Powder Forging of Rapidly Solidified hi-Si Alloy with Back Pressure

Kohno, T.,Kawase, K.,Otsuki, M.,Morimoto, K. The Korean Powder Metallurgy Institute 1998 한국분말재료학회지 (KPMI) Vol.5 No.4

Powder forging with a back pressure was investigated for production of automobile and compressor parts made of a rapidly solidified Al-Si alloy powder. Disk-shaped green compacts made of a rapidly solidified Al-Si alloy powder were hot forged, and hubs were formed by loading back pressure on their top. The influences of the back pressure and die temperatures on forgeabiliy and properties of parts made of a rapidly solidified Al-Si alloy powder were examined. This method was also applied to the production of a scroll part. The results of these studies are summarized as follows : 1. A back pressure on the hub top is very effective for consolidation and preventing crack formation in the hub. 2. When a back pressure tess than 98 MPa is applied, the forging pressure increases by the same amount of the applied back pressure. With more than 98 MPa, the forging pressure increases further due to an increased friction at the hub side. 3. Die temperatures higher than approximately 670k are needed in order to consolidate well the hub top without cracks.

Powder Metallurgy for Light Weight and Ultra-Light Weight Materials

Kieback, B.,Stephani, G.,Weiβgarber, T.,Schubert, T.,Waag, U.,Bohm, A.,Anderson, O.,Gohler, H.,Reinfried, M. The Korean Powder Metallurgy Institute 2003 한국분말재료학회지 (KPMI) Vol.10 No.6

As in other areas of materials technology, the tendency towards light weight constructions becomes more and more important also for powder metallurgy. The development is mainly driven by the automotive industry looking for mass reduction of vehicles as a major factor for fuel economy. Powder metallurgy has to offer a number of interesting areas including the development of sintered materials of light metals. PM aluminium alloys with improved properties are on the way to replace ferrous pars. For high temperature applications in the engine, titanium aluminide based materials offer a great potential, e.g. for exhaust valves. The PM route using elemental powders and reactions sintering is considered to be a cost effective way for net shape parts production. Furthermore it is expected that lower costs for titanium raw materials coming from metallurgical activities will offer new chances for sintered parts with titanium alloys. The field of cellular metals expands with the hollow sphere technique, that can provide materials of many metals and alloys with a great flexibility in structure modifications. These structures are expected to be used in improving the safety (crash absoption) and noise reduction in cars in the near future and offer great potential for many other applications.

Representative Volume Element Analysis of Fluid-Structure Interaction Effect on Graphite Powder Based Active Material for Lithium-Ion Batteries

Yun, Jin Chul,Park, Seong Jin The Korean Powder Metallurgy Institute 2017 한국분말재료학회지 (KPMI) Vol.24 No.1

In this study, a finite element analysis approach is proposed to predict the fluid-structure interaction behavior of active materials for lithium-ion batteries (LIBs), which are mainly composed of graphite powder. The porous matrix of graphite powder saturated with fluid electrolyte is considered a representative volume element (RVE) model. Three different RVE models are proposed to consider the uncertainty of the powder shape and the porosity. P-wave modulus from RVE solutions are analyzed based on the microstructure and the interaction between the fluid and the graphite powder matrix. From the results, it is found that the large surface area of the active material results in low mechanical properties of LIB, which leads to poor structural durability when subjected to dynamic loads. The results obtained in this study provide useful information for predicting the mechanical safety of a battery pack.

Effect of Passivation on the Sintering Behavior of Submicron Nickel Powder Compacts for MLCC Application

Jo, Gi-Young,Lee, Kwi-Jong,Kang, Suk-Joong L. The Korean Powder Metallurgy Institute 2013 한국분말재료학회지 (KPMI) Vol.20 No.6

During sintering of Ni-electrode multi-layer ceramic capacitors (MLCCs), the Ni electrode often becomes discontinuous because of its lower sintering temperature relative to that of $BaTiO_3$. In an attempt to retard the sintering of Ni, we introduced passivation of the Ni powder. To find the optimal passivation conditions, a thermogravimetric analysis (TGA) was conducted in air. After passivation at $250^{\circ}C$ for 11 h in air, a nickel oxide shell with a thickness of 4-5 nm was formed on nickel nanoparticles of 180 nm size. As anticipated, densification of the compacts of the passivated Ni/NiO core-shell powder was retarded: the starting temperature of densification increased from ${\sim}400^{\circ}C$ to ${\sim}600^{\circ}C$ in a $97N_2-3H_2$ (vol %) atmosphere. Grain growth was also retarded during sintering at temperatures of 750 and $1000^{\circ}C$. When the sintering atmosphere was changed from wet $99.93N_2-0.07H_2$ to wet $99.98N_2-0.02H_2$, the average grain size decreased at the same sintering temperature. The conductivity of the passivated powder sample sintered at $1150^{\circ}C$ for 8 h in wet $99.93N_2-0.07H_2$ was measured to be $3.9{\times}10^4S/cm$, which is comparable with that, $4.6{\times}10^4S/cm$, of the Ni powder compact without passivation. These results demonstrate that passivation of Ni is a viable means of retarding sintering of a Ni electrode and hence improving its continuity in the fabrication of $BaTiO_3$-based multi-layer ceramic capacitors.

