Constitutive models for high-temperature flow behaviors of a Ni-based superalloy

The high-temperature deformation behaviors of a typical Ni-based superalloy are investigated by hot compression tests under the strain rate of 0.001–1 s⁻¹and temperature of 920–1040 °C. The experimental results show that the deformation behaviors of the studied superalloy are significantly affected by the deformation temperature, strain rate and strain. The flow stress increases with the increase of strain rate or the decrease of deformation temperature. The flow stress firstly increases with the strain to a peak value, showing the obvious work hardening behaviors. Then, the stress decreases with the further straining, indicating the dynamic flow softening behaviors. Considering the coupled effects of deformation temperature, strain rate and strain on the hot deformation behaviors of the studied Ni-based superalloy, the phenomenological constitutive models are established to describe the work hardening-dynamic recovery and dynamic softening behaviors. In the established models, the material constants are expressed as functions of the Zener–Hollomon parameter. The established constitutive models can give good correlations with the experimental results, which confirm an accurate and precise estimation of the flow stress for the studied Ni-based superalloy.     

Formability and numerical simulation of AZ31B magnesium alloy sheet in warm stamping process

The potential process for mass production of magnesium alloy components in vehicles—warm stamping process was investigated systematically in the present study. For analyzing the forming process, an accurate numerical model describing the unique characteristics of magnesium alloy sheets under warm forming is very essential. Aiming at this, hardening/softening model for 1.5 mm thickness AZ31B magnesium alloy sheet were firstly constructed based on uniaxial tensile tests. Secondly, semispherical drawing was carried out under the selected temperature to generate experimental forming limit curve (FLC) for AZ31B sheet. Then, friction coefficient was identified using a high-temperature tribo-tester. Finally, numerical simulation was implemented and formability of AZ31B sheet warm forming was verified with experiment. The result shows that the formability, thickness distribution and equivalent strain distribution in simulation agreed well with the actual specimens, which thus provided a good data base for describing the unique characteristics of magnesium alloy sheets under warm forming.     

The high temperature flow behavior modeling of AZ81 magnesium alloy considering strain effects

In the field of deformation process modeling, constitutive equations are invariably used as a calculation basis to estimate the materials flow responses. Accordingly, in the present study, a constitutive analysis has been conducted on the AZ81 magnesium alloy employing experimental stress–strain data obtained from isothermal hot compression tests. These tests had been done in the temperature range of 250–450 °C under strain rates of 0.003, 0.03 and 0.3 s⁻¹. The effects of the temperature and strain rate on hot deformation behavior have been expressed in terms of an exponent-type Zener–Hollomon equation. Furthermore, the influence of strain has been included in the constitutive equation by considering its effect on different material constants. Consequently, a model to predict the high-temperature flow behavior of AZ81 magnesium alloy has been established. The true stress–true strain curves predicted by the extracted model are in good agreement with the experimental results, thereby confirming the validity of the developed constitutive relation.     

Strain Softening and Hardening Behavior in AZ61 Magnesium Alloy

The deformation behavior of AZ61 Mg alloy during hot deformation has been investigated in wide temperature and strain rate range by a Gleeble simulator. Specimens are deformed in compression in the temperature range of 523～673 K and at strain rates of 0.001～1 s-1. It is found that the flow curves exhibit a peak and then decrease towards steady-state of classical DRX, which decrease with rising temperature and decreasing strain rate. The deformation behavior of the specimens can be attributed to the occurrence of strain hardening and softening. As stress decreases, the strain hardening rate declines at a fast rate when temperature rises or strain rate decreases. The shapes of θ-σ curves indicate some important features such as subgrain formation, the critical stress, the peak stress and steady stress. The onset of DRX can be determined by the point of inflection on θ-σ or Inθ-σ curves.     

Effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy under asymmetrical uniaxial cyclic loading

Magnesium alloys are increasingly used in the automotive and aerospace industries for weight reduction and fuel savings. The ratcheting behavior of these alloys is therefore an important consideration. The objective of this investigation was to study the effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy. The experiments have shown that the extruded AZ31B Mg alloy presented the following characteristic behavior with increasing number of loading cycles: first an apparent cyclic softening was observed, then a cyclic hardening occurred, and finally a stable state was reached. This generic behavior can be explained by the fact that the variation trend of the maximum strain with the number of cycles differs from that of the minimum strain. The extrusion ratio did not influence the cyclic softening/hardening behavior or the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the mean stress and the peak stress. However, the extrusion ratio influenced the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the stress amplitude. Increasing the extrusion ratio also reduced the ratcheting strain and the effects of the load history on the ratcheting behavior of the extruded AZ31B Mg alloy.     

Hot deformation behavior and processing map development of AZ110 alloy with and without addition of La-rich Mish Metal

In order to compare the workability of AZ110 alloy with and without addition of La-rich Mish Metal(MM), hot compression tests were performed on a Gleeble-3500 D thermo-mechanical simulator at the deformation temperature range of 473-623 K and strain rate range of 0.001-1 s-1. The flow stress, constitutive relation, DRX kinetic model, processing map and microstructure characterization of the alloys were investigated. The results show that the flow stress is very sensitive to deformation temperature and strain rate, and the peak stress of AZ110 LC(LC = La-rich MM) alloy is higher than that of AZ110 alloy.The hot deformation behavior of the alloys can be accurately predicted by the constitutive relations. The derived constitutive equations show that the calculated activation energy Q and stress exponent n for AZ110 alloy are higher than the calculated values of AZ110 LC alloy. The analysis of DRX kinetic models show that the development of DRX in AZ110 LC alloy is earlier than AZ110 alloy at the same deformation condition. The processing maps show that the workability of AZ110 LC alloy is significantly more excellent than AZ110 alloy and the microstructures are in good agreement with the calculated results.The AZ110 LC alloys can obtain complete DRX microstructure at high strain rate due to its higher stored energy and weak basal texture.     

铸态AZ31镁合金热压缩过程中的再结晶行为

利用Gleeble-1500在温度200～500℃和应变速率0.001～1s-1范围内对铸态AZ31镁合金进行热压缩实验,并对动态再结晶行为进行研究。基于温度-应变速率的变化规律(Zener-Hollomon参数,Z参数),分析了形变温度和应变速率对铸态AZ31镁合金组织结构的影响规律。结果表明:动态再结晶发生后,再结晶晶粒尺寸随着形变温度的降低而减小。随着Z值的增加,动态再结晶作用增强,形变组织细化。为了便于工程应用的参考,给出了相应的热加工三维图。     

纤维对短切碳纤维/AZ91D复合材料热变形行为和加工性能的影响纤维对短切碳纤维/AZ91D复合材料热变形行为和加工性能的影响

采用等温压缩试验研究了不同碳纤维体积分数的镁基复合材料（CFs/AZ91D）和镁合金（AZ91D）在变形温度310~430℃、应变速率10-3~10-1 s-1范围内的塑性变形行为。根据实验结果建立了CFs/AZ91D和AZ91D的热加工图,分析了纤维对CFs/AZ91D塑性加工性能与变形机制的影响。结果表明:相比ZA91D,纤维在提高复合材料流动应力的同时促进了基体动态再结晶和应变软化,但纤维体积分数对流动应力与应变软化程度影响较小,CFs/AZ91D热变形时表现出比ZA91D更高的应变速率敏感指数和变形激活能;ZA91D热加工图不存在变形失稳区且其高温低速率区变形时的能量耗散效率大于30%,CFs/AZ91D高温低应变速率区变形时的能量耗散效率大于50%,此时纤维激励了基体合金动态再结晶而使复合材料表现出极高的能量耗散效率,但在低温高应变速率变形时,基体合金与纤维之间的界面开裂极易导致CFs/AZ91D出现塑性流变失稳行为。      

Artificial neural network modeling to predict the high temperature flow behavior of an AZ81 magnesium alloy

In the present work, the capability of artificial neural network (ANN) has been evaluated to describe and to predict the high temperature flow behavior of a cast AZ81 magnesium alloy. Toward this end, a set of isothermal hot compression tests were carried out in temperature range of 250–400 °C and strain rates of 0.0001, 0.001 and 0.01 s⁻¹ up to a true strain of 0.6. The flow stress was primarily predicted by the hyperbolic laws in an Arrhenius-type of constitutive equation considering the effects of strain, strain rate and temperature. Then, a feed-forward back propagation artificial neural network with single hidden layer was established to investigate the flow behavior of the material. The neural network has been trained with an in-house database obtained from hot compression tests. The performance of the proposed models has been evaluated using a wide variety of statistical indices. The comparative assessment of the results indicates that the trained ANN model is more efficient and accurate in predicting the hot compressive behavior of cast AZ81 magnesium alloy than the constitutive equations.     

