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The present study was undertaken to understand the effect of annealing on the microstructural features of the melt-pool structure and the associated multiscale mechanical properties of the Al–2.5%Fe–2%Cu alloy manufactured by laser powder bed fusion (LPBF). Microstructural characterizations and tensile tests were conducted for the LPBF-built specimen and those subsequently annealed at various temperatures ranging from 200 to 500 °C. Nanoindentation hardness mapping was used to evaluate the mechanical inhomogeneity of the melt-pool structure and its changes by annealing at different temperatures. The LPBF-manufactured sample exhibited a melt-pool structure containing numerous particles of the Al23CuFe4 phase (oC28, orthorhombic structure) formed because of local melting and rapid solidification during the LPBF process. The relatively coarsened cellular structure localized along the melt-pool boundary resulted in local soft regions. The local vulnerability contributed to the direction dependence of the tensile ductility. A slight variation was observed in the inhomogeneous microstructure of the samples annealed at 200 or 300 °C. The formation of numerous Al23CuFe4 nanoprecipitates in the α-Al supersaturated solid solution prevented strength loss after post-heat treatments. In addition, considerable coarsening of the intermetallic phase after annealing at 500 °C eliminated the melt-pool structure. The tensile performance of the specimens demonstrated a ductile fracture mode, wherein ductile fracture occurred in the α-Al matrix with low hardness while the harder θ-Al13Fe4 stable phase was embedded within it. The anisotropy in the mechanical properties was less pronounced owing to the significant microstructural changes.

This study reports an anomalous temperature-dependent tensile behavior of laser-beam powder bed fusion (PBF-LB) processed Cu–Cr–Zr alloy. The yield strength of the alloy initially decreases as the temperature increases to 200±5 MPa and then increases to 350±11 MPa at 500°C before reducing to 234±6 MPa at 600°C. The microstructure consists of elongated Cu grains with a high concentration of Cr solute (∼1 mass%), resulting from rapid solidification during the PBF-LB process. Transmission electron microscopy for the specimens deformed at 500°C revealed the presence of numerous nanoscale Cr-rich particles embedded inside the supersaturated solid solution of the Cu matrix. Nanoscale particles can act as barriers to dislocation motion, leading to an increase in internal stress during plastic deformation at elevated temperatures. This work provides the high potential of post heat treatments for achieving superior mechanical performance using high solute supersaturation formed by the PBF-LB process.

Achieving strong direct joining between steel and polymers through mechanical interlocking is crucial for developing multi-material structures, particularly in the automotive and aerospace industries. This study synthesized micro-scale structures on a pure Fe substrate (simulating interstitial-free (IF) steel) for mechanical interlocking with thermoplastic parts. Numerous submillimeter-scale Fe/TiB2 composite particles were in-situ synthesized by laser scanning on the Fe-Ti-B powder mixture and well-bonded with the Fe substrate. The effects of powder composition (TiB2 volume fraction) on the morphology, microstructure, and joint strength with PA6 were investigated. A TiB2 volume fraction over 60 % was essential for the formation of the composite particles promoted by a TiB2 skeletal structure. Higher TiB2 volume fractions increased the area fraction of the composite particles and decreased the bonding ratio (adhesive) of the particles with the substrate due to poor adhesiveness at the edge of the laser-scanning line. A wider high-temperature region was generated at a higher TiB2 volume fraction, suggesting that the reaction heat to form TiB2 assisted the bonding of the particles with the substrate at the edge of the laser scanning line. The Fe/PA6 joint strength increased to approximately 30 MPa with increasing the TiB2 volume fraction to 100 % and showed a linear correlation with the product of particle area fraction and bonding ratio. A higher TiB2 volume fraction was preferable for enhancing the joint strength via the micro-scale structures synthesized by laser scanning on the Fe-Ti-B powder mixture. A combination of the micro-structuring process using a high fraction of TiB2 with advanced joining technologies will contribute to manufacturing high-strength Fe/polymer hybrid parts.

To understand the microstructural factors controlling deformability during the forming process, a comprehensive analysis using in-situ scanning electron microscopy (SEM) with digital image correlation (DIC) for hot-dip Zn–6 %Al–3 %Mg alloy coatings on steel sheets in bending deformation was performed. The Zn–Al–Mg alloy coating exhibited two major microstructural constituents: the primary solidified Al (fcc) phase with dendritic morphologies and a fine ternary eutectic (TE) microstructure of the Zn (hcp), Al, and Zn2Mg phases. Major cracks were initiated on the tensile-strained coating surface, where the strain was localized in the TE regions, whereas the softer dendritic Al phases prevented the crack propagation. Microcracks were initiated in the binary eutectic (BE) Zn/Zn2Mg phases (Al-poor region) localized around the dendritic Al phases, whereas they were arrested in the TE region containing fine granular Al phases. The dendritic Al and fine granular Al phases embedded in the TE microstructure contributed to the high crack resistance.

