The primary objective of this study is to clarify the basic characteristics of piercing high-tensile steel plates. The secondary objective is to propose an appropriate piercing condition for obtaining high-quality holes on surfaces. An experiment on piercing a mild steel plate and two types of high-tensile steel plate is carried out using a ceramic-coated Press Working punch (PW punch ) and a ceramic-coated conventional right angle punch, with a range of clearance from 0.5 to 10%t. Here, the PW punch is used for piercing the steel plates and has been developed using cold backward extrusion technology. The main results of this study are given below: 1) If the clearance is small regardless of the shape of the punch, a high rate of shearing surface could be obtained. 2) However, in the case of piercing in a minute clearance by right angle punch, the squeeze force of steel plates to the punch flank increased sharply as the strength of the steel plates increased. 3) On the other hand, in the case of piercing in a minute clearance by the PW punch, no squeeze force after penetrating the steel plate was detected. 4) Thus, the PW punch was found to be an excellent punch for piercing high-tensile steel plates.

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In conjunction with experimental simulation of metal forming, the development of model materials, by which forming simulation can be carried out at a low stress level at room temperature, was performed. The model materials are mixtures of microcrystalline wax, rosin, mineral oil and powder. The flow curves of the model materials show three typical configurations, i.e., work softening type, steady state deformation type and work hardening type, which are observed in metals and alloys. The above configurations of the flow curves of the model materials could be changed and controlled by adjusting powder content. The flow curves of the model material could be predicted accurately using the work hardening rate equation determined in this work. As an application example, experiments on and visio-plasticity analyses of the plane strain backward extrusion of Mg alloy and its model material, which represent the deformation property of work-softening-type materials, were carried out. We confirmed that the material flow and strain conditions in both materials correspond to each other with sufficient accuracy for engineering purpose.

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Recently, the difficulty in improving in hot- and warm-forging die life is an important problem because of the reduction in forging process cost. The major factor affecting die life is wearing. To solve the problem posed by wearing, some types of surface treatment are applied to dies. The most popular surface treatment is nitride treatment. Now the use of a hard coating to deal with high bearing stress has been increasing. To evaluate these different surface treatments, pin-on-disk and Ogoshi wearing tests are normally used as general tests. However, they can not deal with hot- and warm-forging conditions. Thus, we developed a new die damage test that simulated the actual forging process reported in our previous paper. In this study, we investigated the effect of surface treatment on the wearing of a die prepared by hot and warm forging, using our new test. We confirmed that a nitride layer improves wearing resistance because of the wearing rate reduction effect of the nitride layer. Thus, if the nitride layer used is sufficiently, wearing resistance will be good. The effect was excellent for hot-forging die steel that had a relatively low strength at high temperature. In the case of matrix high-speed tool steel that had an initially high hardness and a relatively high softening resistance, the effect of hard coating was excellent.

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A mechanical clinching process for zinc-aluminum alloy coated steel sheets was developed to improve the corrosion resistance of joined sheets. In this process, the sheets are joined by local hemming with a punch and a die without heating; thus, problems with reductions in electrode life and the thickness of the coating layer in resistance welding are solved. The shaving of the coating layer on the side wall of the depression by the punch was prevented by increasing the corner radius of the punch. The stretching of the coating layer on the top of the projection was minimized by flattening the bottom of the die. The thickness of the coating layer and the corrosion resistance of the joined sheets were improved by optimizing the shapes of the punch and die. Although the corrosion resistance of the joined sheets was 55% of that of the initial coated sheet, the resistance became threefold the resistance of the welded sheets.

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A plastic joining method for fixing bars with a hot-forged plate was proposed in previous papers. In this method, a cold bar is directly indented into a hot plate to pierce the plate without lubrication on a press and then fixed to the plate after piercing. In this study, to improve the bonding strength of a bar-plate, the indented bar was oscillated at amplitudes of 0.2 - 2.0 mm immediately after bar indentation into the plate on an AC servo-controlled press. The effect of bar oscillation on the bonding strength of the bar-plate was examined using an A6061 aluminum bar and an A6061 aluminum plate. The obtained shear bonding stress of the bar-plate in the indentation joining with bar oscillation of 0.2 mm of amplitude was approximately 1.5 times higher than that in the indentation joining without bar oscillation at an indentation temperature of 500 °C. The bonding mechanism of the indentation joining with bar oscillation was discussed from the viewpoints of bar-plate seizure and mechanical clamping associated with the method.

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