• Journal of Powder Materials
• /
• v.17 no.3
• /
• pp.190-196
• /
• 2010

In this paper, rapid solidified Mg-4.3Zn-0.7Y (at.%) alloy powders were prepared using an inert gas atomizer, followed by a severe plastic deformation technique of high pressure torsion (HPT) for consolidation of the powders. The gas atomized powders were almost spherical in shape, and grain size was as fine as less than $5\;{\mu}m$ due to rapid solidification. Plastic deformation responses during HPT were simulated using the finite element method, which shows in good agreement with the analytical solutions of a strain expression in torsion. Varying the HPT processing temperature from ambient to 473 K, the behavior of powder consolidation, matrix microstructural evolution and mechanical properties of the compacts was investigated. The gas atomized powders were deformed plastically as well as fully densified, resulting in effective grain size refinements and enhanced microhardness values.

• Journal of Korea Foundry Society
• /
• v.13 no.5
• /
• pp.441-449
• /
• 1993

SiC particulate reinforced magnesium matrix composites were fabricated by melt stirring method. The effet of several factors on mechanical properties and the efficiency of melt stirring method from the viewpoint of these properties were investigated. The tensile strength increased and the elongation decreased with decrease of the particle size or the increase of the paticulate volume fraction for pure magnesium matrix and Mg-5%Zn alloy matrix composites. A longer stirring time improved the tensile strength of these composites. The tensile strength of Mg-5%Ca alloy matrix composites which shows no uniform paticulate distribution was a little lower than that of matrix alloy. Rapid solidification rate is preferred for the improved tensile strength of these composites. The pure magnesium matrix and Mg-5%Zn alloy matrix composites have tensile strength of about 400MPa. This value agrees with the tensile strength of some magnesium matrix composites fabricated by liquid infiltration method or powder metallurgy method at the same volume fraction of reinforcements of whisker or particle. Therefore, the melt stirring method which has the advantages of simple process is considered to be efficient in fabricating magnesium matrix composites.

• Journal of the Korean Society for Heat Treatment
• /
• v.33 no.3
• /
• pp.99-106
• /
• 2020

Selective laser melting (SLM) is an additive manufacturing process by melting metallic powders and stacking into layers, and can product complex shapes or near-net-shape (NNS) that are difficult to product by conventional processes. Also, SLM process is able to raise the efficiency of production by creating a streamlined manufacturing process. For manufacturing in SLM process using Ti-6Al-4V powder, analysis of microstructural evolution and evaluation of mechanical properties are essential because of rapid melting and solidification process of powders according to high laser power and rapid scan speed. In addition, it requires a post-processing because the soundness and mechanical properties are degraded by defects such as pore, un-melted powder, lack-of-fusion, etc. In this study, hot isostatic press (HIP) was conducted as a post-processing on SLM-printed Ti-6Al-4V alloy. Microstructure of post-processed Ti-6Al-4V alloy was compared to as-built Ti-6Al-4V, and the evolution of quasi-static (Vickers hardness, room temperature tensile characteristic) and dynamic (high-cycle fatigue characteristic) mechanical properties were analyzed.

• Journal of Powder Materials
• /
• v.3 no.4
• /
• pp.271-278
• /
• 1996

In this study, the effect of the transition elements on the microstructure and mechanical properties of rapidly solidified Al-Mg-X alloys was investigated. As a result of the rapid solidification processing, fine equiaxed grains with a mean diameter of 2 $\mu$m were observed in these alloys. Many fine particles were found to be distributed rather homogeneously throughout the matrix with relatively large particles occasionally at grain boundaries. The ultimate tensile strengths of Al-Mg-X alloys were found to decrease rather remarkably at 150 $^{\circ}C$ without the gain of the ductility at 150 $^{\circ}C$, which may result from segregation of $\beta$ ($Al_{3}Mg_{2}$) precipitates. Fine dimples were observed on the fracture surfaces for all alloy systems and the variation of the size and shape of dimples was not observed upon alloy systems. The ductility at 530 $^{\circ}C$ was found to be ~100%, suggesting that grain boundary sliding did not contribute to ductiliy despite he grain size stabilization. The absence of superplastic behavior may be associated with low boundary misorientation in rapidly solidified Al-Mg-X alloys.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2000.05a
• /
• pp.771-774
• /
• 2000

RP techniques have inherent disadvantages caused by their working principles: stair-stepped surface of parts due to layer-by-layer stacking of layers, low build speed caused by line-by-line solidification to finish one layer, and post processing to improve surface finish, etc. The objective of this study is to propose a new RP technique, variable deposition manufacturing (VDM), which can make up for the disadvantages of the existing RP techniques, and to develop an apparatus to implement the technique. The proposed process can greatly reduce the building time and improve the surface finish of parts generated. The experiments are carried out to obtain the range of temperature of molten material to maintain its fluidity and to investigate the effect of gas cooling on the preservation of the slopes. Based on the results, some simple shapes such as a line-shape. an S-shape, and a circle-shape were fabricated from Ethylene Vinyl Acetatecopolymer (EVA). In order to examine the applicability of VDM to more general shapes, a tensile specimen and a yo-yo shape were manufactured by the proposed RP method using EVA material as the first trial approach. The present basic study has shown the possibility of a practicable utilization of the proposed VDM process to prototyping of a general three-dimensional shape.

