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Kim, B.J.,Van Tyne, C.J.,Lee, M.Y.,Moon, Y.H. Elsevier 2007 Journal of materials processing technology Vol.187 No.-
<P><B>Abstract</B></P><P>The hydroformability of an extruded aluminum alloy at elevated temperatures was investigated in this study. To properly analyze the process, it is necessary to account for the variation in the mechanical properties of the aluminum that depend on the forming temperature and the heat conduction during warm hydroforming. Simulations coupling the plastic deformation and temperature distribution in the warm hydroforming process were performed and compared with experimental data. The multi-purpose finite element code DEFORM-2D can handle the calculations, but requires significant computation time if contact heat transfer between the die, the tube, and the pressure medium occurs. Experiments were conducted with a high temperature tribometer (pin-on-disk) allowing the measurement of the friction coefficient for the aluminum alloys at several temperatures. These friction results are applied to the coupled simulation. From the simulations, the optimal process parameters, such as internal pressure and preset temperature on hydroformability, can be found. The comparison of the finite element analysis with the experimental results shows that the hydroformability trends (given by bulge height) and the temperature distribution of the tube specimen agree well with one another. The finite element results also showed that the temperature distribution did not strongly affect the hydroformability of the aluminum tubes.</P>
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Analysis of warping during flat rolling of bimetallic slabs
Lee, CH,Park, JP,Tyne, CV,Moon, YH Professional Engineering Publishing Ltd 2015 Proceedings of the Institution of Mechanical Engin Vol. No.
<P>The current study analyzes the warping behavior of bimetallic slabs during the flat rolling process. An analytical model that predicts the degree of warping during the rolling of bimetallic slabs is developed, based on differential bending, which occurs due to the mismatch in longitudinal elongation. The warping model focuses on the mismatch of the lengthwise elongation, due to differences in the deformation resistances of constituent materials in the bimetallic slab. With this analytical model, warping can be predicted for various processing parameters, such as initial thickness ratios and rolling reductions. To validate the model, flat rolling experiments using Al1050/Al6061 bimetallic slabs, produced by continuous clad casting, were conducted. The deformation characteristics of the bimetallic slabs during rolling were also assessed by a finite element analysis using the commercial code, FORGE™. The results from the current study show that the warping behavior of bimetallic slabs during flat rolling can be characterized with a high degree of reliability.</P>
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A model for an adaptable axisymmetric extrusion die with a bearing length
Gordon, W.A.,Van Tyne, C.J.,Moon, Y.H. Elsevier 2007 Journal of materials processing technology Vol.191 No.1-3
The adaptable design method uses upper bound models to determine die shapes that meet specified criteria, such as minimizing distortion in the product. This method has been developed for the axisymmetric extrusion process. In order to extend the methodology to the three-dimensional extrusion of a non-axisymmetric shape, the deformation zone in the upper bound model would need to be extended into the bearing region of the extrusion die. The necessary equations and conditions needed to make such an extension are presented in this paper. Finite element modeling (FEM) is used to compare the results of extrusion through dies designed by the adaptable die design method, for a model with the deformation zone extended into the bearing region, to a model without such an extension. The results indicate that the upper bound model incorporating a bearing length provides a realistic flow field. The results also demonstrate that the upper bound model can be used to analyze a multi-sectioned die, so long as: (1) the die surface and first derivative of the surface are continuous between sections, and (2) calculation of the internal power of deformation is made for each section separately. The results provide further support for the findings that the average effective strain and the volumetric effective strain rate deviation are robust criteria, which can be used to determine optimal adaptable die shapes with an upper bound model.
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Yoon, S.,Van Tyne, C.J.,Lee, H. Elsevier 2014 Current Applied Physics Vol.14 No.7
The electrical properties of 9 mol% MgO-ZrO<SUB>2</SUB> (Mg-PSZ) with 1 mol% Al<SUB>2</SUB>O<SUB>3</SUB> and the mechanisms for electrical degradation were investigated using structural, morphological, and electrochemical analyses. The addition of Al<SUB>2</SUB>O<SUB>3</SUB> 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 μm and an intergranular layer composed of the Mg-rich phase were identified in a specimen sintered at 1600 <SUP>o</SUP>C. This specimen exhibited a minimum of ionic conductivity (4.98 x 10<SUP>-4</SUP> S cm<SUP>-1</SUP> at 700 <SUP>o</SUP>C) due to the grain boundary resistance (245 Ω cm<SUP>2</SUP>), which dominated the overall resistance. A similar trend was observed over the entire temperature range (600-1500 <SUP>o</SUP>C). An intergranular siliceous impurity (SiO<SUB>2</SUB>) 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 Al<SUB>2</SUB>O<SUB>3</SUB> addition.
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Overview of adaptable die design for extrusions
Gordon, W.A.,Van Tyne, C.J.,Moon, Y.H. Elsevier 2007 Journal of materials processing technology Vol. No.
<P><B>Abstract</B></P><P>The term “adaptable die design” is used for the methodology in which the tooling shape is determined or modified to produce some optimal property in either product or process. The adaptable die design method, used in conjunction with an upper bound model, allows the rapid evaluation of a large number of die shapes and the discovery of the one that produces the desired outcome. In order for the adaptable die design method to be successful, it is necessary to have a realistic velocity field for the deformation process through extrusion dies of any shape and the velocity field must allow flexibility in material movement to achieve the required material flow description. A variety of criteria can be used in the adaptable die design method. For example, dies which produce minimal distortion in the product. A double optimization process is used to determine the values for the flexible variables in the velocity field and secondly to determine the die shape that best meets the given criteria. The method has been extended to the design of dies for non-axisymmetric product shapes.</P>
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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.
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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.
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