Additive manufacturing (AM) has undergone rapid development in the past decade. Owing to its capacity to produce complex and functional parts in various industries, it has been recognized as a remarkable scientific and industrial technique. It is used to enhance weight-saving production and reduce the number of parts, with metal-based AM techniques in particular being recognized as the most promising AM techniques developed thus far. This is because of their high potential for direct production through the selective solidification of metal materials from three-dimensional, computer-aided design data. The medical, aerospace, and part molding industries are some of the many expected to reap particular benefit from the ability to produce high-quality models with reduced manufacturing costs and lead times.

The objective of this special issue is to collect recent research works focused on recent trends in AM. This issue includes 7 papers covering the following topics:

- Metal-based powder bed fusion using a laser beam (PBF-LB/M)

- Wire and arc-based AM (WAAM)

- Fused deposition modeling (FDM)

This special issue is expected to help readers understand the recent trends in AM, leading in turn to further research on AM.

We deeply appreciate the contributions of all authors and thank the reviewers for their incisive efforts.

Snap-fit allows plastic products to have assembly and disassembly capabilities without the use of screws, bolts, or other additional parts. For this reason, snap-fit is used in all kinds of plastic products from stationery to automotive parts. Because the mechanical and other functions of a snap-fit are greatly affected by its shape and material properties, it is desirable to fully evaluate them at the design stage. In addition, as the assembly and disassembly of products by snap-fit is generally performed by people, it is important to evaluate not only virtually but also with actual plastic parts. Therefore, there is a strong need to make a prototype and evaluate the feel of the product during assembling and disassembling, before finalizing on the shape and materials. In the past, making precise prototype required expensive molds, but in recent years, additive manufacturing has made it possible to make prototype efficiently and at low cost. In additive manufacturing, fused deposition modeling (FDM) is considered suitable for snap-fit prototype because it can use the same materials as mass-produced products. Thus, it may be possible to make a snap-fit prototype with rigidity, strength, and other characteristics similar to those of mass-produced products. However, FDM has various processing conditions such as tool path, nozzle temperature, and height of one layer. They are expected to have a significant effect on the snap-fit characteristics. Snap-fit is required to meet various requirements depending on the plastic products. The requirements can be divided into three major categories: in assembly, in disassembly, and when to use. In this study, we investigated the effect of FDM processing conditions on snap-fit characteristic in assembly.

Laser powder bed fusion with metallic materials as a heat source (PBF-LB/M) is an additive manufacturing (AM) technique that has been applied in various industrial fields to reduce component weight, improve functionality, lower manufacturing costs, and reduce lead times. However, detailed characterization of the PBF-LB/M phenomenon is challenging because of the mutual influence of laser parameters and chamber environment. In PBF-LB/M, the powder is repeatedly melted and solidified by laser irradiation. However, the hot spatter generated in the process causes defects and insufficient melting. In this study, we use a high-speed camera to observe hot spatter ejected from the laser-irradiated area of a commercial PBF-LB/M system and investigate the effects of inert gas flow and laser scanning strategy on hot spatter behavior. We found that the ejection velocity of hot spatter immediately after ejection from the melt pool decreases as the particle size increases and is not affected by gas flow velocity. Furthermore, we observed that hot spatter is always ejected behind the laser scanning direction, but the ejection direction of the hot spatter changes over time. Particularly, when the laser scanning direction follows the gas flow direction, the spatter ejected in the backward direction of the scanning direction may follow a large curve over time to the front of the scanning direction and deposit on the build part. Based on the results of these investigations, we drew conclusions on the effect of the laser scanning direction with respect to the gas flow direction on the build quality and found that scanning the laser in the opposite direction to the gas flow is more effective in improving the surface quality.

