• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1997년도 춘계학술대회논문집
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• pp.287-290
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• 1997

각 스탠드 유한요소해석 결과인 기계적-열적 변수들을 이용하여 재결정계산을 한다. 이때 온도변화를 정확히 반영하기 위해 재결정부피율 계산과 결정성장 계산에 additivity rule을 도입하였다. 또한 여러단계의 압연공정 각각에서의 재결정거동을 계속 추적하기 위해 또한 이때의 재결정의 영향을 재료유동에 반영하기 위해 substructure의 개념을 도입하였다. 이러한 과정을 거쳐 7패스후의 최종 두께방향으로의 결정크기분포를 얻을 수 있다. 본 연구 에서는 이때의 최종결정크기 분포를 균일화 시킬 수 있는 공정을 유전 알고리즘을 이용하여 찾아보았다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2000년도 춘계학술대회논문집
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• pp.179-182
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• 2000

The dynamic softening behavior of type 430 ferritic stainless steel could be characterized by the hot torsion test in the temperature range of 900-110$0^{\circ}C$ and the strain rate range of 0.05-5/sec. It is found that the continuous dynamic recrystallization (CDRX) was a major dynamic softening mechanism. The effects of process variables strain ($\varepsilon$) stain rate($\varepsilon$)and temperature (T) on CDRX could be individually established from the analysis of flow stress curves and microstructure. The effect of CDRX individually established from the analysis of flow stress curves and microstructure. The effect of CDRX increased with increasing strain rate and decreasing temperature in continuous deformation. The multipass deformation processes were performed with 10 pass deformations. The CDRX effect occurred in multipass deformatioon. The grain refinement could be achieved from multipass deformation The grain refinement increased with increasing strain rate and decreasing temperature. Also the CDRX in multipass deformation was affected by interpass time and pass strain. The total strain was to be found key parameter to occur CDRX.

• 소성∙가공
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• 제5권4호
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• pp.305-319
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• 1996

In this paper a finite element based system is presented for the prediction of the distributions of the recrystallized grain sizes in the workpiece in hot forging. The system adopts a fully coupled finite element thermo-mechanical model for predicting plastic deformation and heat transfer occurring in the workpiece and employs existing metallurgical models relating the recrystalliza-tion behavior with the thermo-mechanical variables such as temperatures strain and strain rate. The system is applied to upsetting of cylindrical preform. The predicted grain sizes are compared with the measurements. It is further applied to forging of a complex-shaped product.

• 소성∙가공
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• 제8권6호
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• pp.576-582
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• 1999

The high temperature behavior of Al 6061 alloy was characterized by the hot torsion test in the temperature ranges of 400∼550℃ and the strain rate ranges of 0.05∼5/sec. To decide optimum deformation condition, three types of deformation maps were individually made from the critical strain (εc). deformation resistance(σp) and deformation efficiency (η). The critical strain(εc) for dynamic recrystallization (DRX) which was decided from the inflection point of strain hardening rate(θ) - effective stress (σ) curve was about 0.65 times of peak strain (εp). The relationship among deformation resistance (peak stress, σp), strain rate (ε), and temperature (T) could be expressed by ε=2.9×1013[sinh(0.0256σp]7.3exp (-216,000/RT). The deformation efficiency (η)which was calculated on the basis of the dynamic materials model (DMM) showed high values at the condition of 500∼550℃, 5/sec for 100% strain. The results from three deformation maps were compared with microstructures. The best condition of plastic deformation could be determined as 500℃ and 5/sec.

• 소성∙가공
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• 제10권1호
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• pp.35-41
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• 2001

The high temperature deformation behavior of SCM 440 can be characterized by the hot torsion test in the temperature ranges of $900^{\circ}C$~$1100^{\circ}C$ and strain rate ranges of 0.05/sec~5/sec. The aim of this paper is to establish the quantitative equation of the volume fraction of dynamic recrystallization (DRX) as a function of processing variables, such as strain rate ($\varepsilon$), temperature (T), and strain ('$\varepsilon$). During hot deformation, the evolution of microstructure could be analyzed from work hardening rate ($\theta$). For the exact prediction of dynamic softening mechanism the critical strain ($\varepsilon_c$), the strain for maximum softening rate ($\varepsilon^*$ and Avrami' exponent (m') were quantitatively expressed by dimensionless parameter, Z/A, respectively. The transformation-effective strain-temperature curve for DRX could be composed. It was found that the calculated results were agreed with the experimental data for the steel at any deformation conditions.

