Abstract
The effect of the shielding gas concentration on the weld shape is studied for the moving bead-on-plate TIG welding of SUS304 stainless under He-O2 mixed shielding. The small addition of oxygen to the helium base shielding gas can effectively adjust the oxygen content in the liquid pool and the weld shape. Oxygen is a surface active element for stainless steel. When the oxygen content in the liquid pool is over the critical value, around 70ppm, the weld shape suddenly changes from a wide shallow shape to a deep narrow shape due to the change in the Marangoni convection from the outward to inward direction on the liquid pool surface. Weld shape variations at different welding parameters including welding speed, welding current and electrode gap under pure He and He-0.4%O2 mixed shielding are systematically investigated. The results showed that the final TIG weld shape depends to a large extent on the pattern and strength of the Marangoni convection on the pool surface, which is determined by the combinations of the oxygen content in the liquid pool, temperature coefficient of the surface tension (dσ/dT) and the temperature gradient on the pool surface (dT/dr). Different welding parameters will change the temperature distribution and gradient on the pool surface, and thus, affect the strength of the Marangoni convection and final weld shape.