High Strength Nanostructured Metastable Alloys

Eckert, Jurgen,Bartusch, Birgit,Schurack, Frank,He, Guo,Schultz, Ludwig The Korean Powder Metallurgy Institute 2002 한국분말재료학회지 (KPMI) Vol.9 No.6

Nanostructured high strength metastable Al-, Mg- and Ti-based alloys containing different amorphous, quasicrystalline and nanocrystalline phases are synthesized by non-equilibrium processing techniques. Such alloys can be prepared by quenching from the melt or by powder metallurgy techniques. This paper focuses on one hand on mechanically alloyed and ball milled powders containing different volume fractions of amorphous or nano-(quasi)crystalline phases, consolidated bulk specimens and, on the other hand. on cast specimens containing different constituent phases with different length-scale. As one example. $Mg_{55}Y_{15}Cu_{30}$- based metallic glass matrix composites are produced by mechanical alloying of elemental powder mixtures containing up to 30 vol.% $Y_2O_3$ particles. The comparison with the particle-free metallic glass reveals that the nanosized second phase oxide particles do not significantly affect the glass-forming ability upon mechanical alloying despite some limited particle dissolution. A supercooled liquid region with an extension of about 50 K can be maintained in the presence of the oxides. The distinct viscosity decrease in the supercooled liquid regime allows to consolidate the powders into bulk samples by uniaxial hot pressing. The $Y_2O_3$ additions increase the mechanical strength of the composites compared to the $Mg_{55}Y_{15}Cu_{30}$ metallic glass. The second example deals with Al-Mn-Ce and Al-Cu-Fe composites with quasicrystalline particles as reinforcements, which are prepared by quenching from the melt and by powder metallurgy. $Al_{98-x}Mn_xCe_2$ (x =5,6,7) melt-spun ribbons containing a major quasicrystalline phase coexisting with an Al-matrix on a nanometer scale are pulverized by ball milling. The powders are consolidated by hot extrusion. Grain growth during consolidation causes the formation of a micrometer-scale microstructure. Mechanical alloying of $Al_{63}Cu_{25}Fe_{12}$ leads to single-phase quasicrystalline powders. which are blended with different volume fractions of pure Al-powder and hot extruded forming $Al_{100-x}$$(Al_{0.63}Cu_{0.25}Fe_{0.12})_x$ (x = 40,50,60,80) micrometer-scale composites. Compression test data reveal a high yield strength of ${\sigma}_y{\geq}$700 MPa and a ductility of ${\varepsilon}_{pl}{\geq}$5% for than the Al-Mn-Ce bulk samples. The strength level of the Al-Cu-Fe alloys is ${\sigma}_y{\leq}$550 MPa significantly lower. By the addition of different amounts of aluminum, the mechanical properties can be tuned to a wide range. Finally, a bulk metallic glass-forming Ti-Cu-Ni-Sn alloy with in situ formed composite microstructure prepared by both centrifugal and injection casting presents more than 6% plastic strain under compressive stress at room temperature. The in situ formed composite contains dendritic hcp Ti solid solution precipitates and a few $Ti_3Sn,\;{\beta}$-(Cu, Sn) grains dispersed in a glassy matrix. The composite micro- structure can avoid the development of the highly localized shear bands typical for the room temperature defor-mation of monolithic glasses. Instead, widely developed shear bands with evident protuberance are observed. resulting in significant yielding and homogeneous plastic deformation over the entire sample.

Effect of Mechanical Alloying on Combustion Densification of MoSi$_2$

Park, Hyung-Sang,Park, Jin-Seong,Ka, Mi-da,Shin, Kwang-Seon,Kim, Yong-Seong The Korean Powder Metallurgy Institute 1998 한국분말재료학회지 (KPMI) Vol.5 No.4

The effect of the mechanical alloying of elemental Mo and Si powders on the combustion densification behavior of MoSi$_2$ was investigated. The ignition temperature of the combustion reaction of the mechanically alloyed powder was measured to be significantly lower than that of the powder mixture prepared by the low energy ball milling process. The densification of the products after the combustion reaction under compressive pressure from the mechanically alloyed powders, however, was found to be poorer than that of the products from the ball milled powder.

