Motoo Asakawa

Investigation on Twisting and Side Wall Opening Occurring in Curved Hat Channel Products Made of High Strength Steel Sheets

High strength steel sheets are becoming increasingly important for the weight reduction of automotive bodies to meet the requirements for reduced environmental impact. However, dimensional defects resulting from springback are serious issues, and effective methods of predicting and reducing such defects are necessary. In this study, we numerically and experimentally analyzed the mechanisms of dimensional inaccuracies caused by springback occurring in curved hat channel deep drawing products. The analysis was based on the static explicit FEM software “TP-STRUCT” (the solver part is known as “STAMP3D”). The results of the experiments and simulations similarly show that the twist angle is positive (right-hand system) when the drawing height is relatively large. We calculated the twist torque around the longitudinal axis using the stress distributions obtained by FE analysis. Through the investigation of twist torque and its transition during the drawing and die removal processes, we found that the negative torque generated by side wall opening occurring in the die removal process is the dominant factor of the positive twist. Knowing such mechanisms of twist in cases with a relatively large drawing height, we attempted to explore methods of reducing side wall opening by giving the side wall a stepped shape with the eventual aim of reducing twist. Consequently, we concluded that the stepped shape on the side wall has marked effects of reducing side wall opening, mainly through the elimination of bending-unbending effects on die shoulders, which was verified by observing the stress distribution obtained by FE analysis.

Reduction of Springback in Hat Channel with High-Strength Steel Sheetby Stroke Returning Deep Drawing

This study investigated the twisting phenomenon in curved hat channel products made of dual-phase 980-MPa-class high-tensile-strength steel sheets. The stroke returning deep drawing (SRDD) method was proposed to deal with twisting. In this new method, after the punch reaches the bottom dead point, it returns to a certain drawing height without the blank holder being removed. With the application of the SRDD method, twisting hardly occurred, but sidewall curl increased. A two-step SRDD was then proposed to reduce the sidewall curl of SRDD products. In the two-step SRDD method, a stroke returning process is carried out in two steps under different conditions. The results showed that the two-step SRDD method reduced the sidewall curl and twist simultaneously.

Fatigue crack propagation property of friction stir welded 2024-T3 aluminum alloy

Fatigue crack propagation tests on friction stir welded aluminum alloy sheets were carried out to investigate the effect of residual stress on the crack propagation rate. In these tests, the effect of the inclination angle of the weld line on the direction of crack propagation was also investigated. The test results show that crack propagation rate is accelerated by high tensile residual stress around the weld line. The angle of the weld line does not affect the crack propagation rate and direction. This behavior of crack propagation cannot be explained by the consideration of residual stress value alone. The estimation of effective stress range from the crack opening stress is required to satisfactorily evaluate the crack propagation rate of friction stir welded specimens.

Fatigue Crack Growth of Friction-Stir-Welded Aluminum Alloy

Crack propagation tests were conducted to clarify the effect of stress ratio on crack growth rate in friction-stir-welded 2024-T3 aluminum alloy. The effect of the distance between the weld line center and the center of the specimen was also evaluated. The result indicates that the peak of the acceleration is not at the area of maximum tensile residual stress. The modified stress ratio that considers the residual stress identifies that the location of the peak modified stress ratio corresponds to that of the peak maximum acceleration. The relationship between stress ratio and crack growth acceleration in friction-stir-welded panels is also evaluated by an analysis using the stress intensity factor range with and without residual stress and the crack growth rate for the master curve obtained by the base material with a different stress range. The maximum acceleration decreases with the increase of the distance between the weld line center and the center of the specimen and with the increase of the stress r...

Improvement of ductility with maintaining strength of drawn high carbon steel wire

The effect of areal reduction for one pass on mechanical properties of high carbon steel wire drawn using wet-type non-slip drawing machine was investigated. The wires of 0.443 mm in diameter with carbon 0.98% were drawn to 0.06 mm in diameter by reducing the sectional area of the wire by 14 % and 27 %. Tensile strength increased monotonically with increasing drawing strain and there were very few differences of tensile strength by pass schedule. Elongation had the minimum value at 2.5 of drawing strain and reduction of area also had the maximum value at 1.2 of drawing strain. Elongation and reduction of area were improved in the region of drawing strain more than about 3 by decreasing areal reduction for one pass. Therefore, the wire can be drawn with maintaining strength and ductility under small areal reduction for one pass at latter pass regardless of areal reduction at former passes.

