Effect of alumina addition on the microstructure and grain boundary resistance of magnesia partially-stabilized zirconia

윤상현,Chester J. Van Tyne,이희수 한국물리학회 2014 Current Applied Physics Vol.14 No.7

The electrical properties of 9 mol% MgOeZrO2 (Mg-PSZ) with 1 mol% Al2O3 and the mechanisms for electrical degradation were investigated using structural, morphological, and electrochemical analyses. The addition of Al2O3 caused an increase in both the monoclinic and the Mg-rich phases at the grain boundaries in the Mg-PSZ. Coarse grains larger than 20 mm and an intergranular layer composed of the Mg-rich phase were identified in a specimen sintered at 1600 C. This specimen exhibited a minimum of ionic conductivity (4.98 104 S cm1 at 700 C) due to the grain boundary resistance (245 U cm2), which dominated the overall resistance. A similar trend was observed over the entire temperature range (600e1500 C). An intergranular siliceous impurity (SiO2) was present in conjunction with the Mg-rich phase. This impurity and the Mg-rich phase acted as a barrier layer for oxygen ion diffusion. The presence of the intergranular phases (i.e. the monoclinic and Mg-rich phases) contributed to the degradation of the ionic conductivity in Mg-PSZ with an Al2O3 addition.

Feasibility studies of laser surface nitriding on Ti-6Al-4V alloy using a nitric acid solution

황태우,우영윤,Chester J. Van Tyne,문영훈 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.9

Feasibility studies for laser surface nitriding in a nitric acid solution were performed on a Ti-6Al-4V alloy. The susceptibility to surface cracking, hardness, bead shape, and the microstructure of the laser nitrided layers obtained at various concentrations of nitric acid were investigated over a wide range of processing variables. The results show that the hardness of the laser nitrided layer can be increased by increasing both the concentration of nitric acid and the energy density of the laser. The proposed process window shows that a high scanning speed, coupled with higher laser power, can result in crack-free nitriding, and also the hardness of the nitride layer can be finely controlled within this process window. Laser surface nitriding in nitric acid solution can increase the surface hardness without processinduced surface cracking. The proposed process is shown to be feasible and can be performed with a high degree of reliability.

Strain Hardening Behavior During Manufacturing of Tube Shapes by Hydroforming

박현규,이혜경,Chester J. Van Tyne,문영훈 대한금속·재료학회 2009 METALS AND MATERIALS International Vol.15 No.6

Safe and robust process design relies on knowledge of the evolution of the mechanical properties in a tube during hydroforming. The manufacturing of tubular shapes generally consists of three main stages: bending, preforming, and expansion. The latter is usually called hydroforming. As a result of these three steps, the final product’s strain hardening history is nonlinear. In the present study, the strain hardening behavior during hydroforming was experimentally investigated. The variation of local flow stress and/or local hardness was used as an index of the strain hardening during the various steps and the local flow stress and/or local hardness were used with respective correlations to determine the effective strain. The strain hardening behavior during hydroforming after preforming has been successfully analyzed by using the relationships between hardness, flow stress, and effective strain for variable pre-strains prior to hydroforming. The comparison of predicted hardness with measured hardness confirms that the methodology used in this study is feasible, and that the strain hardening behavior can be quantitatively estimated with good accuracy. Safe and robust process design relies on knowledge of the evolution of the mechanical properties in a tube during hydroforming. The manufacturing of tubular shapes generally consists of three main stages: bending, preforming, and expansion. The latter is usually called hydroforming. As a result of these three steps, the final product’s strain hardening history is nonlinear. In the present study, the strain hardening behavior during hydroforming was experimentally investigated. The variation of local flow stress and/or local hardness was used as an index of the strain hardening during the various steps and the local flow stress and/or local hardness were used with respective correlations to determine the effective strain. The strain hardening behavior during hydroforming after preforming has been successfully analyzed by using the relationships between hardness, flow stress, and effective strain for variable pre-strains prior to hydroforming. The comparison of predicted hardness with measured hardness confirms that the methodology used in this study is feasible, and that the strain hardening behavior can be quantitatively estimated with good accuracy.

Inter-granular phase formation and flexural strength of MgO partially stabilized zirconia by Al2O3 additions

김부영,전설,Chester J. Van Tyne,박현,이희수 한양대학교 세라믹연구소 2016 Journal of Ceramic Processing Research Vol.17 No.5

Phase transformation and inter-granular phase formation of 9 mol% MgO partially stabilized zirconia (Mg-PSZ) accordingto Al2O3 addition were investigated in terms of crystallography and morphology, and its flexural strength was measured. Sintered bulk specimens of 9 mol% Mg-PSZ ball-milled with Al2O3 (1, 5, 10 mol%) were evaluated by X-ray diffraction,transmission electron microscopy and EDS. Monoclinic phase in Mg-PSZ, which has a larger lattice than that of cubic andtetragonal phases, increased as a function of Al2O3 addition. The apparent density of Mg-PSZ was decreased with increasingAl2O3 addition, and flexural strength was also exibited the lowest value with 10 mol% Al2O3 addition. Mg-rich phase, forsterite(Mg2SiO4), and spinel (MgAl2O4) were observed along grain boundary of Mg-PSZ with Al2O3 additon by TEM-EDS. It wasconsidered that destabilization of Mg-PSZ by Al2O3 addition led to decrease the apparent density, and countinuous intergranularphase formation caused the decrease of flexural strength.

