Investigation of the Microstructure and Nanoindentation of Processed Ti6Al4V-5ZrO<sub>2</sub>-xSi<sub>3</sub>N<sub>4</sub> Ternary Composites Using Powder Metallurgy

Frequency effect on pulse electric current sintering process of pure aluminum powder

Pure Al2O3 powder was sintered by a pulsed electric current sintering (PECS) process with different pulse current waveforms from two types of power generators: an inverter (frequency 16 kHz) type and a pulsed direct current (DC) (frequency 300 Hz) type. The sample temperature during the PECS process as well as the sintering behavior was investigated. The effects of pulse current waveform on the sintering process were discussed. For the same die temperature ranging from 1100 to 1400°C at holding period, the sample temperature under experiments using the inverter was higher than that under experiments using the pulsed DC. Densification and grain growth were predominated by sample temperature and did not depend on the pulse current waveform directly.

Al-1.0 mass% Mg alloy powders were sintered using the pulse electric current sintering (PECS) process at various temperatures. The microstructure at the interfaces between powder particles and the effect of sintering temperature on interface characteristics were investigated using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The precipitates were observed at the interfaces between powder particles of the compacts. The amounts of the precipitates increased and the compositions changed with an increase in sintering temperature: MgO for the compact sintered at 613 K, MgAl2O4+MgO for those at 663 K and 713 K, and MgAl2O4 for those above 763 K. Comparing the results obtained by the PECS process with those of diffusion bonding experiments and thermodynamic calculation, it was suggested that the temperature at the interfaces between the particles was higher than that of the particles sintered by the PECS process.

A porous structure with a porosity gradient can be applied to the preparation of continuous FGM, where liquid or chemical vapor of the second phase is infiltrated into the graded pores. It also has applications in skeletal implant materials and ultrafiltration media. An attempt was made to fabricate a porous material with a porosity gradient by means of a pulsed electric current sintering (PECS) process. The present work describes not only the measured value of the temperature difference between the upper and lower part of the specimen, which brings about a gradual change in pore distribution, but also the sintering characteristics of the porous structure obtained by the pressureless PECS process.

Sintering behavior of metallic and ceramic powders during pulsed electric-current sintering process

The reduction mechanism of particle surface oxide films on Al–Mg alloy specimens sintered by the pulse electric current sintering (PECS) process was investigated via transmission electron microscopy, energy dispersive x-ray spectroscopy, and thermodynamic calculation. The reduction products were either MgAl2O4or MgO or both, which is dependent on the sintering temperature and Mg content in Al–Mg alloy. Comparing the experimental temperature of the reduction products with that from thermodynamic calculation, it was suggested that the temperature at interfaces between particles was higher than that inside particles. This difference of temperature enhanced reduction of surface oxide films of Al–Mg alloy powders and hence accelerated the sintering in the PECS process.

In the present work, FeMn13-40 wt.% TiC composites was fabricated by pulsed electric current sintering (PECS) process at different temperatures between 990 and 1020°C under a pressure of 60 MPa with a holding time of 5 min in the vacuum. Phase identification was done using the X-ray diffraction. The relative density, the microstructure and the hardness of the samples were characterized. The results showed that the relative density of FeMn13-TiC composites increased with the increase of sintering temperature. The lowest porosity (3.84%) and the highest hardness (70.54 HRC) of the sample were achieved by PECS process, namely sintering at the temperature of 1020°C under the applied pressure of 60 MPa for 5 min.

Utilization of deoxidization mechanism of magnesium (Mg) is an effective method to remove the oxide films at aluminum (Al) alloy powder surface in pulse electric-current sintering (PECS) process. The continuous amorphous oxide film at Al alloy surface are broken and removed by deoxidization of Mg. Crystalline particles of MgAl2O4 or MgO, or both of them, are formed, which depend on Mg content in Al alloy powder and sintering temperature. After that the metal/metal contact is caused, and solid state sintering of Al alloy powder is facilitated. The electrical resistivity and tensile properties of powder compacts are improved by Mg addition. Based on the analyses of electrical resistivity, tensile properties and microstructures of the sintered specimens, optimum amount of Mg addition to improve the sintering properties of Al powder is determined to be 0.3–2.5 mass%.