This study investigated the influence of oxygen concentration in the building environment and the degree of oxidation of maraging steel powder on spatter generation behavior during powder bed fusion (PBF) process. The powders were oxidized at various heat treatment temperatures, and their degree of oxidation was evaluated using Auger electron spectroscopy. The spatter generation behavior of the powders at oxygen concentrations of 1.0×10² ppm (99.99% purity) to 5.0×10⁴ ppm (95% purity) in the building atmosphere was then investigated. The results indicated that the presence of oxygen in the building environment had a greater effect on spatter generation than the oxide film on the maraging steel powder. The oxygen concentration affected the velocity and angle of spatter particles. At an oxygen concentration of 5.0×10⁴ ppm, the number of spatter particles was 2.5 times greater than that of 1.0×10² ppm. A higher oxygen concentration resulted in an increase in the number of fume particles adhering to the spatter surface, reducing its reusability. The oxide film on the powder did not significantly affect the vapor jet behavior, but it altered the powder’s flowability, impacting the spatter generation. To decrease spatter generation and obtain a high-quality spatter surface, it is recommended that the oxygen concentration in the building environment should be maintained at 1.0×10² ppm.

Hybrid manufacturing (HM), which combines additive manufacturing (AM) and subtractive manufacturing (SM), is effective for the fabrication of thin-walled complex shapes, such as impeller blades. Generally, a process planning for HM is to build a near-net shape through AM and finish its surface through SM. However, in this approach, the cutting tools are limited with long tool lengths and small tool diameters to avoid collisions between the cutting tool and workpiece. In addition, the fabrication shapes are also limited. Therefore, one possible solution is to alternate between AM and SM processes multiple times. In this approach, the workpieces are built gradually as the process progresses. Therefore, the cutting tool can easily avoid collision with the workpiece. However, melting penetration and temper color remain on the finished surfaces using the conventional process planning method with alternate multiple switching. In this process planning, AM and SM processes are alternated. Thus, the finished surfaces are remelted in the subsequent AM process. This heat input causes melting penetration and temper color. These thermal effects must be prevented because these can lead to unfinished part and deterioration of the appearance of the workpieces. Therefore, in this study, a novel process planning method that allows alternate multiple switches without thermal effects is proposed. In addition, a process planning support system that simulates SM process was developed. The SM simulation can detect collision between the cutting tool and workpiece. Using the proposed process planning method, the system plans a process in which thermal effects will not occur. In addition, a case study was conducted using a simulated impeller blade geometry. The results of the case study showed that the developed system could plan by using several cutting tools and parameters of the machining head. The system can estimate the processing time based on the cutting tool path, deposition path, SM process conditions, and AM process conditions. The results validated the developed system and demonstrated its usefulness.

Metal-based powder bed fusion with a laser beam (PBF-LB/M) can be applied to fabricate high-accuracy structures compared with other metal additive manufacturing (AM) methods. The rapid solidification of metal powder formed by laser irradiation introduces heterogeneous residual stress, which causes deformation and cracking of the structure. This, in turn, results in the deterioration of quality. In this study, the influence of the laser scan strategy on the residual stress distribution and warpage of the structure was investigated. Using maraging steel powder with an average particle size of 32.5 μm, the structures were constructed using several laser scan strategies at a wavelength of 1070 nm. The residual stress distributions on the surface of the structures were measured by the cosα method by applying X-ray diffraction (XRD). In addition, the warpage of the reverse side of the substrate as a foundation of the structure was measured by a stylus-type surface roughness measuring instrument. The results clarified that the structures constructed by unidirectional scan directions had a tensile residual stress that was generated parallel to the laser scan direction. Meanwhile, the compressive residual stress was generated perpendicular to the laser scan direction. The large warpage was aligned with the laser scan direction and tensile residual stress. When the laser scan direction was rotated by 90° for each layer, the residual stress distribution was generated with a cruciform shape. It was indicated that this residual distribution was caused by a laser scan on the top surface and a lower layer. The anisotropic residual stress distribution and reduction of warpage could be corrected by rotating the laser scan direction by 15° in each layer.

