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Conductive polymer composites (CPC) have been remarkably developed, demonstrating their potential in various applications such as sensors, wearables, and even the field of biomedical. Furthermore, the recently attracted multi-material additive manufacturing (MMAM) technology indicates the potential for expansion into a wider range of applications. However, the characteristics studies of printable composites are still in progress, and this implies that the applicability of composite materials is limited in the near future. It is also necessary to consider an investigation of conductive composites with unexplored wide ranges of filler contents. CPCs are easier to print when the conductivity filler content is low. On the contrary, increasing the conductivity filler content to enhance electrical conductivity leads to a decrease in polymer characteristics, making it challenging to print. In this study, TPU (thermoplastic polyurethane)/MWCNT(multi wall carbon nanotube) filaments incorporating relatively high content MWCNT were manufactured. The filaments were extruded by the melting mixing method and the printing characteristics were discussed using fused filament fabrication (FFF). It was confirmed that TPU/MWCNT filaments with a maximum MWCNT content of 20% could be produced, but printing was limited to 15% MWCNT content. As the MWCNT content increases, the nozzle temperature also needs to be increased to avoid print quality reduction. Additionally, the electrical and mechanical properties of the printed TPU/MWCNT composites were investigated.

In this study, a flexible conductive filament was fabricated by mixing thermoplastic polyurethane (TPU), carbon nanotubes (CNTs), and Cu powder. A polymer extruder was used to disperse conductive materials in the TPU matrix. Because the dispersion of CNTs in polymers is difficult, the dispersion process was repeated several times for the homogeneity of the fabricated conductive filament. Cu powder with high electrical conductivity was additionally added to improve the electrical characteristics of the conductive polymer. As TPU generally has excellent ductility and durability, the fabricated filament can secure flexibility. The resistance was measured and compared according to the mixing ratio of CNT/Cu powder. Based on the obtained results, the resistance of the conductive filament decreased as the CNT and Cu powder contents were increased. The addition of metal powders, such as CNTs and Cu powder, however, reduced the ductility of TPU. Finally, the developed conductive filament was used to fabricate a simple closed photodiode circuit.

Additive manufacturing technology has high applicability across various fields without distinction. Since there is no limitation on materials, functional materials also contribute greatly to the development of additive manufacturing technology. The conductive composite, which provide electrical properties by mixing conductive materials with a polymer that is an insulator, represent one of the most popular areas. These conductive composites can act as a sensor by integrating with an actuator. New types of functional actuators are being designed using sensors with mechanical and electrical properties, and processes for manufacturing them are being developed. In this study, a resistance-based sensor was fabricated by mixing thermoplastic polyurethane (TPU) and multi-wall carbon nanotube (MWCNT). The sensor was printed on a soft gripper and evaluated for resistance response according to bending operation. Analysis was conducted considering several variables, and as a result, superior performance was measured in terms of responsiveness to gripper operation. It is revealed that several types of sensors with respect to TPU/MWCNT composition, having various sensitivity can be used to sense the bending deformation of the soft gripper.

Extending geodetic and astrometric Very Long Baseline Interferometry (VLBI) observations from traditional centimeter wavebands to millimeter wavebands offers numerous scientific potentials and benefits. However, it was considered quite challenging due to various factors, including the increased effects of atmospheric opacity and turbulence at millimeter wavelengths. Here, we present the results of the first geodetic-mode VLBI experiment, simultaneously observing 82 sources at 22/43/88/132 GHz (K/Q/W/D bands) using the Korean VLBI Network (KVN). We introduced the frequency phase transfer (FPT) method to geodetic VLBI analysis, an approach for calibrating atmospheric phase fluctuations at higher frequencies by transferring phase solutions from lower frequencies. With a 2 minute scan, FPT improved the signal-to-noise ratio of most fringes, some by over 100%, thereby enhancing the detection rate of weak sources at millimeter wavebands. Additionally, FPT reduced systematic errors in group delay and delay rate, with the weighted root mean squares (WRMS) of the postfitting residuals decreasing from 25.0 to 20.5 ps at the W band and from 39.3 to 27.6 ps at the D band. There were no notable differences observed in calibrating atmospheric phase fluctuations at the K band (WRMS = 12.4 ps) and Q band (WRMS = 11.8 ps). This experiment demonstrated that the millimeter waveband can be used for geodetic and astrometric applications with high precision.

