Tae-Wan Ku

Bio: Tae-Wan Ku is an academic researcher from Pusan National University. The author has contributed to research in topics: Finite element method & Forming processes. The author has an hindex of 15, co-authored 76 publications receiving 789 citations.

Constitutive modeling for the dynamic recrystallization evolution of AZ80 magnesium alloy based on stress–strain data

Abstract: In order to improve the understanding of the dynamic recrystallization (DRX) behaviors of as-cast AZ80 magnesium alloy, a series of isothermal upsetting experiments with height reduction 60% were performed at the temperatures of 523 K, 573 K, 623 K and 673 K, and the strain rates of 001 s−1, 01 s−1, 1 s−1 and 10 s−1 on a Gleeble 1500 thermo-mechanical simulator Dependence of the flow stress on temperature and strain rate is described by means of the conventional hyperbolic sine equation By regression analysis, the activation energy of DRX in the whole range of deformation temperature was determined to be Q = 21582 kJ mol−1 Based on dσ/dɛ versus σ curves and their processing results, the flow stress curves for AZ80 magnesium alloy were evaluated that they have some characteristic points including the critical strain for DRX initiation (ɛc), the strain for peak stress (ɛp), and the strain for maximum softening rate (ɛ*), which means that the evolution of DRX can be expressed by the process variables In order to characterize the evolution of DRX volume fraction, the modified Avrami type equation including ɛc and ɛ* as a function of the dimensionless parameter controlling the stored energy, Z/A, was evaluated and the effect of deformation conditions was described in detail Finally, the theoretical prediction on the relationships between the DRX volume fractions and the deformation conditions were validated by the microstructure graphs

Experimental and Numerical Investigation on Impact Performance of Carbon Reinforced Aluminum Laminates

Abstract: It is known that fiber metal laminates (FML) as one of hybrid materials with thin metal sheets and fiber/epoxy layers have the characteristics of the excellent damage tolerance, fatigue and impact properties with a relatively low density. Therefore, the mechanical components using FML can contribute the enhanced safety level of the sound construction toward the whole body. In this study, the impact performance of carbon reinforced aluminum laminates (CARAL) is investigated by experiments and numerical simulations. Drop weight tests are carried out with the weight of 4.7 kg at the speed of 1 and 2 m/s, respectively. Dynamic non-linear transient analyses are also accomplished using a finite element analysis software, ABAQUS. The experiment results and numerical results are compared with impact load-time histories. Also, energy-time histories are applied to investigate the impact performance of CARAL.

The workability evaluation of wrought AZ80 magnesium alloy in hot compression

Abstract: Workability, an important parameter in metal forming process, can be evaluated by means of processing maps on the basis of dynamic materials model (DMM), constructed from experimentally generated flow stress variation with respect to strain, strain rate and temperature. To obtain the processing maps of wrought AZ80 magnesium alloy, hot compression tests were performed over a range of temperatures 523–673 K and strain rates 0.01–10 s−1. As the true strain is 0.25, 0.45, 0.65, 0.85 respectively, the response of strain-rate sensitivity (m-value), power dissipation efficiency (η-value) and instability parameter (ξ-value) to temperature and strain rate were evaluated. By the superimposition of the power dissipation and the instability maps, the stable, metastable and unstable regions were clarified clearly. In further, in the stable area the regions having the highest efficiency of power dissipation were identified and recommended. The optimal working parameters identified by the processing maps contribute to designing the hot forming process of AZ80 magnesium alloy without any defect.

Finite Element Analysis of Multi-Stage Deep Drawing Process for High Precision Rectangular Case with Extreme Aspect Ratio

Abstract: Deep drawing process for rectangular drawn section is different with that for axisymmetric circular one. Therefore deep drawing process for rectangular drawn section requires several intermediate steps to generate the final configuration without any significant defect. In this study, finite element analysis for multi-stage deep drawing process for high-precision rectangular cases is carried out especially for an extreme aspect ratio. The results of analysis show that the irregular contact condition between blank and die affects the occurrence of failure, and the difference of aspect ratio in the drawn section leads to non-uniform metal flow, which may cause failure. A series of experiments for multi-stage deep drawing process for the rectangular cases are conducted, and the deformation configuration and the thickness distribution of the drawn rectangular cases are investigated by comparing with the results of the numerical analysis. The numerical analysis with an explicit time integration scheme shows good agreement with the experimental observation.

