The probability that you will get the right answer to your question will increase if you write the chemical composition of the welded material

Grain size difference between zones is obvious. Base metal shows equiaxed grains with some twins. Weld zone shows columnar grains by accelerated solidification.

Hi,

First of all, please define the name of the welded material, and describe a bit about the welding process (a type of process, welding temperature, etc.)

As shown in the figures, there are 2 different zones, in my opinion, in the first figure upper zone is related to base metal( base structure ), and it has a layer structure with basic material hardness, while the bottom part of the figure has a different structure that highly dependent on a material basic structure and process conditions. For understanding the structure of this zones, as l told above, please define the name of the material etc.

Bests,

I suppose it's RSR- structure of the SLM process/ I thinc this structure could be named as "sub-dendritic columnar structure" (or "non-dendritic ... .../ Upper slow is substrate of SLM?)

it is stainless steel 316L welded by laser. but my question is about the shape of the microstructure itself not the composition.

Your question is clear but structure with quite similar shapes of grains can be not the same for different steels and alloys. Base metal of stainless steel 316 has equiaxed austenitic grains. Fusion zone usually consists of columnar and equiaxed dendrites with d-ferrite at the interface. Please see the attached papers, as I understand your micrographs are quite similar.

Hi,

As mention in other answers, I am agree with this that The fusion zone (lower zone in first figure) seems that is the Dendrites.

For more explanation, please refer to below book:

Metal Forming

Authors: Robert M. Caddell, William F. Hosford

Bests,