Thin-walled copper tubes of various shapes are required to reduce the environmental impact of products by improving the performance of their heat exchangers. Tubes of various outer diameters along the longitudinal direction are conventionally manufactured by rotary swaging. However, the shape of the die is just transferred and thin-walled copper tubes are prone to torsion due to friction. To solve these problems, we applied the flexible ball-spin forming (FBS) method, in which ball-spin tools are pressed against a rotating circular tube at an arbitrary axial position. The FBS method was suitable for the flexible diameter reduction of thick tubes, whereas it caused polygonal or torsional defects in the case of thin tubes. Therefore, in this paper, we propose a method of flexible spinning with a torus-shaped tool (FST) for thin tubes. FST can hold tubes in the circumferential direction and maintain a near-circular shape. As a result, the tube diameter was effectively reduced and a reduction ratio of up to 57.1 % was achieved.

In this paper, we describe a drawing method for improving the dimensional accuracy of bars. The proposed method improves the straightness of the bars drawn with the initial curvature and drawing angle. The straightness of the bars is improved using eccentric path line. The eccentric path line is achieved by offsetting the guide installed on the entry side of the die in a direction perpendicular to the drawing direction. The effects of die geometry on the relationship between the amount of guide offset and the curvature of the drawn bar were investigated through experiments and finite element analysis. The results showed that the smaller the contact length between the bar and the die at the approach portion, the more effective the path line eccentricity, and a contact length of about 0.25 relative to a die hole is desirable. From these results, high straightness was obtained when bars with the initial curvature and drawing angle were drawn with path line eccentricity using a contact length of 0.24 relative to a die hole. On the basis of these results, it is possible to produce very straight bars with a small amount of path line eccentricity by selecting the appropriate die geometry.

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.

Sequential press forming is a method used to form a workpiece while changing its location and to manufacture large structures, such as storage tanks. To optimize the sequential press forming conditions, accurate and efficient predictions of product shapes using numerical simulations are required. However, such predictions are currently difficult because the computational cost is large when sequential press forming processes are simulated. In this study, a new prediction model for product shape in sequential press forming was proposed. The detailed ideas are as follows. First, a machine learning model that predicts a product shape formed by a press forming process was developed. The model represents the product shape using a radial basis function network. Then, the predicted results were used as the input data for the next press forming process, and this procedure was repeated to predict the entire sequential press forming process. The predicted results were compared with experimental results, and it was confirmed that the predicted results had good accuracy. Because this procedure allows the prediction of sequential processes by using the machine learning model for a press forming process, these results showed that the proposed model enables efficient predictions of product shape in sequential press forming.

Cold forged parts are typically subjected to heat treatment to improve their mechanical properties. However, it is desirable to eliminate the heat treatment process to reduce their manufacturing cost. In this study, a strength prediction method using a Chaboche combined hardening model was investigated for predicting the strength of cold forged parts that are not subjected to heat treatment after cold forging, especially those which are subjected to tensile loads during operation in the opposite direction of compressive loads during cold forging. Simple shear tests were performed to obtain stress-strain curves with reversed loading over a wide range of strains. Carbon steels were used as test materials. Simple shear tests assisted in identifying the material parameters of the Chaboche combined hardening model so as to reproduce the stress-strain behavior over a range of strains where cold forged parts are subjected to reversed loads during operation: this facilitated the accurate prediction of the strength of the cold forged parts.

Reliable fracture simulations are expected to contribute to the longer life cycle of structures. The phase-field fracture (PFF) model is anticipated to be a robust approach to reproducing crack propagation and branching. Measurement techniques for detecting cracks and fractures have also advanced and are widely used for health monitoring. In the field of structural analysis, a data assimilation method using the full-field measurement data obtained by digital image correlation has been proposed. The data assimilation method is attracting attention as a method of improving the simulation accuracy by utilizing the time-series measurement data obtained from such measurement techniques. In this study, we apply for the first time the nonsequential data assimilation method minimizing the cost function using treestructured Parasen estimator (DMC-TPE method) to assimilate full-field strain measurement data into PFF simulations. In this paper we validate the proposed data assimilation method through numerical experiments, where data assimilation is performed using the pseudo-measurement data obtained from PFF simulations conducted with parameters assumed as true values. The results demonstrate that the proposed method enables the simultaneous inverse estimation of multiple parameters included in the PFF model. The validation results revealed that, although the DMC-TPE depends on the initially estimated values, the parameters that significantly impact the simulation results were accurately estimated.

The effects of shot velocity, shot diameter, and square plate size (width and thickness) on the shape of aluminum alloy A5052-H34 plates after peen forming were investigated by numerical simulation. Numerical simulation was conducted in three steps: in Step 1, shot impact was calculated for a small area using the dynamic explicit finite element method (FEM); in Step 2, the nodal coordinates, stress, and strain distributions calculated in Step 1 were used to calculate the plastic strain distribution in free deformation using static implicit FEM; and in Step 3, the plastic strain distribution obtained in Step 2 was used to calculate plate deformation using the static implicit FEM. The FEM results of the peenformed shapes agreed well with the experimental results, confirming the validity of the numerical simulation method. As shot velocity was increased, the plate shape shifted from a spherical to a cylindrical surface. When the plate width (=length) was large or the thickness was small, the shifting shot velocity decreased side. The strain in the center of the thickness at the plate edge ad a limit for each plate size, because peen forming cannot apply compressive strain to the plate. Above the limited compressive strain, a shift from a spherical to a cylindrical surface occurred.

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.

In this study, with the aim of developing titanium fiber plates (TFPs) that do not generate contamination, the effect of shear distance on the mechanical properties of a TFP fabricated by cold compression shearing using a single titanium fiber was clarified. The occurrence of contamination assuming in vivo use was also evaluated. The results showed that as the shear distance increased, the flexural modulus, flexural strength, and 0.2 % proof stress of a long-fiber TFP increased, and the porosity decreased. Furthermore, both the long- and short-fiber TFPs have bending modulus close to that of a compact bone (10 to 30 GPa), and the porosity changes from about 5 to 25 % depending on the shear distance. Moreover, fatigue tests using molded bodies cut with metal scissors revealed that fibers did not fall off in the long-fiber TFP. On the basis of these results, in this study, we developed a TFP with a bending elastic modulus comparable to that of a compact bone, which does not cause fibers to fall off after being cut.

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 forward rod-backward can combined extrusion test was performed to investigate the tribological properties of duplex stainless steel during cold forging in this study. Two types of duplex stainless steel and an austenitic stainless steel were employed. The specimens were coated with a MoS₂ single-bath-type environmentally friendly lubricant and oxalate and stearic acid conversion coatings. After forming, the geometries of the specimens were measured and plotted to a calibration diagram created by the finite element method to identify the Coulomb friction coefficient at the interface between a specimen and a tapered die surface. The results showed that the environmentally friendly lubricant had a lower friction coefficient for all materials. For the chemical conversion coatings, differences in friction coefficients among the materials, namely the friction coefficient of austenitic stainless steel was higher than the others were observed. This phenomenon is presumed to be due to the differing insoluble components generated during the coating process.