Junehyung Kim

Investigation into manufacturing of very long cups by hydromechanical deep drawing and ironing with controlled radial pressure

Abstract High-quality cups of deep drawing ratio of more than four cannot be simply drawn by conventional drawing and redrawing process. In the present study, after the first deep drawing process, subsequent hydromechanical reverse redrawing with controlled radial pressure and final ironing to control the thickness and outer surface appearance are employed. In order to increase the deep drawing ratio much more than four, the radial pressure is controlled independently of the chamber pressure and thus an optimum forming condition can be found by varying the radial pressure. The final ironing process ensures the fine surface quality as well as improved deep drawing ratio without inducing any forming defect such as slight puckering and surface waviness. The process has been subject to finite element analysis by using the rigid-plastic material modeling considering all the frictional conditions induced by the hydrostatic pressure. The comparison of the computation with the experiment has shown that the finite element modeling can be conveniently employed for the design of the process with reliability from the viewpoint of formability.

Effect of hardening laws and yield function types on spring-back simulations of dual-phase steel automotive sheets

In order to simulate the spring-back of DP-steel automotive sheets, the effect of hardening laws and yield function types has been studied based on finite element simulations performed for 2-D draw bending and S-rail tests. Specifically, the performance of the combined isotropic-kinematic hardening based on the modified Chaboche model was compared with those of the pure isotropic hardening and kinematic hardening laws, along with the non-quadratic anisotropic yield (Yld2000-2d) and Mises yield functions. As for the 2-D draw bending test, numerical results were compared with experimental results for verification purposes.

Improvement of Formability and Spring-Back of AA5052-H32 Sheets Based on Surface Friction Stir Method

A process to improve formability and spring-back was developed for AA5xxx-H temper sheets based on the surface friction stir (SFS) method. In the SFS method, a rotating probe stirs the sheet surface so that material flow and heat, which result from plastic deformation and friction, change the microstructure and macroscopic mechanical properties of the stirred zone and therefore, ultimately, the formability and spring-back performances of the whole sheet. When applied to AA5052-H32 sheets, the process improved formability and spring-back, as experimentally and numerically confirmed in the limit dome height and unconstrained bending tests.

Experimental Formability Investigation for FSW Sheets with Respect to Base Material`s Directional Combination

In order to investigate the formability of friction stir welded(FSW) tailor welded blanks(TWB) with respect to the base material`s directional combination, aluminum alloy AA6111-T4 sheets were welded with three different conjoining types: RD-RD, TD-RD and TD-TD. Here, RD and TD represent rolling and transverse directions, respectively. For experimental formability study, three tests with gradual complexity were performed: the simple tension test with various weld line directions for uni-axial elongation, the hemisphere dome stretching test for biaxial stretching and the cylindrical cup deep drawing test. As a result, all three forming tests showed that RD-RD type samples exhibited the best formability, while TD-TD type sheets showed the least formability performance.

Discharge flocking, a new method to disperse nano-particles

One of the big challenges in the nano-field is how to effectively disperse nanoscale particles, especially CNTs and carbon blacks, which are strongly agglomerated by intermolecular van der Waals forces. Solution-based dispersion methods such as sonication and three-roll milling require lots of dispersion time and have re-agglomeration problems after processing. This study suggests a new method, i.e. discharge flocking, to disperse the particles effectively. Firstly nano-scale particles located between two electrodes, a negative electrode and a ground electrode, are charged by Townsend discharge. Next the negatively charged particles flow toward a target substrate by electric field generated by a positive electrode behind the substrate, and they are finally attached on the substrate. In contrast with the solution-based methods, this process makes nanoscale particles disperse in the air and repel each others because of their negative charges. Therefore, the re-agglomeration problem of processed particles could be prevented. In this study, carbon fiber reinforced epoxy composites including CNTs and carbon blacks were fabricated by the discharge flocking and the solution-based dispersion methods, respectively, and then their mechanical and electrical properties were compared with each other.

Crash Performance Evaluation of Hydro-formed DP-steel Tubes Considering Welding Heat Effects, Formability and Spring-back

In order to numerically evaluate hydro‐formed DP‐steel tubes on crash performance considering welding heat effects, finite element simulations of crash behavior were performed for hydro‐formed tubes with and without heat treatment effects. Also, finite element simulations were performed for the sequential procedures of bending and hydro‐forming of tubes in order to design process parameters, particularly for the boost condition and axial feeding, considering formability and spring‐back. Effects of the material property including strain‐rate sensitivity on formability as well as spring‐back were also considered. The mechanical properties of the metal active gas (MAG) weld zone and the heat affected zone (HAZ) were obtained utilizing the continuous indentation method in this work.