Hello Mr. Kazasidis,

The formation of the microstructures during solidification depends primarily on the thermal gradient in the melt that is being solidified.

Typically, ahead of the solid-liquid interface, the solute concentrations vary from point to point and therefore there exists a gradient in their liquidus temperature. The obtained microstructure can be understood by comparing the temperature of the melt and the liquidus temperature of that alloy at that solute concentration.

To predict the microstructure developed, a critical thermal gradient equation is developed which is calculated from the phase diagram and depends on the solidification velocity, diffusion of the solute in the liquid, concentration of the liquid at that point etc.

If the thermal gradient of the melt is higher than the critical thermal gradient then it will lead to planar solidification. But generally this is not the case in welding. The gradient will be less than the critical gradient and based on its magnitude you will get the cellular, columnar dendrites and equiaxed dendrites subject to how low the thermal gradient is.

At the fusion zone, since it is in the center, the heat taken out from the zone will be less than the outer zones and one can assume that the gradient is the lowest at this point. This will probably cause the growth of equiaxed dendrites and in some cases columnar dendrites also.

You can please refer to the following journal paper:

Article Numerical and experimental analysis for solidification and r...

Regards

Raghav

There are many textbooks that explain the different solidification modes in welding as a function of the welding speed quite nicely. One is for example Fundamentals of welding metallurgy by Granjon