Seong-Chan Heo

Numerical and Experimental Study on Plate Forming Process using Flexible Die

A flexible forming apparatus is composed a number of punches which have spherical pin tip shape instead of conventional solid die. The flexible forming tool consisted of punch array in a matrix form was proposed as an alternative forming method to substitute the conventional line heating method which use heat source to induce residual stress along specified heating lines. In this study, application of the flexible forming process to the small scale curved plate forming was conducted. Numerical simulations for both solid and flexible die forming process were carried out to compare the shape of the products between flexible and conventional die forming process. In addition, spring-back analysis was conducted to figure out the feasibility of the flexible forming process comparing with the die forming process in view of final configuration of the specimens. Moreover, experiment was also carried out to confirm the formability of the process. Consequently, it was confirmed that the flexible die forming method has capability and feasibility to manufacture the curved plates for shipbuilding.

Study on Application of Flexible Die to Sheet Metal Forming Process

Flexible forming process for sheet material using reconfigurable die is introduced based on numerical simulation. In general, this flexible forming process using the reconfigurable die has been utilized for manufacturing of curved thick plates used for hull structures, architectural structures and so on. In this study, numerical simulation of sheet metal forming process is carried out by using flexible dies model instead of conventional matched die set. The numerical simulation and experimental verification for sheet metal forming process using a flexible forming machine that is more suitable for thick plate forming process are carried out to confirm the appropriateness of the simulation process. As an elastic cushion, urethane pads are utilized using hyperelastic material model in the simulation for smoothing the forming surface which is discrete due to characteristics of the flexile die. In the flexible forming process for sheet metal, effect of a blank holder is also investigated according to blank holding methods. Formability in view of occurrence of dimples is compared with regard to the various punch sizes. Consequently, it is confirmed that the flexible forming for sheet material using urethane pad has enough capability and feasibility for manufacturing of smoothly curved surface instead of conventional die forming method.

Development of Stretch Forming Apparatus using Flexible Die

A stretch forming method has been widely used in sheet metal forming process. Especially, this process has been adopted in aircraft and high-speed train industries for skin structure forming having a variety of curvature. Until now, solid dies, which are designed with respect to the specific shapes and manufactured as a single piece, have been usually applied to stretch forming process. Therefore, a great number of solid dies has to be developed according to the shapes of the curved skin structure. Accordingly, a flexible die is proposed in this study. It replaces the conventional solid dies with a set of height adjustable punch array. A usefulness of the flexible die is verified through a formability comparison with the solid die using finite element method considering an elastic recovery and the stretch forming apparatus with the flexible die is developed.

Evaluation of Strain, Strain Rate and Temperature Dependent Flow Stress Model for Magnesium Alloy Sheets

The formability of magnesium alloy sheets at room temperature is generally low because of the inherently limited number of slip systems, but higher at temperatures over . Therefore, prior to the practical application of these materials, the forming limits should be evaluated as a function of the temperature and strain rate. This can be achieved experimentally by performing a series of tests or analytically by deriving the corresponding modeling approaches. However, before the formability analysis can be conducted, a model of flow stress, which includes the effects of strain, strain rate and temperature, should be carefully identified. In this paper, such procedure is carried out for Mg alloy AZ31 and the concept of flow stress surface is proposed. Experimental flow stresses at four temperature levels (, , , ) each with the pre-assigned strain rate levels of , and are collected in order to establish the relationships between these variables. The temperature-compensated strain rate parameter which combines, in a single variable, the effects of temperature and strain rate, is introduced to capture these relationships in a compact manner. This study shows that the proposed concept of flow stress surface is practically relevant for the evaluation of temperature and strain dependent formability.

Experimental Study on Non-Axisymmetric Rectangular Cup using Multi-Stage Deep Drawing Process

For multi-stage deep drawing process including ironing operation and biaxial forming in this study, tool developments are achieved, and the developed tool sets are applied to experimental investigations. In process and tool designs, a contact condition between intermediate blank and lower die is considered as the sequential one. In this study, the material used is cold-rolled thin sheet (SPCE) with the initial thickness of 0.4mm. From the experimental approaches, several failures such as tearing, localized thickening and thinning, are observed. To solve these failures, the contact surface on the lower die is modified. As the experimental results by applying the modified lower die, it is investigated that the failures are not occurred, and the excessive deformation behavior due to the thinning and thickening effects are decreased. Furthermore, the thickness distributions on the major axis and the minor axis of each intermediate blank are investigated to be already satisfied the target (ironing) thickness, respectively. By this systematic approach, it is confirmed that the experimental results show good agreements with the designed and required configuration of each deformed and final products.