• Journal of Korea Foundry Society
• /
• v.15 no.5
• /
• pp.459-468
• /
• 1995

For high performance aerospace structures, the properties of highest priority are low density, high strength, and high stiffness(modulus of elasticity). Addition of beryllium decrease the density of the aluminum alloy and increase the strength and the stiffness of the alloy. However it is very difficult to produce the Al-Be alloy having useful engineering properties by conventional ingot casting, because of the extremely limited solid solubility of beryllium in aluminum. So, rapid solidification processing is necessary to obtain extended solid solubility. In this study, rapidly solidified Al-6 at% Be alloy were prepared by twin roll melt spinning process and single roll melt spinning process. Twin roll melt spun ribbons were extruded at $450^{\circ}C$ with reduction in area of 25 : 1 after vacuum hot pressing at $550^{\circ}C and 375^{\circ}C$. The microstructure of melt spun ribbon exhibited a refined cellular microstructure with dispersed Be particles. As advance velocity of liquid/solid interface increase, the morphology of Be particle vary from rod-like type to spherical type and the crystal structure of Be particle from HCP to BCC. These microstructural characteristics of rapidly solidified Al-6at.%Be alloy were described on the basis of metastable phase diagram proposed by Perepezko and Boettinger. The extruded ribbon consisted of recrystallized grains dispersed with Be particles and exhibited improved tensile property compared with that of extruded ingot.

• Transactions on Electrical and Electronic Materials
• /
• v.12 no.1
• /
• pp.20-23
• /
• 2011

Amorphous Ti-(50-x)Ni-xCu (at%) (x = 20, 25, 27, 30) alloy ribbons were prepared by melt spinning. Subsequently, the crystallization behavior of the alloy ribbons was investigated by X-ray diffraction and differential scanning calorimetry. ${\Delta}T$ (the temperature gap between $T_g$ and $T_x$) increased from 33 K to 47 K and the wavenumber ($Q_p$) decreased from 29.44 $nm^{-1}$ to 29.29 $nm^{-1}$ with increasing Cu content from 20 at% to 30 at%. The activation energy for crystallization decreased from 188.5 kJ/mol to 170.6 kJ/mol with increasing Cu content from 20 at% to 25 at%; afterwards, the activation energy remained near constant. Crystallization occurred in two-stage: amorphous-B2-$TiCu_2$ in Ti-Ni-Cu alloys with Cu content less than 25 at%, while it occurred in three-stage; amorphous-B2-TiCu-$TiCu_2$ in Ti-Ni-Cu alloys with Cu content more than 27 at%.

• Transactions of Materials Processing
• /
• v.14 no.6 s.78
• /
• pp.502-508
• /
• 2005

Hypereutectic Al-Si alloys have been regarded attractive for automotive and aerospace application, due to high specific strength, good wear resistance, high thermal stability, low thermal expansion coefficient and good creep resistance. Spray casting of hypereutectic Al-Si alloy has been reported to provide distinct advantages over ingot metallurgy (IM) or rapid solidification/powder metallurgy (RS/PM) process in terms of microstructure refinement. In this study, hypereutectic Al-25Si-2.0Cu-1.0Mg alloy was prepared by OSPREY spray casting process. The change of strain rate sensitivity and Creep transition were analyzed by using the load relaxation test and constant creep test. High temperature deformation behavior of the hypereutectic Al-Si alloy has been investigated by applying the internal variable theory proposed by Chang et al. Especially, the creep resistance of spray casted hypereutectic Al-Si alloy can be enhanced considerably by the accumulation of prestrain.

• Transactions of Materials Processing
• /
• v.25 no.6
• /
• pp.353-358
• /
• 2016

Direct energy deposition (DED) is an additive manufacturing technique that involves the melting of metal powder with a high-powered laser beam and is used to build a variety of components. In recent year, it can be widely used in order to produce hard, wear resistant and/or corrosion resistant surface layers of metallic mechanical parts, such as dies and molds. For the purpose of the hardfacing to achieve high wear resistance and hardness, application of high speed steel (HSS) can be expected to improve the tool life. During the DED process using the high-carbon steel, however, defects (delamination or cracking) can be induced by rapid solidification of the molten powder. Thus, substrate preheating is generally adopted to reduce the deposition defect. While the substrate preheating ensures defect-free deposition, it is important to select the optimal preheating temperature since it also affects the microstructure evolution and mechanical properties. In this study, AISI M4 powder was deposited on the AISI 1045 substrate preheated at different temperatures (room temperature to $500^{\circ}C$). In addition, the micro-hardness distribution, cooling rates, and microstructures of the deposited layers were investigated in order to observe the influence of the substrate preheating on the mechanical and metallurgical properties.

• Journal of the Korean Society for Precision Engineering
• /
• v.17 no.12
• /
• pp.185-194
• /
• 2000

Rapid Prototyping(RP) techniques have their unique characteristics according to the working principles: stair-stepped surface of parts due to layer-by-layer stacking, low build speed caused by line-by-line solidification to build one layer, and additional post processing to improve surface roughness, so it is required very high cost to introduce and to maintain of RP apparatus. The objective of this study is to develop a new RP process, Variable Lamination Manufacturing using linear hotwire cutting technique and expandable polystyrene foam sheet as part material(VLM-S), and to investigate characteristics of part material, cutting characteristics by using linear hotwire cutting system and bonding. Experiments were carried out to investigate mechanical properties of part material such as anisotropy and directional tensile strength. In order to obtain optimal dimensional accuracy, surface roughness, and reduced cutting time, addition experiments were performed to find the relationship between cutting speed and cutting offset of hotwire, and heat generation of hotwire per unit length. So, adhesion strength tests according to ASTM test procedure showed that delamination did not occur at bonded area. Based on the data, a clover-shape was fabricated using unit shape part(USP) it is generated hotwire cutting. The results of present study have been reflected on the enhancement of the VLM-S process and apparatus.