Laser powder bed fused Al-10Si-0.4Mg (mass%) alloy exhibits low and anisotropic thermal conductivity. To elucidate dominant microstructural features generating the characteristic thermal conductivity, thermal conduction analysis was performed by an image-based finite element method (FEM) using scanning electron microscope (SEM) images. The serial sectioning images of as-fabricated and 300ºC/2 h heat treated Al-10Si-0.4Mg alloys were obtained by focused ion beam SEM (FIB-SEM) and were used for constructing three-dimensional models. The homogenization method was applied to calculate average thermal conductivity of these models. When the thermal conductivity of α-Al phase was set at the thermal conductivity of pure aluminum, calculated thermal conductivity of as-fabricated model was much higher than experimentally measured thermal conductivity of as-fabricated Al-10Si-0.4Mg alloy, indicating that α-Al phase in the as-fabricated Al-10Si-0.4Mg alloy had much lower thermal conductivity than pure aluminum. In addition, the anisotropy of thermal conductivity appeared only in the as-fabricated model in which the α-Al/Si interfacial thermal resistance was taken into account. These results were used for discussing dominant contributors of low and anisotropic thermal conductivity of laser additive manufactured Al-10Si-0.4Mg alloy.

Laser powder bed fusion (LPBF) has attracted attention in recent years because it can produce complex and fine three-dimensional structures that cannot be produced by conventional processing methods. However, owing to the high residual stress that is induced in LPBF products, its reduction is an important issue. Because the residual stress is significantly affected by the cooling rate during the LPBF process, the design of the laser scanning pattern is an important factor in its control. In this study, to investigate the effect of a laser scanning pattern on residual stress in the LPBF method, test pieces were prepared using 4 types of scanning patterns using Al-10%Si-0.5%Mg. By measuring the residual stress via X-ray residual stress analysis, it was found that the average residual stress is at a minimum in a pattern with a uniform laser scanning direction and a shorter scanning vector. It was also found that the residual stress distribution becomes non-uniform when the scanning direction is orthogonal and the scanning vector is longer.

We have investigated microstructural characteristics and solute Si element in the α-Al (fcc) matrix of Al-12%Si binary alloy additive-manufactured by laser powder bed fusion (L-PBF) process in terms of different locations in the melt-pool structure (inside melt-pool or around melt-pool boundaries). Their variations by post heat-treatments at various temperatures were examined. The fraction of Si phase and lattice parameter of α-Al phase were quantified by XRD measurements combined with sample rotation and oscillation systems. In the L-PBF manufactured sample, the columnar α-Al phases contained highly concentrated Si element in solution (above 3 mol%) inside melt-pools, whereas the measured Si content was approximately 2 mol% in the relatively coarsened α-Al phase along melt-pool boundaries. The location dependence of solute Si content would be associated with a continuous change in growth rate of solid phase in solidification by local heating in the L-PBF process. The solute Si content was significantly reduced by heat-treatments and its location dependence appeared less pronounced. The as-manufactured sample exhibited a lower lattice parameter of α-Al phase with concentrated Si in solution in comparison with the heat-treated samples. The fraction of Si phase became higher in the heat-treated samples. These results were rationalized by microstructural change at elevated temperatures.

Much attention has been paid to laser powder bed fusion (LPBF) as a manufacturing process to combine new functionalities to mechanical components since it can achieve complex shapes that cannot be processed by conventional methods. LPBF-processed aluminum alloys exhibit high strength due to their peculiar microstructure; however, LPBF parts contain substantial residual stresses, which are generally removed by annealing at the expense of strength. Therefore, there is a need to develop aluminum alloys that do not require annealing for stress relief after LPBF processing. In this study, we investigated the simultaneous achievement of strength enhancement and residual stress reduction by aging treatment on LPBF parts by adding manganese (Mn) to a high-strength aluminum alloy. Although the existence state of Mn has not been clarified, Mn delayed the loss of strength due to heat treatment by inhibiting the spheroidization of silicon-rich eutectic network and extended the time to peak aging from 8 to 24 hours at 150°C and from 1 to 8 hours at 180°C. The reduction in residual stresses ranged from 20 to 33%, depending on temperatures. Our results demonstrated the addition of Mn allows for longer times or higher temperatures on aging while increasing strength and reducing residual stress.

In this study, the influence of laser process parameters on the observed geometrical imperfections in a lattice structure were investigated experimentally. The imperfections were measured by using the X-ray CT machine, and the compressive response of lattice structures fabricated by our in-house metal 3D printer based on the powder bed fusion (PBF) process were investigated. In particular, the effects of the observed geometrical imperfections on the mechanical properties (initial stiffness and plastic yield strength) of the structure were discussed. The cross-section of the inclined strut in a lattice becomes ovalized which is due to the excessive heat flowing through the metal powder around the cross-section. The ovalization becomes significant as the laser scanning speed increases. From the mechanical properties predicted based on geometric imperfections, the best conditions for enhancing the initial stiffness and the plastic yield strength for a lattice block were found that the laser power p＝150W and the scanning speed v＝700 mm/s.

