The research is focused on influence of shielding gas on GMA welding process of precipitation hard enable Al alloys (6xxx and 7xxx). Pure Ar of different gas flow 0-35 l∙min-1, different Ar+He mixtures, up to 70 % He, active Ar+CO2 gases were used. The optimum Ar shielding gas flow was found to be 15 l∙min-1, where under this value porosity and over this value decrease of weld penetration have been observed. He content in Ar+He shielding gas mixtures is changing heat input and therefore weld shape. Increase of He content improves heat transfer efficiency, weld penetration, weld width etc. Influence of theoretically known properties of gases on welding process (e.g. ionization potential, thermal conductivity etc.) is quite complex, so experimentally researched influence of gases and their mixtures done in this research with exact values and practical results are valuable for praxis.