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Song, Yuepeng,Zhu, Yanmin,Gao, Dongsheng,Guo, Jing,Kim, Hyoung Seop The Korean Powder Metallurgy Institute 2013 한국분말재료학회지 (KPMI) Vol.20 No.5
Recently, self-propagating high-temperature synthesis (SHS), related to metallic and ceramic powder interactions, has attracted huge interest from more and more researchers, because it can provide an attractive, energy-efficient approach to the synthesis of simple and complex materials. The adiabatic temperature $T_{ad}$ and apparent activation energy analysis of different thermit systems plays an important role in thermodynamic studies on combustion synthesis. After establishing and verifying a mathematic calculation program for predicting adiabatic temperatures, based on the thermodynamic theory of combustion synthesis systems, the adiabatic temperatures of the NiO/Al aluminothermic system during self-propagating high-temperature synthesis were investigated. The effect of a diluting agent additive fraction on combustion velocity was studied. According to the simulation and experimental results, the apparent activation energy was estimated using the Arrhenius diagram of $ln(v/T_{ad}){\sim}/T_{ad}$ based on the combustion equation given by Merzhanov et al. When the temperature exceeds the boiling point of aluminum (2,790 K), the apparent activation energy of the NiO/Al aluminothermic system is $64{\pm}14$ kJ/mol. In contrast, below 2,790 K, the apparent activation energy is $189{\pm}15$ kJ/mol. The process of combustion contributed to the mass-transference of aluminum reactant of the burning compacts. The reliability of the simulation results was experimentally verified.
Finite Element Analysis of the Effect of Friction in High Pressure Torsion
Yuepeng Song,Wenke Wang,Dongsheng Gao,Eun Yoo Yoon,이동준,김형섭 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.3
High pressure torsion (HPT) is one of the most important techniques among various methods that createsevere plastic deformation in the production of bulk materials with nano/ultrafine grained microstructures. Since the driving force in deforming the workpiece in HPT is surface friction, understanding of the frictioneffect is critical for successful application of HPT. In this study, the friction effect in HPT was analyzed usingthe finite element method. The distribution of effective strain on the contact surface of the HPT samplesunder different friction conditions was investigated. The friction force influenced the effective strain morein the middle and edge regions than in the central region. The condition for the minimum friction factor thatcould achieve a sticking condition between the surfaces of the dies, and the samples in the middle and edgeregions, was investigated. There was a critical friction coefficient in which the effective strain varies sharplywith an increasing friction coefficient.
Inhomogeneous Deformation of Interstitial Free Steel during the High Pressure Torsion Process
Yuepeng Song,Miaomiao Chen,Wenke Wang,Baoyan Xu,Dongsheng Gao,Shuai Zhang,Hyoung Seop Kim 대한금속ㆍ재료학회 2017 대한금속·재료학회지 Vol.55 No.10
Interstitial free (IF) steel disks were subjected to various degrees of revolution during application of the high-pressure torsion (HPT) process, and the resulting distributions of hardness and microstructure during the early torsion stage of high-pressure torsion (HPT) were investigated using experimental and simulation approaches. The results indicated that the deformation in the HPT-processed IF steel disk was inhomogeneous, producing low hardness in the center and high hardness in the edge region. The experimental results, including the hardness and microstructure distributions, indicated that the severe deformation zone proceeds gradually from the center to the edge of the HPT disks in the early torsion stage, and also confirmed verify that the deformation on the upper surface of the disks lags behind that on the bottom surface. Simulation results from a finite element method analysis strongly supported the experimental conclusions. (Received December 30, 2016; Accepted July 2, 2017)
Fabrication of W-Cu Alloy via Combustion Synthesis Infiltration Under an Ultra-Gravity Field
Yuepeng Song,Qian Li,Jiangtao Li,Gang He,Yixiang Chen,김형섭 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.6
Tungsten copper alloy with a tungsten concentrate of 70 vol% was prepared by self-propagating hightemperaturesynthesis in an ultra-gravity field. The phase structures and components of the W-Cu alloyfabricated via this approach were the same as those via traditional sintering methods. The temperature andstress distributions during this process were simulated using a new scheme of the finite element method. The results indicated that nonequilibrium crystallization conditions can be created for combustion synthesisinfiltration in an ultra-gravity field by the rapid infiltration of the liquid copper product into the tungstencompact at high temperature and low viscosity. The cooling rate can be above 100,000 K/s and high stresses intungsten (~5 GPa) and copper (~2.6 GPa) were developed, which passivates the tungsten particle surface,resulting in easy sintering and densifying the W-Cu alloy. The reliability of the simulation was verified throughtemperature measurement and investigation of the microstructure. The W-Cu composite-formation mechanismwas also analyzed and discussed with the simulation results.
Yuepeng Song,Wenke Wang,이동준,정혁재,이성학,김형섭 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.1
Distinction between plastic deformation occuring in compression and compression–torsion stages isimportant for understanding the properties and microstructures of materials processed by high-pressure torsion(HPT). In the present study, remarkable through-thickness inhomogeneities of hardness and microstructurewere found in the samples processed by compression stage of HPT. Three regions on the radialdirection plane of compressed disks were defined to display the inhomogeneity: edge zone (high hardness),radial medium zone (uniform hardness) and center zone (low hardness near the surface and highhardness in the thickness central plane). A low hardness region in the center near the surface was detectedand found to stretch along the upper and bottom surfaces of the disks compressed by low pressure. Thislow hardness region was also found to decrease with increasing the pressure. Not only the hardness butalso the microstructure through-thickness inhomogeneity is attributed to stress and strain distribution in thedisk as well as to a huge friction between the anvil and the disk during processing.
( Yuepeng Song ),( Miaomiao Chen ),( Baoyan Xu ),( Dongsheng Gao ),( Jing Guo ),( Lingfeng Xu ),( Zheng Wang ),( Hyoung Seop Kim ) 대한금속재료학회(구 대한금속학회) 2016 대한금속·재료학회지 Vol.54 No.11
Herein, we report the results of our investigation on the effect of friction and anvil design on the heterogeneous plastic-deformation characteristics of copper during the compressive stage of high-pressure torsion (HPT), using the finite element method. The results indicate that the friction and anvil geometry play important roles in the homogeneity of the deformation. These variables affect the heterogeneous level of strain in the HPT compressed disks, as well as the flash in the disk edge region. The heterogeneous plastic deformation of the disks becomes more severe with the increasing depth of the cavity, as anvil angle and friction coefficient increase. However, the homogeneity increases with increases in the wall angle. The length of flash and the area of the dead metal zone increase with the depth of the cavity, while they decrease at a wall angle of 180°. (Received March 17, 2016; Accepted May 25, 2016)
Wenke Wang,Yuepeng Song,Dongsheng Gao,Eun Yoo Yoon,이동준,이종수,김형섭 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.5
High pressure torsion (HPT) is useful for achieving substantial grain refinement to ultrafine grained/nanocrystalline states in bulk metallic solids. Most publications that analyzed the HPT process used experimental and numerical simulation approaches, whereas theoretical stress analyses for the HPT process are rare. Because of the key role of compression stage for the deformation of HPT, this paper aims to conduct a theoretical analysis and to establish a practical formula for stress and forming parameters of HPT process using the slab analysis method. Three equations were obtained via equations derivation to describe the normal stress states corresponding to the three zones of plastic deformation for HPT process as stick zone, drag zone and slip zone. As to the compression stage of HPT, the stress distribution results using the finite element method agree well with those using the slab analysis method. There are drag and stick zones on the contact surface of the HPT sample, as verified by the finite element method (FEM) and slab analysis method.