The optimization of processing parameters is indispensable for the laser powder bed fusion (L-PBF) process. The deposited energy density (DED) is one of the process indexes for the L-PBF process and has a simplified formula of P·v^−0.5, where P is the laser power, and v is the scan speed. This parameter describes the change in the relative density and the melt pool morphology with laser power and scan speed well, whereas it does not include the effect of other processing parameters, e.g., hatch spacing (S). In the present study, an attempt was made to incorporate the effect of S into DED. Al-12Si (mass％) alloy cube samples were fabricated by L-PBF under various P, v, and S, for evaluating the relative density and the melt pool morphology. The melt pool depth and width of L-PBF-manufactured Al-12Si alloy increased linearly with P·v^−0.5 and did not exhibit a clear correlation with S. Based on the experimental observation, the effect of hatch spacing on DED was estimated to be S^−0.5, and a new index of P·v^−0.5·S^−0.5 was proposed. This index described the change in the relative density of the L-PBF-manufactured Al-12Si alloy with laser conditions (P, v, and S) well when the thermal conduction mode melting was dominant. This study also indicated the limitation of the applicability of P·v^−0.5·S^−0.5 under the keyhole or transition mode melting.

Mater. Trans. 64 (2023) 1099-1106に掲載

Aluminum matrix composites reinforced with ceramic particles have recently received considerable research interest owing to their light weight, high modulus, and high strength. This study fabricated SiC reinforced Al-10 wt％Si-0.35 wt％ Mg (AlSi10Mg) alloy matrix composites with high relative density using a laser-based powder bed fusion (LPBF) process. The reaction phase of Al₄SiC₄ from the interface of the SiC particles occurred under LPBF and its fraction increased with increasing energy density. A high compressive stress of >600 MPa was obtained for the AlSi10Mg/SiC composite produced via LPBF at an optimum energy density (250 J/mm³). This study analyzed the mechanical properties of the LPBF composite hierarchically from micro to macro levels. A higher Young’s modulus (evaluated using a compression test) was obtained for the AlSi10Mg/SiC composite fabricated using LPBF than the AlSi10Mg alloy. From the micro level analysis on mechanical properties, the increased Al₄SiC₄ formation with an increase in energy density improved the hardness and Young’s modulus compared to those of the AlSi10Mg alloy. However, SiC in the composite is more effective for being high strength and Young’s modulus than Al₄SiC₄.

Mater. Trans. 64 (2023) 1125-1134に掲載

Powder bed fusion using a laser beam (PBF-LB) was performed for Ti-6Al-4Nb-4Zr (mass％) developed by our group to improve the oxidation resistance at temperatures greater than 600℃ by adding Nb and Zr to near-α alloys. Microstructure evolution of the PBF-LB samples by heat treatment was investigated, especially for heat treatment duration in the α + β phase, cooling rate, and heat treatment in the β phase. The equiaxed α phase formed during heat treatment along the melting-pool boundaries. The high volume fraction of the α phase and high Nb contents in the β phase was obtained by slow cooling (furnace cooling) compared with fast cooling (air cooling). The α/β lamellar structure formed in the melting pool boundaries with 100 µm in size and no equiaxed α phase formed along the boundaries by heat treatment in the β phase regime. Creep life at 600℃ and 137 MPa was similar for the air-cooled and furnace-cooled samples, but the slightly slower deformation was obtained in the furnace-cooled sample. Creep life of the sample heat treated in the β phase region drastically increased due to the absence of the equiaxed α phase. Dominant deformation mechanism of creep was grain boundary sliding. The small equiaxed α phase accelerated grain boundary sliding.

Mater. Trans. 64 (2023) 1175-1182に掲載．AbstractおよびFigs. 5-9のキャプションを修正．

In this study, a multi-phase-field (MPF) framework for predicting epitaxial grain growth in selective laser melting (SLM) additive manufacturing (AM) with multi-track and multi-layer scanning was developed. The spatiotemporal change in temperature was approximated using the Rosenthal equation, which provides a theoretical solution for the temperature distribution due to a moving point heat source. The powder bed was modeled as a polycrystalline layer. Large-scale MPF simulations for SLM-AM were performed using parallel computing with multiple graphics processing units. Using the MPF framework developed herein, we simulated SLM-AM with four tracks and four layers for 316L stainless steel. By observing the epitaxial grain growth process on two-dimensional cross-sections and in three dimensions, we clarified a typical growth procedure of grains with characteristic 3D shapes. The MPF framework will potentially enable a systematic estimation of the material microstructures formed during SLM-AM.

