Materials Science Forum Vols. 534-536

Abstract: A novel variant of the three-dimensional printing technique has been developed and used to manufacture NiTi parts. Instead of metal powder the process uses granules, which consist of a mixture of metal powder and organic binder. These granules are spread on a working table. Then a solvent is dispensed with a printer head to consolidate a selected area of the granule bed and the table is moved down. The "solvent on granule" printing process is repeated until a threedimensional green body is obtained. The green part is finally debinded and sintered to obtain a dense and fully metallic part. NiTi parts have been successfully produced by this technique with densities of about 95% of the theoretical density. Detailed information on the microstructure has been obtained by X ray diffraction. Sintered parts exhibit shape memory effect, which has been measured during thermal cycling under tensile stress.

Abstract: The error function can be calculated based on the Simpson method through a subroutine program. An integration program by FORTRAN language was made for diffusion equations of extended source with infinite extent and limited extent. The distribution curves of alloying elements could be driven from calculated results written in ASCII code. The results on some alloying elements such as C, Co, Cr, Mn, Mo, Ni and V’s diffusion in iron, showed the diffusion distance for Ni and Mo can only be 1~3 μm and more distance for Co at common sintering temperature of 1120°C. Increasing the sintering temperature up to 1300°C, distribution curve could be improved. To refine the particle size of the added elements down to a scale of micrometers is an effective way to get homogeneous distribution.

Abstract: Calcium-hexaluminate phase(CA6) phase is known to be effective for the crack shielding due to the spinel block crystal structure. In this study, we focused to control the CA6 morphology for the good damage tolerance behavior in alumina and zirconia/calcium-hexaluminate(CA6) composites. calcium-hexaluminate(CA6) composites were prepared from zirconia, alumina and calcium carbornate powders. Calcium-hexaluminate(CA6) phase was obtained by the solid reaction through the formation of intermediate phase(CA2). CA6 phase showed column type abnormal grain grown behavior composed of small blocks. Due to the typical microstructure of CA6, alumina and zirconia/calcium-hexaluminate composites provide well controlled crack propagation behavior.

Abstract: In the present work, the sintering behavior of high strength Al-5.6Zn-2.5Mg-1.6Cu (in wt.%) alloy compacts was investigated. Green parts made by pressing of mixed elemental powders were sintered at different temperatures between 400 and 600 °C. The effect of the sintering temperature on density, dimensional change and microstructure of the Al alloy was studied. Microstructural evaluation was accompanied by XRD and DSC methods in order to determine the temperature and chemical composition of the liquid phases formed during sintering. It was found that three transient liquid phases are formed at 420, 439 and 450 °C. Microstructural study revealed the progressive formation of sintered contacts due to the presence of the liquid phases, although the green compact expands as a result of the melt penetration along the grain boundaries. While Zn melts at ~420 °C, the intermetallic phases between Al and Mg were found to be responsible for the formation of liquid phase and the dimensional change at higher temperatures.

Abstract: The decrease of the distance between particle centers due to the growth of the sinter necks can be explained by the well known two-particle model. Unfortunately this model fails to provide a comprehensive description of the processes for 3D specimens. Furthermore, there is a significant discrepancy between the calculated and the measured shrinkage because particle rearrangements are not considered. Only the recently developed analysis of the particle movements inside of 3D specimens using micro focus computed tomography (μCT), combined with photogrammetric image analysis, can deliver the necessary experimental data to improve existing sintering theories. In this work, μCT analysis was applied to spherical copper powders. Based on photogrammetric image analysis, it is possible to determine the positions of all particle centers for tracking the particles over the entire sintering process and to follow the formation and breaking of the particle bonds. In this paper, we present an in-depth analysis of the obtained data. In the future, high resolution synchrotron radiation tomography will be utilized to obtain in-situ data and images of higher resolution.

