ok. There are several possibilities for porosity;
1. Check your gas supply. I presume you are using a welding grade of Argon gas. Depending on usage, I have seen problems with gas contaminants that could result could in porosity of the weld during solidification.
2. Make sure the weld joint is clean of oil, obvious surface oxides and other contaminants PRIOR to welding. Aluminum oxide can absorb moisture. During welding, hydrogen that is produced from moisture is absorbed by the weld metal resulting in porosity.
3. Aluminum is readily weldable by the GTAW process. Make sure you are using an AC power setting to provide for cleaning action of the aluminum oxide during welding.
4. You might want to check the condition of the welding electrodes. Bare wire that is laying around the shop can develop an oil film that will cause porosity. Wipe down the welding electrodes before using.