Mater. Trans. 64 (2023) 1150-1159に掲載．式（1）を修正．

Additive manufacturing has attracted much attention as a new technology for producing lightweight and high-strength materials. The multi-phase-field method has been used in powerful numerical simulations to predict solidification microstructure formation in additive manufacturing. To verify the non-equilibrium multi-phase-field (NEMPF) model that can consider strong out-of-equilibrium solid/liquid interfacial conditions, the NEMPF model coupled with the CALPHAD-based thermodynamic database was used to simulate the solidification process in 316L stainless steel (SS) under a rapid cooling condition. The results show that interstitial carbon atoms tend to segregate at the solid/liquid interface, whereas no concentration gradient is observed at the grain boundary between the solid phases. Conversely, substitutional Cr, Mo, and Mn atoms were segregated in the solid grain boundaries. The effects of interfacial mobility and interfacial permeability on the solidification behavior were investigated. The results show that these parameters strongly influence the solidification rate and distribution of the solute concentration at the solid/liquid interface.

Mater. Trans. 64 (2023) 1160-1168に掲載

We first fabricated the single crystal of 316L stainless steel by laser powder bed fusion (L-PBF), focusing on the applicability of the µ-Helix scanning strategy with narrow pitch as a method for obtaining a single crystal. Various combinations of laser power and laser scanning speed were examined. The cubic block samples were orientated to <100> in the X-laser scanning direction, to <110> in the Y-laser scanning direction, and to <110> in the building direction, which is contrary to that obtained by the µ-Helix scanning strategy in electron beam melting (EBM), in which the X- and Y-laser scanning directions are orientated to <110>, and Z-direction is oriented to <100> direction. Also, it has been demonstrated that melt pool monitoring by on-axis and off-axis dual photodiodes can detect the nonequivalence of ±X-scanning and ±Y-scanning, which is responsible for the unexpected crystal orientation.

Mater. Trans. 64 (2023) 1135-1142に掲載

The morphology of microstructure and tensile properties of Ti-48Al-2Cr-2Nb (at％) alloy rods fabricated by the electron beam powder bed fusion (EB-PBF) process were investigated with a particular focus on the influence of scan speed of the electron beam. Homogeneous near lamellar structure composed of the α₂ and γ phases can be obtained in the rod fabricated under the slowest scan speed. The fine lamellar spacing which contributes to the high strength of the alloy is derived from the fast-cooling rate of EB-PBF. On the contrary, a layered microstructure comprising a duplex-like region and an equiaxed γ grain layer (γ band) perpendicular to the building direction is obtained when increasing scan speeds. We observed for the first time that an increase in the scan speed results in a narrow width of the γ band. We also found that these microstructural changes have a significant influence on the mechanical properties of the rods. The near lamellar structure exhibits higher strength compared to the layered microstructure. Whereas, the rods with the layered microstructure show large ductility at room temperature. The elongation of each rod strongly depends on the width of the γ band owing to the preferential deformation of the γ band.

Mater. Trans. 64 (2023) 1112-1118に掲載

Laser irradiation of β-NaFeO₂ shows local melting and crystal growth from the melt pool and sodium ion conductivity. Local melting, densification, and crystal growth were confirmed on the β-NaFeO₂ surface through laser irradiation with a wavelength of 1 µm. Scanning electron microscopy and electron backscatter diffraction analysis confirmed anisotropic grain growth and grain boundary shrinkage in the laser-irradiated area. X-ray diffraction results suggested that the laser-irradiated area was in a low crystalline state. In addition, the precipitation of Fe₃O₄ in laser-irradiated β-NaFeO₂ indicates a reduction reaction, suggesting that the laser-irradiated area is heated to 1500℃ or higher. We confirmed that the structure change region could be controlled down to a thickness of about 10 µm by laser irradiation with varying laser power to β-NaFeO₂.

