Additive manufacturing (AM) with metals is currently one of the most promising techniques for 3D-printed structures, as it has tremendous potential to produce complex, lightweight, and functionally-optimized parts. The medical, aerospace, and automotive industries are some of the many expected to reap particular benefits from the ability to produce high-quality models with reduced manufacturing costs and lead times. The main advantages of AM with metals are the flexibility of the process and the wide variety of metal materials that are available. Various materials, including steel, titanium, aluminum alloys, and nickel-based alloys, can be employed to produce end products.

The objective of this special issue is to collect recent research works focusing on AM with metals. This issue includes 5 papers covering the following topics:

- Powder bed fusion (PBF)

- Directed energy deposition (DED)

- Wire and arc-based AM (WAAM)

- Binder jetting (BJT)

- Fused deposition modeling (FDM)

This issue is expected to help readers understand recent developments in AM, leading to further research.

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

The powder bed fusion (PBF) technique is a metal-based additive manufacturing (AM) method in which metal powder is deposited on a substrate and melted by selective laser-beam irradiation. Given that the process and parameters of metal-based AM are complicated, there are various problems in high-precision fabrication. One of these is that although metal-based AM can be used for fabrication of high-density parts, pores can easily form inside the fabricated structure owing to process instabilities. Pore formation degrades the mechanical strength of the fabricated structure. Therefore, this study investigated the pore formation mechanism inside a structure fabricated by PBF. Pore suppression by controlling the substrate temperature was also evaluated. Small- and large-sized pores with diameters of 10 μm and more than 50 μm, respectively, were found. Furthermore, differences in pore formation in the cross-section of the fabricated structure were observed owing to a variation in the volume-specific energy density and substrate temperature. At a substrate temperature of 25°C, the number of pores decreased more at the upper position than at the lower position owing to repeated melting and solidification under the laser-beam irradiation. At a substrate temperature of 200°C, the number of pores decreased significantly more than at 25°C. Furthermore, as the substrate temperature increased, the wettability of the molten metal improved, resulting in smaller contact angles of the fabricated structure in the single-line track. In PBF, multiple lines are fabricated in each layer. At low substrate temperatures, interstices were formed between the lines owing to the low wettability of the molten metal. These interstices acted as the origins of pores when the next layer was fabricated. Heating the substrate made the surface of the structure smooth owing to the high wettability of the molten metal and a reduction in the number of pores. Therefore, the formation of large pores could be reduced by controlling the substrate temperature.

The joining of dissimilar materials is an important process to produce a large production. In other words, the reliability of such a production is determined by the joining technique because the joint interface often becomes the weakest point against stress. In case of metals, welding and riveting are popular approaches for joining dissimilar materials. However, these techniques generally involve manual and complex operations; therefore, the production quality cannot be maintained, because the accuracy and efficiency of these operations strongly depend on the worker’s skill. From this viewpoint, additive manufacturing (AM) can be useful to produce parts using a combination of dissimilar metals. Metal AM has attracted considerable attention from aerospace and automobile industries recently because of its flexibility and applicability in the production of various complex-shaped parts. Directed energy deposition (DED), one such metal AM method, forms a deposit of powder material and simultaneously irradiates a laser beam on the baseplate. DED can be applied to cladding and repairs as it can be conducted on the surface of the part. In particular, a combined part of dissimilar metals can be easily and directly produced from scratch by changing the powder material of the process. A graded material can also be produced by blending different powders and changing their ratios appropriately. In order to realize such applications of DED, the mechanical properties of the produced part must be evaluated in detail. In this study, a part combining a nickel-based superalloy (Inconel 625) and a stainless steel alloy (SUS316L) is produced using DED; the produced part is evaluated through a tensile strength test, Vickers hardness measurement, metal structure observation, and element distribution analysis. In addition, a graded material is also produced to evaluate the basic characteristics of the joint produced using DED. The experimental results show that the produced joint is sufficiently stiff against tensile stress and its hardness is increased because of the solid solution of niobium in the stainless steel area. The results of the elemental distribution analysis and the Vickers hardness test indicate that a graded joint of Inconel 625 and SUS316L can certainly be produced using DED.

