Papers by Author: Dong Geun Lee

Abstract: The effect of post weld heat treatment on mechanical properties of friction welded Alloy 718 and SNCRW was investigated. Friction welding tests were carried out at a constant rotation speed and pressure. Optimum friction condition was found to be the friction pressure of 25kg/cm2, friction time of 40sec, upset pressure of 80 kg/cm2, and dwell time of 5sec. After friction welding tests, post weld heat treatments were performed in the temperature range of 500-900°C for 8hrs in order to investigate the microstructure and mechanical properties of weld joint. Specimens with the post weld heat treatment at 720°C for 8hrs show optimal mechanical properties. Residual stress of post weld heat treated specimens was measured to weld joint in the same temperature range. After friction welding tests on samples with a diameter of 80mm, tensile properties of post-weld-heat-treated and non-heat-treated samples were compared.

Abstract: The process design of profile ring rolling for a large-scale turbine diaphragm was made using the calculation method and three dimensional finite element method (FEM). The design criteria are to achieve uniform distributions of strain and temperature, and defect-free profiled ring products. Based on the compression test results of the low alloy steel, deformation processing map was generated using the combination of the dynamic materials model (DMM) and stable or unstable forming criteria. The processing map was used to determine the optimum ring rolling temperature and feed rate of the mandrel. FEM analysis was simulated to predict the formation of rolling defects and deformed shape in the profile ring rolled diaphragm product. Finally, optimum process design to obtain a sound large-scale turbine diaphragm without forming defects was suggested and its validation was made by the comparison between the experimental data and FE analysis results.

Abstract: Dynamic deformation and fracture behavior of Zr-based bulk metallic glass (BMG) and BMG composite containing dendritic β phases was investigated in this study. Dynamic compressive test results indicated that both maximum compressive stress and total strain of the BMG and BMG composite decreased with increasing test temperature because shear bands could propagate rapidly as the adiabatic heating effect was added at high temperatures. Above the glass transition temperature, total strain decreased more abruptly due to crystallization of amorphous phases. Maximum compressive stress and total strain of the BMG composite were higher than those of the BMG because β phases played a role in forming multiple shear bands. The BMG composite having more excellent dynamic properties than the BMG can be more reliably applied to the structures or parts requiring dynamic properties.

Abstract: A methodology for evaluating and predicting component lives in creep-fatigue interaction region was investigated for Waspaloy. A unified viscoplasticity constitutive equation including multi-back stresses was used to describe cyclic material behaviors. Also, a continuum damage model coupling with the creep-fatigue damage rules was established based on the analysis of creep and low cycle fatigue behavior. Multi-axial fatigue and creep equivalent stress concepts were employed to predict three dimensional component lives. Notched cyclic tests under various stress conditions in the creep-fatigue interaction region were carried out to validate the life prediction methodology with FEM simulation based on the continuum damage model. The comparison of experimental data and prediction results indicates that the continuum damage model is a powerful approach for the prediction of component lives.

Abstract: Dynamic compressive tests were conducted on Zr-based amorphous alloys and amorphous matrix composite containing dendritic β phases. Dynamic compressive tests were conducted using a compressive Kolsky bar and then the test data were analyzed in relation to microstructure and fracture mode. Under dynamic loading, the maximum shear stress and ductility of the amorphous alloy and composite were considerably lower than those under quasi-static loading because of the decreased resistance to fracture. Deformation under dynamic loading lowered strain and compressive strength because of reduced fracture resistance, and the alloy containing dendritic β phases showed better compressive strength and ductility than the monolithic alloy.