• Proceedings of the Korean Powder Metallurgy Institute Conference
• /
• 1997.10a
• /
• pp.7-7
• /
• 1997

During sintering of very porous green bodies, as obtained by compaction of hard powders - such as tungsten carbide or ceramics - or by injection moulding, important shrinkage occurs. Due to heterogeneous green density field, gravity effects, friction on the support, thermal gradients, etc., this shrinkage is often non-uniform, which' may induce significant shape changes. As the ratio of compact dimension to powder size is very high, the mechanics of continuum is relevant to model such phenomena. Thus numerical techniques, such as the finite element method can be used to simulate the sintering process and predict the final shape of the sintered part. Such type of simulation has much been developed in the last decade firstly for hot isostatic pressing and next for die compaction. Finite element modelling has been recently applied to free sintering. The simulation of sintering should be based on constitutive equations describing the thermo-mechanical behaviour of the material under any state of stress and any temperature which may arise within the sintering body. These equations can be drawn either from experimental data or from micromechanical models. The experiments usually consist in free sintering and sinter-forging tests. Indeed applying more complex loading conditions at high temperature under controlled atmosphere is delicate. Micromechanical models describe the constitutive behaviour of aggregates of spheres from the deformation of two-sphere contact either by viscous flow or grain boundary diffusion. Such models are not able to describe complex microstructure and mechanisms as observed in real materials but they can give some basic information on the formulation of constitutive equations. Practically both experimental and theoretical approaches can be coupled to identify the constitutive equations. Such procedure has been performed for modelling the sintering of compacts obtained by die pressing of a mixture of tungsten carbide and cobalt powders. The constitutive behaviour of this material during sintering has been described by a linear viscous constitutive model, whose functions have been fitted from results of free sintering and sinter-forging experiments. This model has next been introduced in ABAQUS finite element code to simulate the sintering of heterogeneous green compacts of various geometries at constant temperature. Examples of simulations are shown and compared with experiments.

• Journal of Powder Materials
• /
• v.9 no.6
• /
• pp.381-393
• /
• 2002

Recent developments in nanocrystalline and nanocomposite rare earth-transition metal magnets are reviewed and emphasis is placed on research work at IFW Dresden. Principal synthesis methods include high energy ball milling, melt spinning, mold casting and hydrogen assisted methods such as reactive milling and hydrogenation-disproportionation-desorption-recombination. These techniques are applied to NdFeB-, PrFeB- and SmCo-type systems with the aim to produce high remanence magnets with high coercivity. Concepts of maximizing the energy density in nanostructured magnets by either inducing a texture via anisotropic HDDR or hot deformation or enhancing the remanence via magnetic exchange coupling are evaluated. With respect to high temperature applications melt spun $Sm(Co_{0.74}Fe_{0.1}Cu_{0.12}Zr_{0.04})_{7.5}$ ribbons were prepared, which showed coercivities of up to 0.53 T at 50$0^{\circ}C$. Partially amorphous $Nd_{60}Fe_xCo_{30-x}Al_{10}(0{\leq}x{\leq}30)$ alloys were prepared by copper mold casting. The effect of transition metal content on the glass-forming ability and the magnetic properties was investigated. The $Nd_{60}Co_{30}Al_{10}$ alloy exhibits an amorphous structure shown by the corresponding diffraction pattern. A small substitution of Co by 2.5 at.% Fe results In the formation of Fe-rich crystallites embedded in the Nd-rich amorphous matrix. The Fe-rich crystallites show hard magnetic behaviour at room temperature with a coercivity value of about 0.4 T, relatively low saturation magnetization and a Curie temperature of 500 K.