Wire arc additive manufacturing (WAAM) is a crucial technique in the fabrication of 3D metallic structures. It is increasingly being used worldwide to reduce costs and time. Generally, AM technology is used to overcome the limitations of traditional subtractive manufacturing (SM) for fabricating large-scale components with lower buy-to-fly ratios. There are three heat sources commonly used in WAAM: metal inert gas welding (MIG), tungsten inert gas welding (TIG), and plasma arc welding (PAW). MIG is easier and more convenient than TIG and PAW because it uses a continuous wire spool with the welding torch. Unlike MIG, tungsten inert gas welding (TIG) and plasma arc welding (PAW) need an external wire feed machine to supply the additive materials. WAAM is gaining popularity in the fabrication of 3D metal components, but the process is hard to control due to its inherent residual stress and distortion, which are generated by the high thermal input from its heat sources. Distortion and residual stress are always a challenge for WAAM because they can affect the component’s geometric accuracy and drastically degrade the mechanical properties of the components. In this paper, wire-based and wire arc technology processes for 3D metal printing, including their advantages and limitations are reviewed. The optimization parametric study and modification of WAAM to reduce both residual stress and distortion are tabulated, summarized, and discussed.

Additive manufacturing (AM) using metal materials (metal AM) is useful in the fabrication of metal parts with complex shapes, which are difficult to manufacture via subtractive processing. Metal AM is employed in the manufacture of final products as well as in prototyping. Recently, certain metal-AM machines have been commercialized. Powder-bed fusion and direct energy deposition are the main types of metal AM; they require the use of a high-power laser or electron beam and most of them are highly expensive. On the other hand, AM machines of the material-extrusion (ME) type can fabricate metal parts at a low cost. ME is the method of extruding materials from a nozzle and fabricating thin layers. By mixing a metal filler with a base material, it is possible to impart various mechanical properties to the extruded material, such as electrical or thermal conductivity. If the extruded material is baked in a furnace after fabrication, the object can be sintered. During the sintering process, the fabricated objects always shrink and dimensional errors occur. One of the reasons for the shrinkage is that voids are generated inside the object after the degreasing process and collapse during the sintering process. Because the void is generated as a space by replacing a binder that becomes vaporized during the degreasing process, the shrinkage may be controlled by decreasing the content in polymers. In this study, the effect of the metal filler density on the shrinkage in shape was investigated through experiments using two types of metal ME AM. One type is the fused filament fabrication (FFF), in which a material that consists of a metal filler and fused plastics is extruded; the other type is the ultrasonic vibration-assisted ME (UVAME) device, in which a metal powder suspension with a small amount of thickening polymer is extruded. In the latter method, materials with an extremely high density in metal fillers were used; it was considered that degreasing was not required. Two types of specimens were fabricated using AM devices; they were then degreased and sintered. The resulting shapes of the objects were measured with a 3D scanner and were compared. The experimental results showed that the shrinkage of the material with a high density of metal fillers was less than that of the material with a low density of metal fillers.

3D printers that use the fused deposition modeling (FDM) method are generally based on a three-linear-axis mechanism. However, because the posture of the workpiece is limited, the shape of the model that can be generated by this type of 3D printer is restricted. The 3D printer makes 3D models by stacking up materials on a plane. Because of this principle, a base to support the laminated material is necessary, and it is impossible to develop a model shape with an overhang without support parts. Although the problem is solved by making a foundation using a support material, it takes time to shape and remove the material. Therefore, this conventional method is time consuming. The purpose of this research is to laminate and make shapes that are difficult to laminate with a three-axis 3D printer without using support material. Therefore, a new five-axis 3D printer was developed with the FDM method, and its control program was designed. In addition, hardware consisting of the mechanical structure and the servo control system was developed, and the laminating path, which can exert the effect of the five-axis mechanism, was calculated. The posture of the workpiece can be controlled by mounting the B-axis, which tilts the lamination table, and the C-axis, which rotates the lamination table added on the three-axis configuration 3D printer. Furthermore, a five-axis synchronization control program was developed to control the motion of the five-axis synchronous motion. Furthermore, to correct the nozzle position due to the posture change of the workpiece, a mathematical model of shape creation theory was applied to derive the offset command value. As a result of the laminating experiments of the overhang shape model, the five-axis mechanism and laminating path were sufficiently effective, and the five-axis synchronous control of the 3D printer demonstrated the creation of the overhang shape. However, in experiments using a conventional three-axis mechanism 3D printer with the same lamination path, resins did not adhere and dripped, making shaping impossible. Because of these results, the machining time of the five-axis controlled 3D printers was shorter than that of conventional three-axis-controlled 3D printers. Here, the basic configurations and control system are reported.

