Makoto Kobashi

The wettability and the reaction for SiC particle/Al alloy system

The incorporation process and the wettability for an SiC particles/aluminium alloy system were measured. The wettability between SiC particle and liquid aluminium was evaluated by the time required for the particulate incorporation. The incorporation time could be measured from a stirring time-melt temperature chart. Magnesium and titanium shortened the incorporation time of α -SiC particles into liquid aluminium and improved the wettability because of their strong affinity for SiC. Copper and zinc prolonged the incorporation time and no reaction products were found in the matrix. Furthermore, surface active elements with weak affinity for SiC (lead and bismuth) extremely prolonged the incorporation time because these elements prevent the reaction at the interface, whereas lithium shortened the incorporation time remarkably.

Porous composite with negative thermal expansion obtained by photopolymer additive manufacturing

Additive manufacturing (AM) could be a novel method of fabricating composite and porous materials having various effective performances based on mechanisms of their internal geometries. Materials fabricated by AM could rapidly be used in industrial application since they could easily be embedded in the target part employing the same AM process used for the bulk material. Furthermore, multi-material AM has greater potential than usual single-material AM in producing materials with effective properties. Negative thermal expansion is a representative effective material property realized by designing a composite made of two materials with different coefficients of thermal expansion. In this study, we developed a porous composite having planar negative thermal expansion by employing multi-material photopolymer AM. After measurement of the physical properties of bulk photopolymers, the internal geometry was designed by topology optimization, which is the most effective structural optimization in terms of both minimizing thermal stress and maximizing stiffness. The designed structure was converted to a three-dimensional stereolithography (STL) model, which is a native digital format of AM, and assembled as a test piece. The thermal expansions of the specimens were measured using a laser scanning dilatometer. Negative thermal expansion corresponding to less than −1 × 10−4 K−1 was observed for each test piece of the N = 3 experiment.

Processing of Intermetallic Foam by Combustion Reaction

Intermetallic (Ni–Al) foams were fabricated by combustion reactions. The heats of formations of these intermetallics were high enough to obtain bulk reaction products and pores were formed in the specimen during the combustion reaction. Some processing parameters of the combustion reaction were varied to control porosity and cell morphology of the intermetallic foams. The amount of aluminum in the green compact was an important factor in controlling the porosity of the synthesized nickel aluminide. The relative density of the green compact needed to be more than 0.72 for effective foam formation. The enthalpy change of the combustion reaction was controlled either by adding a reaction- enhancing agent (B4C) or by adding a heat-absorbing agent (TiC). Both the porosity and the cell size of the synthesized intermetallics were successfully controlled by changing the reaction enthalpy (from 10 to 85 % porosity).

Fabrication and analysis of an in situ TiB2Al composite by reactive spontaneous infiltration

Metal matrix composites (MMCs) have been extensively investigated in recent years, particularly in the area of processing techniques. Of the many potential techniques, in situ particulate formation is currently emerging. The aim of the present work is to study an in situ reaction taking place between TiN or TiC{sub x}N{sub 1{minus}x}, B powders and Al melt during spontaneous infiltration for the fabrication of an Al/TiB{sub 2} composite. An experimental setup was also used to perform kinetic studies of the process.

Combustion Synthesis of Porous TiC/Ti Composite by a Self-Propagating Mode

Porous titanium carbide (TiC) and TiC/Ti composites were synthesized by self-propagating high-temperature synthesis (SHS). Titanium and carbon powders were blended by various Ti/C blending ratios. The heat of reaction between titanium and carbon was high enough to induce the self-sustaining reaction of TiC formation on condition that some processing parameters (Ti/C ratio and porosity of the precursor) were appropriately selected. When the Ti/C blending ratio was high, the excess amount of titanium absorbed the heat of reaction. Consequently, the heated zone was not heated up to the ignition temperature. On the other hand, when the Ti/C ratio was low, high thermal conductivity of the precursor prevented an ignition of the heated side of precursors. The pore morphology was controlled by changing the Ti/C ratio and the preheat temperature.

