Seijiro Maki

Warm and Hot Stamping of Ultra High Tensile Strength Steel Sheets Using Resistance Heating

A warm and hot stamping process of ultra high tensile strength steel sheets using resistance heating was developed to improve springback and formability. In this process, the decrease in temperature of the sheet before the forming is prevented by directly heating the sheets set into the dies by means of the electrical resistance, the so-called Joule heat. Since the heating time up to 800°C is only 2 seconds, the resistance heating is rapid enough to synchronise with a press. The effects of the heating temperature on the springback and formability of ultra high tensile strength steel sheets were examined. The springback in hat-shaped bending of the high tensile strength steel sheets was eliminated by heating the sheet. In addition, the ultra sheet having a tensile strength of 980MPa was successfully drawn by the heating. The heating temperature is optimum around 600°C due to the small springback and oxidation and the increase in hardness.

Improvement of Formability by Oscillation of Internal Pressure in Pulsating Hydroforming of Tube

Abstract A new history of internal pressure in the hydroforming processes of tubes is developed to attain high formability. The effect of improvement of formability by the oscillation of internal pressure in a pulsating hydroforming process of tubes is examined using both finite element simulation and experiment. The deformation behaviour during the hydroforming is greatly affected by the oscillation of internal pressure. For a monotonic history of small internal pressure, the wrinkling was caused by insufficient bulging, whereas the necking and bursting occurred for a monotonic history of large internal pressure. The occurrence of these defects can be avoided by oscillating the internal pressure in the pulsating hydroforming. The improvement of formability in the pulsating hydroforming is due to both low pressure and oscillation of pressure. The effects of the amplitude and cycle number of pressure in the pulsating hydroforming on the deformation behaviour are investigated. It is found the oscillation of internal pressure is effective in preventing the occurrence of defects.

Lining of metal plates with foils using hot shot peening

Abstract A method for lining workpieces with dissimilar foils using shot peening is proposed. The foil is welded to the surface of the workpiece due to plastic deformation caused by the hit of many shots. To examine the possibility of welding, point welding using a single shot is performed, and the bond strength is evaluated by tearing the foil from the workpiece. In this experiment, the workpieces are mild steel, commercially pure copper and aluminium, and the foils are commercially pure aluminium and mild steel of 0.1 mm thickness. The lining of the soft metal with the hard metal and the reverse combination are dealt with. The weldability is also heightened by heating the foil and workpiece. By means of peening with many shots, the aluminum foil is successfully welded over the surface of the mild steel workpiece.

3-D Finite Element Simulation of Pulsating T-Shape Hydroforming of Tubes

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.

Lining of light metals with hard powders using shot peening

Abstract A lining of light metals with hard powders using shot peening was carried out. The hard powders are bonded to the surface of the workpieces due to the collision with many shots. To fix the hard powders on the surface of the workpiece, the powder is placed on the uneven surface, and the surface is covered with a pure aluminium foil during the shot peening. To make the fixing of powders easy, the lining of workpiece with the powders sandwiched in between two aluminium foils was also tried. In this experiment, a centrifugal shot peening machine with an electrical heater is employed. The workpieces are aluminium and magnesium alloys, and the hard powders are commercially pure chromium, ceramics and cemented carbide. The effects of collision speed and the processing temperature on the bondability were examined. The wear resistance was evaluated by wear test. The hard powders are successfully bonded over the surface of workpieces by raising the processing temperature.

Hot Stamping and Press Quenching of Ultrahigh Strength Steel Sheet Using Resistance Heating

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.

Resistance Heating Characteristics of High-Strength Steel Sheet Blank for Hot Press Forming

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.

Resistance Sintering of Copper-Graphite Powder Mixture under Pressure

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.

Prevention of Oxidation in Hot Stamping of Ultra High Strength Steel Sheets

Oxidation in hot stamping of ultra high strength steel sheets was prevented by coating the sheets with an oxidation preventive oil. For four types of oxidation preventive oil, the degree of oxidation under natural cooling of the heated sheets without forming was first evaluated. The oil that forms a liquefied film at elevated temperatures exhibited high oxidation prevention, and this oil was chosen for a hot bending experiment. Hot hat-shaped bending of the coated sheets using resistance heating was carried out to examine the properties of the products. The bending load was markedly decreased, the shape accuracy of bent products was very high, the surface roughness was similar to that of the sheet, and the hardness was about 1.5 times larger than that of the sheet before the bending due the die quenching. In addition, the layer remaining on the surface of the formed product could be removed using phosphoric acid. It was found that the hot stamping operation using the oxidation preventive oil is effective in the precision forming of ultra high strength steel sheets.

Semisolid Forging of Cast Iron Using Rapid Resistance Heating

Feasibility of semisolid forging of cast iron using rapid resistance heating was experimentally investigated. Gray pig iron FC250 and spheroidal graphite cast iron FCD600, whose carbon equivalents are both 4.3% in mass, were used for the experiments. Since these cast irons have a narrow semisolid temperature range, an AC power supply with an input electric energy control function was used. In this study, the resistance heating characteristics of the cast irons were firstly examined, and then their semisolid forging experiments were conducted. In the forging experiments, the conditions of the forgings such as microstructures and hardness properties were examined, and the feasibility of the semisolid forging of cast iron using resistance heating was discussed. As a result, it was found that the method presented here is highly feasible.