Abstract: The mechanism of the increasing of A-TIG welding penetration is studied by using the activating flux developed by ourselves for stainless steel. The effect of flux on the flow and temperature fields of weld pool is simulated by the PHOENICS software. It shows that without flux, the fluid flow will be outward along the surface of the weld pool and then down, resulting in a flatter weld pool shape. With the flux, the oxygen, which changes the temperature dependence of surface tension grads from a negative value to a positive value, can cause significant changes on the weld penetration. Fluid flow will be inward along the surface of the weld pool toward the center and then down. This fluid flow pattern efficiently transfers heat to the weld root and produces a relatively deep and narrow weld. This change is the main causation of penetration increasing. And arc shrinking can cause the weld width to become narrower and the penetration to become deeper, but arc shrinking not the main causation of penetration increasing. The simulation results of the weld pool shape accord with the experiment results well.

Key words: Activating flux, A-TIG welding, Penetration increasing, PHOENICS