The relationship between convection mechanism and solidification structure of the iron-based molten pool in metal laser direct deposition

Metal Laser Direct Deposition (MLDD) is an additive manufacturing method involving multiple physical processes. Due to the existence of various complicated physical processes inside the molten pool, it is necessary to study the convection mode and solidification morphology in the molten pool. The convection of the molten pool generates heat and rapid solute transfer in the molten pool, which has an important influence on the solidification morphology of the deposited layer. In this study, the constructed MLDD numerical model was used to analyze the internal convection morphology of the molten pool. The convection form in the molten pool was revealed by setting different physical conditions. The results show that the Marangoni movement is the main factor in the formation of cladding layers. The impact force generated by the protective gas has an influence on the convective morphology, while the influence of gravity on the convection is limited. Through the numerical calculation, the relevant solidification parameters of the molten pool were obtained and verified by the MLDD experiment of the iron-based alloy powder. Numerical analysis and experimental results show that the Marangoni motion caused by the temperature gradient is the decisive factor leading to the solidification structure, and the phenomenon of columnar to equiaxed transition (CET) from the bottom to the top of the molten pool occurs.