加工方式对AZ31变形镁合金热拉伸流变应力的影响

针对不同方法制备的AZ31镁合金薄板,利用热拉伸试验机和金相显微镜对其在不同温度和变形速率下的流变应力进行了实验研究.结果表明:挤压、交叉、热轧和冷轧等方法制备的AZ31镁合金薄板的应力-应变曲线基本特征是相同的.峰值流变应力随变形温度的升高和应变速率的降低而降低,在低温时具有明显的厚度效应;当温度大于350℃时峰值流变应力几乎不随板材厚度变化而变化;应变速率小于1.0×10-2s-1,变形温度大于150℃下所有AZ31薄板的延伸率均δ≥45%;单向轧制薄板的各向异性随温度提高减小.     

GH5188合金板材高温拉伸变形流动行为与本构方程研究

目的 研究GH5188合金板材高温拉伸变形流动行为,为高温合金板材高温成形工艺的制定和优化提供指导。方法 基于GH5188合金板材高温拉伸试验,分析了变形工艺参数对GH5188合金板材高温拉伸变形时真应力、应变速率敏感性指数和应变硬化指数的影响规律,建立了本构模型对其流动行为进行描述和预测。结果 GH5188合金板材高温拉伸变形流动行为受应变硬化、流动软化和应变速率硬化的共同影响,其变形过程分为弹性变形、加工硬化、稳态流动和断裂4个阶段。随变形温度的升高和应变速率的降低,真应力减小。变形温度、应变速率和真应变对GH5188合金板材的应变速率敏感性指数和应变硬化指数具有显著影响。基于Johnson-Cook和Hensel-Spittel模型,建立了考虑应变硬化效应、流动软化效应和应变速率硬化效应耦合影响的GH5188合金板材高温拉伸变形本构模型(JC-HS模型),采用该模型预测的真应力与试验值的平均相对误差为6.02%。结论 建立的JC-HS模型能够较好地描述和预测GH5188合金板材的高温拉伸流动行为。     

Investigation on the effect of friction stir processing on the superplastic forming of AZ31B alloy

Superplastic forming has now become conventional for forming complex parts from sheet metals. In many superplastically formed aerospace components, only a selective region undergoes superplastic forming. In those cases, instead of selecting a material exhibiting superplastic properties, a light weight and low cost material can be chosen and its microstructure can be modified locally by the Friction Stir Processing (FSP) technique. In this work, AZ31B magnesium alloy is chosen, and friction stir processing is performed by varying the process parameters, such as tool axial force, tool traversing speed and tool rotational speed. The process parameter that produced equiaxed grains in the stirred zone with a grain size less than 10 μm is selected. With this parameter, single pass FSP, multiple pass FSP without overlapping and multiple pass FSP with overlapping are performed on the AZ31B magnesium alloy sheets and their superplastic behaviour was examined. Also the theoretical modelling was carried out to determine the strain rate sensitivity for the friction stir processed AZ31B magnesium alloy and for the nonprocessed AZ31B magnesium alloy. It is found that the strain rate sensitivity for the friction stir processed component has increased, when compared to the base metal.     

