Duk-Jae Yoon

TENSILE TEST BASED MATERIAL IDENTIFICATION PROGRAM AFDEX/MAT AND ITS APPLICATION TO TWO NEW PRE-HEAT TREATED STEELS AND A CONVENTIONAL Cr-Mo STEEL

A material identification program AFDEX/MAT is presented in this paper. The program is based on the method for acquiring true stress-strain curves over large range of strains using engineering stress-strain curves obtained from a tensile test coupled with a finite element analysis. In the method, a tensile test is analyzed using a rigid-plastic finite element method combined with a perfect analysis model for its associated simple bar to provide the information of deformation. An initial reference true stress-strain curve, which predicts the necking point exactly, is modified iteratively to minimize the difference in tensile force between the experiments and predictions of the tensile test. It was applied to identifying the mechanical behaviors of two new pre-heat treated steels of ESW95 and ESW105 and a conventional Cr-Mo steel of SCM435. The predictions are compared with the experiments for the tensile test of the three materials, showing an excellent similarity.

Finite Element Analysis of Central Bursting Defects Occurring in Cold Forward Extrusion

An improved approach to simulating the formation of central bursting defects is presented in this paper. The rigid-plastic finite element method and a modified McClintock damage model are employed. An improved node-splitting technique with degenerate element cleaning is proposed. The technique is applied to several cold forward extrusion processes to reveal the effects of reduction of area, die conical angle, and friction on the formation of central bursting defects. A comparison with experimental results and predictions found in the literature shows that the present analysis provides more realistic predictions of the shape of a central bursting defect (i.e., an obtuse V-shape, which is typical).Copyright

Quantitative study of the Bauschinger effect in high-strength low-strain-hardening materials for metal forming applications

In this paper, an experimental study of the Bauschinger effect is carried out in high-strength, low-strain-hardening materials. Pre-heat-treated ESW105 steel, which has a high initial yield stress with negligible strain-hardening capability, is employed in this study. For the sake of comparison, the Bauschinger effect is also investigated in SCM435 steel, which has typical strain-hardening capability. The Bauschinger effect tends to decrease the forming load during cylinder compression of an ESW105 workpiece initially elongated via a drawing process with proper reduction of area, but does not contribute to forming load reduction in the case of SCM435, even under the same conditions, since it is surpassed by the strain-hardening capability of the material.

Additional material feeding method by plunger shape design for hydroforming of triangular tubes

In this study, a hydroforming process is used for the manufacture of a bicycle frame tube, one side end of which is expanded to an acute-angled triangular shape from the initial tube, which has a regular triangular cross section. The end section of the expanding side is designed to be smoothly connected with the initial tube cross section to maintain sealing by the plunger. The tube hydroforming process is controlled using an axial feeding plunger and a pressure intensifier. Various load conditions can be applied to the initial tubes through these units. However, the experimental results showed that forming into the desired shape without bursting at the expanding area or shear cracking at other areas was not possible. These problems seem to occur because the circumferential propagation of plastic deformation is constrained by the triangular cross-sectional profile of the initial tube. To resolve this problem, less axial feeding should be supplied to the compressing zone compared with the expanding zone even in a single tube. In this work, besides conventional axial feeding, an additional axial feeding method by an end cam–shaped plunger is suggested to supply different axial feeding for each circumferential location of the tube. Finite element simulation and experimental studies are performed to verify the effectiveness of the concept.

A PROCESS MAP FOR SEQUENTIAL COMPRESSION-BACKWARD EXTRUSION OF AZ31 MG ALLOYS AT WARM TEMPERATURS

This paper is concerned with development of a process map for sequential compression-backward extrusion of bulk AZ31 Mg alloy at the warm temperatures. In experiments, metal flows and crack initiations are carefully investigated and formability is examined systematically for various forming conditions such as forming temperature, punch speed, and a gap between a specimen and die. Then, a process map for the sequential compression-backward extrusion of bulk AZ31 Mg alloy at the warm temperature is proposed. In order to further understand deformation behaviors and damage evolution during warm forming process, thermo mechanical finite element analyses coupled with damage evolutions are carried out. In general, finite element predictions support experimental observation. Finally, it is concluded that the process map, proposed for the sequential compression-backward extrusion of AZ31 Mg alloy, is valid.

Product Geometry for Process Parameter during Rotary Swaging Process as Chipless Forming Process

This study deals with the dimensional accuracy of outer diameter and geometrical workability in rotary swaged product for various process parameters such as percent reduction in outer diameter and the ratio of thickness to outer diameter of a tube. It is generally known that greater cold strengthening is achieved by rotary swaging process rather than by conventional process such as rolling with respect to the same reduction of cross-sectional area. Percent reduction in the diameter and the ratio are considered and selected as important process parameters because of playing a key role during rotary swaging process. In case of tube under rotary swaging process the ratios have influence on geometrically proper workability without defect for different percent reductions in the diameter. In addition the change of metal flow of workpiece under the swaging process is microscopically and globally observed to analyze the reason why dimensional accuracy of the outer diameter of final product is improved after the rotary swaging process. This work might provide available information for the optimum rotary swaging process.