The experiments of wire arc additive manufacturing (WAAM) with general pulsed gas metal arc welding (GMAW) machine were performed to optimize process parameters. As optimized conditions, 140A welding current, 34 V welding voltage, 40 cm/min welding speed, and 15 mm contact tip to work distance (CTWD), were selected. Welding current and voltage affected heat input. Lower deposition, short circuit mode, and spatters were obtained by insufficient heat input. The melting of previous layer and lower height were obtained by excessive heat input. Welding speed and CTWD affected surface state and arc stability. Unsound surface and bead shape due to short circuit mode and spatters were observed in slow welding speed and short CTWD conditions. Arc instability were also observed in fast welding speed and long CTWD conditions. With the optimized process parameter, curved propeller shape blade was manufactured by the change of robot arm teaching point. It can be considered that the desired sufficient quality was obtained due to curved shape.

Welding process simulation is applied at the design stage to increase the reliability of the assembled final structure. To improve the accuracy of welding thermal deformation analysis, it is important to define an accurate welding heat source model. Plasma arc welding (PAW) shows characteristics of both arc and laser heat sources, making it challenging to select appropriate heat source models and parameters. Due to these difficulties, there are few references for thermal deformation analysis for PAW. This study describes the effect of Gaussian parameters of PAW heat sources on temperature distribution and thermal deformation analysis. A conical heat source was defined by combining a volumetric heat source and a surface heat source, and analyses were performed by changing the Gaussian parameters of the surface heat source. Changes in Gaussian parameters affect the size of the heat source, which in turn affects temperature distribution. As a result, Gaussian parameters influence the concentration of the heat source, the width of the heat-affected zone, and the quantitative magnitude of thermal deformation.

In the automobile industry, there is a push to maximize the mileage of electric vehicles by using lightweight materials. Hence, the use of welded aluminum in electric vehicles is increasing, and there is a clear need for detailed characterization of the fatigue life of these welded parts. In this study, gas metal arc welded AA6061-T6 specimens were fabricated and an ultrasonic fatigue test method for ultra-high cycle fatigue testing was proposed. An S-N fatigue test curve of 109 cycles was obtained, with an infinite life (109 cycles) confirmed at 118 MPa and 105 cycle level fatigue life confirmed at 216 MPa.

To investigate the effect of weld geometry on the fatigue performance of aluminum lap joint arc welding, GMAW experiments and fatigue tests were conducted. The results show that the combination of a smaller toe angle and full penetration with a proper gap size results in the best fatigue performance. A proper gap size can disperse the stress concentration, and the dispersion increases fatigue performance. Because of the reduction in the welded area, partial penetration has a lower fatigue performance than the full-penetration case. In the lap joint, a large toe angle causes a smaller weld reinforcement owing to geometric reasons, and therefore exhibits a lower fatigue performance.

Recently, the development of material technology for application to wearable devices has been promoted. In particular, as core technologies for e-skin, Internet of Things, and AI robots, studies on flexible circuits required for connection and operation between devices are attracting attention. In this study, an epoxy-based flexible electrical conductor (EFEC) that can be applied to flexible circuits was developed. It was made by mixing a 2-µm Ag flake, epoxy resin, epoxy hardener, and IPA at a ratio of 50:10:5:5. Flexibility can be achieved by adjusting the ratio of the epoxy base and hardener. IPA, a volatile solvent, was added to lower the viscosity of the epoxy, and Ag flakes were added to increase the electrical conductivity. It was dispersed with a sonicator and magnetic stirrer for overall resistance uniformity. A specific resistance of 2.85 × 10 - 3 Ω cm was measured. Through scanning electron microscopy and energy-dispersive X-ray spectra analyses, the cause of the low resistance was identified. In addition, 100% strain was achieved in all cases through the tensile test and resistance deviation results. Among them, the lowest resistance deviation was confirmed in CASE 4. Although the specific resistance of the EFEC was approximately 10 times higher than those of electrically conductive adhesives (ECAs) ( 3.6 × 10 - 4 Ω cm ), it could efficiently be used as a conducting material. ECAs become brittle after hardening. However, we observed that the EFEC exhibited flexibility and that hardening did not proceed even after an extended period. Therefore, the applicability of the EFEC’s flexible circuit was confirmed by securing high electrical conductivity and flexibility.

Several studies have been conducted to predict issues caused by the welding process. The finite element method is extremely useful in analysing this welding process. Generally, in the analysis of welded structures, the welded joint is assumed to be rigid to large structure on vehicle-body crash or fatigue. This assumption does not require a theoretical background of a complicated welding process; therefore, it is relatively easy to set up an analysis model, which significantly reduces the analysis time. However, material properties and mechanical behavior of welded structures cannot be expressed, and there is no sufficient verification for this in several industries. Moreover, few studies have reported on strength analysis, including the welding process; therefore, this study presents a simple modeling using commercial program based on the finite element method. The welding process is simulated by identifying the welding-heat-source variable through experiments and a value close to the measured deposited metal and fusion-zone size are obtained. A flow stress–strain curve based on a uniaxial tensile test is obtained and the tensile strength of the welded specimen is analyzed.