Mater. Trans. 64 (2023) 1188-1193に掲載．Fig. 2のキャプションを修正．

Freeze-dry pulsated orifice ejection method (FD-POEM) is a promising approach of fabricating spherical refractory alloy or composite powders for laser powder bed fusion (L-PBF). However, due to the weak particle strength induced by their intrinsic pore characters, the FD-POEM powders were likely crushed during the powder recoating process. Taking MoSiBTiC alloys as an example, in this work, plasma spheroidization (PS) treatment was utilized to strengthen the FD-POEM powders. The MoSiBTiC powders were completely melted and rapidly solidified during PS, leading to the evolution of mesh-pore to a fully dense structure. Correspondingly, the PS powders displayed an increased sphericity of 0.98, while showing a significantly decreased particle size of 53.1 µm. Moreover, the laser absorptivity of PS powder was reduced because of the decreased multiple reflections via the smooth powder surface. Microstructure evaluations illustrated that the PS MoSiBTiC powders mainly consisted of the elemental (Ti, B, and C)-supersaturated Mo phase, as well as small amounts of Mo₅SiB₂, TiC, and Mo₂B, indicating the occurrence of in-situ alloying during PS. Furthermore, the internal and inter-particle microstructures of PS powders were highly homogeneous, attributing to the uniformly dispersed elemental powders of FD-POEM powders. The results of this work suggest that the combination of FD-POEM and PS is an effective approach of fabricating refractory powders for L-PBF.

Mater. Trans. 64 (2023) 1119-1124に掲載

Microscopic stress-strain curves were obtained by applying stress measurements using the HR-EBSD method and strain measurements using the DIC method to the same field of view in SEM in-situ tensile tests. From the analysis of these microscopic stress-strain curves, yield stress map and work hardening rate map were successfully produced. The relationship between these mechanical property value maps and the microstructure is investigated. The yield stress maps confirm the tendency of the local yield stress to show different values for different grains, but the Schmid factor alone cannot explain the magnitude of the yield stress. When adjacent grains with significantly different Schmid factors deformed cooperatively, the yield stress is found to increase as a result of stress partitioning. Localized regions of extreme work hardening rates were observed in the work hardening rate maps. These regions were located close to grain boundaries with low m′ values where slip transfer was difficult and an interruption of the slip bands was observed. In addition, the rate of increase of GND density with strain was large in these regions. From these results, it can be understood that the extreme work hardening rates are due to increased back stresses caused by the accumulation of dislocations on low m′ grain boundaries.

Press hardened steel (PHS) has been applied to many automotive body parts to reduce the weight of the white body, and high corrosive properties are required in PHS. Al-Si coated steel sheets are widely used for protection against scaling and decarburization. Although Al-Si coated steel sheets have better corrosion resistance than bare steels due to the barrier effect, the corrosive protection performance of Al-Si coated steel sheets is not sufficient for use in wet under-body areas compared with galvanized or galvannealed steel sheets. Zn coated PHS is appropriate for hot stamped parts in which corrosive protection is required because of its sacrificial protection property. However, micro-cracks occur in severely deformed areas of Zn-coated parts under some hot stamping conditions, and deep micro-cracks can affect the fatigue property. In this study, several hat-shaped samples were prepared, and the depth of micro-cracks was changed by altering hot stamping conditions. The effects of the micro-crack depth on the fatigue property of Zn-Ni coated PHS were investigated by a plane bending fatigue test. At the average depth of 20 µm, the fatigue life of the Zn-Ni coated samples was almost equal to that of the Al-Si coated sample which was taken from the wall side of the hat-shaped sample after crash forming. Considering the standard deviation of the micro-crack depth in practical use, the Zn-Ni coated steel sheets can be used equally with the Al-Si coated steel sheets in fatigue property when the micro-crack depth was restricted to under 10 µm. Moreover, the impacts of hot deformation conditions on the micro-crack depth were investigated. It was found that the reduction of equivalent plastic strain is effective in lowering the micro-crack depth.

Mater. Trans. 64 (2023) 2270-2277に掲載