塑性と加工 Vol. 58(2017) No. 679

Ultrashort-pulsed laser peen forming is a method of thin-sheet-metal forming using laser-induced shock waves. The authors have applied the process to microsheet parts bending, which is accomplished by repeating line scanning. However, when a severe curvature is requested, nonnegligible reduction of sheet thickness is induced by laser ablation. In order to restrict the thickness reduction, the authors attempted to improve the bending efficiency, which would allow a reduction in the number of necessary pulses. Scanning velocity and scanning pitch were changed, while the total irradiated pulse number was constant. The obtained results showed that a scanning velocity higher than the conventional one is favorable for improving the bending efficiency. In addition, smaller scanning pitches were adopted to reduce the radius of curvature. The influence of the scanning pitch on the bending efficiency was much weaker than that of scanning velocity. From the results of the evaluation of efficiency and scanned surface asperity, a scanning velocity of 20 mm·s^-1 was judged to be the best. The best scanning condition achieved a 40% smaller radius of curvature than the conventional one.

We investigated a method of determining the geometry of the die and punch, which are not allowed to fracture in the forming process of backward extrusion of cup. On the basis of the localized bifurcation theory, it is suggested that final fracture, which is recognized as the separation of a continuum material, may occur in the shear band zone with cleavage. A newly proposed instability condition to the cleaving action in the shear band zone is applied to predict the fracture limit on the forming process. Finite element analysis of the forming process is performed to know the stress state, which is required in the theory. The experimental observation using an optical microscope is also conducted to recognize fracture occurrence during the forming process. Results show that the combination of the punch shoulder radius and the die shoulder radius ensures forming without fracture.

In this paper, we describe the joining property of welded sheets obtained by magnetic pulse welding. In this method, it is possible to weld aluminum alloy sheets to a low-carbon sheet, for example, A6061-T6 or A2017-T3. An electromagnetic force deforms the aluminum sheet, and the height of the sheet is governed by both the force and the properties of the sheet. Collision times are examined in various gap lengths for the four kinds of aluminum sheets in 2.0 kJ discharge energy experiments, and collision velocities are calculated using the collision times. The relationship between shearing load of the welded sheet and the gap length is also examined. The collision times of the four sheets are approximately equal at gap lengths less than or equal to 2.48 mm and increase in accordance with the materials property values of quasi-static process at gap lengths more than 2.48 mm. Magnetic pulse welding is phenomenon in a high strain rate and the strain rate of the sheet exceeds 10000 s^-1 as a rough estimate. It is difficult to weld the sheets with a large flow stress to the low-carbon sheet, and the welding depends on the flow stress. Therefore, the flow stress of aluminum sheet also is a factor influencing the joining property of welded sheets.

In-plane tension/compression tests of an annealed steel sheet were carried out to investigate the work-hardening behavior under two-stage loading paths. The two-stage loading paths consist of plane-strain tension/compression for the 90° direction from the rolling direction followed by unloading and subsequent uniaxial tension/compression in the 0° and 90° directions from the first loading direction. It was found that the work-hardening behaviors were significantly affected by the inner product of the strain rate mode tensors for the first and second loadings and that the effect of the deformation mode （tension/compression） was small. A new work-hardening equation that can reproduce the work-hardening behavior in the second loading was proposed. Experiments and finite element analyses of pipes with bending deformation history under internal-pressure and axial compression were performed. As work-hardening behaviors for those analyses, compression in the axial direction of the pipe or tension in the circumferential direction of the pipe was applied in accordance with the proposed equation. The calculated result for tension applied in the circumferential direction was in the closest agreement with the experimental result.