Compressive torsion processing (CTP) is a unique severe deformation process in which a work piece is simultaneously subjected to compressive and torsional loadings without a change in its cylindrical shape. Two types of torsional loading were investigated: cyclic reverse loading with various torsional angles and monotonic rotational loading with various rotations. The effects of the different torsional loading methods and torsional angle or rotational repetition on the refined microstructure distributions of pure aluminum were investigated. As a result, refined microstructures with a grain size less than 10 μm were easily formed near both basal planes of the cylindrical specimen; however, the refinement was not easy at middle regions between the basal planes. In the cyclic reverse loading, increasing torsional angle increased the size of the refined microstructure region, but it was impossible to form refined microstructures near the center axial and middle regions. On the other hand, in the monotonic rotational loading, a homogeneous refined microstructure was obtained, even with a small rotation repetition. This indicates that monotonic rotation loading is much more effective for obtaining a homogenous refined microstructure.

View full abstract

The features of sheared edges in warm and hot punching of an ultra high strength steel sheet using resistance heating were examined. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased. The rollover depth and burr height of the sheared edge markedly increased from about 800°C. The roughness level of the fracture surface became high above 650°C, whereas that of the burnished surface was almost constant. The punching load was considerably reduced by the heating, i.e., 40% of the cold punching load at 650°C and 15% at 1070°C. The effects of the punching speed and the clearance between the punch and the die on the surface quality of the sheared edge became marked at high temperatures. It was found that the warm and hot shearing of ultra high strength steel sheets is effective for improving the surface quality of the sheared edge and in reducing the shearing load.

View full abstract

There are a number of instances in which wrought Mg alloys show marked work hardening followed by work softening during compressive deformation in a warm temperature range. Work softening is unfavorable for the warm forging of wrought billets, because it induces plastic instability. In this study, the effect of initial texture on work hardening and work softening has been examined using a hot-extruded AZ61 alloy bar. The hot-extruded bar had a strong fiber texture of {0001} parallel to the extrusion axis. By upsetting it with a low strain rate on the 10^-4 s^-1 order at 400°C, the fiber texture was almost completely randomized. In the warm compression of the randomly textured AZ61 alloy, the flow stress became steady without work softening after showing a low degree of work hardening. It has been confirmed that high degrees of work hardening and work softening in the warm compression of hot-extruded specimens are caused by stress concentration on grain boundaries and stress relaxation by dynamic recrystallization, respectively. In steady-state deformation, the microstructures and textures of both specimens showed almost no differences, resulting in nearly the same flow stresses.

View full abstract

The design assignment of connecting portions on a paperboard is important in order to stabilize the transport of the paperboard during die cutting. A mechanical estimation of various deformations of connecting portions is desired to predict an allowable assignment of connecting portions. The main purpose of this study is to reveal the elongation properties of the connecting portion of a paperboard with respect to several primary parameters such as nick width and crushed blade tip thickness. The result shows that there is a lower bound width of the connecting portion; the tensile strength of the connecting portion strongly depends on the cutting direction and grain (machine) direction of the paperboard. It is also clarified that the presence of string like dust affects the tensile strength of the connecting portion and that increasing blade tip thickness reduces the tensile strength of the connecting portion.

View full abstract

In the surface finishing of high-hardness materials such as die materials, it is common for a polishing process to be used. If burnishing is feasible for the surface finishing of a die using a CAD/CAM system and a machining center, it will become a useful working method for the following reasons: (1) reduction of the time required to make work arrangements, (2) improvement of the precision of shape and dimensions, and (3) improvement of the mechanical properties of the surfaces. In this paper, the relationships between working conditions and surface roughness are examined when the finish burnishing of a die material is carried out. The burnishing experiments were performed on prehardened steel with a SiC ceramic ball having a 9.525 mm diameter. Under the conditions of a burnishing force of 400 N and a pitch of 0.195 mm, finish burnishing was performed on a milled surface having a roughness of Ra1.4 μm, and a mirror surface having a roughness of Ra0.1 μm was obtained. In addition, in the case of the product having a free curved surface, a good mirror surface was obtained.

View full abstract

In-plane tension and compression experiments on a copper alloy sheet (phosphor bronze) 0.25 mm in thickness for electronic parts were carried out using a specially designed testing apparatus. The testing apparatus was equipped with comb-shaped dies so that we could observe the stress-strain curves of sheet metal subjected to tension followed by compression, and vice versa, without causing the buckling of the specimen, as well as those for the monotonic tension and compression of the as-received material. A difference in the flow stresses of tension and compression was observed for the as-received alloy sheet. The Bauschinger effect was measured and the measured stress-strain curves were precisely approximated using Voce′s function. As a second part of the experiment, bending moment-curvature diagrams were observed for the as-received and prestrained specimens and compared with those calculated using the tensile and compressive stress-strain curves observed for them. The observed moment-curvature diagrams were in good agreement with the calculated ones when the strength differential effect was accurately modeled for the as-received specimens and when the Bauschinger effect was accurately modeled for the prestrained ones. Thus, we conclude that the accurate modeling of the strength differential and Bauschinger effects of the copper alloy is crucial for predicting the alloy′s bending behavior accurately.

View full abstract