Hideaki Kuramoto

Observation of particle behavior in copper powder compact during pulsed electric discharge

Abstract The behavior of spherical copper powder particles of uniform size (average diameter: 550 μm) in a powder compact was observed under an optical microscope during a single-pulse electric discharge of 500 ms duration. The morphologies of necks formed between powder particles were observed under a scanning electron microscope, and their diameters were measured. The results obtained are summarized as follows: pressure and pulsed current density determine whether or not a spark occurs. Spark is more likely to occur at interparticle contacts under low pressure and high current density. Where a spark occurs, particles are joined together by melting. Regardless of whether or not a spark is observed, necks are formed at points of contact between particles and neck diameter increases with pulsed current density. These results suggest that microscopic sparking, melting, and vaporization occur by means of extremely high temperature attained by local heat generation at the interparticle contacts in the initial stage of compaction.

A case study for production of perfectly sintered complex compacts in rapid consolidation by spark sintering

Abstract The distribution of voltage, temperature and titanium-compact density in the spark sintering was investigated for clarification of phenomena in the sintering stage governed by plastic deformation mechanism. The largest heat source in a punch–die-two step cylindrical compact system was in the punch with a smaller sectional area. The heat flow was mainly from the smaller punch to the compact. The sintering was mainly promoted by the mechanisms of plastic deformation and power law creep during the continuous current discharge. The effect of the plastic deformation mechanism on the final relative density of compacts was large in the press method and flanged compact used in this study. A procedure, particularly the press method, was proposed on the basis of this result for the production of homogeneously perfect sintered products. This proposed procedure could produce perfectly sintered complex compacts with their designed final dimension.

Melting and solidification of TiNi alloys by cold crucible levitation method and evaluation of their characteristics

Abstract The addition of Re, Fe and Cr into Ti–50 mol.-%Ni has been carried out to improve the oxidation and mechanical properties. The mono phase consisting of TiNi with the B2 type structure was identified in micro-alloyed materials proposed on the basis of the d-electrons concept. Experimentally, TiNi alloys were melted and solidified by the cold crucible levitation melting (CCLM) method. The TiNi–(Cr, Fe, Re) alloys with high purity and without contamination from a crucible were prepared, and the homogeneous microstructure was achieved by the diffusion mixing effect of CCLM even in the as-cast alloys which contained Re and Cr with higher melting temperatures and different specific gravities. The transformation from austenite to martensite phases occurred in all alloys below or above room temperature. Some alloys had the ability of shape memory even at room temperature. Ternary alloys showed a higher flow stress level compared with the binary TiNi alloy. On the other hand, the oxidation at 1273 K was promoted by the formation of titanium oxides (TiO2) on the alloy surfaces. The oxidation resistance was improved by the formation of the continuous Cr2O3 film in TiNi–Cr alloys. The alloying effects by ternary elements (Re, Fe, Cr) in the intermetallic TiNi as well as metallic materials were explained well using two parameters used in the d-electrons concept.