Mechanical component failure is usually caused by metal fatigue originating from small defects in metallic materials. Thus, it is important to precisely capture the fatigue properties of materials containing small defects. Fatigue tests of materials with artificial surface defects introduced by drilling have been conducted. Using the resulting data, an equation for predicting the material fatigue limit has been proposed on the basis of the √area parameter model, and its effectiveness has been confirmed for various materials. However, for additive manufactured (AM) materials that contain internal defects resulting in failure, controlling the size of the defect where the fracture originates is extremely difficult. Therefore, verification of the predictive ability of the √area parameter model for AM materials is impossible, in contrast with other materials that fail because of surface defects. In this context, developing a technique to intentionally introduce internal defects with arbitrary sizes at arbitrary locations can provide insights that help predict the fatigue limit of AM materials. This study aimed to establish a technology for quantitatively evaluating the effect of internal defects on the fatigue properties of AM materials by introducing internal defects with arbitrary sizes at arbitrary locations via AM. Specimens with different defect sizes and locations were prepared. Prior to the fatigue tests, the defect sizes and locations were measured non-destructively via X-ray computed tomography (CT). The fatigue tests were conducted in air at room temperature. All the specimens failed because of the intentionally introduced internal defects, and the fatigue lives became shorter with increasing defect sizes, except for the specimens with defects adjacent to the surface. In those cases, fatigue cracks easily reached the surface; therefore, the fatigue lives were speculated to be shorter than those of the specimens with the same defect sizes. Moreover, the defect sizes determined from the fracture surfaces by scanning electron microscopy were nearly consistent with those determined by X-ray CT.

This paper focuses on the hybrid process combining metal additive manufacturing (AM) and subtractive processing developed for application to injection molds. The basic concept is a combination of laser powder bed fusion of metal powder and subtractive processing. This process is characterized by alternating buildup and milling processes. Even the inner surface of deep grooves, which conventionally required electrical discharge machining, can be machined with small-diameter tools with a short flute length. Therefore, molds with complex shapes that previously required electrical discharge machining can be manufactured in a single process. Moreover, a dimensional accuracy and surface roughness of levels equal to those achieved by machining with the machining center can be ensured. In the hybrid process, it is necessary to minimize the surplus solidified area (which is the area milled by the small-diameter tool). Therefore, the formation mechanism of the surplus solidified region is verified. It is shown that the power distribution of the laser beam significantly affects the size (width and depth) and density distribution of the excessively solidified region. In addition, the effective value of metal AM mold is introduced. The 3D cooling circuit improves the efficiency of the injection molding process. If the temperature balance between the cavity side and core side is achieved, the distortion of the molded product would be suppressed. If the cooling effect is promoted, the molding cycle would be shortened substantially. Second, the effect of the gas vent function by a permeable structure is explained through actual examples. The effect of the gas vent function by the permeable structure is explained. It is indicated that stable molding can be achieved. In addition, the appearance defects of molded products can be reduced when the air inside the cavity is exhausted sufficiently from the mold through the permeable structure.

In this paper, we propose a method of manipulation planning for cable wiring. The method enables to take into account the deformation of cables while connector incorporation process. Using a physical simulation that predicts the shape of a cable based on the pose of both ends of the cable, we generate a connector moving path that avoids intereference between the cable and surrounding structures. We conducted experiments under several different environments and several different lengths of cables, and confirmed that actual cable manipulation can stably be achieved by using the proposed method.

This study proposes a novel process called peeling of nano-particle (PNP) to remove material locally on a hard material surface, such as silicon carbide (SiC), diamond, and gallium nitride (GaN), using the magnetic nano-particles in an aqueous solution controlled by magnetic fields. By the concept of the PNP process, magnetic fields are generated by two solenoid coils, which are sandwiched between the hard material sample, to pull the magnetic nano-particles to adhere to and then peel the material from the sample surface. In this experiment, iron (II, III) oxide (Fe₃O₄) particles with a diameter size in the range of 50–100 nm were dispersed in water, and the pH value was adjusted to 10 by potassium hydroxide (KOH). The particles were magnetically controlled on the silicon carbide (4H-SiC) surface by the magnetic fields at approximately 17 mT. To confirm the contact phenomenon of the Fe₃O₄ particles on the 4H-SiC surface during the PNP process, an optical system was developed by applying evanescent field microscopy to limit the observation range to approximately 300 nm from the 4H-SiC surface. According to the experimental observed results, the control phenomenon of two examples of Fe₃O₄ particles could be observed through their scattering light, which relates to the magnetic field generating sequence wherein the particles were magnetically pulled in and out of the 4H-SiC surface in the limit range of the evanescent field. During the particle pull to the surface, particles were able to be tracked in the X–Y directions during the approach to the 4H-SiC surface. The Brownian motion ranges in all directions of the particles decreased when the particles approached close to the surface due to the pulling magnetic field. Moreover, the magnetic field enforced the magnetic moment of the particle and limited their rotation.