Abstract: A-TIG welding experiments of 6 mm 6061 aluminum alloy were carried out to investigate the effects of activating fluxes on weld appearance and macroscopic morphologies by using five kinds of single-component activating fluxes such as SiO₂,TiO₂,CaF₂,Cr₂O₃ and BaCl₂. The mechanism to increase weld penetration was analyzed by considering the change of arc shape in A-TIG welding process. The experimental results indicate that all the above activating fluxes improve weld penetration. The most remarkable effect is obtained when the flux is SiO₂,but the effect of CaF₂ on penetration is not obvious. The weld appearance is poor when the base metal is coated with SiO₂,while good weld appearance can be obtained when the flux of TiO₂ is used. The phenomenon of arc contraction is not remarkable when the arc moves into the flux zone. The significant improvement of weld penetration may be the result of increase in heat input caused by the change of resistance in conducting channel.