High temperature mechanical properties and performance of Ni-Fe-based superalloys depend very much on the grain structure such as average grain size and size distribution of grains. The grain structure of hot-forged Ni-Fe-based superalloys, and consequently its mechanical properties can be controlled by proper design of the hot forging processes including heating, preform and forging shape, workpiece and die temperature, ram speed, and so on. Therefore it is essential for the optimization of the high temperature mechanical properties and performance of forged parts to predict the grain structure of hot-forged Ni-Fe-based superalloys, which is evolved by dynamic recrystallization taking place during hot forging. In addition, thorough investigation for dynamic recrystallization mechanism is necessary for improving the understanding of dynamic recrystallization phenomena as well as the accuracy of the prediction.
In order to predict the evolution of grain structure during hot forging of Alloy 718 that is a representative Ni-Fe-based superalloy, semi-empirical equations for the critical strain ($ε_c$), dynamically- recrystallized grain size ($D_{drx}$) and the fraction of dynamic recrystallization ($X_{drx}$), which are 3 variables describing the dynamic recrystallization behavior were quantitatively formulated.
For the formulation of the semi-empirical equations, a series of isothermal compression tests were carried out up to an axial strain of ~0.7 within the temperature range 927℃ to 1066℃ and strain rate range $5×10^(-4) s^(-1)$ to $10 s^(-1)$ under the constant strain rate condition. The initial average grain size, 15 - 320 ㎛ was chosen to investigate its effect on dynamic recrystallization.
For the microstructure prediction, the semi-empirical equations for dynamic recrystallization were implemented into the commercial FE code, DEFORM-3D. The evolution of grain structure during actual hot forging of Alloy 718 was simulated using the 3-D FE simulator in combination with the microstructure prediction module, and the predicted microstructure was validated by comparing the simulated grain structure with that of the actual forgings. For the actual hot forging, a two-step forging route was employed, i.e. upset forging of the cylindrical billets with a tapered end in the first step, and the final forging in the following step. The forging temperature was 1010℃. A non-isothermal rigid-plastic FE code was employed for the simulation of the forging processes and its microstructure.
The critical strain for the onset of dynamic recrystallization can be obtained from the σ-ε curves. The critical strain for the onset of dynamic recrystallization increased with increasing the initial grain size and Zener-Hollomon parameter(Z). The empirical equation for the critical strain was expressed as a function of initial grain size ($D_o$) and Zener-Hollomon paramete(Z). The volume fraction ($X_drx$) and size of recrystallized grains ($D_{drx}$) were analyzed by observing the hot-compressed samples. The variations of volume fraction of dynamic recrystallization with strain showed Avrami-type sigmoidal curves. Therefore they were fitted to an Avrami-type equation, and expressed as a function of Do and Z. Dynamically recrystallized grain size was expressed as a function of Zener-Hollomon parameter only. The formulated equations for dynamic recrystallization were implemented into a 3-dimensional FEM forging simulator for prediction of microstructure evolution during hot forging of Alloy 718. The local microstructure of actual hot-forged parts were investigated and compared with the predicted one. Good agreements between the measured and the predicted grain structure were observed for dynamically-recrystallized grain size as well as the fraction of dynamic recrystallization. Average grain size calculated by applying the rule of mixture also showed good agreement between the measured and the predicted. The microstructure prediction tool was successfully validated so that it can be effectively used in optimizing the thermo-mechanical processes including hot forging etc.
Dynamic recrystallization(DRX) phenomena was also investigated for the uniaxially-compressed Alloy 718 tested at temperature, 1010℃ and 1066 ℃, and strain rates, $0.5 s^(-1)$ and $0.005 s^(-1)$. TEM observation revealed corrugation and bulge of initial grain boundary in the sample compressed up to true strain of 0.05. This implies that the nucleation of DRX grains on initial grain boundary is due to the formation of bulges on initial grain boundary by grain boundary shearing. The misorientation angle distributions of boundaries of as-compressed samples were also analyzed with deformation condition using EBSD system. From EBSD analysis, it was observed that the frequency of low angle boundary(LAB) with misorientation over 5˚ increased in deformed samples when compared with the samples only annealed for 5 minutes at the compression temperatures. This tendency was more obvious in partially-recrystallized samples that have the unrecrystallized old grains. In this condition, LAB frequency also decreased smoothly with misorientation angle. The misorientation angle distributions of as-compressed samples were also presented cumulatively up to 70˚ of misorientation angle. While the plateau regions were observed in as-annealed samples, cumulative frequencies were continuously increased, especially in partially- recrystallized samples, between 10˚ and 20˚ of misorientation angle. These variations in cumulative frequencies support that the formation of high angle boundaries is closely related to the presence of low angle boundaries formed in unrecrystallized old grains. The variation of misorientation angle of subgrains in a unrecrystallized old grain was investigated using TEM. It was observed that the nearer the subgrain to DRX front, the higher the misorientation angle of subgrain boundary. In case of subgrain adjacent to DX front, it ws inceased up to about 12˚ that is almost high angle boundary. From these observations, it was concluded that after initial grain boundaries were completely covered by DRX grains, dynamic recrystallization progresses by the repeated nucleation of new DRX grains on DRX front caused by progressive subgrain rotation during hot deformation.
