensation from a water-cooled welding torch.



4. Unclean shop compressed air.
   
5. High relative humidity.
   
6. Hydrogen in shielding gas.