Tendency Analysis of Shape Error According to Forming Parameter in Flexible Stretch Forming Process Using Finite Element Method

A shape error of the sheet metal product made by a flexible stretch forming process is occurred by a various forming parameters. A die used in the flexible stretch forming is composed of a punch array to obtain the various objective surfaces using only one die. But gaps between the punches induce the shape error and the defect such as a scratch. Forming parameters of the punch size and the elastic pad to prevent the surface defect must be considered in the flexible die design process. In this study, tendency analysis of shape error according to the forming parameters in the flexible stretch process is conducted using a finite element method. Three forming parameters, which are the punch size, the objective curvature radius and the elastic pad thickness, are considered. Finite element modeling using the punch height calculation algorithm and the evaluation method of the shape error, which is a representative value for the formability of formed surface, are proposed. Consequently, the shape error is in proportion to the punch size and is out of proportion to the objective curvature radius and the elastic pad thickness.

Numerical and Experimental Study for Improvement of Formability in Flexible Forming Process

In this article, the design of the flexible forming process considering die shape compensation using an iterative over-bending method based on numerical simulation was conducted. In this method, the springback shape obtained from the final step of the first forming simulation is compared with the desired objective shape, and a shape error is calculated as a vector norm with three-dimensional coordinates. The error vector is inversely added to the objective surface to compensate both the upper and lower flexible die configurations. The flexible die shapes are recalculated and the punch arrays are adjusted according to the over-bent forming surface. These iterative procedures are repeated until the shape error variation converges to a small value. In addition, experimental verification was conducted using a 2000-kN flexible forming apparatus for thick plates. Finally, the configuration of the prototype obtained from the experiment was compared with the numerical simulation results, which had springback compensation. It is confirmed that the proposed method for compensating for the forming error could be used in the design of flexible forming of thick-curved plates.

Statistical Study on Correlation Between Design Variable and Shape Error in Flexible Stretch Forming

A flexible stretch forming process is useful for small quantity batch production because various shape changes of the flexible die can be achieved conveniently. In this study, the design variables, namely, the punch size, curvature radius and elastic pad thickness, were quantitatively evaluated to understand their influence on sheet formability using statistical methods such as the correlation and regression analyses. Forming simulations were designed and conducted by a three-way factorial design to obtain numerical values of a shape error. Linear relationships between the design variables and the shape error resulted from the Pearson correlation analysis. Subsequently, a regression analysis was also conducted between the design variables and the shape error. A regression equation was derived and used in the flexible die design stage to estimate the shape error.

Finite Element Analysis and Validation for Dimpled Tube Type Intercooler Using Homogenization Method

Three-dimensional finite-element methods(FEM) have been used to analyze the thermal stress of an exhaust gas recirculation(EGR) cooler due to thermal and pressure load. Since efficiency and capability of the heat exchanger are mainly dependent on net heat transferring area of the EGR cooler system, the tube inside the system has a numerous dimples on the surface. Thus for finite element analysis, firstly the dimple-typed tube is modeled as a plain element without the dimple, and then the equivalent thermal conductivities and elastic modulus are calculated. This work describes the numerical homogenization procedure of the dimple-typed tube and verifies the equivalent material properties by comparison of a single unit and the actual full model. Finally, the homogenization scheme presented in this study can be efficiently applied to finite element analyses for the thermal stress and deformation behavior of the EGR cooler system with the dimple-typed tube.

Evaluation of Formability on Hydroformed Part for Automobile Based on Finite Element Analysis

Tube hydroforming process is generally consisted with pre-bending, preforming and hydroforming processes. Among forming defects which may occur in tube hydroforming such as buckling, wrinkling and bursting, the wrinkling and bursting by local instability under excessive tensile stress mode were mainly caused by thinning phenomenon in the manufacturing process. Thus the accurate prediction and suitable evaluation of the thinning phenomenon play an important role in designing and producing the successfully hydroformed parts without any failures. In this work, the formability on hydroformed part for automobile, i.e. engine cradle, was evaluated using finite element analysis. The initial tube radius, loading path with axial feeding force and internal pressure, and preformed configuration after preforming process were considered as the dominant process parameters in total tube hydroforming process. The effects on these process parameters could be confirmed through the numerical experiments with respect to several kinds of finite element simulation conditions. The degree of enhancement on formability with each process parameters such as initial tube radius, loading path and preform configuration were also compared. Therefore, it is noted that the evaluation approach of the formability on hydroformed parts for lots of industrial fields proposed in this study will provide one of feasible methods to satisfy the increasing practical demands for the improvement of the formability in tube hydroforming processes.