Abstract: The weld penetration increased by SiO₂ is the most obviously when some single-component fluxes are used as surface activating flux in AC A-TIG(alternating current activating-tungsten inert- gas)welding of aluminum alloy.The mechanism of SiO₂ increasing weld penetration of AC A-TIG welding was studied.SiO₂ has little effect on weld appearance of electron beam welding with positive defocus.A series of DCSP A-TIG welding experiments were carried out to investigate the effects of SiO₂ on welding arc.It is found that SiO₂ can constrict arc polarity area, broaden arc column and improve arc voltage.AC A-TIG welding with helium shielding gas was carried out.It is found that SiO₂ constricts evaporation area, weakens evaporation of activating flux or base metal, and depresses weld pool surface.During the welding process, SiO₂ coating is always on the weld pool surface.Only there is very narrow gap on the central region of depressed weld pool.Vapor concentrates in the lower middle part of arc.The results indicate that arc polarity area constriction and input heat increase are the main mechanism to increase weld penetration by SiO₂ in AC A-TIG welding for aluminum alloy