In warm forging above a temperature of 800°C, the dominant die failure of more than 70% is said to be adhesive wear. In order to reduce the production cost under warm and hot forging conditions, the precise prediction of die wear and countermeasures to minimize it are desired. Many research works on the prediction of die have been carried out; one of them is Archard formulation, which considers accumulated frictional work and die softening. Archard formulation was applied to the prediction of the wear of a warm backward extrusion punch, and the predicted tendencies of wear volume and profile were not identical to the test results. In this research work, we proposed the new prediction formulation for die wear using the plastic flow criteria, Dc value of die steel and Usui's formulation, which is developed for cutting tools. This new formulation for die wear is applicable to a wide temperature range, a wide area on the punch, and different die materials. We confirmed that the results of punch wear prediction obtained using the new formulation and FE analysis agreed with the experimental results of the die damage test of the warm backward extrusion type.
The precision of the measured stress and strain in a compression test was evaluated numerically and experimentally. Attention was focused on a simple compression test on a cylindrical specimen whose geometry is specified by ASTM E9. Elastic-plastic finite element analyses were carried out to examine the uniformity and uniaxiality of stress and strain. The stress distribution was evaluated in the cross section at the centre of the specimen and the strain distribution was evaluated on the outside surface of the specimen and also in the radial direction in the specimen corresponding to each evaluation point on the outside surface. The parameters changed in the numerical analyses were the yield stress and work-hardening ratio of the material, the aspect ratio of the specimen, and the friction coefficient. Numerical results showed that the parameter most strongly affecting for the precision of the measured stress and strain is the work-hardening ratio of the material, and that the lower its value the poorer the uniformity and uniaxiality. Experimental results using steel with a low work-hardening ratio showed good agreement of the axial strain on the outside surface of the specimen with the numerically predicted results. The recommended aspect ratio of the cylindrical specimen is over 2.5, and a ratio of 3 is sufficient enough to ensure the precision of measured results.
Fundamental studies on burrs of a thick metal plate in blanking are carried out to elucidate the burr forming mechanism of a blank cut using a blanking die without putting a blank holder on the plate on the scrap side. In this study, we examined the effects of scrap width, tool shape, wearing at the tool edge and blanking speed on burr shape and height experimentally. The following results are obtained. Even if the cutting edge of a punch does not show any wear, burrs grow up in the case of working under tensile force at connecting areas between the blank side and the scrap side during a blanking process, that is, narrow scrap width and rotating deformation of scrap caused by a punch with triangular groove. Wear at the cutting edge of a punch induces burrs and also burr shape varies with tool shape owing to the difference in the deformation process on the scrap side. In particular, sharp burrs are formed in the case of blanking using a punch with a flat face. In the case of blanking using a wear-free triangular groove punch, the burr height at a high blanking speed is less than that at low blanking speed.
To improve the stretch flangeability of ultra high strength steel sheets having a low ductility, flanging with a punch having gradual contact was developed. The tensile stress on the corner of the sheet flange was reduced by using angle of inclination of the punch. Although the stretch flanging ratio decreased as the angle and width of the punch inclination increased, the fracture on the corner of the sheet flange appeared at the limiting stretch flanging ratio. The large camber on the upper surface of the sheet after flanging with the punch was observed. The optimum angle and width of the punch inclination for the prevention of the occurrence of fracture were calculated using finite element simulation under a limited punch stroke. The limited flange length of the sheet with the optimum punch showed an increase of 30% for the 980 MPa grade ultra high strength steel sheet.
The use of extruded aluminum alloy sections is advantageous for reducing the weight of many types of structure. In general, a secondary forming process such as bending is required when these materials are applied on structural members. However, in the bending process, undesirable deformation such as flattening and wrinkling arise easily. In this study, an improved rotary bending process is used for fabricating light gauge square tubes of A6061S and 63S-O. Flattening distortion on the cross section is restrained using an elastic laminated mandrel. In this study, we show that applying optimum axial tension is effective for suppressing wrinkling in order to reduce compression strain. For example, bend degree reaches 4.3 at the minimum R0/H0 without any defects, under the working conditions using a thickness ratio of A6061S and 63S-O tubes of t0/H0=0.038, where R0, H0 and t0 are the radius of the central plane, and the height and thickness of the square tube, respectively. Moreover, applying axial tension is effective for increasing the working limit, especially for thin-walled tubes, because in the thickness ratio of A6063S-O of t0/H0=0.025 the working limit is 10.3-fold that under non axial tension.
A forming experiment has been carried out to develop a new incremental forming method for the high-strength titanium alloy sheet Ti-6Al-4V, using a bar tool with a spherical head. As a pre-experiment, a blank sheet was heated locally using a high-frequency induction coil located beneath the sheet. The temperature of the sheet was adjusted and kept constant by controlling induction current depending on the feedback from infrared thermometer reading. With this system, the energy of heating depends on the distance between the sheet and the induction coil. This property enables the separation of the heated and non heated areas on the sheet throughout the forming process. The heated area has better condition to deform by material softening. The non heated area is expected to not crack as a result of the recovered strength of lower temperature, even though the area has been thinned by the deformation. Finally, a truncated pyramid, with a 40% reduction in the thickness of the side wall, was formed successfully, under some suitable forming conditions. It was found that this heating method was effective for forming titanium alloy sheets, and that the pattern of thickness variation was the same as that in a conventional forming process.
Tube bending without the use of a roll die or a mandrel recently been developed. In this bending method, straight pipes are clamped and bent by the bending arm, while they are reduced in diameter by squeezing with a conical die. The stress induced by bending and squeezing prevents the flattening or wrinkling of pipes in small-radius bending. Moreover, the bending moment is controlled by adjusting two parameters, the length [LR] between the center of the rotating arm and the exit of the conical die, and the ratio [λ] of feeding velocity by the pusher to the circumferential velocity by the rotating arm. In this study, the deformation behaviors of the intrusion bending with squeezing are clarified by experimental tests and finite element analysis.
Structural channels have advantages of low cost and light weight because the geometrical moment of inertia is large despite the light weight. However, the stiffness of the flange in the cross-sectional plane of the channel is small. To increase the stiffness, a reinforcement is welded. Therefore, the authors invented a structural channel with a rib. In this paper, we clarify the effects of constraint conditions on the undesirable deformation of the rib formed on the structural channel by finite element analysis. The process of rib forming is described as follows: (1) a structural channel is set on a die, (2) forming tools hold the web and the flange of the structural channel near the rib is formed, and (3) a punch moves toward the flange opening side from the web side. Warpage occurs because of the in-plane bending of the flange and the drawing of the web; therefore, holding the flange edge is an effective way of decreasing the warpage.