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Extrusion Deformation Behavior of Magnesium Alloys : 마그네슘합금의 압출변형거동에 대한 연구

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Authors

최기순

Advisor
신광선
Major
공과대학 재료공학부
Issue Date
2015-02
Publisher
서울대학교 대학원
Keywords
Magnesium alloysExtrusionTextureFEMVPSC
Description
학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 2. 신광선.
Abstract
본 연구는 상용 AZ31합금과 신개발 ZAM631합금의 압출변형 중에 발달하는 집합조직 예측을 통해 마그네슘합금의 성형성을 획기적으로 향상시키기 위한 연구이다.
온도변화에 따른 미시적 소성변형기구의 고찰을 위하여 여러 슬립 및 쌍정모드의 임계전단응력을 도출하였다. 이를 위하여 미소 역학에 기초한 visco-plastic self-consistent-genetic algorithm (VPSC-GA) 전산 모사를 수행하였다. Basal 슬립과 tensile twin 모드의 임계전단응력은 온도변화에 민감하게 반응하지 않으며, 반대로 prismatic, 슬립과 compressive twin모드의 임계 전단응력은 온도변화에 크게 의존 하는 것으로 관찰되었다.
AZ31합금과 ZAM631합금에 대하여 고온압축실험을 수행하여 유동응력에 미치는 온도 및 변형율속도의 영향을 분석하였다. 모든 변형조건에서 변형 중 연화현상이 발생했으며 미세조직 분석을 통해 동적재결정이 원인임을 규명하였다. 또한 동적재결정립의 크기와 Z(zener-hollomon parameter)의 관계를 정립하였고 변형 중의 연화현상을 반영한 유동응력 방정식을 구하였다. 유동응력 방정식으로 구한 응력곡선은 실험결과와 잘 일치하였다.
압출공정의 거시적 소성변형 해석을 위해 상용 유한요소 해석 프로그램인 DEFORM-3D를 사용하였다. AZ31합금 판상(3t)과 ZAM631합금 봉상(16Ф) 압출재의 간접압출 모사를 수행하였고 압출비는 각각 56:1과 25:1이었다. 압출변형 중 빌렛의 온도분포, 유효변형율속도 및 유효변형율분포를 분석하고 압출재 잔여부분에 해당하는 위치에서의 변형거동을 각각 비교하였다.
압출변형 중에 발달하는 집합조직을 예측하기 위해 유한요소해석 수행으로 획득한 거시적 압출변형거동을 미시적 변형거동에 연계시킨 VPSC-FE모델을 사용한 새로운 집합조직발달 예측 기법을 개발하였다. 현재까지 제안된 집합조직발달 예측기법들은 소재의 변형이력을 집합조직발달에 반영하는 데에 한계가 있었다. 특히 Voce경화법칙을 사용하는 VPSC모델의 경우에 압출변형 중에 발생하는 연화현상을 모사할 수 없는 한계로 인해 변형 중 연화 현상이 발생하는 압출변형으로 인한 집합조직발달 예측에 한계가 있었다. 개발한 집합조직발달 예측기법을 사용하여 AZ31합금 판상 (3t) 및 ZAM631합금 봉상(16Ф) 압출재의 집합조직발달을 예측 하였다.
In this study, the extrusion deformation behavior of wrought magnesium alloys such as commercial AZ31 alloy and newly developed ZAM631 alloy was studied by using crystal plasticity and finite element method to improve the formability by controlling texture evolution. The study focused on investigating the microscopic deformation mechanism and the macroscopic extrusion deformation behavior of the alloys and intensively on predicting texture evolution of the alloys during extrusion.
When the critical resolved shear stress (CRSS) values and hardening parameters were investigated, the number of parameters for VPSC model could be more than 50 depending on the number of slip and twinning modes. Therefore optimization technique, Genetic Algorithm, was used to find the optimum CRSS values and hardening parameters of the alloys. To date, there are so many works that have been carried out to find the CRSS values for understanding the deformation mechanisms of magnesium and its alloys. But most of the works have been performed for 3 or 4 deformation modes at room temperature or single temperature without any temperature change. Because the plastic deformation of the alloys is greatly influenced by various parameters such as temperature and deformation modes, the considered deformation modes and the temperature range were very important to understand correctly the deformation mechanisms of the alloys. In this study, five deformation modes were considered to be active per grain i.e. basal slip, prismatic slip, second order pyramidal slip, tensile twin and compressive twin at the temperature range of 533~653K. Visco-plastic self-consistent-genetic-algorithm (VPSC-GA) optimizations were performed to find the optimal CRSS parameter sets corresponding to the process temperature and to investigate the microscopic deformation behavior. During compressive deformation, the basal slip and tensile twin were mainly activated. The contribution of the basal slip decreased and that of the tensile twin decreased with increasing temperature. The CRSS values decreased with increasing temperature in all deformation modes. The CRSS values of the prismatic slip, the slip and the compressive twin were strongly dependent on temperature, whereas the basal slip and the tensile twin were weakly dependent on temperature.
The hot compression tests of AZ31 and ZAM631 alloy were performed at the temperature range of 533~653K and the strain rate range of 10-3~1/s using Gleeble thermo-mechanical simulator. The peak value of flow stress increased with increasing strain rate at constant temperature and decreased with increasing temperature at constant strain rate. The flow stress-strain curves showed distinct flow softening at all deformation conditions. The microstructures of samples sectioned parallel to deformation direction were examined by optical microscopy. The results showed that the flow softening were due to dynamic recrystallization (DRX) during deformation. The size of DRX grains was formulated as a function of the Zener-Hollomon parameter. The flow stress equation characterizing DRX has been derived. Good agreements between the predicted and measured flow stress were achieved.
A non isothermal three-dimensional finite element simulation of the indirect extrusion process was carried out by using the commercial rigid plastic software DEFORM-3D V11.0 to investigate the macroscopic extrusion deformation behavior of AZ31 and ZAM631 alloy during extrusion. Indirect extrusion processes were simulated at a reduction ratio 56:1 at the ram speed 1.2 cm/min for extruded plate (3t) and at a reduction ratio 25:1 at the ram speed 0.8 cm/min for extruded rod (16Ф), respectively. In order to reduce the computational time, only a quarter model was used considering the rotational symmetry in geometry and loading condition. The flow stress model was implemented as a function of plastic strain, strain rate and temperature defined by the compressive stress-strain curves measured using a Gleeble thermo-mechanical simulator at the temperature range of 533~653K. The evolution of temperature, strain rate and strain during extrusion were predicted.
These macroscopic variables are not enough to predict quality of extruded product which is more strongly related to the microscopic variables such as slip activity and texture change. The VPSC model can predict microscopic evolution but only with simple loading condition at constant temperature. To monitor microscopic behavior during the non-isothermal extrusion process new methodology was designed to connect macroscopic deformation history to the microscopic model, VPSC. The modified VPSC-FE subroutine provided by VPSC7 released year 2007 is modified to accommodate different CRSS parameter sets at different temperature. In this study, 4 set of CRSS parameters are generated for each temperature using VPSC-GA and applied VPSC-FE model. The texture evolutions during the plate (3t) extrusion of AZ31 alloy and the rod (16Ф) extrusion of ZAM631 alloy were predicted by using the new methodology, respectively. The prediction showed good qualitative agreement with the experimental results. The new methodology can be successfully applied to infer the effects of temperature on the texture evolution and used to improve the formability of magnesium alloys by controlling the texture evolution during extrusion.
Language
English
URI
https://hdl.handle.net/10371/117997
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