In additive manufacturing of metal products, predicting deformations induced during the process is indispensable for improving the quality of the products and reducing the post-process machining time. Finite element analysis (FEA) based on the inherent strain method using a voxel mesh is an effective prediction method due to its reasonable analysis time, and to efficiently implement the prediction, a method of easily and accurately defining the inherent strain value is required. In this study, an analysis method based on multi-layer model theory was proposed to evaluate the inherent strain applicable to electron beam melting. The inherent strains obtained by the analysis were -0.835% and -3.42% for 12Cr steel and Co alloy, respectively. As a verification result using the FEA based on the analyzed inherent strain, the out-of-plane deformation of the base plate and the thickness of the manufactured parts were evaluated with accuracies of ±2.0mm and ±0.5mm, respectively.

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Laser Ultrasonic Technique (LUT) is applicable even in a high temperature state such as during welding because the laser is irradiated remotely without contact. Recent studies have reported that LUT is applicable during welding and it’s effective for in-process monitoring. However, in the previous studies, there was a problem with the portability of the measurement device especially in a generation laser, so the scope of its application was limited. In this study, we constructed a robotic measurement system of laser ultrasonic with a small microchip laser mounted on a robotic arm. Microchip laser, which is prototype product (1064nm, 20mJ/pulse, 1.5ns, 20Hz), was used to generate ultrasonic, and a laser interferometer (532nm, 1W) was used to detect the surface micro vibration caused by arrival of ultrasonic wave and was scanned using a mechanical stage. After acquiring the B-scope, which is composed of a generation-point and multi detection-points, the synthetic aperture focusing technique (SAFT) is applied. SAFT image obtained by in process measurement was evaluated to confirm the presence or absence of welding defect indication. Then, during Gas Metal Arc (GMA) welding process in the mid-pass of multi-layer welding for a single bevel groove, we attempted to detect welding defects generated by introducing slits before welding. As a result, in the second pass, defects were indicated in all areas where defects existed, and no indications were found in areas without defects. Therefore, it was shown that the robotic laser ultrasonic measurement system constructed in this study can detect defects in process of welding. In conclusion, we were able to show the possibility of evaluating the welding quality in process of welding even at the construction site by selecting the appropriate generation position and detection range according to the required accuracy and laser specifications.

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The weldability of cast iron depends on several factors such as the welding method, chemical composition of the filler material, and the presence or absence of preheating/post-weld heat treatment (PWHT). In the past, the authors of this study have proposed the utilization of mild steel and austenitic stainless-steel filler materials for cast iron. They advocated the use of welding methods such as shielded metal arc welding (SMAW) and gas metal arc welding (GMAW). However, these studies did not examine preheating or PWHT. There have been no studies to date on the effects of heat treatment on the metallographic structure and mechanical properties of welded joints obtained using mild steel and austenitic stainless-steel filler materials. The aim of this study is to investigate the effect of PWHT on joints obtained when mild steel filler material and austenitic stainless steel-filler material are used to join spheroidal graphite cast iron. For the welding method, SMAW and GMAW were applied as in the previous studies. After welding, each test piece was maintained at 850℃ for 1h and air-cooled. Subsequently, metallographic observation, hardness testing, and tensile tests were performed on the welded joint. For the test pieces with mild steel filler material, the ledeburite that appeared during welding decomposed, and the maximum hardness of the joint was significantly reduced. For the test piece with austenitic stainless-steel filler, ledeburite did not decompose and the maximum hardness of the joint did not decrease. In the tensile test, the tensile strength, 0.2% proof stress, and elongation increased. These results suggest that PWHT is effective in improving the tensile strength, proof stress, and elongation of the joint.

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A non-iterative method is proposed to evaluate surface tension and contact angle simultaneously from a sessile drop observation in gravity using two universal figures. A theory of the sessile drop in gravity is used assuming an axisymmetric profile of the drop. Numerical calculation is carried out to prepare the two figures. The first one of the figures is a relation between the normalized curvature at the top of the drop and the geometry ratio of height to radius of a point on the drop surface. The second one of the figures is a relation between the normalized curvature and the normalized height of the point. The position and the tangential angle of the point on the drop surface are obtained from the sessile drop observation. Using the first figure, the normalized curvature can be determined from the position and the tangential angle of the point. Using the second figure, the normalized height of the point can be determined from the normalized curvature determined. Since the height and the normalized heights can be obtained from the measurement, the normalizing factor can be obtained. Because the factor is a function of the gravity and the surface tension of the liquid, we can evaluate the surface tension of the liquid from the point on the sessile drop surface. By taking average using several points, we can improve accuracy of the surface tension evaluated. Also, the first figure can be used to evaluate the contact angle from the position of the drop edge without measurement of angle. The present method is demonstrated using a pure water droplet on a water-repellent surface. It shows a successful evaluation of the surface tension and the contact angle, simultaneously from the droplet observation, without any iterative calculation.

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This paper describes a high-current Gas Metal Arc Welding (GMAW) process using a constant voltage power source developed for high-efficiency welding of thick stainless steel plates. A short arc length as a welding condition is preferable to concentrate the arc and obtain a deep penetration for thick plate welding. However, it is easily found that an unstable arc derived from a short circuit happens irregularly by using a general purpose GMAW with constant voltage characteristic. A “low-frequency modulated voltage control” was applied for stabilizing a high-current arc that can maintain the short arc length without short circuiting. The welded joint using the developed welding process showed the similar corrosion resistance as that using the submerged arc welding, however, porosity was observed in the welded joints using the developed welding process in the current region of 400A or above. Consequently, a new current waveform control that changes the welding current dynamically and periodically has been developed, which realized reducing porosity and single-pass full-penetration welding of 9mm-thickness type 304 stainless steel plates with square groove.