Effect of Additional Shear Strain Layer on Microstructure and Tensile Strength of Fine Wire

The mechanical and electrical applications of fine wires (D = 0.1 mm) has become more widely spread. In general, it is well known that fine drawn wires have high tensile strength while maintaining ductility. It has been determined that a hardened layer of around 0.04 mm in depth, referred to as the “additional shear strain layer,” is generated beneath the surface layer of the wire, and this additional shear strain layer affected the tensile strength of the fine wire. As an origin of this phenomenon, it was ascertained that the microstructure of surface layer was finer than that of center layer. The purpose of this paper is to investigate the effect of die angle on the microstructure and the tensile strength of the additional shear strain layer. The tensile test was performed as the surface layer was thinned by electro-polishing, and the crystal orientation and the crystal grain were measured via EBSD. As a result, it was ascertained that die angle affected the tensile strength and crystal grain refinement of the additional shear stray layer.

Development of Shearing with Torsion for Restricting Work Hardening by Torsion

Shearing is a commonly used method of cutting materials for former from bars or wires. However, the cut face shape of a material after shearing is poor because droop is formed at the edges of the cut face. It is necessary to alter the cut face shape after shearing for use in former. In this research, we developed a new shearing method in which shearing is combined with twisting to achieve a short cutting time with a length high accuracy and a cut surface without droop. Annealed low-carbon steel bars (diameter: 1.96 mm) were used for the experiment. The specimens were twisted after a static shear force was applied to the bar. As a result, the droop height decreased markedly. In addition, to restrict the work hardening area by applying torsion, the bar was twisted after shear stress, which was slightly less than the yield stress, was generated at the clearance with the loading of a large static shear force. The equivalent stress at clearance exceeded the yield stress, when the torsion was small because the shear stress was large. Therefore, the twisted area became narrow, and work hardening was restricted.

Evaluation of Fatigue Crack Growth Behavior in FSW Joint by Experiment, Analysis and Elasto-Plastic FEM

Elasto-plastic FEM is used to examine the crack opening stress of a 2024-T3 Aluminum alloy sheet and a friction stir welded (FSW) panel. To investigate the effect of plastic deformation around the crack on the crack opening stress, the effects of the following two parameters, the distance between the weld line and the center of the crack starter, and the magnitude of the tensile residual stress on the crack opening stress, are evaluated. The da/dN-ΔK curves and a-N curves for the FSW plate are obtained numerically using an experimental da/dN-ΔK curve for the base material and the calculated crack opening stress. In addition, the da/dN-ΔK curves where ΔK is evaluated by correction factor and its a-N curves are obtained analytically. Comparison of these numerical results with the results of empirical tests shows while that the FEM result conforms to experimental data, the calculations based on the correction factor show poorer correspondence. These results demonstrate that FEM can reasonably predict the da/dN-ΔK curves and a-N curves for an FSW panel.

Improvement of Straightner in Tension Annealing and Rotational Blade Straightner for Superfine Wire

Higher straightness of superfine wires of 0.1 mm or less in diameter is in demand for recent development of electronic devices.A rotational blade straightener is effective for straightening superfine wires. It is particularly effective for straightening coiled wires because the straightener can bend wires along the longitudinal and circumferential directions. We found that a higher rotational speed of blades, lower feeding velocity, and gradual decrease in curvature ratio during straightening were very effective in producing excellent straightness in rotational blade straightening.Tension annealing is also one of the popular methods to straighten superfine wires. The tension annealing has a higher level of straightening effect without surface damage. The phosphor bronze and the tungsten superfine wires are used in this study. We observe that the initial radius of curvature, roundness and work hardening exponent are very important factors to make excellent straightness in tension annealing.

Numerical Investigation on Formability of Ellipse Deep Drawing by Sheet Hydroforming

This paper presents the numerical investigation on the formability of the sheet hydroforming by a finite element method. The finite element method code for sheet hydroforming simulation was developed based on the static‐explicit finite‐element method code STAMP3D. Experiments and Simulations of ellipse deep drawings were performed. The simulated thickness strain distribution which has two local thinnings at the punch shoulder and at the upper part of the side wall well agreed with the experimental one, thus demonstrating the validity of the developed code. Moreover, the effects of the friction increasing effect which is one of the main features in the sheet hydroforming on the formability were examined by comparing the results between the simulations with the friction coefficient μ=0 and those with μ=0.15. When μ=0 was employed, the local thinning at the upper part of the side wall disappeared. This showed that the occurring the two local thinnings in the sheet hydroforming was originated in the friction ...