A new methodology to generate instruction commands for prompt machine control as a replacement for the previously prepared numerical control (NC) programs is developed to realize an innovative intelligent machine tool. This machine tool can eliminate NC program preparation, achieve cutting process control, reduce the production lead time, and realize an autonomous distributed factory. In this study, the innovative intelligent machine tool based on the computer-aided manufacturing-computer NC integrated concept is developed. The special feature of this system is to generate instruction commands in real time for prompt machine control instead of using NC programs. Digital Copy Milling, which is a digitized version of traditional copy milling, is realized by using only the computer-aided design model of the product. In this system, the cutting-force simulation is performed simultaneously with the real-time tool path generation. Then, the tool feed rate can be controlled according to the predicted cutting force. Therefore, both the improvement of the machining efficiency and the avoidance of machining problems can be achieved. The instantaneous cutting force model predicts the cutting force. In this system, the work material is represented by the voxel model, and the uncut chip thickness is calculated discretely from the number of voxels removed. Thus, it is possible to predict the cutting force in the case of non-uniform contact between the tool and the work material. In this study, a machining simulation is conducted to validate the proposed method. The results of the simulation show successful tool feed speed adaptation based on the predicted cutting force. The results also show the effective reduction of the machining time. A case study of a custom-made product for dental prosthetics is examined as a good application of both the proposed adaptive control and the Digital Copy Milling system. Through this method, it is possible to improve the machining efficiency and prevent tool breakage.

The improvement in the positioning accuracy of machine tools necessitates reliable friction models for compensation. Friction and damping are primarily caused by mechanical contacts, and they have a wide influence on the dynamics of machine tools. Particularly in the linear motor driven axis, linear bearings induce majority of the friction; contact is observed between the ball and raceway in linear bearings. Based on the Hertzian contact theory and a tangential force model, a model is developed for the friction behavior during the contact between the ball and raceway. This model determines the stick and slip areas, and the relative velocity at the contact surface. Hence, the calculation of the friction force, its hysteresis characteristics, and the stick and slip portions becomes possible.

The flexible job shop scheduling problem (FJSSP) is an extension of the classical job shop scheduling problem (JSSP) that allocates jobs to resources while minimizing the maximum completion time of all the jobs. Machine assignment and job sequence are determined in the FJSSP. To efficiently solve the FJSSP, which is a non-deterministic polynomial-time hard problem, a heuristic method must be used. In previous studies, the FJSSP has been solved using neighborhood algorithms that employ various metaheuristic methods. These approaches constrain the neighborhood operation to jobs on a critical path and simultaneously change the machine assignment and job sequence. Branches on the critical path are easily generated in the FJSSP search processes; this branch structure can improve the efficiency of the FJSSP. This study investigates two neighborhood search algorithms used for changing the machine assignment and job sequence via a critical path. The first method changes the machine assignment and job sequence simultaneously, whereas the second method changes them independently. In this study, we propose an efficient neighborhood generating method using a branch block of critical path.

In this study, a phosphor was coated on a microstructured film to achieve light control. This process resulted in a large-area phosphor film and enabled the microstructure to be packaged directly into the LED body. Thus, the LEDs retain their air and water barrier functions, control light, achieve higher forward luminous intensity, and have a wider scope of applications. Roll-to-roll processing was performed to mold a microstructure and phosphor on polyethylene terephthalate (PET) film by applying ultraviolet light. This approach expedited the preparation of a large-area phosphor film and enabled the precise control of the thickness and evenness of the phosphor layer, thus ensuring uniform light distribution and eliminating the yellow halo within the light body induced by the uneven thickness of the phosphor layer. The experimental results revealed that the luminous intensity of the LED to which the microstructured PET film was attached at 0° (center) increased by 11.88% relative to the luminous intensity of the LED without the film. Moreover, at 30° to −30°, the luminous intensity of the LED with the film improved by 10.36%. Therefore, the device retained its color uniformity and achieved higher forward luminous intensity.

