A new physical simulation method for evaluating the effect of shear deformation on the evolution of a microstructure is proposed using niobium-bearing steel. The effects of the processing conditions, such as pre-heat treatment, strain rate, and cooling rate, on the formation of ultrafine grains were experimentally examined by the shearing method named the “interrupt shearing test”. This method utilizes a high-speed compression testing machine and is capable of simulating the formation of fine grained steel in the phase transformation procedure. Large shear deformation has been reproduced by the proposed method, and a grain size of approximately 1μm has been obtained by controlled mist cooling.
Product defects have been predicted by numerical simulations in cold forging. Various ductile fracture prediction equations have been proposed, some of which have already been implemented in commercial finite element codes. However, there are still some cases where the fractured region of actual products cannot be predicted using the existing ductile fracture prediction equations. In the present paper, a new equation for predicting ductile fracture was proposed, which can improve the prediction accuracy of ductile fracture was proposed. The proposed equation was expressed as a second-rank symmetric tensor, which is a product of the stress tensor and strain increment tensor. The effectiveness of the proposed equation was confirmed in a uniaxial tension test of a notched round bar. Next, the proposed equation was applied to the cold forging of a hollow shaft. As a result, the calculated most-damaged region was corresponded to the actual fractured region of the product.
During the multipass hot processing of alloy steel, restoration processes such as static and dynamic recrystallization and recovery occur. Owing to these phenomena, flow stress characteristics are affected by various deformation conditions, such as temperature and accumulated strain. The objective of this research is to develop a mathematical model for predicting flow stress to represent these effects by simply considering the processing history and restoration. Hot compression experiments were conducted and the results were compared with the predicted results. It was found that the prediction accuracy can be improved by simultaneously considering the effects of accumulated strain and restoration.
To reduce axial load in cold backward extrusion, torsional oscillation with a maximum frequency of 1.5 Hz around the extrusion axis was superposed with axial deformation. A cylindrical aluminum workpiece was simultaneously extruded in the axial direction and twisted in the circumferential direction between extrusion and knockout punches with a maximum axial speed of 0.1 mm/s, an alternating amplitude of 5°, and a maximum angular speed of 5 rpm at room temperature. The relationship between the torsional oscillation conditions and the axial load was investigated in backward extrusion with reductions in cross-sectional area of 0.17-0.69. The maximum reduction in axial load was obtained with approximately 20% in backward extrusion with a reduction in cross-sectional area of 0.51 under rotation/extrusion speeds higher than 60°/mm. The influence of the friction condition, the work-hardening characteristic of the workpiece, and the reduction in cross-sectional area on the reduction in axial load was discussed using the results of finite element simulation.
The surface quality and shape of an extruded part of a workpiece were investigated in cold backward cup extrusion with torsional oscillation. In this extrusion process, a cylindrical aluminum workpiece lubricated with a mineral oil was simultaneously extruded in the axial direction and twisted in the circumferential direction between extrusion and knockout punches with a maximum axial speed of 0.1 mm/s, an alternating amplitude of 5°, and a maximum angular speed of 0.5 rpm at room temperature. The deformation behavior of the workpiece at the inner surface of the container was investigated by experiment and finite element simulation. The sidewall of the workpiece was axially extruded and circumferentially rotated with approximately 25% lower contacting pressure with the container, so that an extruded workpiece with a uniform and smooth sidewall surface was obtained. Furthermore, the circumferential profile of the sidewall height of the extruded workpiece was close to uniform because the workpiece was uniformly lubricated by the circumferential rotation of the workpiece. The influence of the non-uniform friction at the container-workpiece contact on the sidewall height of the extruded workpiece was discussed using the results of finite element simulation.