Constitutive model for casting magnesium alloy involving spherical void evolution

Casting magnesium alloys are highly heterogeneous materials inevitably containing numerous voids.These voids will evolve during material deformation and markedly affect material behaviors,so it is important to investigate the equation of the void evolution and the constitutive relation involving the void evolution.By assuming the voids in casting magnesium alloys were spherical,the growth equation of the voids was obtained from the incompressibility and continuity conditions of material matrix. Through c...     

Constitutive description of casting aluminum alloy based on cylindrical void-cell model

Casting aluminum alloys are highly heterogeneous materials with different types of voids that affect the mechanical properties of the material. Through the analysis of a cylindrical void-cell model the evolution equation of the voids was obtained. The evolution equation was embedded into a nonclassical elastoplastic constitutive relation, and an elastoplastic constitutive relation involving void evolution was obtained. A corresponding finite element procedure was developed and applied to the analyses of the distributions of the axial stress and porosity of notched cylindrical specimens of casting aluminum alloy A101. The computed results show good agreement with experimental data.     

Constitutive description for casting magnesium alloy involving void evolution

In order to investigate the effect of microvoids on the mechanical behavior of casting magnesium alloy, a spherical void-cell model of the material was presented. The velocity and strain fields of the model were obtained from the assumption that the material matrix is homogeneous and incompressible. The hardening and softening functions, which respectively reflect the deformation-hardening and void-softening behaviors of the material, were presented and introduced to an endochronic constitutive equation for describing the mechanical behavior of the material including microvoids. The corresponding numerical algorithm and finite element procedure were developed and applied to the analyses of the elastoplastic response and the porosity of casting magnesium alloy ZL102. The computed results show satisfactory agreement with experimental data.     

基于圆柱形孔洞体胞模型分析的铸造镁合金ML316本构描述

假设在铸造镁合金材料中的微孔洞具有圆柱形的形状,然后隔离一个圆柱形微孔洞,建立圆柱形微孔洞体胞模型,在基体不可压条件下,得到圆柱形微孔洞的演化方程.根据圆柱形微孔洞体胞模型的速度场得到其应变场,进而基于材料内时本构理论,定义与材料孔洞弱化相关的内蕴时间,得到考虑变形强化和孔洞长大弱化的材料弹塑性本构方程.发展相应的有限元分析程序及数值算法,用其描述铸造镁合金试件在拉伸载荷作用下的应力与应变、塑性变形与材料孔隙率的关系以及试件缺口前缘孔隙率的变化,分析结果与实验结果有较好的一致性.     

Effect of void nucleation and growth on forming limit diagrams of textured aluminum alloy sheets

A numerical code has been developed to calculate Forming Limit Diagrams (FLDs) of textured aluminum alloy sheets. This code is based on the Marciniak-Kuczynski (M-K) model, but allows for void nucleation and growth so that limit strains and fracture strains can be predicted. The strain induced void nucleation model was employed together with the Cocks and Ashby’s void growth model. The influences of initial texture, texture evolution, and void nucleation and growth during deformation on the FLDs of an Al-Mg alloy were all investigated. Satisfactory agreement was obtained between the predictions and measured data, It was also shown that the introduction of void damage into the old M-K model can lead to more reasonable and accurate predictions.     

Molecular dynamics simulation on plasticity deformation mechanism and failure near void for magnesium alloy

The plastic deformation processes of magnesium alloys near a void at atomic scale level were examined through molecular dynamics (MD) simulation. The modified embedded atom method (MEAM) potentials were employed to characterize the interaction between atoms of the magnesium alloy specimen with only a void. The void growth and crystal failure processes for hexagonal close-packed (hcp) structure were observed. The calculating results reveal that the deformation mechanism near a void in magnesium alloy is a complex process. The passivation around the void, dislocation emission, and coalescence of the void and micro-cavities lead to rapid void growth.     

