Grain Structure Control of Additively Manufactured Metallic Materials

Abstract

Grain structure control is challenging for metal additive manufacturing (AM). Grain structure optimization requires the control of grain morphology with grain size refinement, which can improve the mechanical properties of additive manufactured components. This work summarizes methods to promote fine equiaxed grains in both the additive manufacturing process and subsequent heat treatment. Influences of temperature gradient, solidification velocity and alloy composition on grain morphology are discussed. Equiaxed solidification is greatly promoted by introducing a high density of heterogeneous nucleation sites via powder rate control in the direct energy deposition (DED) technique or powder surface treatment for powder-bed techniques. Grain growth/coarsening during post-processing heat treatment can be restricted by presence of nano-scale oxide particles formed in-situ during AM. Grain refinement of martensitic steels can also be achieved by cyclic austenitizing in post-processing heat treatment. Evidently, new alloy powder design is another sustainable method enhancing the capability of AM for high-performance components with desirable microstructures.

Microstructure, Solidification Texture, and Thermal Stability of 316 L Stainless Steel Manufactured by Laser Powder Bed Fusion

“…TEM EDX analysis (Figure 3d) showed that the particles contained an enhanced concentration of Si, O, and Mn. These observations were consistent with the data obtained by others [22,23,25,[29][30][31] who have also investigated LPBF 316 L stainless steel. Saeidi [22] detected similar elements in nanoparticles observed by TEM and denoted the particles as silicates.…”

“…Even after 15 min at 1050 • C, no remarkable colony growth was observed. These observations were in agreement with the results published in [22,25,64], where recrystallization and grain growth in LPBF 316 L material were observed after annealing for 30-60 min at temperatures between 1060-1400 • C. The experimentally observed recrystallization temperatures of LPBF 316 L steel were higher than those in conventional materials where static recrystallization starts after annealing for 30-60 min at temperatures 750-900 • C [61]. This can be explained by stabilization of the colony boundaries due to the segregation of solute atoms.…”

“…Not all newly nucleated colonies had the same color as the adjacent grains in the substrate, meaning that that not all substrate grains satisfied the conditions for epitaxial nucleation. Similar epitaxial nucleation between the layers was observed by EBSD in different AM materials [5][6][7]25,26]. Figure 2c also shows that in many colonies, the orientation in the inner region was generally the same.…”

“…In [23], amorphous particles containing Cr, Mn, Si, and O were studied by TEM. MnO-SiO 2 nanoscale rhodonite particles were reported in [25]. Si-enriched oxide nanoparticles were also reported in [29].…”

“…This is an important factor causing the large scatter of the experimentally measured hardness values. Other factors that influence the hardness of PLBF stainless steels are, for example, the interaction of dislocations with the particles [22,23,25,29], residual stresses, material porosity, and defects. These factors may result in a strengthening or weakening effect, thus increasing spread in the experimentally measured data.…”