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In order to clarify the relationship between the shielding gas flow and the shieldability during a lap-fillet arc welding, oxygen concentration measurements on a water-cooled copper which imitated a lap-joint in tungsten inert gas welding were conducted. Moreover, the shielding gas flow and the high-speed plasma flow during the welding were visualized by the schlieren method and the observation method using a tube tungsten electrode and xenon gas, respectively. As a result, a low-oxygen concentration region, where the oxygen concentration was less than or equal to 100ppm, was formed on the water-cooled copper. It was suggested that this was because the high-speed plasma flow was separated into the upper side and the lower side of a step which imitated the lap-joint part, and the intrusion of the atmosphere was prevented by the increase of the velocity of a shielding gas flowing around the plasma flow. When the arc length was set to be long, although the position of the low-oxygen concentration region was changed, there was small difference of the shieldability caused by the difference of the torch angle. However, when the arc length was set to be short and the torch was tilted, the plasma flow was blocked by the step and it did not flow to the upper side of the step. It caused that the shieldability became worse because the low-oxygen concentration region became narrow due to the low-speed shielding gas flowed on the upper side of the step. These results obtained in this study suggested that the shieldability during the welding could be improved by controlling the arc length.

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Submerged arc welding is widely used for butt welding of thick plates in large steel structures because of its high deposition rate and high weld quality. With thickness of 150mm or more, multi-layer welding is required and it takes a lot of time to finish build-up. Narrow gap welding with 1 to 3 degrees of groove angle is effective process to reduce the welding time. Recently, ultra-narrow gap welding with approx. 0 degree of groove angle has been studied. However, ultra-narrow gap SAW is hardly applied because of its risk of welding defects such as lack of fusion and undercut. In this study, it was clarified the influence of distance L between welding torch and groove wall on the welding defect with ultra-narrow gap SAW. Undercut is caused by arc gouging effect and excessive melt of groove wall. And relationship between lack of fusion and weld shape was elucidated. It was established the model for judging lack of fusion defined by heat input to groove HG and distance squared between heat source and groove corner r2 which calculated by heat input Q, bead width WB and cross sectional area of deposited metal AR. And with the model, appropriate range of L for each welding condition was determined.

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The authors focused on a buried-arc phenomenon for a high-efficiency thick-plate welding and developed a “low-frequency modulated voltage control” in order to stabilize a buried arc at high-current range, which repeats high and low voltage terms using a welding power source with constant-voltage characteristic. When the control was applied to high-current buried-arc welding using solid wires of φ1.2mm and φ1.4mm, a rotating transfer mode was observed at a high-voltage term. It was clarified that a high-current buried-arc phenomenon was stabilized by supporting the buried space with a rotating arc. On the other hand, when the low-frequency modulated voltage control was applied to a solid wire of φ1.6mm, it was revealed that a rotating transfer mode was not observed although the control was effective in stabilizing a high-current buried-arc phenomenon. Our study shows its stabilization mechanism based on the experimental observation of the difference in arc phenomena during a buried-arc welding with and without low-frequency modulated voltage control, in this paper. Applying this result, we additionally report a high-efficiency thick-plate welding with a low wire feed rate condition.

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This paper reports on the effect of carbon, silicon, manganese, chromium and aluminum on solidification cracking susceptibility of high manganese steel. The solidification cracking susceptibility of high manganese steel has been evaluated by using trans-Varestraint test and BTR that is one of the evaluation index of the solidification cracking susceptibility was obtained and compared. According to quenched microstructure observation by EPMA and EBSD analysis, solidification mode at all material might be an austenite phase (single-phase), in addition MnS was observed in the weld metal of all tested samples with different chemical composition and M3P particle was observed only for 13%Cr material. A numerical analysis approach for solidification cracking susceptibility of high manganese steel depending on some solute elements was carried out to quantitatively evaluate the temperature range and confirm the validation of BTR and influence of the solidification phenomenon. Based on both temperature ranges obtained experimentally and analytically, the solidification cracking susceptibility increased with increasing carbon, silicon and manganese, and these doesn’t change by chromium content in the high manganese steel. And then, aluminum decreased the solidification cracking susceptibility, but it might be caused ductility-dip cracking in the steel.

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The purpose of this study was to develop a high-efficiency and low-heat-input CO2 arc-welding process using hot-wire feeding. A previous paper showed that the proposed hot-wire CO2 arc-welding process has the potential to simultaneously achieve both high efficiency and low heat input. This paper investigated the production of a sound joint with only two welding passes on a butt joint of 20mm-thick steel plates with no defects or unstable welding phenomena using the developed hot-wire CO2 arc-welding process. Welding condition optimization was investigated using high-speed imaging and cross-sectional observations. The optimized conditions, which were the combinations of the welding current and hot-wire feeding speed as 350A and 7.5m/min, 400A and 5m/min, 450A and 5m/min, 500A and 5m/min, derived to avoid the presence of defects and molten metal precedence, which achieved sound welded joints possessing adequate properties of strength and toughness.

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