For high cycle production of carbon-fiber-reinforced plastics, press forming using a carbon-fiber-fabric-reinforced thermoplastic sheet has been performed. The deformation of crossed fibers during forming was investigated. In the press forming of a hemispherical conical cup, die clearance along the conical vertical line was determined according to the thickness increase estimated from the concentration of fibers during forming. Owing to the die clearance, the pressed profile was in good agreement with the die profile, and the deviation was only about 0.1mm. The thickness was constant along the circumference of the conical surface. There were no voids in the pressed material. Investigation of the deformation of carbon fiber fabrics and die clearance design will be important for press forming using a carbon-fiber-fabric-reinforced thermoplastic composite sheet.
A nonlinear friction coefficient model considering contact pressure, sliding velocity, and sliding length has been developed to improve prediction accuracy of the formability of steel sheets. The effects of contact pressure and sliding velocity under the condition of mixed lubrication were estimated using a friction test, in which a long steel sheet was sandwiched between dies and then drawn. The effects of sliding length under the condition of lack of lubricants were estimated using another friction test, in which a long tool bar was sandwiched between steel sheets and then drawn. The results of the cup drawing experiments and FE forming analysis using the friction model developed showed that the model improves the prediction accuracy of the formability of steel sheets determined by monitoring the change inpress load with press stroke and the thickness distribution on the cup wall. Finally, the formability in press motion control with a servo press was predicted with the model developed, and the effects of press motion control on the improvement of the formability of steel sheets were estimated.
A nonlinear friction coefficient model was applied to finite element (FE) forming analysis in order to predict the formability in stamping steel sheets by press motion control with a servo press. This nonlinear friction coefficient model considers the effects of contact pressure, sliding velocity and sliding length on the coefficient of friction between steel sheets and dies in stamping. The results of cup drawing experiments showed that the formability in stamping by press motion control was predicted accurately by the developed method of FE forming analysis. A suitable press stroke position for applying press motion control to cup drawing was calculated to improve the wall thickness reduction at the punch shoulder portion effectively. By applying the developed method to the prediction of the effective press motion, motion control with a servo press can be efficiently applied in mass production systems such as automotive stamping lines.
There are many demands for cup-shaped parts with a gear shape in the automotive industry and a large load is often required for the complete filling of the gear portion in conventional progressive forming. In this study, a new method was proposed for solving the problem using a multi-action press. This method features the operation of depressing the cup wall to provide an appropriate shape for gear filling with the cup corner prior to gear forming. Experiments and FE analysis were carried out, varying the amount of cup wall depression. The filling ratio of the gear increased with increasing amount of cup wall depression and complete filling was achieved with a low load compared with progressive forming. An increase in load and burr formation were observed after the cup corner reached the bottom of the die. Accordingly, the optimum conditions for cup wall depression should be determined so that the unfilled portion is minimized at the stroke that the cup corner reaches the bottom of the die.
A micro-hydromechanical deep-drawing (MHDD) apparatus for manufacturing a micro-complex-shape components and increasing of drawn cup accuracy has been developed in this study. This apparatus with a simple forming process mechanism can achieve high dimensional accuracy using servo press mechanics with a double-action type, one-stroke forming process without transferring and positioning, force control, and fine flow rate control of the pressure medium. It is confirmed that the MHDD apparatus developed can prevent wrinkling by applying an appropriate constant gap and stably generate the counterpressure. Micro-drawn cups of 0.8mm diameter are successfully fabricated. Also, the effects of counterpressure on drawability and dimensional accuracy at the bottom of the cup are investigated for phosphor bronze, stainless-steel, and pure titanium foils with a thickness of 50μm. The appropriate counterpressure applied in MHDD results in wrinkling constraint and a reduction in frictional drawing force. Consequently, it is concluded that the forming limit and dimensional accuracy can be improved by MHDD.
Hot semi-punching and cold scrap removing processes were developed to make holes in hot-stamped steel parts having high strength. For hot semi-punching, additional channels for taking punching scraps out of dies are not required. Achieving the minimum amount of residue without detachment of punching scraps and no clearance between the die and punch were the best conditions for the hot semi-punching process. The hot punching and cold removing loads were much smaller than the cold punching load of quenched sheets, the quality of the sheared edge was high and the delayed fracture around the sheared edge was prevented. In addition, quenching of punching portions was prevented by sandwiching these portions between ceramic tools during rapid resistance heating to facilitate cold punching of hot-stamped parts. The cold punching load for the local prevention of quenching was half of that without local prevention and the delayed fracture was also prevented, whereas the drop in hardness around the sheared edge became larger than that measured for laser cutting.
The deformation behavior of 6000 series aluminum alloy sheets under biaxial tension was precisely measured for linear stress paths, using servo-controlled biaxial tensile tests with both cruciform and tubular specimens. The tubular specimens were fabricated by bending and laser-welding as-received flat sheet materials. Differential work hardening (DWH) behavior was observed; the shapes of the contours of the plastic work constructed in the principal stress space changed with an increase in plastic work. A new constitutive model that can reproduce the DWH behavior was proposed; the model is based on the Yld2000-2d yield function with the exponent and material parameters changing as functions of plastic work. Finite element analysis of hydraulic bulge forming was performed using both the proposed DWH model and the conventional isotropic hardening model based on selected yield functions. The calculated results based on the DWH model were in closest agreement with the experimental results. Thus, the new constitutive model has been verified to be effective for improving the accuracy of the FEA.
In a study of the deformation behavior of stainless clad steel resulting from the difference in flow stress between hot stainless steel and mild steel at the leading and tail ends, the effects of various rolling conditions on the alteration of mild-steel protrusion were evaluated by experimental hot rolling and two-dimensional FEM analysis. The experimental and analytical results showed that the mild-steel protrusion at the tail end is reduced when using rolls with different diameters and lubricated rolling in comparison with rolling under conditions of equivalent roll diameters, equivalent roll velocities, and non lubricated rolling, because the former conditions promote uniform deformation of the mild-steel protrusion at the tail end. On the other hand, the mild-steel protrusion is facilitated by rolling at different velocities, as this enhances shear deformation in the roll bite. The inlet angle of the clad steel on the entrance side of a roll bite materially affects the mild-steel protrusion. As the inlet angle increases, the mild-steel protrusion at the tail end undergoes a large deformation owing to the insertion of the clad steel into the roll bite at an inclined angle. Furthermore, under the condition of constraint of the clad steel at the roll bite entrance (roll bite angle=0°), the use of different roll diameters and lubricated rolling has a large effect in suppressing mild-steel protrusion.