Marangoni convection and gas tungsten arc weld shape variations on pure iron plates

Abstract

Bead-on-plate welds are made on pure iron using Ar-O2 mixed shielding gas in TIG welding with the O2 content in the shielding from 0.1 to 3.0%. The weld shapes are analyzed by optical microscopy and the weld metal oxygen content is measured using an oxygen/nitrogen analyzer. Results showed that the weld shape was sensitive to the concentration of the shielding gas. When the O2 content in the shielding gas is in the range of 0.3 to 1.0%, the weld shape is narrow and deep and the weld depth/width ratio reaches 0.5. Outside this range, the weld shape is shallow and wide. The small addition of oxygen to the argon shielding gas in TIG welding provides a means of adjusting the oxygen content in the liquid pool. The oxygen in the pure iron pool plays an important role as an active element, which directly affects the Marangoni convection mode in the pool. When the oxygen content in the pure iron pool is between 80 ppm and 250 ppm, an inward Marangoni convection occurs on the pool surface. The weld beads are significantly oxidized with the increasing oxygen content in the shielding gas. However, this oxide layer will not become a barrier for oxygen absorption into the liquid pool. © 2006 ISIJ.