Flow stress and microstructural evolution in as rolled AZ91 alloy during hot deformation

1　INTRODUCTIONMgalloysarethelightestconstructionmaterialsformanyengineeringcomponentsduetotheirlowdensity ,higherductilityandsuitablestrength[13] .However ,Mgalloyshavepoorformabilityandlimit edductilityatroomtemperaturebecauseofitsHCPstructure[4 ] .Therefore ,itisrequiredforMgalloystobedeformedatwarmtemperature(>4 98K ) [5] ,es peciallyatelevatedtemperature ,whenprismaticslipisactivatedsothattheformalityofMgalloyscanbeimprovedintermsofhighductilityandmakesiteasytosimplifythedesignandm…     

AZ80镁合金热变形流变应力研究

在应变速率为0.001s-1～10s-1,变形温度为200℃～400℃条件下,在Gleeble-3800热模拟机上对AZ80合金的流变应力进行了研究。结果表明,AZ80合金的流变应力强烈地受变形温度的影响,当变形温度低于300℃时,其峰值流变应力呈现幂指数关系;当变形温度高于300℃时,其峰值流变应力呈现指数关系。在该文实验条件下,AZ80合金热变形应力指数n=8.43,热变形激活能Q=165.83kJ/mol。     

镁合金AZ61热流变应力模型的两种建模方法

为确定镁合金AZ61热变形特性与制定合理的成形工艺参数,利用Gleeble-1500热模拟试验机研究该材料在变形温度523K~673K和应变速率0.001s-1~1s-1下的流变应力行为。根据实验数据,确定热变形激活能,建立峰值应力与温度和应变速率的关系式。采用两种不同方法,分别建立任意时刻流变应力与温度、应变速率和应变的关系式,并验证了流变应力方程的准确性。研究结果表明,直接考虑应变对应力的影响模型相对误差为5.46%,通过动态再结晶分数间接考虑应变对应力的影响模型相对误差为5.42%,两种模型的预测值均与实验值较吻合。     

铸造Mg-4Al-2Sn-Y-Nd镁合金的热压缩行为与加工图

利用热压缩实验研究一种新型的具有优异室温塑性的Mg-4Al-2Sn-Y-Nd镁合金的高温流变行为,变形温度为200～400℃,应变速率为1.5×10-3～7.5 s^-1。结果表明：合金的应变速率敏感因子（m）在不同变形温度下均明显小于AZ31镁合金的m值,因此该合金适合在高应变速率下进行热加工。在真应力-应变曲线基础上,建立Mg-4Al-2Sn-Y-Nd 镁合金高温变形的本构方程,并计算得到合金的应力指数为10.33,表明合金在高温下主要的变形机制为位错攀移机制。同时,利用加工图技术确定合金的最佳高温变形加工窗口,即变形温度在350～400℃之间,应变速率在0.01～0.03 s^-1。     

Flow stress and microstructure evolution of semi-continuous casting AZ70 Mg-alloy during hot compression deformation

To evaluate and predict flow stress and set up hot forging process of AZ70 magnesium alloy, hot compression tests of AZ70 magnesium alloy were carried out on Gleeble 1500D thermo-mechanics tester at 300-420℃ and strain rates of 0.001-1 s^-1 with different compression degrees. It is indicated that temperature and strain rate are the main factor affecting the flow stress and microstructure. Stress increases but average grain size decreases with temperature decreasing and strain rate increasing. The stress model, constituted by introducing temperature-compensated strain rate, the Zener-Hollomon parameter, has a good fitness with the proof stress value under the experimental condition. The reciprocal of grain size at true strain of 1.0 has a linear relation with natural logarithm of Z parameter, and the correlation coefficient, R=0.95, is very significant by examination. The hot deformation activation energy Q of AZ70 alloy is 166.197 kJ/mol by calculation.     

AZ31B镁合金薄板的单向拉伸行为实验研究

在Gleeble-3500热模拟试验机上对AZ31B镁合金薄板(0.6 mm)拉伸试样在100～350℃的温度范围和1×10-1～1×10-3s-1的应变速率范围内进行了的单向拉伸实验,根据实验结果对AZ31B镁合金薄板的力学性能进行了分析.结果表明:AZ31B镁合金薄板在较低变形温度100～150℃时,应变速率对流动应力的影响不大;相比之下应变速率对AZ31B镁合金的断裂伸长率却有一定的影响,提高应变速率会降低材料的伸长率;在较高变形温度(200℃以上)时,应变速率对流动应力的影响比较明显,表现出显著的应变速率敏感性.     

Effect of thermomechanical parameters on dynamically recrystallized grain size of AZ91 magnesium alloy

Dynamic recrystallization of the AZ91 alloy was studied by conducting hot compression tests at temperature range of 325-400 °C and strain rate of 0.001-1 s⁻¹. The influence of the hot deformation variables on flow stress as well as recrystallized grain size was investigated. The results showed that by decreasing temperature and increasing strain rate, flow stress increases while dynamically recrystallized grain size decreases. A power-law relation developed between the characteristic peak strain and Zener-Hollomon parameter and the exponent was determined as 0.17. Besides, the linear regression between the Zener-Hollomon parameter and dynamically recrystallized grain size developed another power law equation, with a stress exponent equivalent to −0.13.     

