Papers by Keyword: Densification Mechanism

Abstract: In this paper, obtaining by PM of aluminum based materials, characterization of them and Finite Element Analysis (FEA) of compaction were investigated. Sintered aluminum alloys (Al-Cu and Al-Mg-Si) were tested from physical and mechanical point of view and the obtained experimental results were compared with those of sintered bronze powder materials. We studied the compressibility and densification mechanism of Al-Cu mixed powders and for prediction of compaction behavior we used FEA. The data was obtained on the stress distribution in the compacted material and on the deformations occurring throughout the mixed metal powder of the compacted samples. The results of FEA were compared with those obtained experimentally

Abstract: Alumina ceramics with good mechanical and corrosion resistance are the ones of the most widely used engineering ceramics. The aluminum has high strength, high conductivity and high plasticity etc. so that aluminum ceramics are used in more and more industries. In this paper, the mass fraction of 25% Al₂O₃ powder and the mass fraction of 75% Al powder were mixed in the blender. Mixer speed is100r/min with mixing time of 3.5 h. Forming, sintering and a series of processes for preparing the alumina/aluminum metallic ceramic materials, through performance testing and analysis, found that the density of the sample firstly increased and then decreased with the increase of sintering temperature. A melting point is close to the sintering temperature and the density of the cermet can be made relatively high. When the sintering temperature is about 600°C and 700°C, the macro performance of sample is better. The cermet is sintered at 700°C and its microstructure is relatively better.

Abstract: The densification kinetics of copper powder during sintering has been investigated using scanning electron microscopy analysis and density measurements. A constitutive model was proposed to predict the densification kinetics as a function of sintering condition by considering the densification parameter to obtain more accurately predicted results. The activation energy for copper densification kinetics was calculated from experimental data and compared with activation energies associated with different densification mechanisms. We found that the lattice diffusion mechanism acted as the primary densification mechanism for copper powder during sintering.

Abstract: Alumina ceramics with good mechanical and corrosion resistance, is one of the most widely used engineering ceramics. The aluminum has a high strength, high conductivity, high plasticity, etc. than aluminum ceramics used in more and more industries. In this paper, aluminum and alumina powder as raw material, mixing, forming, sintering and a series of processes for preparing the alumina/aluminum metallic ceramic materials, through performance testing and analysis can be found in the ratio of raw materials 50wt% Al, 50wt % Al₂O₃ relatively good moldability. After sintering, after measuring the density contrast is found better density in the pressing process pressure of 20MPa and holding pressure time for 20min. By comparing the sintering process, after the interface structure by scanning electron microscopy and found help improve density through the secondary sintering metallic ceramic materials.

Abstract: Uranium dioxide powders were surface pre-oxidation treated. Phases and properties of the powders after pre-oxidation treatment were investigated by DSC-TG、XRD、BET and Dilatometer. The results show that there is a little of U₃O₇ in uranium dioxide powders by pre-oxidation treatment at 240°C for 8h in air, UO₂ powders will transform into U₃O₈ at 382°C for 8h in air. The shrinkage temperature of the uranium dioxide pellets after pre-oxidation treatment decreased from 1200°C to 580°C, densification rate (ΔL/L) increased also from 1.52×10^-4/K to 3.08×10^-4/K. The mechanism of low temperature sintering to pre-oxidation UO_2+x pellets was explained by simplified point defect model and densification equation. The UO_2+x diffusion coefficient, , is much higher than which of UO₂. The densification equation is expressed by , and the factor A is according to , a=16.23658, b=0.04247 and c=-2.18802×10^-5.

Abstract: Movement behavior of AISI 316L stainless steel powder under hot isostatic pressing (HIP) were investigated using finite element analysis (FEA). The analysis, which was based on the porous metal yield criterion, was carried out in the FEA program. Density distributions, deformations and displacements of compact were discussed. The evolution of displacement for some typical positions in the compact was also studied in this paper. The calculation results show that thermal expansion plays an important role at the early stage of HIP. There are large displacements for powder compact during the ramp stage and the early holding stage in the HIP cycle. Correspondingly, the improvement of densification increases significantly. Simulation results for the shape change and average density of a sample were also compared with experiment.

Abstract: Warm compacting behaviors of pure titanium powders were studied. The results show that warm compaction can be applied to titanium powders. The green densities obtained through warm compaction are generally higher than obtained through cold compaction at the same pressure. The optimal warm compacting temperature is about 140 . At the compacting pressure of 500 Mpa, the ejection force of titanium powders through warm compaction is 32.4% lower than through cold compaction. At the same pressure, the effective compression force through warm compaction is bigger than one through cold compaction. In addition, the densification mechanism of warm compaction was discussed.

Abstract: The densification mechanism during the park-plasma-sintering (SPS) processing was examined in high purity MgAl2O4 spinel. As the density ρt increases, that is, as the effective stress σeff decreases, stress exponent n evaluated from σeff dependence of densification rate varies from n ≥ 4 in the low ρt region, n ≈ 2 in the intermediate ρt region to n ≈ 1 in the high ρt region. TEM observation shows that significant stacking faults caused by partial dislocations are observed in the low ρt region, but limited in the high ρt region. The ρt dependent densification behavior and microstructure suggest that the predominant densification mechanism during the SPS processing changes with ρt from plastic flow by partial dislocation motion for the low ρt region (n ≥ 4) to diffusion-related creep for the high ρt region (n ≈ 1).

Abstract: The densification mechanism in park-plasma-sintering (SPS) processing was examined in MgAl2O4 spinel. As the relative density ρt increases, that is, as the effective stress σeff decreases, stress exponent n evaluated from effective stress-densification rate relationship continuously varies from n  4 to n  1. TEM observation shows that significant stacking faults caused by partial dislocations are frequently observed in the low ρt region. The results suggest that, for spinel, the predominant densification mechanism in SPS processing changes with ρt from plastic flow by a partial dislocation motion in the low ρt region (n  4) to diffusion-related creep in the high ρt region (n  1).

Abstract: Warm compaction is a low cost process to make high density and high performance iron base powder metallurgy parts. Based on results obtained from the dynamic compacting curve, ejection force curve, X-ray diffraction, micro-hardness of iron powder, friction condition and lubricant properties, densification mechanism of warm compaction can be drawn. In the initial stage, the rearrangement of powder particles is the main factor. It contributes more in the densification of warm compaction than that in cold compaction. However, in the later stage, the plastic deformation of powder particles is the primary factor. The increase in plasticity at high temperature can harmonize the secondary rearrangement of powder particles. During the compaction, the polymer lubricant has great contribution to the densification of the powder, since it improves the lubricating condition and effectively decreases the friction in the forming process and thus enhances the compact density. The dynamic compacting curve of warm compaction can be divided into three phases. The first is the particle rearrangement dominant phase; the percentage of particle rearrangement in warm compaction is higher than that in cold compaction by 15-31%. The second is the elastic deformation and plastic deformation dominant phase. The third is the plastic deformation dominant phase. The study of the powder densification mechanism can direct engineers in designing and producing warm compaction powders for high density parts.