For a better understanding of the effect of the strength difference between soft and hard phases on the Bauschinger effect of dual-phase (DP) steels, we experimentally investigated two types of DP steel, i.e., one with a large strength difference and the other with a small difference. Reversed simple-shear tests revealed that the steel with a larger strength difference showed earlier re-yielding under reverse loading; thus, the strength difference between phases enhances the Bauschinger effect. Furthermore, the actual morphology of the steel with a larger strength difference was reproduced in a finite element model of which hard and soft phases were represented by high and low flow stresses, respectively. By changing the flow stress for the hard phase, the effect of the strength difference was analyzed by finite element analyses. In the simulations, the observed difference of the Bauschinger effect was qualitatively reproduced. It was also shown that strains given to the soft phase in the vicinity of the hard phase are relatively low and those areas first re-yield immediately after reverse loading. This was pronounced in the case with a larger strength difference. The earlier re-yielding for the larger strength difference is possibly ascribe to not only the higher back stresses but also the lower strains.

The formability of die-based press forming (stretch forming) and that of incremental forming (IF) were compared using the ductile fracture equation and stress triaxiality to investigate whether IF’s superior formability could be accurately evaluated. Experiments were conducted to determine the fracture conditions of the test material in each forming method, and finite element method analysis was performed on the basis using experimental results to obtain the stress triaxiality. In press forming, the stress triaxiality remained nearly constant during forming, indicating that the evaluation using the ductile fracture equation was feasible. In IF, the stress triaxiality around the tool was found to be the same as that in press forming, making it difficult to evaluate its high formability using the ductile fracture equation. Focusing on the formed areas of the test material, the stress triaxiality was not constant and changed over time during the forming process. Damage analysis using GISSMO, which accounts for temporal variations in stress triaxiality, was performed. This analysis revealed that both IF and press forming could be effectively evaluated, and that IF’s superior formability could also be appropriately represented.

The possibility of controlling the bending angle in press V-bending using shot peening (SP) in the die was confirmed using aluminum alloy A5052 with 1 mm thickness. The bending radius and angle were 10 mm and 90°, respectively. After press bending, the bending angle is greater than 90°. By controlling air pressure and shot mass, the bending angle was reduced using SP. The reduction in bending angle using SP was greater when SP was performed during press loading than after unloading. Under the conditions of this experiment, the reduction in bending angle using SP was small for thicknesses of 2 mm or greater. Future experiments will be planned to use a direct-pressure system with a high shot velocity.

There are two main types of body-frame structures for automobile bodies: monocoque structures, which are made by welding several panel parts together, and space-frame structures, which are made by joining closed-section tubes together. Space-frame structures have high strength and stiffness. Therefore, that can reduce vehicle weight. Furthermore, if the dimensions of the rectangular cross section of the curved tube can be varied in the longitudinal direction, further weight reduction is expected. The bent tube with variable cross section can be formed by the existing method, hydroforming. However, hydroforming requires large dies that fit the shape of the product, and the forming machine is large and expensive, because of the large clamping force required. In this paper, we adopt the roll forming method, which is smaller than the conventional method, has smaller forming load, and can form square tubes continuously. Then, we propose the novel die-less tube bending method in which the straight circular tube is continuously bent and formed into various cross section by means of the rectangular opening formed by the four forming rolls installed in two directions. Using a prototype device, we verified that the bent tube with variable cross section in the longitudinal direction can be formed.

To improve the accuracy of sheet forming analysis, it is necessary to model an anisotropic yield surface on the basis of the measurement of biaxial stress status. To measure the plastic anisotropy, uniaxial tensile tests with an angle inclined to the rolling direction are carried out. Biaxial tensile tests with cruciform specimens are also preferred to be carried out with an angle between the axes of principal stress and the anisotropy. However, the accuracy of measured stress and the direction of plastic strain increment in the inclined biaxial tensile tests has not been verified. In this study, cruciform biaxial tensile tests with an inclined angle were analyzed using the finite element method. Using the finite element analysis as a virtual experiment, the stress points on equal plastic work contours and the directions of plastic strain increment were obtained from the tensile load and local strain. The results of virtual experiments were compared with the yield surface and normal direction derived from the yield function used in the finite element analysis. It was verified that the yield surface can be measured properly in biaxial tensile tests using cruciform specimens with an inclined angle.

