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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.

Resistance spot welding is widely used in automobiles and ships, and especially, it accounts for 70% of the body welding process. However, resistance spot welding is limited when welding the closed structure of a vehicle body. As welding is performed by placing a spot gun in a closed structure, a passage is required for the insertion of the spot gun. This passage is called a “passing hole”, and the structural design of the passing hole and the cover that is installed to block the passing hole is complicated, resulting in durability problems as the weight of the vehicle body increases. This study aims to develop a plasma arc spot welding (PASW) process that can replace resistance spot welding. “Part 1 - Development of Plasma Arc Spot Welding Process” determines whether this process can replace the resistance spot welding process through experiments by adjusting the existing process variables of the plasma arc welding process as well as additional process variables. The experimental results of applying the PASW process are then applied to the development of a finite element analysis model for predicting the fracture strength of the PASW process in Part 2.

Homogeneous and heterogeneous PAW (plasma arc welding) experiments were conducted to obtain weld beads with tensile strength of 100% or more compared to that of the base metal in lap joint fillet welding of CP (complex phase) 780 MPa grade high strength steel sheet. Homogeneous and heterogeneous welding mean welding technic without and with welding wire in each. Common basic welding parameters of both homogeneous and heterogeneous PAW wire were applied, including welding speed of 21 cm/min, arc length of 3.5 mm, weaving amplitude of 3 mm, work angle of 30°, and Ar 99.99% shielding gas with 12 L/min. In homogeneous PAW, the optimized additional welding conditions were a welding current of 90 and 95 A, push angle of 20°, and weaving frequency of 5 Hz. The fracture strengths of all the samples were less than that of the base metal, and all fractures occurred at the weld bead fusion zone due to insufficient reinforcement. In heterogeneous PAW, the optimized additional welding conditions were a welding current of 125 A, push angle of 0°, weaving frequency of 4 Hz, and wire feed speeds of 1.53 and 1.38 m/min. Moreover, the welding wire was fed from behind the arc. The fracture strengths of all the samples were greater than that of the base metal, and fractures occurred at the base metal or fusion zone boundary. The reason for higher tensile in heterogeneous PAW wire is deduced as the large cross-section area of reinforcement provided by the welding wire comparing to homogeneous welding. It is well known that small radius of welding heat source clearly produces narrow heat affected zone. Therefore, the PAW produces relatively narrower heat affected zone, then resulted as fully enough strength of weldment. This is another reason of the current achievement.

Conventional fluorescent dyes have the property of decreasing fluorescence due to aggregation-caused quenching effects at high concentrations, whereas aggregation-induced emission dyes have the property of increasing fluorescence as they aggregate with each other. In this study, diketopyrrolopyrrole-based long-wavelength aggregation-induced emission dyes were used to prepare biocompatible nanoparticles suitable for bioimaging. Aggregation-induced emission nanoparticles with the best morphology and photoluminescence intensity were obtained through a fast, simple preparation method using an ultrasonicator. The optimally prepared nanoparticles from 3,6-bis(4-((E)-4-(bis(40-(1,2,2-triphenylvinyl)-[1,10-biphenyl]-4-yl)amino)styryl)phenyl)-2,5-dihexyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DP-R2) with two functional groups having aggregation-induced emission properties and additional donating groups at the end of the triphenylamine groups were considered to have the greatest potential as a fluorescent probe for bioimaging. Furthermore, it was found that the tendency for aggregation-induced emission, which was apparent for the dye itself, became much more marked after the dyes were incorporated within nanoparticles. While the photoluminescence intensities of the dyes were observed to decrease rapidly over time, the prepared nanoparticles encapsulated within the biocompatible polymers maintained their initial optical properties very well. Lastly, when the cell viability test was conducted, excellent biocompatibility was demonstrated for each of the prepared nanoparticles.

