Papers by Author: Makoto Kobashi

Abstract: In this paper, a novel processing method (reactive precursor method) to manufacture high-melting point porous Al-Ti intermetallics is investigated. Especially, morphological control of porous structure is focused. In the reactive precursor process, precursors are made by blending aluminum and titanium powders. The precursor is heated to ignite an exothermic reaction (so called “combustion reaction”) between the elemental powders. Pore formation is a well-known intrinsic feature of the combustion reaction, and we tried to control the pore morphology. Fundamentally, the closed-cell structure can be obtained when the maximum temperature during the reaction exceeds the melting point of the reaction product. By blending the exothermic agent powder in the precursor, the maximum temperature is increased and the reaction products are melted. The porosity is controlled by the maximum temperature. In contrast, an open-cell porous structure can be obtained when the maximum temperature is below the melting point of the reaction product. Microwave heating turned out to be an effective method to create an open cell structure. A powdery substance that does not react with other elemental powders (heat-absorbing agent powder) decreases the temperature during the reaction. Closed, open and bimodal-sized open pores have been achieved by the reactive precursor process so far.

Abstract: Cast AlFe alloys containing several percent iron have low ductility because of their brittle precipitates. Therefore, precipitate refinement is very important for improving their mechanical properties. In recent decades, severe plastic deformation processes have been developed to achieve this grain refinement. For example, our previously proposed severe plastic deformation process, called compressive torsion, is quite effective for not only grain refinement but also precipitate refinement even in brittle materials. In the present work, precipitate refinement of cast Al—Fe alloys by compressive torsion and the resulting improvements in their tensile properties were investigated. Compressive torsion with various numbers of revolutions was applied to Al—Fe alloys at 373 K. Then, the alloys were subjected to tensile testing at room temperature, 473 K, and 573 K. The obtained experimental results indicated that the initial eutectic microstructure of the alloys disappeared after the compressive torsion processing. All large precipitates with sizes of more than 200 μm were refined, and their sizes were reduced to several tens of micrometers. Furthermore, these refined precipitates were dispersed homogenously in the alloy microstructure. In result, the tensile properties of the alloys, namely, their strength and elongation, were improved remarkably. In particular, the elongation reached more than 30% at room temperature.

Abstract: Recently, industrial technology for both improving thermal conductivity and controlling the coefficient of thermal expansion of heat sink materials has became an important issuebecause of the downsizing of electronic devices. We have been investigating the innovative processing method for TiB2 dispersed Cu matrix composite by reactive infiltration process in which the combustion reaction of elemental powders (Ti+2B+Cu → TiB2+Cu) and pressureless infiltration of molten Cu into porous reaction product (TiB2/Cu composite) are combined. By this process, fine TiB2particles (2~3µm) can be dispersed in Cu matrix homogeneously. However, for better thermal conductivity and reduced thermal expansion, 3-dimentionally continuous inter-penetrating structure of TiB2 and Cu phases is suitable. In this study, we researched the effects of Cu powder size and volume fraction in Ti,B,Cu green powder compact on the microstructure of the combustion synthesized TiB2/Cu composite. When Cu powders were smaller than 45µm, TiB2 particles were uniformly dispersed in Cu matrix. However, when Cu powders were larger than 150µｍ, monolithic Cu area without TiB2 dispersion was formed. The monolithic Cu area tended to be connected each other by increasing the amount of Cu powders. This resulted in the formation of 3-dimensionally continuous inter-penetrating TiB2/Cu microstructure.

Abstract: Compressive torsion process (CTP) which was developed by authors is effective process for grain and precipitates refinement of metallic materials with a severe plastic deformation. In the CTP, a cylindrical specimen is subjected to simultaneous compressive and torsional loading without change in its shape. However, metal flow and strain distribution in the processed specimen are not cleared, because the deformation is very large and complicated. In the present work, visualization of internal deformation of specimen processed by CTP was investigated using dual alloy etching technique. Two kinds of aluminum alloy were prepared by cutting on fan-like shape and alternately placed to a cylindrical shape. After CTPing, contrasts in the specimen were observed by polishing and etching. The internal distribution of shear strain was quantified by measuring the displacement of interface between the alloys. As a result, the visualization and quantification of internal deformation was successfully carried out using the technique. The internal strain distribution was varied not only in radial direction but also in longitudinal direction because of frictional constraint on the lateral face. A laminate contrast of the alloys observed on the vertical cross section was well related with the strain distribution in the specimen.