• Journal of the Korean Ceramic Society
• /
• v.54 no.4
• /
• pp.340-348
• /
• 2017

$SiC_f/SiC$ composites were joined using a $60{\mu}m-thick$ $Ti_3AlC_2$ or $Ti_3SiC_2$ MAX phase tape. The filler tape was inserted between the $SiC_f/SiC$ composites containing a 12 wt.% $Al_2O_3-Y_2O_3$ sintering additive. The joining was performed to a butt-joint configuration at $1600^{\circ}C$ or $1750^{\circ}C$ in an Ar atmosphere by applying 3.5 MPa using a hot press. Microstructural and phase analyses at the joining interface confirmed the decomposition of $Ti_3AlC_2$ and $Ti_3SiC_2$, indicating the joining by solid-state diffusion. The results showed sound joining interface without the presence of cracks. Joining strengths higher than 150 MPa could be obtained for the joints using $Ti_3AlC_2$ or $Ti_3SiC_2$ at $1750^{\circ}C$, while those for joined at $1600^{\circ}C$ decreased to 100 MPa approximately without the deformation of the joining bodies. The thickness of initial filler tape was reduced significantly after joining because of the decomposition and migration of MAX phase owing to the plasticity at high temperatures.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2001.04a
• /
• pp.905-909
• /
• 2001

The significance of casting/forging process for reducing the production cost of large components is being noted in these days. This casting/forging process is a method of forging a workpiece preformed by casting into the final shape. In this study, the casting/forging process has been applied in manufacturing a large aluminum flange in order to determine the optimum forging condition of the aluminum flange. The optimum range of forging temperature of Al 5083 was from $420^{\circ}C$ to $450^{\circ}C$. The suitable strain rate was 1.5 $sec^{-1}$. The deformation amount of a preform in a forging process is key role in the mechanical properties of casting/forging products. In order to find the change of mechanical properties according to effective stain of cast aluminum billets, a hot upsetting test were performed with rectangular blocks and then a uniaxial tensile test was performed with specimens cut from the upsetted billets. The tensile strength and the elongation of cast/upsetted aluminum billets were increased largely until the effective strain was 0.7. FE analysis was performed to determine the configurations of cast preform and die for an aluminum flange. In the FE analysis, the forging load-limit was fixed 1500ton for the low equipment cost. The cast preform was designed so that the effective stain around the neck of a flange exceeds 0.7. In the forging experiment for an aluminum flange, it was confirmed that the optimal configuration of the cast preform predicted by FE analysis was very useful. The cast/forged products using designed preform were made perfectly without any defects.

• Journal of the Korean Society for Precision Engineering
• /
• v.23 no.4 s.181
• /
• pp.153-161
• /
• 2006

Micro-forming is a suited technology to manufacture very small metallic parts(several $mm{\sim}{\mu}m$). Micro-forming of $Zr_{62}Cu_{17}Ni_{13}Al_8$ bulk metallic glass(BMG) as a candidate material for this developing process are feasible at a relatively low stress in the supercooled liquid state without any crystallization during hot deformation. In this study, micro- formability of a representative bulk metallic glass, $Zr_{62}Cu_{17}Ni_{13}Al_8$. was investigated for micro-forging of U-shape pattern. Micro-formability was estimated by comparing $R_f$ values ($=A_f/A_g$), where $A_g$ is cross-sectional area of U groove, and $A_f$ the filled area by material. Micro-forging process was simulated and analyzed by applying finite element method. FEM simulation results showed reasonable agreement with the experimental results when the material properties and simulation conditions such as top die speed, remeshing criteria and boundary conditions were tightly controlled. The micro-formability of $Zr_{62}Cu_{17}Ni_{13}Al_8$ was increased with increasing load and time in the temperature range of the supercooled liquid state. Also, FEM simulation using a commercial software, DEFORM was confirmed to be applicable for the optimization of micro-forming process.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.38 no.8
• /
• pp.849-855
• /
• 2014

The GFRP composite is used for hot press flow molding of automotive components, and the different flow rates of fiber and plastic are likely to induce fiber orientation and inhomogeneity in the material. However, very limited systematic research studies are available on composite materials with superior flow homogeneity and optimized fiber orientation. The inhomogeneity and fiber orientation issues of GFRP composites have still not been resolved through research. The plain weaving method applied to the GFRP prepreg can improve its recyclability, inhomogeneity, fiber flow, structural stability, fiber deformation, surface smoothness, degree of impregnation, and other mechanical properties. The need for more detailed and thorough studies is evidenced.

