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In selective laser melting (SLM), temperature gradients and cooling rates are extremely large in comparison to the ordinary directional solidification process. Therefore, the standard analytical methods are not able to predict the dependency of the dendrite arm spacing on the process parameters correctly. In the current research, we use a quantitative multicomponent phase-field model to investigate the arm spacing during the SLM process taking into account the dependency of the tip undercooling on the solidification velocity. It is found that the precision of the phase-field method can be estimated by a stability parameter which is defined as a ratio of the numerical resolution to the solidification velocity and should be chosen larger than a critical value. We show that our developed results are in good agreement with the theoretically obtained ones based on Kurz-Fisher method. We investigate the microstructure evolution and component distribution in FeMn-Al-C solidified alloy during SLM process. The arm spacing and the Mn distribution are in a very good agreement with the experimental results. Additionally, the resulting non-standard dependencies of the arm spacing on the process parameters are compared with analytical calculations, which show excellent agreement between predictions and experimental measurements.