In this study, effects of heterogeneous nucleation site particles on density, microstructure and hardness were studied in the pure aluminum samples additively manufactured by laser-based powder bed fusion (PBF-LB) method with a small amount of TiC particles. It was found that higher density of the sample was found by addition of small amount of TiC particles. It was also found that the sample containing 1.0 vol% TiC particles has the most refined microstructure. In addition, the samples fabricated with 0 vol% and 0.1 vol% TiC particles have <101> texture along building direction, whereas <100> texture was observed for the samples with 0.5 vol% and 1.0 vol% TiC particles. As the volume fraction of TiC particles increases, the hardness of the samples was also increased. Higher hardness in the samples with TiC particles was mainly caused by not reinforcement effects of TiC particles but refinement of grain structure by heterogeneous nucleation.

The mechanical properties of JISAC8A alloys (Al-12%Si-1%Mg-1%Ni-1%Cu alloy) fabricated by laser-based powder bed fusion (LB) or electron-based powder bed fusion (EB) or gravity casting (CAST) were studied by means of tensile tests at room temperature and 300°C. These alloys were under two types of heat-treatment sequences. One was a T6 treatment, which was corresponding to the heat treatment at 500°C for 2h and 170°C for 4 h, followed by the annealing at 300°C for 10 h. Another was directly annealed at 300°C for 10 h after the fabrication. In the annealed specimens without T6 treatment, the LB alloy showed the highest strength and best elongation at room temperature and 300°C, because eutectic Si particles and Al-Ni-Cu-Fe compounds formed very fine network structure in Al matrix. In the annealed specimens with T6 treatment, the tensile strength at room temperature did not depend on the fabrication methods but the LB alloy showed the best elongation because of finer and isotropic shape of secondary phase particles. The annealed CAST alloy showed the highest strength at 300°C, and the annealed LB alloy showed the lowest strength at 300°C.

The effects of magnesium alloy compositions on the interfacial microstructure, corrosion resistance and mechanical properties of explosively welded magnesium alloys and aluminum alloy were investigated. The bonding interface had a periodic wavy shape. Thin interlayer consists of intermetallic compound, γ-Mg₁₇Al₁₂ phase, was formed at the bonding interface in all samples. When the aluminum concentration in the magnesium alloy is low, the intermediate layer at the bonding interface is thin and intermittently formed, but when the aluminum concentration in the magnesium alloy was high, the intermediate layer was relatively thick and uniformly formed, resulting in high bonding strength. In all samples, brittle fracture surface was observed and the presence of γ-Mg₁₇Al₁₂ phase was confirmed on the fracture surfaces.

In recent years, it has been a challenge for the transportation sector to reduce the weight of vehicles for higher fuel efficiency. This expands the use of light metals such as Al alloys. However, these materials have low hardness and surface strength. In this study, Al-alloy (AC8A; Al-Si-Cu-Ni-Mg) composites reinforced with Al₂O₃ fibers were used as base materials and were coated with multilayer diamond-like carbon (DLC) films to improve their properties. The multilayer DLC films were deposited on the Al alloy composites by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) using methane and acetylene gases as the raw materials. As the number of DLC film layers increased, the Young's modulus of the samples decreased and the following ratios of hardness (H) to elastic modulus (E) also increased: H/E, H³/E², H²/E. In addition, the critical number of peeling slides increased as the composite volume fraction and the number of layers increased.

In this study, we attempted to fabricate aluminum foam precursor by friction stir processing with zero-degree tilt angle and foam the obtained precursor. If precursors can be fabricated with zero-degree tilt angle, it is expected that the size of the aluminum foam can be increased, productivity can be improved, and the cost can be reduced by simplifying the equipment. It was shown that the precursor can be fabricated with zero-degree tilt angle. In addition, aluminum foam can be obtained by foaming the precursor fabricated with zero-degree tilt angle similar to that fabricated with three-degree tilt angle.

The work hardening of aluminum alloys depends on the dislocation substructure in addition to the dislocation density. In this study, in-situ XRD measurements during tensile deformation were conducted on cold-rolled 1200 aluminum and change in the dislocation substructure was evaluated by analysis using Convolutional Multiple Whole Profile method. As a result, the dislocation cell structure was formed before tensile test and when the stress exceeded 32 MPa, the dislocation cell structure decomposed and the dislocation dispersion approached uniformly. It is considered that 32 MPa is a micro-yield stress at which the initial dislocations start to move. As the plastic deformation progressed, dislocation arrangements parameter decreased and finally became 0.5, it is considered that the dislocation cell structure was formed again.

The incremental hammering method using an eccentrically rotating tool was developed to markedly reduce the time required for incremental forming. The developed method was applied to forming the end of an aluminum alloy (A5052) circular tube into a square cross section. The experimental results indicate that a square cross section is obtained after two-pass incremental hammering, although springback is observed after one-pass incremental hammering. It was demonstrated that the forming time is≤5% of that of conventional incremental forming.