Interaction between Impurities and Welding Variables in Determining GTA Weld Shape Depending on the trace elements in the base materials, identical changes in welding variables can have an opposite effect on weld shape

Variations in GTA weld shape, or penetration, have been shown in many cases to arise from small changes in trace element content of the base metal. Heiple and Roper have proposed that CTA weld shape is generally deter­ mined by fluid flow patterns in the weld pool, and that certain trace elements can alter the fluid flow patterns by changing surface tension gradients on the weld pool. The surface tension gradients will also be altered by changes in tempera­ ture gradients on the weld pool surface since the surface tension is temperature dependent. We propose that the primary mechanism by which welding parameter variations affect GTA weld shape is by changing the temperature gradient on the weld pool surface through changes in the input power density. A welding vari­ able change which increases the input power density will magnify the tempera­ ture gradient, and thus the surface ten­ sion gradient, thereby driving the existing fluid flow pattern more strongly. Identical changes in welding variables can there­ fore produce opposite changes in weld shape in materials with different trace element content. The effects of current, travel speed, arc length, electrode tip angle, and torch gas composition on GTA weld shape are reported for high purity and sulfur or selenium doped 304 stain­ less steel. All weld bead geometry changes caused by changing welding variables were found to be consistent with the surface tension driven fluid flow model.