Published papers

Disturbance observation is a sensorless process monitoring technique that has not yet been applied to the monitoring of tool collision in the use of machine tools. This paper presents a monitoring algorithm that detects collisions that involve the breakage of drills through observation of the disturbance force change rate. A comprehensive experimental study of diverse operator-induced collisions was conducted to analyze the collision detection potential of the disturbance observer. It was found that collision monitoring by the disturbance observer is more sensitive and responsive than that using a sensorless jerk observer.

Tool fracture detection is important to avoid tool breakage and ensure cutting accuracy. However, conventional tool condition monitoring methods use additional sensors that are expensive, increase the failure rate, and reduce the machine-tool stiffness. This study proposes a novel in-process method to detect tool fracture based on disturbance observer theory. It uses only servo information in a ballscrew-driven stage control system. Furthermore, a rotational digital filter is developed and applied to drilling tests to enhance the detection accuracy. Tool fracture is successfully detected without any external sensors by the proposed method.

Selective laser melting (SLM) is one of the additive manufacturing (AM) methods which is applicable to metal. Although this technology enables to form complex inner shapes such as lattice structure, steep temperature gradients due to local laser irradiation and uneven heat conduction during the process may cause bending and cracking of the product. In this study, the influence of laser power and scanning speed on the mechanical characteristics of the support structure and lattice structure formed by SLM is investigated. In the tensile test of the support structure, a positive correlation with volumetric energy density was confirmed. In the compression test of the lattice structure, it was clear that larger amount of heat energy from the laser makes struts of lattice structure thicker and its Young's modulus higher.

Many studies have focused on the stabilization of additive manufacturing (AM) in microgravity for its use in various space projects. Nevertheless, this paper presents a vital clue for innovating metal AM technologies from the perspective of high gravity. High-gravitational powder bed fusion has an excellent potential to address various challenges in AM, such as density enhancement, spatter suppression, and precise fabrication. This study summarizes an analogy among phenomena in different gravitational fields and establishes a combined machine for centrifuge and powder bed fusion. The results confirm the spatter suppression and fine-powder availability in high gravity, both theoretically and experimentally.

Directed energy deposition (DED), which is one of the metal-powder additive manufacturing processes, is a promising method suitable for repairing. Because the carrier gas that transports the metal powder and the shield gas that protects the optical system are supplied in DED, it is difficult to estimate the transport of metal powder due to the complex turbulence around the processing point, resulting the powder supply efficiency gets lowered. This study computationally analyzed the fluid dynamics of two types of DED nozzles for improving the powder supply efficiency by changing the number of powder channels, working distance and channel diameter.

Polymer science plays a crucial role in the understanding and numerical study of material extrusion processes that have revolutionized additive manufacturing (AM). This study investigated the impact of high-gravity conditions on material extrusion and conducted a numerical study by referring to the development of a high-gravity material extrusion system (HG-MEX). In this study, we evaluated the polymeric characteristics of HG-MEX. By analyzing the interplay between polymer behavior and gravity, we provide insights into the effects of high gravity on extrusion processes, including filament flow, material deposition, and the resulting fabrication characteristics. The established numerical study of high-gravity material extrusion in additive manufacturing is a meaningful and valuable approach for improving the quality and efficiency of the process. This study is unique in that it incorporates material surface characteristics to represent the performance and contact with polymer science in additive manufacturing. The findings presented herein contribute to a broader understanding of polymer science and its practical implications for HG-MEX under various gravitational conditions.

In-situ resource utilization has become a significant issue in recent space projects. For example, various studies have focused on lunar manufacturing using regoliths available on the moon's surface. Although regolith manufacturing will be conducted under lunar gravity, most studies have not considered the adverse effects of low gravity on additive manufacturing processes. This study evaluates the gravitational effect on lunar regolith powder bed fusion (PBF) by applying centrifugal acceleration to the PBF process. The experimental results show that gravitational acceleration influences the density, hardness, and transparency of the fabricated regolith blocks.

Directed energy deposition (DED) is one of additive manufacturing processes attracting attention from various industries. Foam metal is a material having beneficial characteristics, but is difficult to produce with conventional manufacturing system. This study proposes a simple foam metal fabrication method by applying DED. The proposed method produces foam stainless alloy in DED by adding a foaming agent, titanium hydride (TiH2), into the powder material. This study evaluates the foaming behavior of porous stainless in DED by changing the particle size of TiH2 and the number of layers per line, and obtains the optimal number of layers to improve the porosity dispersion.

