Recently, there have been increasing demands for high-throughput, high-resolution and cost-effective fabrication techniques for glass owing to its excellent optical properties (e.g., high refractive index and low UV absorption level) and chemical/thermal stability, which are essential for high-performance optical, biological, and chemical micro- electro-mechanical system (MEMS) applications. Glass hot-embossing techniques are expected to meet the requirements mentioned above and thus are being actively studied. The motivation of this work is the notion that replicated quartz glass may be applied not only to the fabrication of MEMS devices but also to mold replication processes as molds. Although a quartz mold is generally used for the UV-imprint process, it can also be applied to glass embossing with the aid of adequate mold release treatment. In this study, quartz molds were prepared by hot embossing with carbon molds, and applied to the hot embossing of borosilicate. First, two types of glassy carbon (GC) master were prepared by dicing and Q-switched Nd:YVO4 laser machining to produce quartz molds by hot embossing. Then, quartz molds were replicated from the GC masters by hot embossing, and their surfaces were sputtered with molybdenum. Finally, micro-hot-embossed test structures were developed in borosilicate with high fidelity. The examined method showed great potential for fabricating quartz molds efficiently and at a very low cost.
The deep drawability of tailored blanks (TBs) composed of thick and thin sheets was investigated by performing square-cup drawing experiments. For a square TB, the weld bead line at the bottom of the cup moves toward the thick-sheet side during drawing, whereas the weld bead lines at the flange are fixed by a step placed on the blank holder. The drawability of TB is considerably lower than those of their component sheets, since the plane-strain stretching mode is more likely to occur in such TB under constraint. To improve the drawability, two different types of TB, namely, TB with corner cuts and nonsymmetric rectangular TB, were examined. It was found that the drawabilities in both cases were higher than that of the square TB. In particular, the increase in limiting cup height was remarkable in the latter case.
In sheet metal working, the accuracies of bending angle and flange length have a significant effect on product quality. Hence, it is necessary to accurately predict the springback and bend deduction of a sheet metal. The effect of punch track radius on the mechanism of L-bending was clarified by a 2D finite element method in this study. Because both springback and spring forward exist in L-bending, it is difficult to predict bending angle. Hence, firstly the validity of numerical analysis was confirmed by comparison with experimental results. Secondly, the effects of punch track radius on bending force, bending radius, bend deduction, springback and spring forward were elucidated. Finally, the new L-bending mechanism was clarified.
Recently, a warm forging process has been found to be effective from the points of view of near net shaping and the abolishment of additional heat treatment by microstructure control. The die life in hot forging is only about 1/10∼1/100 of that in cold forging, because of the serious heat damage involved. The die life in warm forging is normally about the same as that in hot forging. There are many important factors in real forging such as cooling, lubrication, tool material properties and surface treatment; thus, it is difficult to determine the optimal combination for forging. To consider the above-mentioned phenomena, we have developed a new punch damage test using parts former machine for warm forging. In this study, we investigated the effects of tool material, forging temperature and lubricant condition on die damage. It was shown that a tool material with high yield strength at a high temperature reduces the magnitudes of wear and plastic flow, and that the reduction in forging temperature or lubricant flow increases them. This new test is useful for clarifying the mechanism of hot- and warm-forging die damage.
Hole expansion using a flat-bottomed cylindrical punch was carried out to clarify the fracture behavior of high-strength steel sheets by changing the initial hole diameter, d0. Experimental results showed that the hole expansion ratio, λ, became larger as the initial hole diameter, d0, increased. Moreover, the fracture site around the hole shifted considerably with the initial hole diameter, d0, in the case of high-strength steel sheets with high r-value. On the other hand, the analytical results exhibited poor coincidence with experimental ones when the initial hole diameter was small, whereas good correlation was observed with a large initial hole diameter. As a result, it was clarified that plastic deformation behavior during stretch flange forming performed using a flat-bottomed cylindrical punch is influenced not only by the planar anisotropy of the mechanical properties of steel sheets but also by the initial hole diameter.
A forming process of tailor blanks having local thickening for the deep drawing of square cups was developed to improve the drawability. A sheet having uniform thickness is bent into a hat shape of two inclined portions, and then is compressed with a flat die under restraint of both edges to thicken the two inclined portions. The bending and compression are repeated after a right-angled rotation of the sheet for thickening in the perpendicular direction. The thickness of the rectangular ring portion equivalent to the bottom corner of the square cup is increased, particularly the thickening at the four edges of the rectangular ring undergoing large decrease in wall thickness in the deep drawing of square cups becomes double. The degree of thickening can be adjusted by controlling the punch stroke in the bending. By using the tailor blanks having local thickening, not only the decrease in wall thickness at the bottom corner of the square cup was prevented, but also the limiting drawing height of the cup without fracture was increased to 28.3 mm, whereas that for the uniform blank was 21.3 mm.
Local die quenching in the hot stamping of quenchable steel sheets was developed to form ultra high strength steel products having a strength distribution. Local portions of the heated sheet were quenched by holding grooved tools at the bottom dead center during the stamping. Non-contact portions without the quenching were generated in the sheet by grooving the tools, i.e. the strength in the contact portions is high owing to the quenching and that in the non-contact portions is low owing to the lack of the quenching. The hardenability for the local die quenching was first examined by sandwiching limited portions of the heated sheet between a large steel die and sheet holder without deformation. Next, hot hat-shaped bending with grooved tools was performed to form products having high strength around corners in contact with the tools. In addition, quenching in the flange portions of the bent sheet was prevented by steatite plates having low conductivity placed in the flange portions of the tools. Hat-shaped products having a tensile strength of approximately 1.5 GPa only at the corners were formed, and the punching loads in the bottom and flange portions were considerably lower than those for the non-grooved tools.