Laser-beam powder bed fusion (PBF-LB) technique was used to produce an Al–2.5 %Fe–2 %Cu ternary alloy, featuring a two-phase eutectic composition of α-Al/Al23CuFe4 in non-equilibrium solidification, as determined by thermodynamic calculations. The specimen manufactured by PBF-LB exhibited a high tensile strength exceeding 350 MPa and a low thermal conductivity of approximately 140 W m−1 K−1. Subsequent annealing at 300 °C improved the thermal conductivity to 175 W m−1 K−1 without compromising the strength. This improvement was attributable to forming numerous Al23CuFe4 nanoprecipitates, which consumed solute elements. By appropriately managing the factors contributing to strengthening, a superior strength–conductivity balance can be achieved by implementing post-heat treatments.

Mechanical coupling in architectured materials has been traditionally investigated in the context of generalized continuum mechanics and is often assumed to be non-existent in the framework of classical continuum mechanics. In this paper, we challenge this misconception and study an anisotropic architectured material exhibiting shear-shear and shear-normal coupling from the standpoint of classical continuum mechanics. The material is non-regular tetrahedron lattice, a potential candidate for biomedical implants, but the lack of understanding about its anisotropic behavior and mechanical couplings has limited its application. We exploited the unit-cell definition with periodic boundary conditions and performed elastic and elastoplastic homogenizations. Non-zero coupling sub-matrices appeared in the homogenized elasticity matrix, which we further transformed into material’s natural coordinate system using elastic distance function. This allowed for anisotropy identification and determination of all the coupling parameters. Next, compression tests are conducted on laser powder bed fused Al-12Si (mass%) lattice samples with different relative densities and spatial orientations. Employing test data, mechanical anisotropy and shear-normal couplings are experimentally characterized. Both numerical and experimental results confirmed the presence of mechanical couplings and predicted a similar anisotropic tendency in the material. Finally, the role of manufacturing defects in deterioration of as-designed mechanical properties is discussed.

Metal additive manufacturing (AM) enables the fabrication of lattice-structured heat sinks with enhanced heat transfer properties. The lattice structural features for enhancing heat transfer and suppressing pressure loss need to be clarified. To describe the structural features dominating heat transfer and pressure loss, eleven structural parameters consisting of three groups, namely the area, hydraulic diameter, and effective flow path, are proposed and used as inputs for the neural network (NN) surrogate model. The surrogate model precisely and quickly predicts the heat transfer property and pressure loss simulated by computational fluid dynamics. The random forest feature importance is used to select necessary and sufficient structural features. Four parameters describing the bottleneck in the flow pathway, surface area, and intricate structures are selected. Even when only the selected four parameters are used, the NN model maintains a high prediction performance, suggesting that the four parameters are sufficient for describing the heat transfer and pressure loss. Four parameters are used to discuss the design concept for lattice-structured heat sinks. An appropriately intricate lattice structure exhibits improved heat transfer and reduced pressure loss compared to a simple lattice structure. This study provides new insights into the design of lattice-structured heat sinks fabricated by AM.

We examined the variation in the solidification microstructure of AlSi10Mg alloy powder over a wide range of cooling rates controlled by using the fast scanning calorimetry (FSC) technique. Two exothermic peaks derived from the primary solidification of the α-Al phases and the α-Al/Si eutectic reaction were detected. These exothermic peaks became broader and shifted to lower temperatures at higher cooling rates. Focus ion beam (FIB) milling enables microstructural characterization of AlSi10Mg powder solidified at controlled cooling rates above 102 °C·s−1 in the FSC equipment. The sample solidified at 4 × 104 °C·s−1 exhibited a fine microstructure consisting of a primary α-Al phase surrounded by lamellar-shaped Si phases. The fraction of the Si phase continuously decreased with increasing cooling rate, indicating higher content of solute Si element in the α-Al phase solidified at a higher cooling rate. The secondary dendrite arm spacing (λ) of the primary α-Al phase decreased in the sample solidified at a higher cooling rate (dT/dt). The relation follows a general equation of λ = A (dT/dt)-n (A: 38.4, n: 0.33) in wide range of cooling rate (10-1 ∼ 104 °C·s−1). The results were utilized to discuss the cooling rate of Al alloys during the laser powder bed fusion (L-PBF) process.

