This study characterizes the microstructure and its associated crystallographic features of bulk maraging steels fabricated by selective laser melting (SLM) combined with a powder bed technique. The fabricated sample exhibited characteristic melt pools in which the regions had locally melted and rapidly solidified. A major part of these melt pools corresponded with the ferrite (α) matrix, which exhibited a lath martensite structure with a high density of dislocations. A number of fine retained austenite (γ) with a orientation along the build direction was often localized around the melt pool boundaries. The orientation relationship of these fine γ grains with respect to the adjacent α grains in the martensite structure was (111)_γ//(011)_α and [-101]_γ//[-1-11]_α (Kurdjumov–Sachs orientation relationship). Using the obtained results, we inferred the microstructure development of maraging steels during the SLM process. The results depict that new and diverse high-strength materials can be used to develop industrial molds and dies.

In this study, we systematically investigated microstructures and tensile properties of an Al–12mass%Si alloy additive-manufactured by laser powder bed fusion (LPBF) process and subsequently annealed at various temperatures. Microstructure of the as-fabricated sample was characterized by a number of melt pools consisting of α-Al phases surrounded by Si eutectic phases. Fine Si precipitates were observed in the α-Al phase in the sample annealed at 200 °C. The eutectic Si phase appears to agglutinate, resulting in a coarsened Si phase formed at high temperatures above 300 °C. The initial cellular microstructure completely disappears and a number of coarsened Si phases and plate-shaped intermetallic phases (β-AlFeSi) were formed in the sample annealed at 530 °C. However, the grain morphology of the α-Al matrix slightly changed after the annealing at high temperatures. The as-fabricated specimen showed a high strength above 400 MPa and a low ductility of below 10% in total elongation. The tensile ductility varied depending on the tensile direction. The annealed specimens exhibited a lower tensile strength and larger elongation, whereas the direction dependence of the tensile properties was less pronounced in the specimens annealed at higher temperatures. The anisotropic tensile ductility can be rationalized by preferential fractures occurred around melt pool boundaries.

In the present study, in order to elucidate geometrical features dominating deformation behaviors and their associated compressive properties of lattice structures, AlSi10Mg lattice structures with three different unit cells were fabricated by laser powder bed fusion. Compressive properties were examined by compression and indentation tests, micro X-ray computed tomography (CT), together with finite element analysis. The truncated octahedron- unit cell (TO) lattice structures exhibited highest stiffness and plateau stress among the studied lattice structures. The body centered cubic-unit cell (BCC) and TO lattice structures experienced the formation of shear bands with stress drops, while the hexagon-unit cell (Hexa) lattice structure behaved in a continuous deformation and flat plateau region. The Hexa lattice structure densified at a smaller strain than the BCC and TO lattice structures, due to high density of the struts in the compressive direction. Static and high-speed indentation tests revealed that the TO and Hexa exhibited slight strain rate dependence of the compressive strength, whereas the BCC lattice structure showed a large strain rate dependence. Among the lattice structures in the present study, the TO lattice exhibited the highest energy absorption capacity comparable to previously reported titanium alloy lattice structures.

The present study addressed the effect of heat treatment process on microstructure of an AlSi10Mg lattice structure with a body-centered cubic unit cell manufactured via laser powder bed fusion (LPBF). The as-manufactured lattice specimen exhibited a unique cellular structure composing of primary α-Al phases bounded by α-Al/Si eutectic microstructure. A gradient microstructure (continuous microstructural changes) was found in the node and strut portions composed of the lattice specimen. The microstructure appears more equiaxed and coarser with approaching the bottom surface of both portions. The continuous microstructural changes contributed to a variation in hardness measured at different locations in the as-manufactured lattice specimen. Si particles finely precipitate in the primary α-Al phases, and eutectic Si particle coarsening occurs at an elevated temperature of 300 °C. The microstructural coarsening is more pronounced at a higher temperature. A number of significantly coarsened Si particles and a stable Fe-containing intermetallic phase (β-AlFeSi) were observed at all locations in 530 °C solution-treated specimen. The homogenous microstructure results in a constant hardness value independent of the location in the lattice specimen. These results provide new insights to control the compressive properties of the AlSi10Mg lattice structure manufactured via LPBF by subsequent heat treatment processes.

Centimeter-sized samples of hypereutectic Al–15 mass% Fe alloy were manufactured by a laser powder bed fusion (L-PBF) process while systematically varying laser power (P) and scan speed (v). The effects on relative density and melt pool depth of L-PBF-manufactured samples were investigated. In comparison with other Al alloys, a small laser process window of P = 77–128 W and v = 0.4–0.8 ms⁻¹ was found for manufacturing macroscopically crack-free samples. A higher v and P led to the creation of macroscopic cracks propagating parallel to the powder-bed plane. These cracks preferentially propagated along the melt pool boundaries decorated with brittle θ-Al₁₃Fe₄ phase, resulting in low L-PBF processability of Al–15%Fe alloy. The deposited energy density model (using P·v^−1/2) would be useful for identifying the optimum L-PBF process conditions towards densification of Al–15%Fe alloy samples, in comparison with the volumetric energy density (using P·v⁻¹), however, the validity of the model was reduced for this alloy in comparison with other alloys with high thermal conductivities. This is likely due to inhomogeneous microstructures having numerous coarsened θ–Al₁₃Fe₄ phases localized at melt pool boundaries. These results provide insights into achieving sufficient L-PBF processability for manufacturing dense Al–Fe binary alloy samples.

The samples of the Al–15Fe (mass%) binary alloy that were additively manufactured by laser powder bed fusion (L-PBF) were exposed to intermediate temperatures (300 and 500 °C), and the thermally induced variations in their microstructural characteristics were investigated. The L-PBF-manufactured sample was found to have a microstructure comprising a stable θ-Al₁₃Fe₄ phase localized around melt-pool boundaries and several spherical metastable Al₆Fe-phase particles surrounded by a nanoscale α-Al/Al₆Fe cellular structure in the melt pools. The morphology of the θ phase remained almost unchanged even after 1000 h of exposure at 300 °C. Moreover, the nanoscale α-Al/Al₆Fe cellular structure dissolved in the α-Al matrix; this was followed by the growth (and nucleation) of the spherical Al₆Fe-phase particles and the precipitation of the θ phase. Numerous equiaxed grains were formed in the α-Al matrix during the thermal exposure, which led to the formation of a relatively homogenous microstructure. The variations in these microstructural characteristics were more pronounced at the higher investigated temperature of 500 °C.