The conventional incremental sheet forming method of the two-point-support type, in which a bar tool with a spherical head is used, usually requires a special die to support the blank sheet. On the other hand, a new concept for this forming process, which requires no die or support tool, has been proposed. The new process can enable the production of many kinds of products using only one elastomer plate, which is a substitute for the special dies for each individual product. At first, a blank sheet placed on the base is pressed with the spherical head of the bar tool to the surface of the elastomer base. Then the tool is driven along a preset path on the depth-level plane. The sheet is bulged continuously along the tool path, and rises up into a 3D shape as a result of the accumulation of deformation caused by each contour path. In the present paper, forming experiments were carried out by the new process using mild steel sheet. Finally, some forming conditions that result in the shapes of good condition were successfully determined.
In the ironing of stainless steel cups using a cermet die having lubricant pockets, the conditions of optimum lubrication for attaining low friction were investigated. On the surface of the die, fine lubricant pockets are made by polishing of the shot-peened surface. The effects of lubricants and ironing speed on the ironing load and limit were obtained. The thickness of remaining lubricant on the sidewall of the ironed cup was measured by mixing a fluorescence oil in the lubricant. When the amount of lubricant was sufficient and the ironing speed was high, the ironing load was lower than that of a die having a polished surface and greater thickness of remaining lubricant on the sidewall of the ironed cup. In repeated ironing, the surface roughness of the ironed cup for the die having the lubricant pockets was almost constant, whereas the roughness for the die having a polished surface increased.
The numerical analyses and experiments of press-forming were carried out in order to clarify the mutual effects of the punch shoulder radius Rp, flange height H and mechanical properties on flange-up formability for the flange in the longitudinal edge. The results showed that the formability is strongly mutually affected by not only Rp and H but also the r value and plastic tangent modulus normalized by yield stress H’/YP. When Rp is small and H is high, the r value and H’/YP also significantly affect the formability. As a result, the formability of high-tensile-strength steels, whose r value and H’/YP are relatively small, is inferior to that of mild steel. Moreover, it was clarified that when Rp is small and H is high, the bending radius of sheets around the base of the flange becomes smaller than Rp during forming, which causes strain concentration in the flange edge. Therefore, it is suggested that the bending radius of sheets during forming should be controlled to be larger than Rp to obtain good formability.
Numerical analyses of press-forming were carried out to analyze the mutual effects of forming methods and the shape of formed parts, such as width W and wall angle θw on flange-up formability for a joint flange within the edge of the formed part, when the bending curvature of the blank sheet around the base of the flange, 1/Rm, was focused on. It was clarified that a decrease in 1/Rm in the early forming stage affects the suppression of the thickness decrease in the flange edge, leading to wrinkling around the flange base, unless the excessive material volume is reduced before the press stroke reaches the bottom. Then, the effects of the newly developed pad, the shape of which corresponds to the punch shoulder part, on the formability, which can control the bending behavior, were investigated by both numerical analyses and experiments of press-forming. The results showed that the newly developed pad improves the formability and enables flange-up forming even for ultra high strength steel with sheet shape optimization.
Tab leads used as the terminals of laminated-type lithium-ion batteries are fabricated by cantilever-type shearing of aluminum sheet coils. There are several drawbacks in the shearing of tab leads owing to the effect of tool wear, such as an increase in burr height at the cut surface with increasing number of shearing passes. To solve this problem, we designed and fabricated a zero-clearance cantilever-type shearing die equipped with high-wear-resistance polycrystalline diamond (PCD) tools (a punch and a die) (Die 1). The die has a mechanism that can prevent the increase in the clearance of the punch and die during shearing. We also designed and fabricated a zero-clearance blanking die used for the precision blanking of foils (Die 2). The demand for foils as materials used for electronic devices has been expanding recently. Die 2 was fabricated by shaving the side of a powder-alloy die using a PCD punch. The results of a shearing experiment using Die 1 indicate that the height of burrs generated during the cantilever-type shearing of aluminum sheets can be suppressed to 5 µm or less. Moreover, the results of a blanking experiment of 20 µm-thick stainless-steel foils using Die 2 reveal that a high-precision cut surface can be obtained over the entire blanked contour.
Warm forming at temperatures from 200 to 600℃ has several advantages compared with cold forming at room temperature: better shape accuracy, better stretch flange formability and lower press load. In this study, the stretch formability in warm forming of uncoated high-strength-steel sheets and galvannealed high-strength-steel sheets were investigated by spherical stretch forming tests from room temperature to 600℃. The experimental results showed that the maximum formable dome height without cracks in warm forming is lower than that in cold forming despite the lower material strength at elevated temperatures. In order to clarify the factors leading to poor stretch formability when using warm forming, surface temperatures and strains were examined in the spherical stretch forming tests. Also, sliding tests of strips of the uncoated steel sheets and galvannealed steel sheets were carried out under the same heating conditions as the spherical stretch forming tests. The results suggested that two factors could cause the degradation of stretch forming performance during warm forming: differences in material strength between high- and low-temperature areas, especially at high initial forming temperatures of more than 400°C, and high coefficients of friction, particularly at a low initial forming temperature of 200℃. It would be necessary to take both factors into account during warm forming.
The hole expansion test using a conical punch was carried out to clarify the effects of geometries such as vertical angle, tip shape of the punch and sheet thickness on the stretch flangeability. The results showed that the hole expansion ratio depended on the fracture limit strain and the localized deformation behavior on the hole edge accompanied by the geometries. The fracture limit strain was influenced by the work hardening around the hole edge with contact between the punch and the hole edge. In particular, with little work hardening, the fracture limit strain depended only on the sheet thickness, independent of the vertical angle. Moreover, the localized deformation behavior was influenced by the gradient of the circumferential stress along the radial direction around the hole edge. This gradient changed with the vertical angle. When the vertical angle was sharp, the gradient of the circumferential stress showed a steep slope up to the late stage of the hole expansion process, and the deformation was slowly localized. Consequently, the hole edge was deformed more uniformly and the hole expansion ratio became large. When the hole edge deformed uniformly, the hole expansion ratio significantly depended on the fracture limit strain.
High-strength steel sheets are increasingly applied to automotive bodies for weight reduction in order to meet the needs for reduced environmental impact. However, dimensional defects resulting from springback are serious issues, and effective methods of predicting and reducing such defects are required. In this study, we propose and validate the advantage of the stroke-returning deep-drawing process, in which stroke is returned to the specified depth after drawing to the bottom dead point. The following results were obtained. Firstly, by the stroke-returning deep-drawing process, the twist angle became approximately zero. Secondly, the occurrence of bottom curvature made the sidewall curvature increase. Thirdly, when bottom curvature did not occur, sidewall curvature decreased in accordance with the equation of wide direction stress. Finally, the twist angle and sidewall curvature decreased concurrently upon deeply drawing and the greatly returning stroke process.