Abstract: In order to further reveal the characteristic of U-TIG arc, a hole drilling method is used to determine the difference of arc distribution between Ultrasonic-TIG (U-TIG) and conventional TIG arc, and the influence of welding current, arc length and nozzles height on distribution characteristic is also discussed. It shows that the arc distribution of U-TIG welding periodically changes with arc length increasing from 2 mm to 6.2 mm, and arc pressure reaches maximum with arc length being 4.4 mm. This is different from conventional TIG welding whose arc pressure decreases continuously. Besides, nozzle height is not a process parameter in conventional TIG welding, however, it greatly influences the distribution of arc pressure in U-TIG welding：the peak pressure decreases from 170.8 Pa to 60.5 Pa with nozzle height rising from 2 mm to 5 mm. The experiments indicate that the U-TIG welding can obtain higher arc pressure, and the pressure distribution is close to Gaussian distribution, beneficial to welding penetration and productivity.

Key words: Arc pressure, Hybrid arc, Influencing factor, Ultrasonic- tungsten inert gas