Abstract: In this paper, a new attempt of transfer heat foaming was examined on the precursor method to fabricate long scale aluminum foams. In this new method, the induction coil heater was moved along the longitudinal direction of a rod precursor to foam a part of the precursor continuously. Long scale aluminum precursor was successfully foamed by the transfer heat foaming in which heating coil was moved along the precursor to control the temperature of heated part constantly.

Abstract: The present study is aiming at investigating the possibility of producing a magnesium foam from machined chips. To produce highly porous magnesium foam, precursor producing process was investigated by hot extrusion and compressive torsion processing (CTP). The CTP could realize well-consolidated precursors and homogeneous distribution of a blowing agent. The precursor made of machined chips satisfactorily expanded, and the porosity were comparatively high by optimizing processing parameters of the CTP.

Abstract: It is favorable to disperse fine strengthening particles under 1μm to expect the effective dispersion strengthening mechanism of metal matrix composites. In this research, TiB2 particle was synthesized in Al matrix by a combustion reaction and the influence of the powder blending ratio was examined in detail. The mole mixture ratio of Ti and B powder was fixed to B/Ti=2, and the blending ratio of Al powder was varied from 40 to 70vol%. The compacted blended powder was heated under an Ar atmosphere in an induction furnace, and heating was stopped immediately after the combustion reaction took place. The synthesized TiB2 particle became finer by increasing the blending ratio of Al, and the dispersion of particles about 0.3μm was achieved. However, large quantity of Al-Ti intermetallic compounds remained when 70vol% Al was blended, indicating that the combustion reaction was not completed in this specimen.

Abstract: Reactive infiltration is a manufacturing process of metal matrix composites with low cost and low environmental impacts. In this study, reactive infiltration of a NiO/Ti blended powder preform with molten Al was examined. Titanium powder as an infiltration aid was mixed with NiO powder by various blending ratios. The preform and the Al ingot were then heated together up to 1273K ~1673K and held at these processing temperatures for 60 minutes by an induction furnace in N2 gas atmosphere. After the heating process, the vertical cross section was observed to see whether the infiltration and the in situ reaction occurred successfully. Spontaneous infiltration of molten aluminum into the powder preform did not occur when either processing temperature or blending ratio of titanium was not sufficiently high enough. Spontaneous infiltration occurred when processing temperature and volume fraction of titanium were 1273K, 1373K and 15%, 20%. But when volume fraction of titanium was 25%, the preform exploded by an extremely high. It was confirmed that Al3Ti, Al3Ni2 and Al2O3 were formed after the infiltration.

Abstract: A compressive torsion processing (CTP) was applied to hypereutectic Al-Si alloy in order to raise ductility and formability by microstructure refinement of the alloy. The CTP is a unique severe plastic deformation process and it can easily apply large strain to a work piece without change in shape. In the present work, influence of compressive torsion processing temperature on microstructure refinement and tensile property of hypereutectic Al-Si alloy is dealt with. When the CTP was applied on the Al-Si alloy, primary and eutectic Si particles were refined more effectively at lower processing temperature. Total tensile elongation of CTPed alloy was four times as large as that of non CTPed one. Distribution of the total elongation was quite uniform in the whole CTPed specimen.

Abstract: Compressive torsion combined loading that uses relatively low compressive pressure has a great advantage of microstructure refinement of cylindrical metal blocks without changing their shape while processing. In the present work, effects of processing temperature and rotation times on homogeneity of the refined microstructure were investigated for Al-5%Mg alloy. Although lower processing temperature was effective to obtain fine grains, it was difficult to obtain homogeneous refinement at lower temperature. Higher processing temperature was favorable to obtain homogeneous microstructure, for instance, in the cylindrical specimen of φ25×10 mm the homogeneous refinement could be obtained at higher temperatures than 373K. Increasing rotation times was also effective to obtain homogeneous refined microstructure for thicker specimens.