Ni-Fe기 초내열합금 열간 단조품의 기계적 특성과 성능은 열간 단조과정에서 발생되는 동적 재결정 현상에 의해 지대한 영향을 받는다. 동적 재결정은 단조 과정에서 부품의 결정립 조직 미세화를 통해 소재의 기계적 특성을 향상시킬 수 있는 반면에 불균일한 미세조직을 발생시킴으로써 부품의 성능과 수명을 저하시키는 원인이 될 수 있다. 따라서 부품의 수명과 성능을 향상시키기 위해서는 동적 재결정에 의해 발생되는 미세조직 변화를 미리 예측하고 제어하는 기술은 매우 중요하다. 따라서 본 연구에서는 대표적인 Ni-Fe기 초내열합금인 Alloy 718 합금을 이용하여 동적 재결정 거동을 정량적인 수식으로 나타내고 이를 이용하여 실제 열간 단조품의 미세조직을 예측하였다. 또한 미세조직 예측 결과의 정확성을 향상시키기 위해서는 무엇보다도 동적 재결정 기구에 대한 이해가 요구되기 때문에 이에 대한 연구를 병행하였다.
다양한 고온 변형 조건, 즉 소재의 온도와 변형률 및 변형속도 조건에서 718 합금의 고온 압축 실험을 수행하고 이들 압축 시편에 대한 미세조직 분석을 통해 동적 재결정 거동을 수식화하였다. 동적 재결정 거동은 동적 재결정의 발생 여부를 판단하기 위한 임계 변형률, 동적 재결정된 결정립의 크기 및 동적 재결정 분율의 변화를 소재의 초기 결정립 크기와 변형조건, 즉 소재 온도, 변형률 및 변형률 속도의 함수로 나타내었다. 시험조건은 온도 $927^{\circ} C$ - $1066^{\circ} C$, 변형률 속도 $5 x 10^(-4) s^(-1) - 10 s^(-1)$ 의 범위에서 수행하였고 변형율은 최대 0.7까지 변형하였다. 또한 초기 결정립 크기의 효과를 관찰하기 위해 $53\mu m$ 와 $320\mu m$ 로 초기 결정립 크기가 다른 소재를 이용하여 동적 재결정 현상을 조사하였다.
이와같은 실험을 통해 얻어진 수식을 이용하여 실제 718 합금 열간 단조품의 미세조직을 예측하고 실 단조품의 미세조직과 예측 결과를 비교한 결과 매우 정확히 미세조직을 예측할 수 있었다. 이들 미세조직 예측 시스템은 향후 다양한 형상을 갖는 단조품의 미세조직 예측, 열간 단조품의 부위별 특성 제어 등을 통해 단조 공정을 최적화 하는데 효율적으로 활용될 수 있다.
한편 본 연구에서는 동적 재결정 기구에 대한 이해를 위해 고온 압축 변형된 시편의 미세조직을 광학현미경, EBSD 및 투과전자현미경을 이용하여 분석하고 특히 고온 변형에 따른 결정립 또는 아결정립 입계의 특성 변화를 관찰함으로써 동적 재결정 기구를 제시하였다. 분석 결과 초기 결정립계에서 발생되는 동적 재결정립의 경우 결정립계 미끄럼에 의한 bulging mechanism을 통해 동적 재결정립의 핵생성이 이루어짐을 확인할 수 있었다. 한편 초기 결정립계가 동적 재결정된 결정립으로 완전히 뒤덮힌 뒤에 발생되는 새로운 재결정립의 핵생성은 고온 변형과정에서 나타나는 아결정립들의 점진적인 회전(progressive rotation)에 의해 이루어지는 것으로 확인되었다.