铸态AZ80镁合金高温热变形行为研究

在Gleeble-1500热模拟机上对铸态AZ80镁合金在应变速率为0.005s-1~5s-1、变形温度为200℃~400℃条件下的高温热压缩变形行为进行了研究。结果表明,材料真应力-真应变曲线呈现动态再结晶特征。在温度T≥250℃,试样流变应力行为对应变速率敏感;在低温下T=200℃,应变速率对流变应力影响不大。高温下试样流变应力符合幂指数函数关系,应力指数n为6,热变形激活能Q为220kJ/mol。在高应变速率条件下,试样在变形中的温升是应变量的函数。实验中,Zener-Hollomon参数值大的试样温升明显,而Zener-Hollomon值小的试样变形温度基本保持不变。     

Compressive deformation behavior of AZ31 magnesium alloy under quasi-static and dynamic loading

Compressive properties of AZ31 alloy were investigated at temperatures from room temperature to 543 K and at strain rates from 10-3to 2×10 4s-1.The results show that the compressive behavior and deformation mechanism of AZ31 depend largely on the temperature and strain rate.The flow stress increases with the increase of strain rate at fixed temperature,while decreases with the increase of deformation temperature at fixed strain rate.At low temperature and quasi-static condition,the true stress-true strain curve of AZ31 alloy can be divided into three stages(strain hardening,softening and stabilization) after yielding.However,at high temperature and high strain rate,the AZ31 alloy shows ideal elastic-plastic properties.It is therefore suggested that the change in loading conditions(temperature and strain rate) plays an important role in deformation mechanisms of AZ31 alloy.     

AZ80镁合金变形特性及管材挤压数值模拟研究

利用Gleeble热模拟机研究了AZ80合金的高温变形特性。结果表明,流变应力取决于变形温度和变形速率。当应变速率一定时,流变应力随变形温度的升高而降低;当温度一定时,流变应力随着应变速率的升高而增大。根据AZ80镁合金真应力-真应变曲线,建立了其流变应力模型。采用刚塑性有限元法对AZ80镁合金管材挤压过程进行热力耦合数值模拟,并分析了高温挤压成形过程中变形力及金属流动规律,着重探讨了变形温度和挤压速度等挤压工艺参数对挤压力、应变场以及应力场的分布及变化情况的影响。模拟的结果为AZ80镁合金管材挤压工艺参数的制定、优化提供了科学依据。     

变形条件对AZ61镁合金力学行为的影响

用热模拟实验机对AZ61镁合金在变形温度为150℃～400℃,应变速率为0.01s-1～10s-1的条件下进行压缩变形,研究不同变形条件下AZ61镁合金的力学响应。结果表明,AZ61镁合金压缩变形时表现出动态再结晶特征,随温度上升,再结晶容易发生且应力峰降低;随变形速率增加,发生再结晶转变的临界应变增大。相比之下,变形温度对AZ61合金力学行为的影响要大于应变速率的影响。     

AZ61镁合金热压缩流变应力及晶粒尺寸预测

通过在Gleeble-1500D热模拟试验机上对AZ61镁合金进行热模拟试验,获得等温恒速单轴方向热压缩变形过程的流变应力-应变关系曲线.经过分析和计算曲线的特征值,利用线性和非线性数值回归方法建立峰值应力、峰值应变、再结晶晶粒尺寸等特征值与Zener-Hollomon参数即带温度补偿的应变速率因子Z的定量关系.流变应力的预测计算值与实测值误差在10%以内,晶粒尺寸计算值与实测值误差在5%以内.     

Deformation Mechanism and Hot Workability of Extruded Magnesium Alloy AZ31

Using the flow stress curves obtained by Gleeble thermo-mechanical testing, the processing map of extruded magnesium alloy AZ31 was established to analyze the hot workability. Stress exponent and activation energy were calculated to characterize the deformation mechanism. Then, the effects of hot deformation parameters on deformation mechanism,microstructure evolution and hot workability of AZ31 alloy were discussed. With increasing deformation temperature, the operation of non-basal slip systems and full development of dynamic recrystallization(DRX) contribute to effective improvement in hot workability of AZ31 alloy. The influences of strain rate and strain are complex. When temperature exceeds 350 °C, the deformation mechanism is slightly dependent of the strain rate or strain. The dominant mechanism is dislocation cross-slip, which favors DRX nucleation and grain growth and thus leads to good plasticity. At low temperature(below 350 °C), the deformation mechanism is sensitive to strain and strain rate. Both the dominant deformation mechanism and inadequate development of DRX deteriorate the ductility of AZ31 alloy. The flow instability mainly occurs in the vicinity of 250 °C and 1 s-1.     

