Tracer investigation of convection in weld pool under TIG welding process

Tungsten particles were chosen as the trace-element to investigate the pattern of the Marangoni convection on the pool surfaces of conventional TIG and double shielded TIG weld and to study the mechanism of the TIG weld shape variation. Before welding, the 300-500 micron size tungsten particles were uniformly smeared on the surface of work piece which were inserted two tungsten plates at the pool bottom to separate the weld pool center area and the edge region. For conventional TIG welding process, outward Marangoni flow existed on the pool surface and the tungsten particles concentrated in the two edges at the pool bottom. For double shielded TIG process, an inward Marangoni convection on the molten pool surface and tungsten particles located on the pool center between the two tungsten plates. Tracer test proved the flow direction of the weld pool in the TIG process and verified the active element could change the direction of the Marangnon convection.