Herein, as the fabrication of the microchannels for fluid flow using La(Fe,Si)13 magnetic refrigerant material is required while maintaining its superior magnetocaloric performance, sinter molding of the microchannel-designed La(Fe,Si)13 was investigated using a dissolutive mold comprised of NaCl to develop a molding process without contamination by carbon and other light elements. In addition, to avoid the thermal decomposition of fine particles of La(Fe,Si)13, a solid-state reaction was conducted during sintering using a mixture of α-Fe and La-rich compounds as the initial powder. After sintering, the sample shape exhibited channel widths and depths of 200 μm and pillar widths of 300 μm, which were originally designed for the NaCl dissolutive mold. The metallographic microstructure mostly comprised the La(Fe,Si)13 phase, and the volume fractions of the extra phases, such as α-Fe and La2O3, were suppressed to 1.1 % and 3.2 %, respectively. The maximum of the magnetic entropy change in a single pillar reached −19 J/kg K. Direct observation of magnetic nucleation/growth confirmed the smooth emergence of the first-order transitions in multiple pillars. Consequently, this fabrication approach promoted micro-channel-designed La(Fe,Si)13Hy while maintaining a large magnetocaloric effect via low carbon contamination.

In order to manage both the strength and ductility of Al–Si alloys additively manufactured by laser powder bed fusion (L-PBF), an attempt was made to introduce nanoscale precipitates within the α-Al (fcc) supersaturated solid solution using the L-PBF manufactured Al–12%Si alloy by artificial aging treatments. The aging treatment at 120°C improved both the tensile strength and ductility (total elongation) of the L-PBF manufactured specimen. The simultaneous strength–ductility enhancement is due to the homogeneously distributed nanoscale Si precipitates that enhanced the strain hardening. These results provide new insights for overcoming the strength-ductility trade-off in the L-PBF manufactured Al alloys.

Surface micro−structure for joining with thermoplastic parts was additively manufactured on an aluminum alloy (A5052) substrate by laser scanning on Al−Ti−C powder mixtures. The micro−structure consisted of particle−shaped protrusions with a few hundred micrometers. The effect of C content in Al−Ti−C powder mixtures on the micro−structure and constituent phases was investigated. The fraction of particle−shaped protrusion reached maximum when the C/Ti molar ratio was 0.8. Adhesiveness between the particle−shaped protrusions and A5052 substrate decreased drastically when the C/Ti molar ratio increased from 0.6 to 0.8. Changes in these structural factors were related to the formation of TiC and Al3Ti phases after the laser−irradiations. The A5052 substrate was joined with Polyamide−6 sheets via the micro−structure by hot−pressing. When the C/Ti molar ratio was 0.6, the joint strength reached maximum since both the fraction of particle−shaped protrusions and adhesiveness between particle−shaped protrusions and A5052 substrate were high. The constitute phases (TiC and Al3Ti phases) in the micro−structures were assessed by Scheil simulation for Al−Ti−C system to calculate the component partitioning and solidification path during solidification. These results were used to discuss the relation between the surface micro−structure and powder compositions.

In this study, an attempt is made to control the WC-Co two-phase microstructures by laser powder-bed fusion (LPBF) process using cemented carbide powders with different compositions of WC-25Co (mass%) and WC-17Co (mass%) and post heat-treatment at 1380 °C. The as-built cemented carbide samples had different microstructures from those of the cemented carbides fabricated by conventional manufacturing methods. The microstructures of the as-fabricated WC-25Co and WC-17Co cemented carbides were characterized by two types of regions. Region 1 consisted of W2C and W3Co3C phases, including numerous pores and defects. Region 2 consisted of WC and Co phases. Post heat-treatment of the LPBF-built WC-25Co sample at 1380 °C led to the formation of a defect-free WC-Co two-phase microstructure. In contrast, the W3Co3C phase remained in the LPBF-built WC-17Co sample even after heat treatment. The addition of carbon to the WC-17Co powder prevented the formation of the W3Co3C phase in the cemented carbide processed by LPBF and subsequent heat treatments. The addition of carbon to the WC-17Co powder had a slight effect on the hardness of as-fabricated samples, but reduced the hardness of heat-treated samples. The reduced hardness was attributed to the increased volume fractions of the WC and W3Co3C phases.

The formation of shear band in metallic lattice structures under compression prompts an unstable stress–strain curve, resulting in a significant reduction in the energy absorption capacity. The development and formation of shear band need to be clarified, and versatile approaches to suppress the shear band are required. In this study, we explored the formation mechanism of shear band using a representative Al–12Si alloy lattice structure, consisting of a simple body-centered cubic (BCC) unit cell manufactured by laser powder bed fusion. The X-ray computed tomography observations of an interruptedly compressed lattice specimen and finite element analyses revealed that the crack initiations in the bent struts under the localized tensile stress trigger the formation of shear band. Accordingly, an attempt was made to fabricate various lattice structures consisting of BCC unit cells with different strut diameter ratios to suppress shear band formation. Finite element analyses revealed relaxed tensile stress inside the struts with the controlled diameter ratios, suggesting the suppressed formation of shear bands; this was demonstrated by compression tests. The present results provide new insights for achieving a stable compressive deformation and high energy absorption capacity of lattice structures.