Flow stress and softening behavior of wrought magnesium alloy AZ31B at elevated temperature

1　INTRODUCTIONThewroughtmagnesiumalloyshaveexcellentspecificstrengthandstiffness ,machinability ,dampcapacity ,dimensionalstability ,lowmeltingcostsandare ,hence ,veryattractiveinsuchapplicationsasau tomobile ,aviation ,electronicandcommunicationin dustry[16 ] .Investigationsontheflowstressandsofteningbehaviorofmagnesiumalloysathigherformingtem peratureandstrainratehavebeenanimportantsub jectinwroughtmagnesiumalloysforming[710 ] .InthispapertheflowstressandsofteningbehaviorofAZ31Bdeform…     

Superplasticity in fine-grained AZ61 magnesium alloy

The effect of grain refinement on the superplasticity of ingot-processed magnesium alloy was investigated. From the AZ61 material with a linear intercept grain size of 5 μrn, which was obtained by the multi-rolling process at an elevated temperature, tensile elongation over 400% could be achieved at 10^-3s^-1 at 400°C with a maximum value of 560% at 2x10^-4s^-1 at the same temperature. It was found that grain boundary diffusion controlled grain boundary sliding and pipe diffusion controlled slip creep govern the plastic flow at low and high strain rate ranges, respectively. A deformation map for pure magnesium was constructed to examine the effect of grain size and flow stress on deformation behavior at elevated temperature. The superplastic formability of Mg alloys was demonstrated by forming an AZ61 sheet into a hemi-sphere.     

Effect of microporosity on the tensile properties of AZ91 magnesium alloy

The effect of microporosity on the tensile deformation of as-cast AZ91 magnesium alloy was investigated through systematic experimental approaches and theoretical predictions of a constitutive model for tension instability. The strain rate sensitivity was measured at room temperature by the incremental strain rate change test, and the microporosity was quantitatively obtained by fractography analysis on fractured surfaces. The tensile strength and elongation of as-cast AZ91 alloy have a strong inverse parabolic dependence on microporosity variation. The constitutive model can exactly predict the tensile deformation of AZ91 alloy through a practical estimation of strain-related terms and load carrying capacity by quantitative fractography. The overall plastic deformation of AZ91 alloy depends fundamentally upon not only the variation of load carrying capacity, but also on the strain hardening exponent and strain rate sensitivity. The contribution of strain rate sensitivity to plastic deformation becomes increasingly significant with a decreased strain hardening ability. As the strain rate sensitivity of the conventional material is very low but not zero, the constitutive model for the exact prediction of plastic deformation should take the strain rate sensitivity term into account.     

5A90铝锂合金热态下本构关系研究

进行了5A90铝锂合金在200℃~450℃温度范围和0.3×10-3s-1~0.2×10-1s-1应变速率范围内的单向拉伸试验。结果表明,5A90铝锂合金的流动应力随变形温度的升高而减小,随应变速率的增大而增大;而其最大延伸率的变化趋势与流动应力的相反;最佳的成形温度范围在400℃左右。通过试验数据的计算及拟合,得到了任意温度下5A90铝锂合金应力-应变-应变速率关系的本构方程。     

AZ40M镁合金的Fields-Backofen模型及加工图

通过热压缩模拟试验,研究了AZ40M镁合金在塑性成形过程中,流动应力随应变速率、变形温度及应变的变化。基于试验的真应力-真应变数据,建立了Fields-Backofen模型,通过优化改进,提出了一种新的模型,并绘制了AZ40M镁合金的加工图。结果表明,在峰值应力前AZ40M镁合金的Fields-Backofen模型预测曲线与试验数据比较符合;改进后的新模型,具有较高的精度,相关系数达到了0.9918,平均相对误差为3.84%,在整个变形阶段的预测曲线与试验数据更加一致。通过加工图分析表明,在变形温度为620~673K,应变速率为0.005~0.05s^-1时适宜该合金的热加工。     

AZ31镁合金热变形流动应力预测模型

采用近等温单轴压缩实验获得了AZ3l镁合金变形温度为523 723 K,应变速率为0.01—10 s^-1条件下的流动应力,分析了变形温度和应变速率对流动应力的影响规律.结果表明,AZ31镁合金变形过程中发生了动态再结晶,523 K时形成细小组织;而723 K时动态再结晶和长大的晶粒沿径向拉长.考虑实验过程塑性变形功和摩擦功引起的温度升高,在高应变速率条件下采用温度补偿修正了流动应力.在此基础上,建立了基于双曲正弦模型的峰值流动应力和统一本构关系,该模型利用材料参数耦合应变来描述流动应力的应变敏感性,进一步获得了合金热变形过程中流动应力与变形温度、应变速率和应变的定量关系.采用该本构关系模型预测流动应力具有较高的精度,预测值与实测值相关系数为0.976,平均相对误差为5.07%,实验条件范围内预测的流动应力与实验值几乎能保持一致.     

TiC/AZ91D composites fabricated by in situ reactive infiltration process and its tensile deformation

1 Introduction Relative to aluminum matrix composites, magnesium-matrix composites are receiving interests increasingly in recent years due to their low densities and high specific properties. They are potentially attractive for the applications in aeros…