Behavior of weld pool convection and columnar-to-equiaxed grain transition in gas tungsten arc welds of ferritic stainless steels with different aluminum contents

In gas tungsten arc welding (GTAW), columnar grains typically grow from the fusion line of a weld to its centerline because of high temperature gradient and small growth rate. This columnar grain often causes premature failure during bending or expansion in the center of the fusion zone. Equiaxed grains, on the other hand, reduce the occurrence of cracks and improve the ductility, strength, and toughness in the fusion zone. Microstructural solidification has a significant effect on the mechanical properties; the columnar-to-equiaxed grain transition (CET) is one factor that significantly improves fusion zone mechanical properties. During GTAW, a thermal gradient occurs on a molten pool owing to the uneven mixing of the elements or the unequal compositions of the base materials, leading to a non-uniform surface tension. This is called the Marangoni effect, which creates a flow inside the molten metal and affects the overall weld shape. The Marangoni effect is associated with a coefficient called the surface tension gradient or the thermal gradient of surface tension that affects the direction of molten metal flow. The content of oxygen and flux affect the surface tension gradient. If the content of oxygen and flux are high, the surface tension gradient becomes positive, with a narrow and deep-penetrating pool. In this study, various experiments and a numerical simulation were performed to analyze the weld pool behavior and CET in GTA welding of ferritic stainless steels with aluminum contents of 100 and 300 ppm. In the experiment, 300 ppm of aluminum was observed to have a wider bead width than that observed for the 100-ppm sample; further, the growth of equiaxed grains in the centerline was promoted. In the numerical simulation, it was confirmed that when the aluminum content was 300 ppm, the outward flow occurring in the arc center formed a wide and shallow penetration. Furthermore, it was revealed that aluminum combines with oxygen to form aluminum oxide, thereby reducing the oxygen content. Therefore, as the aluminum content increased, the surface tension gradient was observed to become negative and affect CET.