Heung-Kyu Kim

Evaluation of heat transfer coefficient during heat treatment by inverse analysis

Abstract Reliable prediction of the properties of a heat-treated workpiece requires accurate data of the heat transfer coefficient during heat treatment process. In the present investigation, inverse heat transfer formulation using a two-dimensional finite element method was developed as a tool to evaluate heat transfer coefficient during heat treatment. The formulation has a function of providing a time profile of heat transfer coefficient on various surface locations with measured temperature at proper locations within workpiece under heat treatment. By the present formulation, heat transfer coefficients were evaluated for fan and water cooling of heated solid cylindrical carbon steel specimens. Comparison of the obtained heat transfer coefficients against the existing results on heat treatment shows that the obtained values of coefficients have reasonable accuracy, enough to predict the properties of the heat-treated workpiece.

Finite element analysis of grain-by-grain deformation by crystal plasticity with couple stress

Abstract Rigid–plastic crystal plasticity with the rate-sensitive constitutive behavior of a slip system has been formulated within the framework of a two-dimensional finite element method to predict the grain-by-grain deformation of single- and polycrystalline FCC metals. For that purpose, individual grains are represented by several numbers of finite elements to describe the sub-grain deformation behavior, and couple stress has been introduced into the equilibrium equation to be able to describe the size effect as well as to prevent mesh-dependent predictions. A modified virtual work-rate principle with an approximate interface constraint has been suggested to use a C 0 -continuous element in the finite element implementation, and the couple stress work-rate has been formulated on the basis of an assumed constitutive behavior. Simulated plane-strain compressions of a single crystal cube show that the shearing and the deformation load are closely related to the imbedded lattice orientation of the crystal grain, and that the sub-grain deformation and the load magnitude can be controlled by the couple stress hardening. It is also confirmed that almost the same predictions are obtained for different mesh systems by considering the couple stress hardening. Simulated plane-strain compressions of a bi-crystal show considerably curved grain-by-grain surface profiles after large reduction for several combinations of the imbedded lattice orientation. The high couple stress hardening predicted around grain boundaries is supposed to be related to the grain size effect. It is also supposed that consideration of couple stress is necessary to predict the sub-grain or the grain-by-grain deformation, and the couple stress hardening may be used to describe the state of microstructures in grain.

Evaluation of Contact Heat Transfer Coefficient and Phase Transformation during Hot Stamping of a Hat-Type Part

Using an inverse analysis technique, the heat transfer coefficient on the die-workpiece contact surface of a hot stamping process was evaluated as a power law function of contact pressure. This evaluation was to determine whether the heat transfer coefficient on the contact surface could be used for finite element analysis of the entire hot stamping process. By comparing results of the finite element analysis and experimental measurements of the phase transformation, an evaluation was performed to determine whether the obtained heat transfer coefficient function could provide reasonable finite element prediction for workpiece properties affected by the hot stamping process.

An Optimal Cure Process to Minimize Residual Void and Optical Birefringence for a LED Silicone Encapsulant

Silicone resin has recently attracted great attention as a high-power Light Emitting Diode (LED) encapsulant material due to its good thermal stability and optical properties. In general, the abrupt curing reaction of the silicone resin for the LED encapsulant during the curing process induces reduction in the mechanical and optical properties of the LED product due to the generation of residual void and moisture, birefringence, and residual stress in the final formation. In order to prevent such an abrupt curing reaction, the reduction of residual void and birefringence of the silicone resin was observed through experimentation by introducing the multi-step cure processes, while the residual stress was calculated by conducting finite element analysis that coupled the heat of cure reaction and cure shrinkage. The results of experiment and analysis showed that it was during the three-step curing process that the residual void, birefringence, and residual stress reduced the most in similar tendency. Through such experimentation and finite element analysis, the study was able to confirm that the optimization of the LED encapsulant packaging process was possible.

A Comparative Study of Failure Criteria for Magnesium Alloy Sheet under Warm Press Forming Condition

Magnesium sheet alloys possess limited plastic formability at room temperature but their formability is substantially improved at elevated temperatures and optimum strain rates. In the present paper, three different types of failure criteria, namely, strain-based, stress-based, and work-based criteria, are compared for their applicability to warm press forming of magnesium sheet alloys. Warm deep-drawing experiments were conducted on AZ31 alloy sheet, and the results were used to assess the strength and weakness of the failure criteria.

Forming Analysis for Warm Deep Drawing Process of Magnesium Alloy Sheet

Due to the low densities and high specific strength and stiffness, magnesium alloy sheets are very attractive lightweight materials for automotive and electrical products. However, the magnesium alloy sheets should be usually formed at elevated temperature because of their poor formability at room temperature. For the use of the magnesium alloy sheets for an industrial, their mechanical properties at elevated temperature and appropriate forming process conditions have to be developed. In this study, non-isothermal simulation of a square cup drawing of magnesium alloy sheets have been conducted to evaluate a proper forming process conditions such as the tool temperature, the tool shoulder radius, friction between the blank and the tools. According to this study, appropriate forming process conditions of square cup drawing at elevated temperature from magnesium alloy sheets are suggested.

