Elucidation of solidification mode of Fe-Mn-Si alloy during TIG spot welding using synchrotron X-ray

High-spatial resolution synchrotron X-ray imaging was combined with X-ray diffraction for a detailed investigation of FA mode solidification in Fe-Mn-Si (FMS) alloys during tungsten inert gas (TIG) spot welding. The solidification sequence of X-ray radiographs and X-ray diffraction patterns clearly revealed that the formation of equiaxed γ-Fe dendrite occurred in front of the growing columnar δ-Fe dendrites without any observable phases in the interdendritic regions. The solidification behavior of the FMS alloy was different from the conventional FA mode for the type 304 stainless steel, in which the cellular γ-Fe was independently formed in the interdendritic regions of the primary δ-Fe. During the columnar dendrite growth of δ-Fe for the FMS alloy, the ratio of the temperature gradient to the growth velocity was remarkably decreased and solute enrichment in the remaining liquid occurred, which likely led to the formation of equiaxed γ-Fe dendrite.