• Journal of Welding and Joining
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• 제7권4호
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• pp.38-45
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• 1989

In this study, in order to evaluate the shape and the size of the plastic zone at the notch tip before stable crack growth, a newly developed technique for plastic strain measurement, that is, the recrystallization-etching technique was applied to observe the intense strain zone at the notch tip of weld HAZ. 1) The recrystallized specimens showed that the amount of the intense strain zone, more than 20% plastic zone, was quantitatively observed as the plane strain state during the growth of the plastic zone. 2) The behavior of plastic deformation at midsection are different for parent and weld HAZ. In addition, the micro crack initiation occurs at midsection, parent and weld HAZ when the crack opening displacement(COD) value is .delta.$_{t}$=0.4mm. 3) The plastic zone for parent proceeds in the forward direction at notch tip and for weld HAZ in the right and left direction at the notch tip. 4) The relation between plastic strain energy(Wp) and COD(.delta.$_{t}$) depended on yield stress, gradient and plastic strain size.ize.

• 소성∙가공
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• 제11권6호
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• pp.538-546
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• 2002

Hot deformation behavior of Udiment720Li was characterized by compression tests in the temperature range of 10$25^{\circ}C$ to 115$0^{\circ}C$ and the strain rate range of $0.0005 s^{-1};to;5 s^{-1}$. The combination of dynamic material model (DMM) and Ziegler's instability criterion was applied to predict an optimum condition and unstable regions for hot forming. A dynamic recrystallization model coupled with FEM results was used to interpret the evolution of microstructures. In order to verify the reliability of the present coupled model, isothermal forging was performed in the temperature range 1050~115$0^{\circ}C$ at strain rates of $0.05 s^{-1};and;0.005 s^{-1}$. The present model was successfully applied to the hot forming process of Udimet720Li.

• Nuclear Engineering and Technology
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• 제40권4호
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• pp.305-310
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• 2008

Based on a composition of 99.4 wt% AISI 316L stainless steel, 0.3wt% Ti and 0.3 wt% $Y_2O_3$, an austenitic ODS steel was fabricated by a process of mechanical alloying, hot isostatic pressing and rolling. Fine oxide particles were observed in the matrix, and their chemical formulations were determined to be $Y_2Si_2O_7$ and TiO. Heat treatment of the cold-rolled sample at $1200^{\circ}C$ induced an isotropic tensile behavior at room temperature and at $700^{\circ}C$. This result would be mainly attributed to the equiaxed grains that form as a result of the heat treatment for recrystallization.

• 열처리공학회지
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• 제32권1호
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• pp.1-11
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• 2019

In this study, microstructure evolution and crystallographic orientation are investigated under various deformation conditions in M1 magnesium alloy. M1 magnesium ingot was rolled at 673 K with the rolling reduction of 30%. The compression test specimens were machined out from rolled plate, and then the specimens were annealed at 823 K for 1h. Uniaxial compression tests were conducted at 723 K and under the strain rate ranging from $5.0{\times}10^{-4}s^{-1}$ to $5.0{\times}10^{-2}s^{-1}$ up to a true strain of -1.0. For observation of crystal orientation distribution, EBSD measurement was performed. Occurrence of the dynamic recrystallization and grain boundary migration were confirmed in all case of the specimens. The distribution of the grains is not uniformed in the experimental conditions.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2000년도 춘계학술대회논문집
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• pp.183-186
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• 2000

The high temperature deformation behavior of spray-formed Al-19wt%Si-1.87wt%Mg-0.085wt.%Fe alloy was studied by torsion testing in the strain rate range of 0.001-1 sec-1 and in the temperature range of 300-500 $^{\circ}C$. The relationship between stress temperature and strain rate is expressed using the Power law. the behavior of dynamic recrystallization is showed in 300-35$0^{\circ}C$, 1-0.1sec-1 and the behavior of dynamic recovery is showed in 450-50$0^{\circ}C$, 0.01-0.001sec-1 The size of Si particles is mall when the temperature is low and the strain rate is high. The strain rate sensitivity(m) and the apparent activation energy(Q) indicate the dependence on strain rate and temperature for flow stress respectively. The hot ductility is high when m is high and Q is low. The maps of strain rate sensitivity and apparent activation energy suggest the optimum processing conditions.