Investigation of Shape Accuracy in the Forming of a Thin-walled S-rail with Classification of Springback Modes

This paper aims to evaluate quantitatively the springback characteristics that evolve in the sheet metal forming of an S-rail in order to understand the reasons of shape inaccuracy and to find a remedy. The geometrical springback is classified into six modes: angle change of punch and die shoulders, wall curl, ridge curl, section twist, and axial twist. The measuring method for each springback mode is suggested and quantitative measurements were made to determine the tendency towards shape accuracy. Forming experiments were conducted with four types of steel sheets that have different tensile strengths, which were 340MPa, 440MPa, 590MPa and 780MPa, in order to evaluate the effect of the tensile strength and the bead shape on the springback behavior. Springback tendencies show that they are greatly affected by the tensile strength of the sheet and the shape of the tools. Almost all springback modes except the section twist and the axial twist show a linearly increasing trend as the tensile strength of the sheet increases. The results can be used as basic data for design and for compensation of the press die geometry when forming high strength steels which exhibit large amounts of springback.

Comparison of Springback Modes in the Stamping Process of an S-rail with HSS according to the Hardening Model

In this study, springback amounts of an S-rail are quantitatively compared according to the hardening model using a finite element simulation for the stamping process with high strength steels. For comparison of the hardening models, two types of hardening models were investigated. The two models were isotropic hardening and kinematic hardening. For the analysis with kinematic hardening, the Yoshida-Uemori model was selected. Five kinds of springback modes were measured at designated sections and a comparison was made between the experiment and the analyses with two types of hardening models. The analysis results show that the springback in the flange and the wall curl are predicted more accurately with a kinematic hardening model.

A study on the change of die roll size by the shape of die chamfer in fine blanking die for automobile door latch

Abstract There is always die roll in fine blanking parts which is able to have 100% clean shear surface. In this paper the change of die roll size was studied by fine blanking tryout in order to minimize die roll size. Various die inserts with different die chamfer were machined, fine blanking die was manufactured and tested. The die roll sizes of fine blanking samples were measured and the tendency of thickness directional die roll size was comprehended. This result will be used on the design of die chamfer in order to minimize thickness directional die roll size of fine blanking partsKey Words : Fine blanking, Die roll, Die chamfer, Die insert, Punch 본 논문은 지식경제부의 자동차 핵심 부품 생산 기반 공정 플랫폼 기술 개발 사업(과제번호 : 10-FM-2-0038)의 연구 과제로 수행되었음. * 교신저자 : 김종덕(jdk@kitech.re.kr)접수일 11년 01월 03일 수정일 (1차 11년 02월 08일, 2차 11년 02월 09일) 게재확정일 11년 02월 10일 1. 서론 파인 블랭킹(이하 FB)은 1 회의 블랭킹에서 제품의 전체 두께에 걸쳐 필요로 하는 고운 전단면과 양호한 제품 정밀도를 얻는 프레스 가공 공정으로 더 이상의 기계 가공이 필요 없다. 일반적으로 FB 기술이 적용되는 제품은 사용 목적상 일반 프레스에서 블랭킹 가공할 경우 후속 공정이 많아져 경제성이 없기 때문에 후판 제품(두께 0.5 - 20mm)에 많이 적용되고 있다. 따라서 FB 제품은 주로 자동차의 기능 부품, 전기전자 구조 부품, 일반 기계요소 등과 같이 제품의 정밀도와 절단면의 품질이 동시에 요구되는 후판 제품에 적용된다. FB 제품은 그림 1과 같이 전단 과정이 3 단의 복합 운동 프레스에서 재료를 완전히 클램핑한 상태에서 전단하고 특히 펀치와 다이 사이의 간극(clearance)이 제품 두께의 약 0.5%를 유지하므로 전체 두께의 절단면에 걸쳐 매끄러운 전단면을 얻을 수 있으며, 표면 거칠기는 재료의 두께에 따라 일반 기계 가공에서 얻을 수 있는 Ra 0.4 -16

Finite-Element Analysis of Warm Square Cup Deep Drawing Process of Magnesium Alloy AZ31 Sheet

Magnesium alloys are expected to be widely used fur the parts of structural and electronic appliances due to their lightweight and EMI shielding characteristics. While the die casting has been mainly used to manufacture the parts from the magnesium alloys, the press forming is considered as an alternative to the die casting for saving the manufacturing cost and improving the structural strength of the magnesium alloy parts. However, the magnesium alloy has low formability at room temperature and therefore, in many cases, forming at elevated temperatures is necessary to obtain the required material flow without failure. In the present study, square cup deep drawing tests using the magnesium alloy AZ31 sheet were experimentally conducted at various elevated temperatures as well as room temperature, and the corresponding finite-element simulations, which calculated the damage evolution based on the Oyane`s criterion, were conducted using the stress-strain relations from the tensile tests at various temperatures. The formability predictability by the finite-element analysis was investigated by comparing the predicted damage distributions over the deformed AZ31 sheet at elevated temperatures with the corresponding experimental deformations with failures.