Hot working and crystallographic texture analysis of magnesium AZ alloys

Abstract

The hot working behaviour of magnesium AZ (e.g. AZ31; Al: 3%, Zn: 1%) alloys and their associated crystallographic texture evolution is reviewed. Under hot working conditions, the stress–strain curves show flow softening at all the temperatures and strain rates indicating dynamic recrystallisation (DRX) is predominant. The mean size of the recrystallised grains in all the alloys decreases as the value of Zener–Hollomon parameter Z increases. The hot working range of the alloys dwell between 200 and 500°C and the strain rates between 10^?3 and 5 s^?1. The hot working of AZ series alloy shows discontinuous DRX as the main mechanism. Equal channel angular processing shows continuous DRX. The constitutive equation development shows a linear relationship between the stress and the Z parameter. The activation energy for the alloys ranges from 112 to 169 kJ mol^?1 and Z values range from 10 to 10 s^?1. Textural examinations show basal texture as the predominant orientation.     

Localized deform behavior of AZ31B magnesium alloy by numerical simulation

The dynamic compression behavior of AZ31B magnesium alloy with hat shaped specimen was investigated at high strain rate in this paper. Based on the Johnson‐cook constitutive model and fracture model, the interaction of temperature, stress and strain fields of AZ31B magnesium alloy with hat shaped specimen were numerically simulated by using ANSYS/LS‐DYNA software under different strain rates, which was validated by experiment. It is found that the plastic strain is highly concentrated on the corner of the hat shaped specimen, which leads to large localized deformation. The voids are nucleated and extended by compression stress. Work harden effect is caused by remained plastic strain, which is located around adiabatic shear band. The stress collapse is discovered in gauge section, which is also discovered in experiment. Thermal soften effect is suppressed with the strain rate increased.     

Effect of processing parameters on the hot deformation behavior of as-cast TC21 titanium alloy

Isothermal compression tests of as-cast Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb (TC21) titanium alloy are conducted in the deformation temperature ranging from 1000 to 1150 °C with an interval of 50 °C, strain rate ranging from 0.01 to 10.0 s⁻¹ and height reductions of 30%, 45%, 60% and 75% on a computer controlled Gleeble 3500 simulator. The true stress–strain curves under different deformation conditions are obtained. Based on the experimental data, the effects of deformation parameters on the hot deformation behavior of as-cast TC21 alloy were studied. The deformation mechanisms of the alloy in the whole regimes are predicted by the power dissipation efficiency and instability parameter and further investigated through the microstructure observation. It is found that at the height reductions of 30%, 45% and 60%, the softening of stress–strain curves at high strain rate (>1.0 s⁻¹) is mainly associated with flow localization, which is caused by local temperature rise, whereas at low strain rate, the softening is associated with dynamic recrystallization (DRX). However, the instability showed in flow localization occurs at low strain rate of 0.01 s⁻¹ when the height reduction reaches 75%. In addition, the effects of strain rate, deformation temperature and height reduction on microstructure evolution are discussed in detail, respectively.     

Hot working behavior of AZ31 and ME21 magnesium alloys

The plastic deformation and recrystallization behavior of the commercial magnesium alloys AZ31 and ME21 were analyzed in a wide temperature range. Using the conventional hyperbolic sine equation the flow stress dependence on temperature and strain rate was modeled. The activation energy for plastic deformation significantly increased with increasing temperature and delivered values above 180 kJmol^?1 for both alloys in the very high-temperature regime (400–550 °C). At lower temperatures (250–400 °C) the activation energy of the AZ31 alloy was approximately 108 kJmol^?1 considering the peak stress as well as 120 kJmol^?1 considering the flow stress at a strain of 0.5. The stress exponent varied in a range between 4.5 and 6.5. During the high-temperature compression tests a partial recrystallized microstructure was formed, which was distinctly different in AZ31 compared to ME21 due to the different onset of dynamic recrystallization (DRX) mechanisms.     

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity steel

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity (TRIP) steel was investigated by hot compression tests on Gleeble 3500D thermo-mechanical simulator in the temperature ranges of 900–1100 °C and the strain rate ranges of 0.01–10 s⁻¹. The results show that the flow stress is sensitively dependent on deformation temperature and strain rate, and the flow stress increases with strain rate and decreases with deformation temperature. The peak stress during hot deformation can be predicted by the Zener–Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 387.84 kJ/mol. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and decreasing of Z value lead to more adequate proceeding of DRX.     