• Korean Journal of Materials Research
• /
• v.4 no.2
• /
• pp.241-249
• /
• 1994

We studied on the effects of hot-worm rolling on recrystalized structures and tensile strength in over-aged 7075 A1 alloy, to develop the process for improving properties. It showed more clear effect of the grain refinement with over-aging before plastic deformation. That means, the coarse precipitates from over-aging play a roll as nucleation sites in the course of recrystallization. And on this study, the relations between yield strength and grain size was not satisfied with Hall-Petch equation because of the elongated structure, but the yield strength is proportional to aspect ratio of grains. In TMT process for improving strength and toughness, the worm working is available for increase of those properties than cold working.

• Korean Journal of Materials Research
• /
• v.25 no.11
• /
• pp.636-641
• /
• 2015

Titanium has many special characteristics such as specific high strength, low elastic modulus, excellent corrosion and oxidation resistance, etc. Beta titanium alloys, because of their good formability and strength, are used for jet engines, and as turbine blades in the automobile and aerospace industries. Low cost beta titanium alloys were developed to take economic advantage of the use of low-cost beta stabilizers such as Mo, Fe, and Cr. Generally, adding a trace of boron leads to grain refinement in casted titanium alloys due to the pinning effect of the TiB phases. This study analyzed and evaluated the microstructural and mechanical properties after plastic deformation and heat treatment in boron-modified Ti-2Al-9.2Mo-2Fe alloy. The results indicate that a trace of boron addition made grains finer; this refinement effect was found to be maintained after subsequent processes such as hot forging and solution treatment. This can effectively reduce the number of required manufacturing process steps and lead to savings in the overall cost as well as low-cost beta elements.

• Korean Journal of Materials Research
• /
• v.25 no.6
• /
• pp.269-273
• /
• 2015

The effect of adding Ca on the microstructural and mechanical properties of as-cast Mg-11Li-3Zn-1Sn(wt%) alloys were investigated. Mg-11Li-3Zn-1Sn-0.4Mn with different Ca additions (0.4, 0.8, 1.2 wt%) were cast under an $SF_6$ and $Co_2$ atmosphere at $720^{\circ}C$. The cast billets were homogenized at $400^{\circ}C$ for 12h and extruded at $200^{\circ}C$. The microstructural and mechanical properties were analyzed by OM, XRD, SEM, and tensile tests. The addition of Ca to the Mg-11Li-3Zn-1Sn-0.4Mn alloy resulted in the formation of $Ca_2Mg_6Zn_3$, MgSnCa intermetallic compound. By increasing Ca addition, the volume fraction and size of $Ca_2Mg_6Zn_3$ with needle shape were increased. This $Ca_2Mg_6Zn_3$ intermetallic compound was elongated to the extrusion direction and refined to fine particles due to severe deformation during hot extrusion. The elongation of the 0.8 wt% Ca containing alloy improved remarkably without reduction strength due to the formation of fine grain and $Ca_2Mg_6Zn_3$ intermetallic compounds by Ca addition. It is probable that fine and homogeneous $Ca_2Mg_6Zn_3$ intermetallic compounds played a significant role in the increase of mechanical properties.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.24 no.4
• /
• pp.81-90
• /
• 2020

A textile and capillary tube composite panel(TCP) was developed to simultaneously retrofit the seismic performance and the energy efficiency (e.g. heating or insulation performance) of existing unreinforced masonry (URM) buildings. TCP is a light-weight mortar panel in which carbon textile reinforcements and capillary tubes are embedded. Textile reinforcements plays a role of seismic retrofit and capillary tubes that hot water circulates contribute to the energy retrofit. In this paper, the static cyclic loading tests were performed on the masonry walls with/without TCP to understand the seismic retrofit effect of TCP retrofit and the results were summarized. The results of the test showed that the TCP contributed to increase the capacity of the Shear strength and ductility of the URM walls. In addition, the deformation of the wall after cracking was substantially controlled by the carbon textile.