Deciphering particle dispersion mechanisms at different scales of additively manufactured TiCN/AZ91D composites with enhanced strength

Powder Bed Fusion by Laser Beam Melting (PBF-LB/M) has attracted significant interest for fabricating magnesium matrix composites (Mg-MMCs). Particle agglomeration inevitably leads to a strength-ductility trade-off. However, this issue remains unresolved in the PBF-LB/M fabrication of Mg-MMCs, because the key process parameters and control mechanisms are not well understood. Thus, a multiphysics multiscale framework coupling a microscopic Computational Fluid Dynamics-Discrete Phase Model (CFD-DPM) with a submicroscopic Sharp Interface Model (SIM) is developed to precisely regulate the hatch spacing, which plays a key role in particle dispersion, and to uncover corresponding mechanisms. Accurate design of the hatch spacing demonstrates that it should be 2/5 of the molten pool width, as confirmed by experiments. The SIM simulations emphasize that the particles should not be considered inert. Considering Brownian motion, the insufficient free reorientation of particles ahead of the solid-liquid interface can explain why the particle-pushing mechanism dominates in the experiments rather than the engulfment mechanism from the perspective of interfacial energy. With an optimized hatch spacing of 60 µm, increased tensile strength (∼349.2 MPa) with decent ductility (∼4.3%) is achieved in the as-fabricated TiCN/AZ91D composites. The co-segregation of Al/Zn/Mn solutes enhances particle/matrix interfacial bonding and may delay microcrack initiation. The activation of 〈c+a〉 dislocations near the particle/matrix interface facilitates the release of stress concentration and improves deformation uniformity. The developed multiscale framework and particle-dispersion optimization strategy are expected to facilitate the accelerated PBF-LB/M fabrication of new materials.