Effect of the Surface Defects on Hydroformability of Aluminum Alloys

Kim, B.J.,Choi, K.H.,Park, K.S.,van Tyne, Chester J.,Moon, Young Hoon Trans Tech Publications, Ltd. 2007 Key Engineering Materials Vol.340 No.-

<P>Extruded aluminum alloys, which are highly versatile, have relatively modest prototyping cost, good strength and corrosion resistance. Because there is no weld seam, the circumferential mechanical properties may be uniform and advantageous for hydroforming. However, surface defects such as die lines and pick-up can be generated during the extrusion especially due to imperfections on the die surface. In this study, the extent of the crack propagation caused by die lines is evaluated according to the deformed shape of the tube in hydroforming process. And when forming a extruded aluminum tube, the deformed surface of the tube frequently becomes rougher with increasing plastic strain. This is well known as orange peel phenomenon and it has a significantly effect not only on the surface quality of a final product but also on the forming limit. To evaluate the effects of the orange peel on the hydroformability, the inter-stage polishing has been performed. Through the several tests including hydroforming test, the effect of surface defects on the hydroformabilities are well defined.</P>

우수한 학생들의 이공계 유치 방안

Tracy Morly,Chester J,Van Tyne,Bevlee Watford,김하석,박성욱,백남희,이제일,한경희,호병환,김광선 한국공학교육학회 2006 Ingenium(人材니움) Vol.13 No.2

The effect of mechanical properties on hole flangeability of stainless steel sheets

Min Li,Chester J. Van Tyne,문영훈 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.12

This study investigates the effect of mechanical properties on hole flangeability of stainless steel sheets. Four different stainless steelsheets, SUS202, SUS304L, SUS316L, and SUS430, with a thickness of 3.0 mm, were evaluated. To estimate the hole flangeability, holeexpansion ratio, punch load, flange height, and lip thickness were experimentally investigated. The degree of circumferential strain alongthe periphery of the expanded hole is important in determining the hole expansion ratio. For materials with higher work hardening exponentand anisotropy, there is an increased resistance in wall thickness reduction, which accelerates local strain concentration along theperiphery of the expanded hole in the circumferential direction and lowers hole expansion ratio. The main factors controlling punch load,lip thickness, and height were evaluated through correlation analysis. The flangeability of stainless steel sheets is strongly dependent onboth the work hardening exponent and the material anisotropy.

Characterization of Deposited Layer Fabricated by Direct Laser Melting Process

장정환,Byeong Don Joo,Chester J. Van Tyne,문영훈 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.3

Deposition dimensions are important in the final applications of products made by direct laser melting (DLM). This investigation used a 200 W fiber laser to produce single-line beads from stainless steel 316L powder using a variety of different energy distributions. To investigate the deposited layer, deposition width, height, penetration depth, and side surface roughness were measured. In order to validate the effectiveness of the two main process parameters (laser power and scan rate), multi-layered beads were fabricated by the sequential layering of single lines. It was found that with an increase in linear energy density, the wetting angle was reduced, and the average roughness was also increased with linear energy density. An equation that predicts the deposition height for a multi-layered bead is proposed and experimentally validated in this study. For deposited layer applications, the material properties of the deposited layer, such as contact angle, interfacial contact resistance, and flexural strength are estimated. The rougher deposited layers show higher contact angle and interfacial contact resistance. The flexural strength of the DLM fabricated specimen is above 250 MPa.

Effect of Roll Configuration on the Leveling Effectiveness of Tail-Up Bent Plate Using Finite-Element Analysis

Hyung Seo, Jae,Van Tyne, Chester J.,Hoon Moon, Young ASME International 2016 Journal of Manufacturing Science and Engineering Vol.138 No.7

Thermal and Mechanical Properties of Electro-Slag Cast Steel for Hot Working Tools

Moon Young Hoon,Kang Boo Hyun,Van Tyne Chester J. The Korean Society of Mechanical Engineers 2005 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.19 No.2

The thermal and mechanical properties of an electro-slag cast steel of a similar chemical composition with an AISI-6F2 steel are investigated and compared with a forged AISI-6F2 steel. AISI-6F2 is a hot-working tool steel. Electro-slag casting (ESC) is a method of producing ingots in a water-cooled metal mold by the heat generated in an electrically conductive slag when current passes through a consumable electrode. The ESC method provides the possibility of producing material for the high quality hot-working tools and ingots directly into a desirable shape. In the present study, the thermal and mechanical properties of yield strength, tensile strength, hardness, impact toughness, wear resistance, thermal fatigue resistance, and thermal shock resistance for electro-slag cast and forged steel are experimentally measured for both annealed and quenched and tempered heat treatment conditions. It has been found that the electro-slag cast steel has comparable thermal and mechanical properties to the forged steel.