The aluminium oxide crystal, Al2O3, which contains a small amount of chromium, Cr, is called ruby. Pulsed electric current sintering (PECS) was applied to sinter ruby polycrystals. Cr2O3-Al2O3 powder mixture prepared by drying an aqueous slurry containing amounts of Al2O3 and Cr(NO3)3 was consolidated by PECS process. The PECS process was performed in vacuum at sintering temperature raging from 1100 to 1300°C with heating rate of 2 K/min under applied uniaxial pressure varied from 40 to 100 MPa. This study found that highly densified and transparent Cr-doped Al2O3 can be obtained by the PECS process with the high applied pressure at sintering temperature of 1200°C.

Metallic powders with various thermodynamic stability oxide films (Ag, Cu, and Al powders) were sintered using a pulse electric-current sintering (PECS) process. Behavior of oxide films at powder surfaces and their effect on the sintering properties were investigated. The results showed that the sintering properties of metallic powders in the PECS process were subject to the thermodynamic stability of oxide films at particles surfaces. The oxide films at Ag powder surfaces are decomposed during sintering with the contact region between the particles being metal/metal bond. The oxide films at Cu powder surfaces are mainly broken via loading pressure at a low sintering temperature. At a high sintering temperature, they are mainly dissolved in the parent metal, and the contact regions turn into the direct metal/metal bonding. Excellent sintering properties can be received. The oxide films at Al powder surfaces are very stable, and cannot be decomposed and dissolved, but broken by plastic deformation of particles under loading pressure at experimental temperatures. The interface between particles is partially bonded via the direct metal/metal bonding making it difficult to achieve good sintered properties.

Abstract

Heterogeneity of grain refinement and texture formation during pulsed electric current sintering of conductive powder: A case study in copper powder

Sintering of Cu and thermoelectric Ca3Co4O9 was tried using a modified pulsed electric current sintering (PECS) process, where an electrically nonconductive die was used instead of a conventional graphite die. The pulsed electric current flowed through graphite punches and sample powder, which caused the Joule heating of the powder compact itself, resulting in sintering under smaller power consumption. Especially for the Ca3Co4O9 powder, densification during sintering was also accelerated by this modified PECS process.

It was investigated that <TEX>$Al_2$</TEX><TEX>$O_3$</TEX>/Cu nanocomposite powder could be optimally prepared by dispersion and reduction of Cu oxide, and suitably consolidated by employing pulse electric current sintering (PECS) process. <TEX>$\alpha$</TEX>-<TEX>$Al_2$</TEX><TEX>$O_3$</TEX> and CuO powders were used as elemental powders. In order to obtain <TEX>$Al_2$</TEX>O<TEX>$_3$</TEX> embedded by finely and homogeneously dispersed CuO particles, the elemental powders were high energy ball milled at the rotating speed of 900 rpm, with the milling time varying up to 10 h. The milled powders were heat treated at <TEX>$350^{\circ}C$</TEX> in H<TEX>$_2$</TEX> atmosphere for 30 min to reduce CuO into Cu. The reduced powders were subsequently sintered by employing PECS process. The composites sintered at <TEX>$1250^{\circ}C$</TEX> for 5 min showed the relative density of above 98%. The fracture toughness of the <TEX>$Al_2$</TEX><TEX>$O_3$</TEX>/Cu nanocomposite was as high as 4.9MPa.<TEX>$m^{1}$</TEX>2//, being 1.3 times the value of pure <TEX>$Al_2$</TEX><TEX>$O_3$</TEX> sintered under the same condition.

ABSTRACTThis research shows the development of alternative Cu-based materials for applications where enhanced thermal properties are desired. Cu/AlN composites were fabricated from mixtures of pure Cu and copper plated AlN-Cu composite powders. The ceramic phase was added in amounts of 10, 20 and 30 vol.% and the mixtures sintered by pulsed electric current sintering process (PECS). The results showed that the AlN particles are homogeneously distributed in the copper matrix and that the true contacts between hard particles are reduced because of the deposited copper on their surfaces, improving the connectivity of the matrix phase and bonding at the metal-ceramic interface. The relative density of the Cu/AlN composites was major than 97% in all cases. Thermal conductivity of the composites was high and decreased with the ceramic content from 359 to 194 W/mK, for 10 and 30% AlN, respectively. The coefficient of thermal expansion followed a lineal behavior with temperature and is also reduced with the ceramic content.