Improvement of formability for multi-point bending process of AZ31B sheet material using elastic cushion

Abstract: On multi-point forming process, one of the most obvious limitations is the need for a pliable interpolating material such as elastic cushion between punch element tips and sheet metal to prevent the formation of dimples on the surface of final part. In this study, numerical simulations of multi-point bending process in case of different thicknesses of elastic cushion are performed to obtain a specified final shape as a cylindrical surface with curvature radius of 434.65mm and centre angle parameter of 52.73° by using initial blank with length of 800mm, width of 600mm, and thickness of 2mm, respectively. To find the suitable thickness of the elastic cushion, four evaluating indicators including plastic dissipation energy, stress components, shape error and maximum ductile damage are introduced and analyzed. As the results, each value of four evaluating indicators is decreased, and their distributions become more uniform on the deformed blank by adopting the elastic cushion. Resultantly, it is summarized that the formability of AZ31B magnesium alloy can be improved by using the elastic cushion, and the most proper thickness of the elastic cushion is 4 mm for the multi-point bending process of AZ31B sheet with thickness of 2mm.

Bulk forming of sheet metal

Abstract: Ever increasing demands on functional integration of high strength light weight products leads to the development of a new class of manufacturing processes. The application of bulk forming processes to sheet or plate semi-finished products, sometimes in combination with conventional sheet forming processes creates new products with the requested properties. The paper defines this new class of sheet-bulk metal forming processes, gives an overview of the existing processes belonging to this class, highlights the tooling aspects as well as the resulting product properties and presents a short summary of the relevant work that has been done towards modeling and simulation.

Impact resistance of fiber-metal laminates: A review

Abstract: Combining the suitable properties of metals and fiber reinforced composites, as the idea behind the application of new types of materials, called fiber metal laminates (FMLs), have lead to superior impact properties as well as considerable improvement in fatigue performance. The characteristics of FMLs under impact loading and the ways to improve their properties to withstand this type of loading could be of particular importance in aerospace structures and other applications. This paper reviews relevant literature which deals with experimental evidence of “material related” and “event related” impact resistance parameters as well as the articles related to theoretical and numerical simulation of impact loading of FMLs. Relevant results will be discussed and the recommendations that need to be resolved in the future will be addressed.

Low velocity impact response of fibre-metal laminates – A review

Abstract: This contribution hopes to give a comprehensive review of past and current research work published on the dynamic response of fibre-metal laminates subjected to low velocity impact. The historical development of fibre-metal laminates is first reviewed in details, and notable researchers and their contributions are chronologically tabulated and reviewed. Included are also reviews on published experimental, numerical and analytical work on the low velocity impact of fibre-metal laminates. Detailed discussions on the two main groups of parameters namely geometry and material based parameters that influenced the structural response of fibre metal laminates to low-velocity impact. The review concludes with detailed discussions on the future works needed for fibre-metal laminates subjected to low velocity impact loads.

A review on forming techniques for manufacturing lightweight complex—shaped aluminium panel components

Abstract: Aluminium alloys are being increasingly utilised in the automotive and aerospace industries to reduce the weight of vehicles. Extensive research has been conducted to overcome the poor ductility of aluminium alloys at room temperature and improve formability of the materials, to enable complex-shaped panel components to be manufactured. To this end, this paper contains a comprehensive review of widely used forming processes for aluminium alloys, under cold, warm and hot forming conditions, and the material characteristics and equipment used for each process. Based on a detailed analysis from the view of industrial requirements, recent progress in experimentation techniques are reviewed addressing the limitations and improvements of specific forming processes. Furthermore, material modelling methods at both cold and elevated temperature forming conditions have been presented. In addition, finite element (FE) simulations with the implementation of material models are discussed. This review article intends to provide a systematic guide for process designers to choose the most appropriate sheet forming technique for specific industrial applications.

Formability analyses of uni-directional and textile reinforced thermoplastics Part A Applied science and manufacturing

Abstract: The formability of two different composite materials used in aerospace industry has been investigated for a representative product geometry. The deformations during forming of carbon UD/PEEK and glass 8HS/PPS blanks with a quasi-isotropic lay-up were analysed. The UD/PEEK product showed severe wrinkling in doubly curved areas, whereas the 8HS/PPS product showed better formability in those areas. This can be explained by the relatively high resistance against intra-ply shear for the UD/PEEK material. Moreover, the predictive capability of a finite element based simulation tool was shown. For both materials, the prediction of intra-ply shear and large wrinkles showed good agreement with those observed in the actual product. The smaller wrinkles in the products cannot be accurately represented with the element size used. However, predicted waviness at the corresponding locations could indicate critical areas in the product. The presented modelling approach shows great potential for application in the composite product design process.