Experimental study on tensile property of AZ31B magnesium alloy at different high strain rates and temperatures

As the lightest metal material, magnesium alloy is widely used in the automobile and aviation industries. Due to the crashing of the automobile is a process of complicated and highly nonlinear deformation. The material deformation behavior has changed significantly compared with quasi-static, so the deformation characteristic of magnesium alloy material under the high strain rate has great significance in the automobile industry. In this paper, the tensile deformation behavior of AZ31B magnesium alloy is studied over a large range of the strain rates, from 700 s⁻¹ to 3 × 10³ s⁻¹ and at different temperatures from 20 to 250 °C through a Split-Hopkinson Tensile Bar (SHTB) with heating equipment. Compared with the quasi-static tension, the tensile strength and fracture elongation under high strain rates is larger at room temperature, but when at the high strain rates, fracture elongation reduces with the increasing of the strain rate at room temperature, the adiabatic temperature rising can enhance the material plasticity. The morphology of fracture surfaces over wide range of strain rates and temperatures are observed by Scanning Electron Microscopy (SEM). The fracture appearance analysis indicates that the fracture pattern of AZ31B in the quasi-static tensile tests at room temperature is mainly quasi-cleavage pattern. However, the fracture morphology of AZ31B under high strain rates and high temperatures is mainly composed of the dimple pattern, which indicates ductile fracture pattern. The fracture mode is a transition from quasi-cleavage fracture to ductile fracture with the increasing of temperature, the reason for this phenomenon might be the softening effect under the high strain rates.     

The influence of initial microstructure and temperature on the deformation behavior of AZ91 magnesium alloy

The deformation behavior of AZ91 magnesium alloy has been investigated using uniaxial compression tests at a temperature range of 100–300 °C. The different processing routes including homogenization treatment, hot rolling and annealing have been employed to study the effect of initial microstructure on the compressive mechanical response of the AZ91 alloy. The results show that the hot-rolled material presents an enhanced compressive workability at temperatures as low as 100 °C. The experimental alloy exhibit dynamic recrystallization during compression in any of the initial microstructures. The maximum and minimum DRX (dynamic recrystallization) fraction has been obtained in hot-rolled and homogenized conditions, respectively. The recrystallized fraction increases with raising the temperature. In addition the effect of initial microstructure on the peak stress diminishes with increasing temperature while its effect on the peak strain remains remarkable. The softening fraction has been increased with temperature, where a pronounced effect has been recorded in the case of homogenized (un-rolled) material.     

Quantitative Analysis of Work Hardening and Dynamic Softening Behavior of low carbon alloy Steel Based on the Flow Stress

In this study, the constitutive equation and DRX(Dynamic recrystallization) model of Nuclear Pressure Vessel Material 20MnNiMo steel were established to study the work hardening and dynamic softening behavior based on the flow behavior, which was investigated by hot compression experiment at temperature of 950 °C, 1050 °C, 1150 °C and 1250 °C with strain rate of 0.01 s⁻¹, 0.1 s⁻¹ and 10 s⁻¹ on a thermo-mechanical simulator THE RMECMASTOR-Z. The critical conditions for the occurence of dynamic recrystallization were determined based on the strain hardening rate curves of 20MnNiMo steel. Then the model of volume fraction of DRX was established to analyze the DRX behavior based on flow curves. At last, the strain rate sensitivity and activation volume V^* of 20MnNiMo steel were calculated to discuss the mechanisms of work hardening and dynamic softening during the hot forming process. The results show that the volume fraction of DRX is lower with the higher value of Z (Zener–Hollomon parameter), which indicated that the DRX fraction curves can accurately predicte the DRX behavior of 20MnNiMo steel. The storage and annihilation of dislocation at off-equilibrium saturation situation is the main reason that the strain has significant effects on SRS(Strain rate sensitivity) at the low strain rate of 0.01 s⁻¹ and 0.1 s⁻¹. While, the effects of temperature on the SRS are caused by the uniformity of microstructure distribution. And the cross-slip caused by dislocation piled up which beyond the grain boundaries or obstacles is related to the low activation volume under the high Z deformation conditions. Otherwise, the coarsening of DRX grains is the main reason for the high activation volume at low Z under the same strain conditions.