Papers by Author: Seijiro Maki

Abstract: The feasibility of hot stamping and press quenching of ultrahigh strength steel sheet using resistance heating was investigated by demonstrating experiments. In the experiments, a rectangular blank of high strength steel sheet, SPFC980Y, 1.2mm thick, 20mm wide and 130mm long was used and it was resistance-heated with a pair of parallel electrodes placed 120mm away from each other, using an AC power supply with a function of input energy control. The relationship between input energy and heating temperature was examined and then hot stamping and press quenching performances were examined. The press quenching was also experimented in hot hat-shaped bending, and the influences of the heating temperature on spring-back and hardness property were investigated. The results of the experiments revealed that hot stamping and press quenching using resistance heating are highly feasible.

Abstract: Resistance heating performance in some shaped blanks of high-strength steel sheet is examined by experiment and FE simulation, and the feasibility and effectiveness of the application of resistance heating to the hot press forming of high-strength steel sheet are discussed. The followings are made clear through this study. (1) A uniformly distributed current density is essential to uniform blank heating. (2) For a certain shaped blank, no matter how the electrodes are disposed, uniform blank heating is difficult. (3) A blank shape suitable for uniform heating by a single pair of parallel electrodes is restricted to a rectangle.(4) In case a uniformly heated blank is required, two-piece simultaneous forming with point-symmetric die layout is recommended from the viewpoint of material utilization.

Abstract: The effect of oscillation of internal pressure on the formability and shape accuracy of the products in a pulsating hydroforming process of T-shaped parts was examined by finite element simulation. The local thinning was prevented by oscillating the internal pressure. The filling ratio of the die cavity and the symmetrical degree of the filling was increased by the oscillation of pressure. The calculated deforming shape and the wall thickness are in good agreement with the experimental ones. It was found that pulsating hydroforming is useful in improving the formability and shape accuracy in the T-shape hydroforming operation.

Abstract: Shot peening process of inner surface of machine parts in which rebound shots collide with the inner surface was investigated. The jig used for rebound shot peening has the surface with a slope to the inner surface of machine parts, and the collision direction of shots was vertical to the inner surface. In this process, a compressed air type shot peening machine was employed. The influence of processing conditions was evaluated by the distributions of hardness, residual stress and surface roughness in the test pieces of pipes and coiled springs. The effect of shot peening increases with the increase of shots collision energy. A stress peening process for inner surface of coiled springs using rebound shots was also performed. The coiled spring was provided higher compressive residual stresses. The results of a fatigue test showed that the fatigue life of the spring was improved. The compressive residual stress reduces the tensile stress of coiled spring. It was found that the present process was suitable for improving the inner surface of machine parts.

Abstract: Resistance sintering characteristics of electrolytic and atomized copper powders mixed with graphite powder in steps of 10vol% up to 50vol% were experimentally investigated. In the experiments electrifying voltage and time, and applied pressure were varied. Post-electrification pressurization and insertion of low thermal conductive austenitic stainless steel discs between the sintering powder cake and electrodes were attempted to promote the sintering. The effects of the operational parameters including the two devices on sintering performance are discussed. Sintering strength of the resistance-sintered composites is also discussed.