Undercooling state is metastable and immediately releasable by applying disturbance intentionally. Therefore, a 3D printer with considerably high modeling efficiency expectedly can be realized by using the supercooling phenomenon. This study discussed the applicability of the supercooling phenomenon in ice (H2O) to additive manufacturing. As a result, freezing of the supercooling phenomenon due to impact was observed. The freezing behavior depended on the flow rate, and freezing in the syringe was observed when the flow rate was high. In addition, when the conditions such as the amount of water droplet was adjusted, it was possible to fabricate shapes with simple structures by stacking.

Directed energy deposition (DED), a metal additive manufacturing process, has attracted considerable attention in various industries. In the past few decades, several studies have been conducted to enhance the quality of DED-produced parts. Various studies on powder- and laser-based DED have focused on eliminating the residual pores in the fabricated parts, which decrease the mechanical strength. In contrast, this study aims at intentionally increasing the residual pores to produce a porous structure in metals called as foam metal, which realizes lightweight and excellent energy absorption properties. Despite being well-known for their attractive properties, Foam metals are rarely used due to the technical difficulties in their production, which requires special devices and entails considerable cost. The simplicity of the DED process makes it a promising approach to fabricate foam metals because it is easy to foam at the fabrication point by mixing a foaming agent, such as titanium hydride (TiH2), into the material powder. This study evaluates the DED foaming process in stainless steel alloys by changing the particle size of the TiH2 powder and the scanning path to obtain the optimal conditions for enhancing the pore dispersion. The experimental results show that, when the particle size of the foaming agent is small, the number of pores and their distribution increases. Additionally, the mechanical properties are also evaluated through compression experiments, and the energy absorption ratio is found to be higher when a smaller foaming agent is used.

Additive and subtractive (Add/Sub) manufacturing processes are increasingly being combined to produce complex parts with unique geometries and properties. However, the design of such combined processes is often challenging as it requires a deep understanding of the interaction between the different processes. On the other hand, digital twin (DT) technology has become a powerful tool in recent years for optimizing manufacturing processes. This article explores the use of the digital twin technology for a holistic process planning of combined additive and subtractive processes. The article describes the integration of laser metal deposition (LMD) and micro-milling prediction models of resulting geometry (width and height), hardness, and surface roughness into the digital twin. This is then used for combined process planning to achieve different target values regarding resulting geometry and surface roughness. For the planning of this combined process chain, further criteria such as tool life, energy, and process time are considered in the optimization, showing the potential for sustainable and efficient production. Sensorless cutting force estimation is also used to detect small cutting forces, with the potential to use this as a soft sensor for roughness estimation. The measured width, height, and roughness as a result of the process parameters suggested by the optimization algorithms showed a mean absolute percentage error (MAPE) of 4, 17, and 16%, respectively.

Additive manufacturing (AM) has gained significant attention in recent years owing to its ability to fabricate intricate shapes and structures that are often challenging or unattainable using conventional manufacturing techniques. This high-quality development trend entails higher requirements for the structural design of 3D printers. In this study, polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) filaments were fed through a heated extrusion nozzle, which melted the material and deposited it onto a build platform. This study's objectives are high-gravitational material extrusion (HG-MEX) systems development, analyzing the high gravity influences on the flow behavior of materials during extrusion, and understanding the effects of gravitational on material flow and overall extrusion performance. HG-MEX systems have great potential for addressing various challenges in additive manufacturing, such as precise manufacturing. The highlight of the progress is that we developed an HG-MEX system and applied surface science to material extrusion in different gravity. We established a system and obtained results on different gravity, we analyzed the analogy between different gravity phenomena. We analyzed the interplay between the behavior of the fabricated parts and gravity. We analyzed high gravity effects on extrusion processes. The results confirmed the characteristics and feasibility of the developed system. The results suggest that a material extrusion line operating under 15 G conditions resulted in better printing quality compared to one operating under 1 G conditions. This observation implies that high gravity had a positive effect on the extrusion process, leading to improved material extrusion performance.

Additive manufacturing (AM) has gained significant attention in recent years owing to its ability to quickly and easily fabricate complex shapes and geometries that are difficult or impossible to achieve with traditional manufacturing methods. This study presents the development of a high-gravity material extrusion (HG-MEX) system, which generates a high-gravity field through centrifugal acceleration. In this process, the material is dissolved by heating the nozzle and subsequently deposited on the construction platform. The primary objective of this research is to evaluate the positive effects of gravity on material extrusion (MEX), which is a key aspect of AM. To accomplish this, a combined machine comprising a MEX unit and centrifuge is constructed. This HG-MEX system is used to analyze and reflect the influence of gravity on the material extrusion. The experimental evaluations demonstrate that the application of high gravity is a promising approach to improve the shape accuracy and performance of the parts fabricated through MEX. Notably, our results confirm the feasibility of utilizing MEX under high gravity to enhance performance in AM processes.