Numerical control code is typically used for manufacturing a workpiece using machine tools. Most state-of-the-art approaches decouple the set point optimisation into two steps: the geometry and the feed rate optimisation that does not necessarily result in time-optimal set points for the desired geometry. Given the originally programmed geometry through the numerical control code, dynamic constraints of the machine tool, and maximum permissible contour error for the optimisation, a model predictive contouring control based set point optimisation approach is developed to generate time-optimal set points for machine tools globally. A suitable error definition and its linearisation are used whereby the optimisation problem can be represented by a quadratic programming problem with linear constraints. Compared to most state-of-the-art methods, a direct approach is presented and no previous geometry optimisation step is required. Depending on the demands of accuracy, different maximum contour error constraints and penalisation as well as various maximum permissible axis velocities and accelerations are presented and tested on a test bench. The method is shown to be adaptable to different demands on the set points, and the contour errors can be affected by either the constraints or penalising factors.

A new, multi-dimensional, additive manufacturing process for fine ceramics was proposed and developed as part of a national project in Japan. The process consists of three-dimensional printing and two-dimensional coating of fine ceramics. A new coating process, hybrid aerosol deposition (HAD), was proposed as the ceramic coating process. The HAD process is a hybrid of aerosol deposition (AD) and plasma spray. Such new technologies, however, usually take a long time to move from first discovery to use in producing a commercial product. For example, a past study showed that it took nearly 15 years from the invention of the AD process to the time it became a technology used at an industrial company. Therefore, it is very important to consider how to accelerate the learning and technological transfer of a new process to industry in addition to how to develop new processes once they emerge. In this study, a new scheme, a coating hub, is proposed to promote the transfer of the HAD process to industrial adoption. In the coating hub, a collaboration scheme for companies to get interest of the technology, even in the early stages of technological development, is considered. Here, needs-seeds matching, reliable relationships, intellectual property, and the generalization of technology are considered. Another important scheme of the coating hub is to try to couple design with manufacturing. Here, product design tools for agile production are provided. In order to attract and evaluate consumers for targeted products, a Kansei delight design based on the Kano model is introduced. A delight map viewer is provided to visualize potential consumers’ delight factors. Detailed planning from the early trial stage is introduced with the viewer. A topology optimization tool is also provided in the coating hub as a design tool. In order to validate this coating hub concept, a ceramic frying pan is designed as a case study. The delight map viewer proves effective for those who are not design professionals to consider the attractiveness of products based on user evaluation. The coupling of the topology optimization tool is also useful for the multidimensional additive manufacturing of ceramics proposed in this study. This case study implies that even a small manufacturer could design a new product by utilizing the coating hub concept. It would give many new opportunities not only to big manufactures interested in high-end business-to-business components but also to supporting industries and even to individuals to utilize new emerging coating technologies.

Quadrant glitches are caused by friction and motion loss on the feed axis of machine tools. A previously developed method of compensating for quadrant glitches using the feed axis in which the friction model and time series data are not consistent with the actual friction behavior has some problems, making it difficult to construct a feedback system with a high response problems such as a feed axis with a large lost motion. The ultimate goal of this study is to develop an innovative method of compensating for the quadrant glitches caused by the motion of the feed axis of the machine tool using a newly proposed hybrid spindle system with an active magnetic bearing at the end near the end mill and a ball bearing at the other end in combination with a proportional-integral-derivative controller. This study aims to verify the effectiveness of the proposed quadrant glitch compensation method through experiments on the motion of the end mill using a model experimental device for the hybrid spindle system. Through experiments, a quadrant glitch with a peak of 7 μm without compensation is decreased to 1 μm by applying the proposed method using the hybrid spindle system. The undercut error is also decreased by applying the proposed method.