*3.4. Selection of Time Step*

In order to ensure stability in the calculation of the composition field and the advance rate of the solid–liquid interface, the time step is determined by the following Equation [27]:

$$
\Delta t \le \frac{1}{5} \min(\frac{\Delta x^2}{D\_1}, \frac{\Delta x}{V\_{\text{max}}}) \tag{18}
$$

where Δ*x* is the grid size, *D*<sup>l</sup> is the solute diffusion coefficient in the liquid, and *V*max represents the welding speed.