When the welded part of a vehicle body is considered in an analysis model, the accuracy of the analysis varies depending on whether the welded part is assumed to be a simple rigid body or if its characteristics are included. Particularly, when assuming a simple rigid body, the accuracy of the stress result may be insufficient, resulting in a large error. In this study, titled “part 2-development of finite element method (FEM) analysis model for predicting fracture strength”, a commercial welding simulation program, MSC Simufact welding software, was used to create a finite element method analysis model that includes the material characteristics of the welded part, and the analysis accuracy was determined in comparison with the experimental results presented in part 1. The analysis results showed satisfactory results with an error rate of 0.03%. Thus, the fracture strength and fracture position of the welding specimen could be confirmed using the FEM analysis model. Additionally, assuming that the welded part in spot welding has a simple geometric-shaped structure, a simplification spot welding model was created to determine whether it can replace the plasma arc spot welding experiment and pass the tensile shear test.

Photopolymerization-based three-dimensional (3D) printing techniques such as stereolithography (SLA) attract considerable attention owing to their superior resolution, low cost, and relatively high printing speed. However, the lack of studies on improving the mechanical properties of 3D materials highlights the importance of delving deeper into additive manufacturing research. These materials possess considerable potential in the medical field, particularly for applications such as anatomical models, medical devices, and implants. In this study, we investigated the enhancement of mechanical strength in 3D-printed photopolymers through the incorporation of potassium titanate powder (K2Ti8O17), with a particular focus on potential applications in medical devices. The mechanical strength of the photopolymer containing potassium titanate was analyzed by measuring its flexural strength, hardness, and tensile strength. Additionally, poly(ethylene glycol) (PEG) was used as a stabilizer to optimize the dispersion of potassium titanate in the photopolymer. The flexural strengths of the printed specimens were in the range of 15–39 MPa (Megapascals), while the measured surface hardness and tensile strength were in the range of 41–80 HDD (Hardness shore D) and 2.3–15 MPa, respectively. Furthermore, the output resolution was investigated by testing it with a line-patterned structure. The 3D-printing photopolymer without PEG stabilizers produced line patterns with a thickness of 0.3 mm, whereas the 3D-printed resin containing a PEG stabilizer produced line patterns with a thickness of 0.2 mm. These findings demonstrate that the composite materials not only exhibit improved mechanical performance but also allow for high-resolution printing. Furthermore, this composite material was successfully utilized to print implants for pre-surgical inspection. This process ensures the precision and quality of medical device production, emphasizing the material’s practical value in advanced medical applications.

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.

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.

The effects of various weld geometries on the fatigue performance of T-fillet joint aluminum arc welding were investigated by conducting gas metal arc weld (GMAW) experiments using 6061-T6 material for various weld geometries. The results of the fatigue tests conducted using a 5-ton grade tester demonstrated that the fatigue performance of T-fillet joint aluminum arc welding is primarily affected by the area of the welded part and weld reinforcement. A large welded area can be obtained by full penetration, and a large weld reinforcement can be obtained by a large throat size and a small toe angle. Contrary to expectations, it was observed that optimal size and higher quantity of porosities can increase fatigue performance. This can be attributed to porosities behaving similarly to crack stop holes, which can prevent stress focusing. The deviation in fatigue performance in the case of minute penetration depth and minute throat size is not remarkable. Therefore, deeper penetration and larger weld reinforcement are crucial factors in obtaining higher fatigue performance in the industry.

The use of aluminum in automobile bodies has increased in recent years due to its various advantages. However, the research conducted on the fatigue performance of aluminum arc welding is limited. This study conducts 6061-T6 aluminum GMAW experiments for the various weld geometries to determine their effect on the fatigue performance. These weld geometries, which include full penetration, partial penetration, welding with misalignment, different toe angles, and double butt joints, are obtained by controlling the welding conditions. Full penetration presents the best fatigue performance, and the toe angle exhibits a relatively minimal effect on fatigue performance. Misalignment causes a large reduction in the fatigue performance. In the partial penetration and double butt joint case, the non-welded part, where the stress is concentrated, is located at the bottom. Therefore, these two cases present a relatively lower fatigue performance. In the automobile production process, the use of a backing plate is limited; therefore, controlling the penetration depth significantly improves the fatigue performance, despite the difficulty in obtaining the desired penetration depth. Double butt joints present the advantage of penetration depth control; therefore, a double butt joint can be considered as an appropriate alternative solution for full penetration despite the additional cost involved in the double butt joint preparation.