Papers by Author: Masahiro Fujita

Abstract: This study has investigated the plastic forming of magnesium alloys plate. It is not easy to perform the cold-worked with the usual plastic forming method although magnesium alloys have the advantages in terms of strength-to-weight ratio. Therefore, explosive forming method which is one of the plastic forming methods with a specific forming mechanism has been applied. At first, numerical simulations have been conducted to clarify the optimal combination conditions, and then we have verified practical effectiveness of this proposed method by using experimental study.

Abstract: t is well known as an important merit of the explosive forming process that the delicate figure of the die surface is precisely transferred onto the specimen of a thin metal plate, because the plate is strongly pressed against the die by the very high pressure of underwater shock wave. However, we cannot find any examples of work pieces making the best use of the merit in literatures until now. We tried to form explosively a thin copper plate into shapes of a palm and a back of human hand, as a work piece making the best use of the merit. The palm has many small delicate lines such as fingerprints and the back has interesting figures such as loose skin at the joints of fingers and the borders of fingernails. The object of the present investigation is to make work pieces just like to a real hand. In this investigation, the following process is adopted in order to perform successfully explosive forming ; real hand plaster concave model convex model of Duplicone (material for dental impression) die of Ren Cast (a kind of epoxy resin) explosive forming. Annealed copper plates of 0.3 mm thick were used in the experiments. Since the extension limit of the plate is not so large, the plate is apt to be broken. If the plate is broken, the die is modified not to break the plate. Though the die made of epoxy resin is too hard to be modified, the plaster model can be easily modified with a cutter knife or a chisel. The modified die is obtained through model of Duplicone made by using the modified plaster model. After a few times of repeating modification, the breakage of plate was scarcely produced. On the work piece of palm, a lot of small lines including finger prints clearly appeared and the work piece of the back showed the shape just like to real human hand.

Abstract: The metal jet that is flowed out by the oblique collision between a metal flier plate and a metal block becomes a high velocity. We have been developing the device that makes a material extremely high pressure by using the metal jet. The flier plate of the previous device had been accelerated by using a high explosive. There were several problems in the collection and analysis of the material that had been made the high pressure. Therefore we thought up the new device of which the flier plate was accelerated by a powder gun. The collision process was examined by a numerical simulation because the collision process of the flier plate of this device differs from that of the previous device. LS-DYNA was used for a numerical simulation and the difference of the collision process was clarified.

Abstract: When a few branched holes are made in order to cross a main hole at right angle, burrs are produced on the sidewall of the main hole at end of each branched hole. The principle of the process is as follows. After the specimen of which all holes are closed by using sealing tape is submerged, explosive set at the position a little apart from the entrance of the main hole is detonated. As soon as the generated underwater shock wave is arrived at the entrance of the main hole, the sealing tape is broken and the water stream with very high velocity flows into the main hole. The deburring is performed by the action of the water stream with very high velocity. The deburring is sufficiently succeeded in most case. When the main hole is closed, however, the deburring could not be succeeded at the branched hole very near closed position. In order to improve the insufficiency of the process, we designed new equipment in order that underwater shock wave may be concentrated at the entrance of main hole and the water stream with very high velocity, which is caused by the shock wave, may flow along sidewall on which burr is generated. In the present report, we propose the new method and give full account of the experimental results for deburring obtained by using the equipment.

Abstract: Hot dynamic densification method was developed by combining self-propagating high temperature synthesis (SHS) with explosively shock powder compaction technique. This method is extremely short time processing. The main purpose in this study is to perform from synthesis to densification of TiB₂-TiN system high temperature ceramic composites and TiB₂-TiNi-Cu system functionally graded materials (FGMs) in one step. In TiN-TiB₂ ceramic composites, they showed up to 95% of relative density. It was appeared by TEM observations that both the two phases joined tightly each other. The FGMs also were produced by the same technique. They indicated no interlayer exfoliation and no macro cracks after thermal shock tests from 973 K to room temperature. It was shown that thermoelastic property of intermetallic TiNi phase as intermediate layer between ceramics and metal layers operated effectively.

Abstract: Detonating code, which is a flexible code with an explosive core, is normally used to transmit the ignition of explosives with high detonation velocity 6 km/s. Therefore it is difficult to use detonating code for the explosive welding of common metals toward the detonating direction since the welding velocity exceeds the sound velocity of metals. Hence, an explosive welding method using underwater shock wave generated by the detonation of detonating code was tried. In the present investigation, the details of the experimental setup and results are reported. And the welding conditions are discussed through numerical simulation. From these results it is observed that the above technique is suitable to weld thin metal plates with relatively less explosives.

Abstract: The various techniques of phase transformation of the material have been proposed by many researchers. We have developed several devices to generate the ultrahigh pressure by using high explosive. One of them uses metal jets. It is expected that the ultrahigh pressure occurs by the head-on collision between metal jets, because the velocity of the metal jet is very high. By mixing a powdered material with metal jets, the pressure of the material becomes high. The purpose of this study is to transform the phase of the powdered material by using this high pressure. The powders of the graphite and hBN were applied. The synthesis to the diamond and cBN was confirmed by X-ray diffraction (XRD). In this paper, the mechanism of the generation of the ultrahigh pressure is explained and the results of the observation of the powder by using scanning transmission electron microscope (STEM) are reported.

Abstract: The purpose of our research is to generate the ultrahigh pressure by using high explosive and to transform a phase of a material. The extremely high impulsive pressure generator that has been developed by us uses the head-on collision between metal jets. Because the velocity of the metal jet is very high, the ultrahigh pressure will generate. If a powdered material is mixed to metal jets, it is expected that the material is transformed to a high pressure phase by this ultrahigh pressure. A graphite powder was used to synthesize a diamond. The existence of the diamond was confirmed by X-ray diffraction (XRD). In this paper, the mechanism of the generation of the ultrahigh pressure is explained and the results of the observation of the powder by using scanning transmission electron microscope (STEM) are reported.

Abstract: A burr is most commonly created after machining operations, such as drilling. Drilling burrs, for example, are common when drilling almost any material. When burrs are broken during the operation of a machine including the parts with the created burrs, the broken piece is in fear of disturbing normal operation or damaging the parts of the machine, so that the sufficient deburring is requested because it can affect equipment performance, reliability, and durability. Several deburring method have been developed up to date. In the present report, we proposed a deburring method by means of applying underwater shock wave. The method is as follows: after all entrance of holes is closed with seal tape, the equipment is submerged, so that all passages for running fluid are filled with air. The explosive is set under water near the entrance of the main hole. As soon as the explosive is detonated, the underwater shock wave generated at the detonation point arrives at the entrance of the hole and breaks through the tape. The water flows into the hole with a high speed. The burr is broken by water hummer action of high speed. In the present investigation, the experiments of deburring are performed under some setting conditions of explosive. It is found by experimental results, that the burr is sufficiently removed with the newly proposed method. When the shock pressure is sufficiently high at the entrance of hole, the burr is broken surface is smooth as polished one. When the shock pressure is not sufficiently high, the broken surface of the burr is notched.

Abstract: Detonating code is a flexible code with an explosive core. It is used to transmit the ignition of explosives with high detonation velocity in the range of 5.5 to 7 km/s. However, it is difficult to use detonating code for the explosive welding of common metals since the horizontal point velocity usually exceeds the sound velocity. Hence, in the present work, a new method using underwater shock wave generated by the detonation of detonating code was tried. The details of the experimental parameters and the results are presented. From the results it is observed that the above technique is suitable to weld thin metal plates with relatively less explosives.