Abstract: In the present work it has been studied the corrosion performance of a powder metallurgical aluminum alloy in aeronautical environments as a function of heat treatment. For this purpose an Al-Cu-Mg prealloyed powder was uniaxially pressed at 600 MPa followed by sintering at 590°C in nitrogen for 60 minutes. Subsequently sintered samples were heat treated to the T4 and T6 state. Corrosion behaviour was assessed by means of potentiodynamic polarization (PPT) in Dilute Harrison solution (DHS), which is considered to closely emulate the atmospheric environment for aircraft. PPT results for the equivalent wrought counterpart, AA2024 in its typical heat treatment for aeronautical applications T3, are also presented for comparison. The microstructure of each sample has been examined by Scanning Electron Microscopy (SEM) and Energy-Dispersive analysis of X-ray (EDX). Similar corrosion performance was observed for both the as sintered sample and its equivalent wrought counterpart, while corrosion resistance of the PM materials was improved by the heat treatment, especially in the T4 state.

Abstract: Bulk metallic glass (BMG) composites combining a Cu54Ni6Zr22Ti18 matrix with brass powders or Zr62Al8Ni13Cu17 metallic glass powders were fabricated by spark plasma sintering. The brass powders and Zr-based metallic glass powders added for the enhancement of plasticity are well distributed homogeneously in the Cu-based metallic glass matrix after consolidation. The matrix of the BMG composite remains as a fully amorphous structure after spark plasma sintering. The BMG composites show macroscopic plasticity after yielding, and the plastic strain increased to around 2% without a decrease in strength for the composite material containing 20 vol% Zr-based amorphous powders. The proper combination of strength and plasticity in the BMG composites was obtained by introducing a second phase in the metallic glass matrix.

Abstract: Densification behavior of nano-agglomerate powder during pressureless sintering of Fe-Ni nanopowder was investigated in terms of diffusion kinetics and microstructural development. To understand the role of agglomerate boundary for sintering process, densification kinetics of Fe-Ni nano-agglomerate powder with different agglomerate size was investigated. It was found that activation energy for densification process was lower in the small-sized agglomerate powder. The increase in the volume fraction of inter-agglomerate boundary acting as high diffusion path might be responsible for the enhanced diffusion process.

Abstract: The SPS process is a synthetic technique which enables sinter-bonding to occur at low temperature and within a short sintering time. Furthermore, it produces finer grain size than conventional sintering methods. Fe-TiC system is a relatively light material and is one third the size of tungsten carbide and less than half weight of tool steel. It is cost effective when used in Fe-TiC system due to high mass effect with low density and relatively low cost of changing tools and bearings. Fe-TiC system can be machined easily with conventional equipment. In the present work, SPS was conducted at the temperatures of 1200, 1250 and 1300°C at the sintering time of 3, 5 and 10min. As the sintering temperature and holding time increased, the relative density decreased and the hardness reached the lowest at 1250°C. These are attributed to the more pore being generated by liquid Pool at 1250°C and 1300°C. The result of the microstracture observation showed more M6C and MC carbide by liquid Pool as sintering temperature and holding time increased.

Abstract: The effect of the methods for preparing powder blend by conventional milling (Me+Al particles), attriting (Me/Al/ Me/Al composite particles), and plating of Me by Al (Me/Al composite particles) on the structure, internal stress level, and compactability of the powder blends as well as the structure and phase composition of the MeAl compacts was investigated. The Me+Al→MeAl exothermic reaction of these powders occurs at T≥650°C. The reaction sintering (RS) or hightemperature self-propagation synthesis (HTSPS) occurs through the formation of Al melt (liquidphase reaction) and lower-melting MeAl3, Me2Al3, Me3Al aluminides. An increase in the level of internal stresses (IS) upon attritting activates RS at lower temperatures and decreases the value of high-temperature exoeffect. This suppresses the HTSPS development. A large high-temperature exoeffect ensures the intensity and completeness of the reaction interaction, and the application of pressure upon RS or HTSPS provides a high, near-theoretical density of the compacted material.