SUMMARY. GOAL. Obtain experimental data on the effect of ultrafine silica particles in the welding arc, its penetrating power. METHODS. The arc of DC straight polarity with the current strength in the range 70–400 A burned vertically between tungsten non-consumable electrode (diameter 2.4–4 mm) and the metal surface in inert argon gas (TIG welding). The welding experiments were performed on plates measuring 150×50×(2, 4, 6) mm mild steel 3. Before the A-TIG welding was applied on the surface of the wafer uniform controlled flux activating layer (SiO2- silica) with a width of 5 mm and a thickness of 20 microns. Three flux with different fractional composition were used. RESULTS AND ITS DISCUSSION. The voltage on the arc in the pure argon was 12–15V, in pure argon using a flux activating 16–20 V, welding a gas mixture (30% argon + helium) without activating the flux voltage was 20-26 V. According to the thermal power in argon arc welding with flux-activating approaches the welding arc in a gas mixture, but at its superior penetrating power. The stitches are formed deep and narrow, wedge-shaped. CONCLUSIONS. The use of activating flux of ultrafine silica leads to the maximum the penetrating arc capacity by increasing the effective heat capacity of the welding heat source. As a result of applying the proposed composition of the activating flux penetrating power of the welding arc has increased by 2–4 times, the energy intensity of the process has decreased by 30–50%.