Automobile electrification has increased demand for higher-performance gears in drivetrains with specialized functional requirements beyond what conventional carburizing heat treatment can deliver. While carburizing is effective for mass production, it faces inherent limitations in achieving advanced material properties needed for next-generation applications. Therefore, this study proposes a high-speed coating method on rotational gear surfaces using directed energy deposition (DED). The proposed method can form the coating layer continuously and efficiently on the complex-shaped teeth of the gear, compared to the common method that forms coating along a complicated path matching the part's geometry. Furthermore, a localized heat treatment system integrating laser preheating and liquid cooling is introduced to precisely control material characteristics of the coating layer. Experimental results demonstrate that this combined heat treatment achieves a crack-free, hard coating layer on preformed gears. This method shows significant potential for producing customized, high-performance gears for specialized applications in next-generation vehicles while reducing process complexity.

Directed energy deposition (DED) is an additive manufacturing process wherein a metal powder is fed through a nozzle onto a baseplate, melted, and solidified using a laser. Potential applications of DED have been actively discussed. In this study, laser-based DED (DED-LB) was applied as a high-speed metal-coating technique to a cylindrical pipe. The mechanical properties of DED-fabricated objects were strongly affected by processing parameters such as laser power, powder feed rate, and rotational speed of the cylindrical pipe when using 20MnCr5 and M2 as the coating materials. Hardness measurements showed that the hardness of 20MnCr5 increased with a high heat input, whereas that of M2 increased with a low heat input. The two metals exhibited opposite trends in terms of hardness variation because of the differences in their transformation points. This study analyzed a layer coated with DED-LB from the viewpoint of metal morphology.

Selective laser melting (SLM) is one of the additive manufacturing (AM) methods which is applicable to metal. This technique makes it possible to form complex internal shapes such as lattice structures. The lattice structure is expected to have weight reduction and vibration suppression effects on the product. In addition, a support structure is essential for the SLM fabrication process, which has the role of supporting the product and dissipating heat from the product. However, steep temperature gradients due to local laser irradiation and non-uniform heat conduction during the process can cause bending and cracking of the product. In particular, the support structure and lattice structure consisting of thin metal struts are greatly affected by process conditions, so it is necessary to select conditions suitable for stable and high-precision modeling. In this study, the influence of laser power and scanning speed on the mechanical characteristics of the support structure and lattice structure formed by SLM is investigated. In the tensile test of the support structure, a positive correlation with the volumetric energy density was confirmed, and suitable laser conditions were examined. In the compression test of the lattice structure, it was clear that the amount of energy absorption changed depending on the laser power, and the condition of 240 W achieved the maximum energy absorption.

In recent years, metal 3D printers have been attracting attention not only for building solid parts but also for repairing and coating processes. The goal of this study is to apply the directed energy deposition (DED), which is one of the additive manufacturing, to realize high-quality metal coating by depositing the coating material on a cylindrical object. To evaluate the effect of process parameters, experimental points were set up based on the central composite design and deposition was performed using Inconel 625 as the material powder. The hardness of the coating was measured and statistically analyzed by the response surface methodology. From the analysis results, it was found that laser power has the largest influence on the coating hardness, which was improved by 28% compared to the low rotation speed by depositing under the optimum processing parameters.

In recent years, additive manufacturing (AM) technology that can directly shape objects from three-dimensional data has attractedattention.AM technology is promising as a manufacturing method in space because of its resource and space-saving properties. Some studies have shown that gravity change is viewed as a problem to overcome. On the other hand, there has been no progress in the discussion of attempts to effectively utilize gravity changes for AM technology. In this study, we focus on powder bed fusion (PBF) among AM technologies, using gravity as a control factor. The high gravity PBF (HG-PBF) is expected to be able to solve the problems of the current AM technology such as improvement of shape accuracy and process stability. In this study, we used fine powders and experimentally discuss the effect of the high gravity field on improvement of PBF process.

Additive manufacturing is a technology that enables mass customization of products owing to its flexibility and is one of the key technologies used in Internet-of-Things-based manufacturing systems. In recent years, industrial applications of additive manufacturing have been increasing, and expanding into space manufacturing. However, the controllability of gravitational acceleration has not been well explored. In this study, a high-gravitational powder bed fusion system is developed, which produces a high-gravitational field via the application of centrifugal acceleration. It is evident that the powder bed quality is improved, and the spatter generation is dramatically suppressed on applying a high gravitational acceleration of up to 10+ G. The experimental evaluations demonstrate that a high gravitational field improves the surface quality, density, and hardness of the fabricated parts.