In order to avoid the formation of defects in additive manufacturing (AM) by powder bed fusion (PBF) process, it is crucial to understand the relationship between the quality of powder bed and a powder spreading process. In this study, the influences of conditions of powder raking process on the densities and homogeneity of powder bed have been examined by computer simulation using Discrete Element Method (DEM) and experiment of powder bed formation using a blade-type spreader for Ti-6Al-4V powder by way of example. The results were analyzed with a special focus on the effects of the relative size of powder particles with respect to the gap between the blade and the build platform. It has been clearly shown that the gap needs to be larger than the upper bound of the distribution of powder particles diameter to obtain a high-density powder bed. The DEM simulation indicated that the blade can sweep even powder particles that are not in direct contact with the blade when the powder particles are in a close-packed tetragonal configuration. This is the probable reason for the experimental fact that powder particles smaller than half of the gap are suitable for PBF.

Microstructure and tensile properties of Ti-48Al-2Cr-2Nb (at%) rods fabricated by electron beam powder bed fusion (EB-PBF) process were investigated by changing input energy density (ED) which is one of the important factors affecting formation of the melt pool. We found that unique layered microstructure consisting of an equiaxed γ grain layer (γ band) and a duplex region can be formed by EB-PBF with ED in the range of 13 to 31 J/mm³ . It is interesting to note that the width of the γ band and the volume fraction of the γ phase in the duplex region decrease with increasing ED. On the other hand, the α₂/γ lamellar grain in the duplex region increases with increasing ED. These morphological changes in the layered microstructure are attributed to variation of temperature distribution from melt pool caused by increasing ED. Moreover, we also found for the first time the strength of the alloys can be improved by decreasing width of the γ band and increasing of the α₂/γ lamellar grain in the duplex region. Whereas, the width of the γ band and the fraction of the equiaxed γ grain in the duplex region should be increased to enhance ductility of the alloys.

One of the important physico-chemical properties of materials under metal additive manufacturing process is the interfacial properties of liquid/solid such as the wettability between a liquid and its own solid. However, the data of the wettability between a liquid and its own solid for metal systems is limited. In this study, we propose the evaluation method for the interfacial properties between liquid alloy and its solid alloy based on the morphology of as built surface in powder bed fusion. The liquid/solid contact angle for Ti-6mass%Al-4mass%V is measured based on the sample built by electron-beam additive manufacturing, and its interfacial tension is evaluated by Youngʼs equation. The contact angle and interfacial tension between Ti-6mass%Al-4mass%V liquid alloy and its solid alloy are found to be 12°and 376 mN/m, respectively.

TiAl (Titanium aluminide) alloy is attracting attention in the automobile and aviation industries as a promising lightweight heat-resistant alloy material. Parts used in the high-temperature environment must have ductility and toughness at room temperature in addition to high strengths at high-temperature, because some alloys such as TiAl alloy is known as a difficult material for casting and machining. Recently, manufacturing process using AM (additive manufacturing) have been studied to avoid these problems. In this research, the effect of the energy density during AM process on surface roughness and strengths of the built parts was studied by changing build parameters. As the energy density increased, the porosity and surface roughness decreased. The higher energy density tended to increase the proportion of lamellar structure. It was found, however, that the increase in lamellar structure does not necessarily lead to the improvement in tensile and creep strengths at 750°C. As a result, the constant energy density of 15 J/mm³ showed better tensile and creep properties than those of the standard parameter. The results suggested that it is important to optimize the parameters according to the required properties of the parts.

The densification process plays a critical role in determining the microstructure and performance of Ti matrix composites (TMCs). Herein, a comparative study was performed on a graphene oxide (GO)/Ti-6Al-4V composite fabricated by laser powder bed fusion (L-PBF) and spark plasma sintering (SPS). The flexible GO sheets were homogeneously decorated onto the Ti-6Al-4V powders via an electrostatic self-assembly without significantly changing the particle size or sphericity. Under high-energy laser irradiation, the GO sheets were completely dissolved into the matrix. The L-PBF-produced composite was composed of fine αʼ martensite structures due to the rapid solidification and the solute carbon atoms. In contrast, the GO was reacted with Ti matrix and completely transformed into submicron TiC particles during SPS; the composite consisted of α＋β phases with randomly dispersed TiC. Moreover, the L-PBF-produced composite exhibited a higher hardness of 481 HV as compared with the SPS-produced one of 367 HV, attributing to the fine αʼ microstructures and high residual stresses. The present work offers deep understanding on the structural evolution of GO during high-temperature densifications, and shows new insights for fabrication of high-performance TMCs with tailored microstructures.

The AlSi10Mg alloy was manufactured via the powder bed fusion-electron beam (PBF-EB) melting technology. The present study mainly focused on the influence of post heat treatment on microstructure and mechanical properties. The peak hardness was obtained under optimal aging conditions, such as 240°C for 0.1 ks, 200°C for 1.0 ks, and 160°C for 43.2 ks, respectively. The nanoscale and fine Si phase reprecipitated during aging process, giving rise to the strengthening of as-built AlSi10Mg alloy. The tensile strength increased to>300 MPa while the tensile elongation remained approximately 15%. The present study provides a potential method to regulate the microstructure of light-weight alloy in future.

Porous aluminum is fabricated by heating and foaming a precursor, which is fabricated by adding a foaming agent and a thickening agent to solid aluminum. In recent years, joining technologies have been developed owing to multi-materialization, but from the viewpoint of recycling, it is necessary to separate the joined materials when they are to be disposed. It is well known that the strength of porous material decreases compared with that of the base material. Taking advantage of this fact, we attempted to reduce the strength of the joining area by adding a foaming agent to the joining area when joining dissimilar metals, and then heating and foaming the joining area at the time of disposal for the purpose of easy separation. As a result, it was found that the fracture load and absorbed energy required to fracture the specimen can be reduced just by heat treatment, but they can be further reduced by heating and foaming the joining area compared with those in the case of a nonporous specimen.

Welding of aluminum and aluminum-coated steel plates was performed using magnetic pulse welding. A1050 pure aluminum and Al-Si-coated steel plates were used in this study. The aluminum and the aluminum-coated steel plates were used for a flyer plate and a parent plate, respectively. The welding interface was observed with an optical microscope, scanning electron microscope and scanning transmission electron microscope. Tensile-shear tests were conducted for evaluation of the joint strength. After the welding, thickness of the aluminum coating slightly decreased at the welded area and Si particles containing in the aluminum coating were refined. The welding interface exhibited characteristic wavy morphology. A banded structure consisting of fine aluminum grains with diameters of approximately 500 nm was formed along the wavy interface. When the welding was performed at longer plate gap condition, void formation was observed in the banded structure and fracture occurred at the welding interface by tensile-shear test. The void formation is considered to lead to a decrease in joint strength.