Investigation of thermal and solidification behaviors to predict dendrite microstructure formation during directed energy deposition of AlSi10Mg

Abstract

The characteristics of local high-energy input and rapid cooling of L-DED result in uneven microstructure on the scale of the molten pool. This paper develops a coupled temperature and solid-liquid phase fields model, from which the cooling rate and morphology parameters could be calculated to predict the dendrites size and morphology throughout the entire molten pool. The single-track cladding experiment was carried out to verify the feasibility of the model, showing that the geometrical characteristic (i.e., width and depth of the modelled molten pool match closely with the experimental measurements. It is found that from the bonding zone to the top zone of the molten pool, the microstructure evolution trend from columnar dendrites to fine equiaxed dendrites could be accurately predicted with the decreasing morphology parameters and increasing cooling rate. The morphology parameter drops suddenly at about 0.3 mm away from the bottom boundary, indicating that the dendrite microstructure morphology would show a major change from columnar to equiaxial at this specific location. The dendrite microstructure obtained by the experiment also verified this transformation, and further proofed the DED aluminum alloy suffers from columnar to equiaxial transition in the region with morphology parameters greater than about 2 ×10⁸ K∙s/m². This work greatly reduces the time cost in evaluating the dendrites microstructure of DED cladding layers, which is necessary to control the microstructure and to improve properties of L-DED repaired layers.