Fabrication of an AlN particulate aluminium matrix composite by a melt stirring method

The physical and mechanical properties of metal matrix composites (MMCs) are extensively controlled by the structure and properties of the reinforcement/metal interface. From a metallurgical point of view, the ideal interfacial layer in a composite is expected to have some conditions: (a) the presence of intimate contact between the reinforcement and the matrix through satisfactory wetting of the reinforcement by the matrix to ensure enough adhesion and (b) the low rate of a chemical reaction at the interface to prevent degradation of the reinforcement. There are, however, various methods which can be used to improve the wetting characteristics of MMCs. The addition of alloying elements to the metal matrix has been shown to improve the metal/ceramic wettability by either reducing the metal/ceramic interfacial energy or reacting with the ceramic reinforcement to form interfacial reaction products. Although the thermal conductivity of AlN particles is less than that of SiC particles, it is likely that AlN is chemically more stable than SiC. Thus, the aim of the present paper is mainly to study the incorporation behavior of AlN particulates into aluminium melt.

Synthesis of boride and nitride ceramics in molten aluminium by reactive infiltration

The infiltration of solid powder mixtures with molten aluminium has been investigated as a potential route for the synthesis of ceramic/metal composites. Either titanium or tantalum powder was mixed with boron nitride flakes for the reaction powder mixture. The infiltration occurred spontaneously at 1473K for both [Ti+BN] and [Ta+BN] powder mixtures. Owing to reactions between the starting materials, both boride and nitride ceramics were produced in molten aluminium. TiB2 and AlN were produced from the [Ti+BN] powder mixture, and TaB2 and AlN were produced from the [Ta+BN] powder mixture. When the [Ti+BN] powder mixture was used, a reaction producing Al3Ti took place immediately after the infiltration of the molten aluminium, and a subsequent reaction producing TiB2 and AlN proceeded gradually. The time required to convert BN flakes to TiB2 and AlN particles at 1473K was in the range of 1800–3600 s. On the other hand, when the [Ta+BN] powder mixture was used, there was an initial incubation period to allow the tantalum and molten aluminium to react with each other. The reaction between tantalum, BN and aluminium took place after this incubation period.

Fabrication of intermetallic compound matrix composite by spontaneous infiltration and subsequent in situ reaction processes

Abstract Titanium aluminide intermetallic compound matrix composites are in-situ synthesized by the spontaneous infiltration of liquid alumium into the powder mixture of titanium and Al2O3 particles.

Synthesis of Al2O3 matrix composites by reactive infiltration

Reactive infiltration of a NiO-base blended powder with molten aluminium was attempted at 1673 K in order to obtain Al2O3 matrix composites containing a dispersion of Al3Ni, AlNi and/or AlNi3. The NiO powder was barely infiltrated by the molten aluminium after a 3600 s holding time at 1673 K. A continuous layer of Al2O3 was observed to exist at the infiltration front, which prevented any further infiltration. TiB2 particles were added to the NiO powder in order to absorb the heat of reaction between NiO and aluminium. When the TiB2 particle content in the [NiO+TiB2] powder blend was greater than 20 vol%, spontaneous infiltration occurred completely. Thus, it was shown that the addition of the TiB2 particles assisted in the spontaneous infiltration. The specimens produced by the in situ reaction consisted of Al2O3, TiB2 and Al3Ni. Al3Ni was mainly located between the TiB2 and Al2O3. The effect of the TiB2 addition on the infiltration kinetics was to decrease the maximum attainable temperature caused by the exothermic reaction. This in turn prevented the formation of a continuous Al2O3 film at the infiltration front. This resulted in the production of pathways for the infiltration of the molten aluminium and made possible the complete infiltration.

Microstructure refinement of Al-Si alloy using compressive torsion processing

A Compressive Torsion Processing (CTP) is a unique severe plastic deformation process which can easily apply very large strain without shape change to a work piece. Hypereutectic Al-Si alloys have good properties such as low thermal expansion and high wear resistance. It is important for the alloys to control the size of second phase particles (primary and eutectic silicon, intermetallic compounds) as well as grain size of aluminum matrix. In the present work, the CTP was applied to hypereutectic Al-Si alloy (AA390) to investigate the possibility of microstructure refinement of the alloy and the mechanical property of processed alloy was also investigated by tensile test.