Tensile specimens with equivalent plastic strains of 5 %, 10 %, 15 %, and 20 % were prepared for four specimen types: SS400, A1070-O, SUS304, and AZ31. The relationship between equivalent plastic strain and KAM (kernel average misorientation) was visualized and formulated from the results of EBSD (electron backscattered diffraction pattern) and OIM (orientation imaging microscopy) analyses. The results obtained in this study enabled quantitative understanding of the local strain introduced at each location during fabrication and deformation.

This study investigates the work-hardening behavior of a 5 %-Mn high-strength steel sheet in a warm temperature range up to 200 °C. This study was motivated by the known strong temperature dependence of the transformation-induced plasticity (TRIP) effect in this temperature range and need for accurate work-hardening curves for finite element (FE) simulations in press-forming applications for automobiles. Conventional tensile tests with elongation measurements cannot accurately evaluate the work-hardening curves of this steel because of the nonuniform deformation caused by large Lüders elongation, which exhibits approximately 0.1 yield elongation. To overcome this challenge, we employed real-time diameter change measurements converted to true strain to assess local work-hardening at warm temperatures. Key findings include: (1) strong temperature dependence of work-hardening curves up to 100 °C due to TRIP effect suppression with optimal uniform elongation at 75 °C; (2) temperature-independent stress plateau corresponding to Lüders deformation up to 100 °C; and (3) constant fracture strain across temperatures despite significant improvements in uniform elongation. These results provide crucial insights for optimizing press-forming processes and enhancing FE simulations for medium-Mn high-strength steel sheets.

The effects of die temperature, die gap, plunger speed and molten metal temperature on the flow length of A1070 pure aluminum in a thin die gap were investigated. The die gaps were 0.5, 0.8 and 1.0 mm. The plunger speeds were 0.2, 0.4, 0.6 and 0.8 m/s. The die temperatures were 30, 70, 110 and 150 °C.The molten metal temperatures were 680, 730 and 780 °C. When the die gap was 0.5 mm, the flow length was largest when the die temperature was 30 °C and the plunger speed was 0.2 m/s, which is contrary to conventional expectations. However, when the die gap was 1.0 mm, the flow length increased as the die temperature and plunger speed increased, which aligns with previously reported results. It became clear that when the die gap was 0.5 mm, the effects of die temperature and plunger speed on flow length were opposite to those for a die gap wider than 0.5 mm. The cause of this phenomenon is discussed in terms of the adhesion and peeling of the solidification layer used.

Semisolid high-speed roll casting was developed to improve the ductility of recycled aluminum alloys and aluminum alloys for casting. High-speed roll casting (high productivity) and rapid solidification could be realized using copper rolls, without using a parting material on the roll and at a low pouring temperature of molten metal. As a result, a semisolid strip could be continuously cast at a high speed. The casting speeds (rotation speeds of rolls) in this study were 20, 30 and 60 m/min. Detoxifying the Fe impurity included in recycled aluminum alloys from the outer and inner panels of automobiles was attempted by refining the impurity by rapid solidification. The detoxification of the Fe impurity was shown from results of tensile testing, bending and deep drawing. The possibility of sheet forming of aluminum alloys for casting was shown using deep drawing and three-roll bending. Eutectic Si of aluminum for casting was also refined by rapid solidification. Deep drawing with a limiting drawing ratio greater than 1.7 and three-roll bending were possible using by aluminum alloys for casting. It is shown in this study that the ductility of strips of recycled aluminum alloys and aluminum alloys for casting could be improved by semisolid high-speed roll casting.

A systematic study was performed on two types of welding circuit with different inductances and one welding coil at five discharge energies for each circuit. The inductance of each circuit was determined by adding its respective remaining inductance and one effective inductance. This study demonstrated a condition necessary for fabricating an A6061-T6/DP590 welded sheet, resulting in a high deformation velocity of an A6061-T6 moving sheet and a large impulse generated on the sheet after the first collision. The three elements of a discharge current are the maximum value, oscillating period and damping coefficient. Compared with the maximum value, the time to the maximum value and the period in the measured discharge currents, the corresponding error rates of discharge currents reproduced by calculation are very low. Therefore, both the magnetic pressure and the impulse calculated from the reproduced discharge currents are highly reliable. The condition affecting the joining strength of the welded sheet was verified from the collision velocity and the impulse. It was clarified that the high collision velocity above 300 m∙s -1 and the large impulse from the first collision to approximately 14 μs resulted in the fabrication of the welded sheet with high welding strength.