The need for minute processing is increasing recently, and electromagnetic piercing is again attracting attention. The electromagnetic piercing of metal foils theoretically does not need a punch and is performed using only a die. In this paper, we describe the piercing of both aluminum foil and stainless-steel foil using pressurization foil. The electromagnetic force generated in the aluminum foil is calculated with magnetic pressure and is proportional to the square of the diameter of the die hole. Therefore the piercing for a small hole of less than 1.0 mm diameter needs a high discharge energy. In experiments, the piercing of only a 15 μm-thick aluminum foil is impossible because the magnetic pressure is too low or the foil melts with softening, and the piercing of only a 50 μm-thick aluminum foil is also difficult. If a pressurization foil of 50 μm thick or 100 μm thick is added, a good piercing of these foils is enabled for holes from 0.5 to 2.5 mm diameter. The piercing of a 20 μm-thick stainless-steel foil is enabled by using a similar pressurization foil. However, a section of the pierced foil has both a sheared surface and a fractured surface.
Since magnesium alloys are lightweight, they are expected to be promising as structural materials, particularly in transportation. However, because a strong basal texture is prone to develop parallel to the sheet plane in wrought products, formability at room temperature is poor. In this study, to control the basal texture of AZX612 magnesium alloy and to improve its formability, a lateral extrusion method, namely, friction-assisted extrusion (FAE), was applied, and the microstructure, texture, and mechanical properties were investigated. The FAE process was carried out with the extrusion ratios ranging from 4 to 20 at temperatures of 300 and 350 ℃. The results showed that FAE changes the basal texture of the starting as-rolled material into that inclined by about 15° against the extrusion direction and raises the intensity of texture. The 0.2 % proof stress became significantly smaller than that of the as-rolled material in the extrusion direction but larger in the transverse direction, resulting in the larger anisotropy. This can be understood by the activity of basal slip. Elongation to failure was smaller under all extrusion conditions than in the as-rolled material, which was presumed to be caused by the localization of shear deformation.
Formability in draw bending, i.e., deep drawing without shrink deformation at the flange part, in the hot stamping process was investigated and discussed in terms of the effect of the strength of the side wall near the punch shoulder on deep drawability. Furthermore, hot forging lubricants used in a wet condition in the hot stamping process were examined because the die temperature is below 100 °C for die quenching. The results showed that the forming limit of draw bending increased with a lower forming temperature, a lower forming speed, and a lower blank holder force. Under these forming conditions, the temperature of the side wall near punch shoulder, which is a critical part for fracture, decreased indicating an increase in the strength of the side wall. Accordingly, the temperature of the side wall near the punch shoulder is a dominant factor in the control of the draw bending formability in the hot stamping process. In addition, emulsion-type hot forging lubricant exhibited excellent lubricating ability and improved the forming limit of draw bending in the hot stamping process.
Automobile body parts produced by the hot stamping process show excellent shape fixability with an ultrahigh tensile strength of 1.5 GPa. In the present study, we investigated the effect of forming conditions on stretch formability in the hot stamping process. The stretch formability of a hot-stamped steel sheet was much better than that of a cold- stamped sheet of 1470 MPa class, and equivalent to that of a cold-stamped sheet of the 270 - 440 MPa class. The maximum thinning point was the area not in contact with the tool, where the temperature was higher than that of other areas because there was no heat transfer between the sheet and the tool. Strain concentration was considered to occur in this area because the higher temperature leads to lower deformation resistance. The strain distribution, which is a dominant factor controlling stretch formability, was greatly influenced by tool shape, whereas the influences of the temperature at the start of forming and forming speed were relatively small. On the other hand, the deformation limit, which is another dominant factor controlling stretch formability, was greatly influenced by forming speed, which strongly affected the temperature of the blank. In addition, the heat insulation of the punch greatly contributed to the improvement of formability because strain uniformity was increased.
The hole expansion behaviors of mild steel sheets were investigated by experiments and finite element simulations, using three anisotropic yield functions: Hill’48, Yld2000-2d, and Spline models. Spline yield function was expanded to allow the arbitrary segments given by experimental stresses, strains, and strain increments, which are normal vector on the yield surfaces, in the biaxial stress state. Relationships between the yield function models and prediction accuracy were examined through the comparison of thickness distributions at a certain distance from the hole edge. In this work, it was found that accurate modeling of anisotropy affected the accurate prediction of thickness. It was concluded that Spline yield function, which was given the most numbers of experimental values, is the most accurate model in this research.