In this study, the effects of shearing clearance and shearing angle on stretch-flange formability are investigated. The shearing line used is of the notch type composed of straight and circular arcs. The saddle-type stretch-flange test is adopted for this investigation. The results show that stretch-flange formability is improved in narrow clearances. This tendency is different from that obtained by the hole expansion test, in which stretch-flange formability is improved in large clearances. This improvement might be caused by the increase in the area of the fractured surface in narrow clearances. Generally, the larger the fractured surface, the smaller the work hardening on a sheared edge. In addition, the investigation reveals the existence of an optimum shearing angle for excellent stretch-flange formability. FE simulation shows that the optimum shearing angle increases hydrostatic stress in materials during shearing. Compressive stress across the material thickness is mainly affected by shearing angle. The result of the analysis suggests that the optimum shearing angle increases the area of the fractured surface to improve stretch-flange formability.
A method of using ultrasonic waves for measuring the contact condition between a workpiece and die during stamping has been developed. Ultrasonic waves are reflected by air, so the condition of contact between a workpiece and a die can be examined by measuring the height of ultrasonic waves. We measured time traces of the conditions of contact between the workpiece and the die during actual stamping using a servo press. The experimental results show that we can measure the time traces of the gap between the workpiece and the die using ultrasonic waves. Next, we examined how workpiece thickness and material affect the reflection and transmission characteristics of ultrasonic waves. The height ratio of ultrasonic waves is affected by the sound pressure reflectance, which is defined by the sound impedances of the die and workpiece. Furthermore, the height ratio of ultrasonic waves is also affected by workpiece thickness. Accordingly, the condition of contact between the workpiece and the die during stamping can be measured using ultrasonic waves.
The forming of magnesium alloys is carried out at temperatures above higher than 200-250℃ because of the brittleness of such alloys at room temperature. We proposed a method of microforming magnesium alloys by which a workpiece is heated and formed simultaneously in a forming apparatus. A punch with a punch holder or a stationery forming die with a large heat capacity was heated in advance using an electric resistance heater installed in the apparatus. Then, the workpiece was heated and formed simultaneously in a forming process. A notable advantage of the proposed forming method is that the hot forming of small products of magnesium alloys could be achieved with quite a compact forming apparatus. A pinhead and a miniature gear were formed successfully by the above apparatus. A higher tool temperature is required to form high-quality products at higher forming speeds. In contrast, a lower tool temperature is preferable for a longer tool life and less heat dissipation. Then, an apparatus for forming multiple hexagonal bolt heads in a single press cycle was developed and tested. We confirmed that many high-quality products could be formed with an appropriate combination of tool temperature and forming speed using the above apparatus.
Flow stress is commonly measured by tensile test in the strain range before necking appears. The equivalent strain given in sheet metal forming is higher than the strain measured by tensile test. Although flow stress strongly affects the accuracy of FEM simulation, particularly for fracture simulations in stretch forming, extrapolated flow stress is applied to a strain range higher than the measurable ones. The purpose of this study is to propose a measurement method for the flow stress of thin sheet metals in a wide strain range, using the ring compression test. By considering the strain distribution of the workpiece and the elastic deformation of dies through compression, as well as the friction between the workpiece and the dies, it is possible to measure the flow stress of thin sheet metals up to a strain of 1.0. The validation of the measured data by the ring compression test developed was also conducted. The measured data was compared with the data obtained by the tensile test and upsettability test with specimens cut from the same thick sheet metal. The data of the ring compression test developed corresponds to other two data sets.
Flow stress strongly affects the accuracy of FEM simulation, particularly for the fracture prediction of stretch forming. However, it is only possible to measure flow stress by tensile test in a small strain range before necking appears. The purpose of this study is to propose a measurement method for the flow stress of thin sheet metals in a large strain range, using ring compression test. In our previous study, by considering the strain distribution of the workpiece and the elastic deformation of dies through compression and friction between the workpiece and the dies, it is possible to measure the flow stress of sheet-shaped metals up to a strain of 1.0. In this study, the flow stress of high -strength steels and a mild-strength steel was measured by the method developed with considering plastic anisotropy using Hill's equation. Thus, the method developed is more effective than what for measuring flow stress in a variety of thin sheet metals.
The proposed hexagonal dimple pattern sheet (HDP sheet) is an effective lightweight material for the body and other parts of automobiles, because of its increased bending strength and rigidity compared with flat sheets. In this study, the deformation state of HDP sheets formed by roll forming using aluminum alloy sheets has been experimentally examined, and their press formability has been evaluated. The roll-formed HDP sheets were shortened in the rolling direction but remained unchanged in the transverse direction. The deep drawability and shape fixability of the HDP sheets improved, but the stretchability and stretch-flanging formability deteriorated compared with those of the flat sheets. Nevertheless, as the HDP sheets became thicker, their rates of deterioration decreased.