Development of Carbon Nanotube-copper Hybrid Powder as Conductive Additive

Lee, Minjae,Ha, Seoungjun,Lee, Yeonjoo,Jang, Haneul,Choi, Hyunjoo The Korean Powder Metallurgy Institute 2018 한국분말재료학회지 (KPMI) Vol.25 No.4

A conductive additive is prepared by dispersing multi-walled carbon nanotubes (MWCNTs) on Cu powder by mechanical milling and is distributed in epoxy to enhance its electrical conductivity. During milling, the MWCNTs are dispersed and partially embedded on the surface of the Cu powder to provide electrically conductive pathways within the epoxy-based composite. The degree of dispersion of the MWCNTs is controlled by varying the milling medium and the milling time. The MWCNTs are found to be more homogeneously dispersed when solvents (particularly, non-polar solvent, i.e., NMP) are used. MWCNTs gradually disperse on the surface of Cu powder because of the plastic deformation of the ductile Cu powder. However, long-time milling is found to destroy the molecular structure of MWCNTs, instead of effectively dispersing the MWCNTs more uniformly. Thus, the epoxy composite film fabricated in this study exhibits a higher electrical conductivity than 1.1 S/cm.

Microstructure and Soft Magnetic Properties of Fe-6.5 wt.%Si Sheets Fabricated by Powder Hot Rolling

Kim, Myung Shin,Kwon, Do Hun,Hong, Won Sik,Kim, Hwi Jun The Korean Powder Metallurgy Institute 2017 한국분말재료학회지 (KPMI) Vol.24 No.2

Fe-6.5 wt.% Si alloys are widely known to have excellent soft magnetic properties such as high magnetic flux density, low coercivity, and low core loss at high frequency. In this work, disc-shaped preforms are prepared by spark plasma sintering at 1223 K after inert gas atomization of Fe-6.5 wt.% Si powders. Fe-6.5 wt.% Si sheets are rolled by a powder hot-rolling process without cracking, and their microstructure and soft magnetic properties are investigated. The microstructure and magnetic properties (saturation magnetization and core loss) of the hot-rolled Fe-6.5 wt.% Si sheets are examined by scanning electron microscopy, electron backscatter diffraction, vibration sample magnetometry, and AC B-H analysis. The Fe-6.5 wt.% Si sheet rolled at a total reduction ratio of 80% exhibits good soft magnetic properties such as a saturation magnetization of 1.74 T and core loss ($W_{5/1000}$) of 30.7 W/kg. This result is caused by an increase in the electrical resistivity resulting from an increased particle boundary density and the oxide layers between the primary particle boundaries.

Influence of Processing Conditions on PVDF Properties

Lebedev, S.M.,Gefle, O.S.,Semenikhin, M.V. The Korean Powder Metallurgy Institute 2011 한국분말재료학회지 (KPMI) Vol.18 No.2

Study of the main properties of PVDF films produced by two processing technologies such as hot pressing from a melt or solution casting was the aim of this paper. All samples were prepared of as-received PVDF powder. First group of samples was prepared by the hot pressing. Second group of samples was prepared by the solution casting method. PVDF powder was dissolved in dimethylformamide. To characterize properties of samples, different experimental methods such as FRA (dielectric spectroscopy), IR-spectroscopy and DSC/TGA analysis were used in this work. It was found that IR-spectra of both studied groups do not change compared to that for virgin PVDF powder. It confirms that molecular structure is practically independent on the processing technology of samples. The only difference has been found that new band centered at $1723\;cm^{-1}$ appears for samples prepared by the hot pressing method. This absorption band is related with formation of C=C bonds in samples prepared by the hot pressing method in contrast both to PVDF powder and samples prepared by the solution casting method.

Synthesis and Spark-plasma Sinetring of Nanoscale Al/alumina Powder by Wire Electric Explosion Process

Kim, Ji-Soon,Kim, H. T.,Illyin, A. P.,Kwon, Young-Soon The Korean Powder Metallurgy Institute 2005 한국분말재료학회지 (KPMI) Vol.12 No.5

Nanoscale Al powder with thin layer of alumina was produced by Wire Electric Explosion (WEE) process. Spark-Plasma Sintering (SPS) was performed for the produced powder to confirm the effectiveness of SPS like so-called 'surface-cleaning effect' and so on. Crystallite size and alumina content of produced powder varied with the ratio of input energy to sublimation energy of Al wire ($e/e_s$): Increase in ($e/e_s$) resulted in the decrease of crystallite size and the increase of alumina content. Shrinkage curve during SPS process showed that the oxide surface layer could not be destroyed near the melting point of Al. It implied that there was not enough or no spark-plasma effect during SPS for Al/Alumina powder.