压铸镁合金微观组织模拟

采用CellularAutomaton微观模型 ,对压铸镁合金的三维微观组织演变进行了模拟。在模拟过程中 ,采用连续形核模型处理液态金属的异质形核现象 ,通过正态分布函数描述形核质点密度随温度的分布关系 ,在给定过冷度时对分布函数积分可得到该时刻的形核密度 ,晶粒生长模型则考虑了枝晶尖端生长动力学和择优生长方向 <10 10 >,也考虑了温度场的影响 ,并完成了宏观温度场和微观计算的耦合。最后通过压铸镁合金标准拉伸试棒金相试验 ,对模拟结果进行了验证 ,试验结果和模拟结果在平均晶粒尺寸方面符合较好     

用Cellular Automaton方法模拟压铸镁合金AM50的微观组织

基于Cellular Automaton(CA)方法的基本原理,建立了压铸镁合金AM50形核和等轴晶粒生长的二维数学物理模型,能够对任意晶向的等轴晶晶粒生长进行模拟。模型耦合了宏观传热与微观组织模拟计算,镁合金AM50阶梯块压铸件的温度场通过热传导反算法计算求得。对阶梯块压铸件不同阶梯表面的微观组织进行了模拟,并与金相试验结果进行了对比,它们在晶粒尺寸上吻合较好。     

低压铸造ZL114A铝合金微观组织模拟

在大量实验数据统计分析的基础上,建立了低压铸造ZL114A铝合金形核模型,得到了相关热力学,动力学参数.采用了一种改进的Celluar Automaton方法耦合有限差分法对合金的宏观凝固及微观组织演化过程进行了数值模拟.考虑了固/液界面的溶质再分布、微观固相分数、曲率以及各向异性因素.利用阶梯铸件研究了不同冷却速度下晶粒度、二次枝晶臂间距以及共晶组织含量的变化规律,并将模拟值与实验值进行了对比验证.     

Microstructural evolution,flow stress and constitutive modeling of Al−1.88Mg−0.18Sc−0.084Er alloy during hot compression

To explore the hot compression behavior and microstructural evolution, fine-grained Al?1.88Mg?0.18Sc? 0.084Er (wt.%) aluminum alloy wires were fabricated with Castex (continuous casting?extrusion) and ECAP-Conform, and their hot compression behavior was investigated at temperatures of 673?793 K and strain rates of 0.001?10 s^?1; the microstructures were characterized by optical microscope, X-ray diffractometer, transmission electron microscope, and electron backscattered diffractometer, and the flow stresses were obtained by thermal compression simulator. Microstructural evolution and flow curves reveal that dynamic recovery is the dominant softening mechanism. Continuous dynamic recrystallization followed by dynamic grain growth takes place at a temperature of 773 K and a strain rate of 0.001 s^?1; the yielding drop phenomenon was discovered. Hyperbolic sine constitutive equation incorporating dislocation variables was presented, and a power law constitutive equation was established. The stress exponent is 3.262, and the activation energy for deformation is 154.465 kJ/mol, indicating that dislocation viscous glide is the dominant deformation mechanism.     

镁合金铸造成形过程宏观／微观模拟

通过研究镁合金压铸过程中界面热,采用热传导反算法确定压铸过程的界面换热系数,研究镁合金压铸过程中工艺参数及凝固过程对铸件界面换热系数的影响规律,建立镁合金压铸过程界面换热边界条件的处理模型,以实现镁合金压铸过程中凝固过程的准确预测。通过实验研究镁合金压铸过程中凝固组织,建立了镁合金压铸过程中形核模型。采用CA方法,建立了镁合金枝晶生长模型,以实现镁合金凝固组织的预测。采用相场方法研究了镁合金枝晶生长形貌。     

ZK60镁合金热压缩变形流变应力行为与预测

在变形温度为523---673 K, 应变速率为0.001---1 s^-1的条件下, 采用Gleeble--1500热模拟试验机对ZK60镁合金的热变形行为进行了研究. 结果表明, ZK60镁合金流变应力随变形温度升高和应变速率的降低而减小. 其高温压缩流变应力曲线可描述为加工硬化、过渡、软化和稳态流变4个阶段, 但在温度较高和应变速率较小时, 过渡阶段不很明显. 建立了一个包含应变的流变应力预测模型, 模型中的9个独立参数可以通过非线性最小二乘法拟合求得, 预测的流变应力曲线与实验结果吻合较好.     

Using incremental forming to calibrate a void nucleation model for automotive aluminum sheet alloys

A novel method for the quantification of void nucleation rates in sheet material is presented. An incremental sheet forming process is employed to create large regions of homogeneous deformation, such that material density changes can then be used to quantify the evolution of void volume fraction with applied strain. This technique is employed to calibrate the void nucleation behaviour of three automotive aluminum sheet alloys (AA5182, AA5754 and AA6111) for incorporation into finite element method models which employ the Gurson–Tvergaard–Needleman (GTN) constitutive softening equations.     

基于元胞自动机AZ31镁合金微观组织模型

以AZ31镁合金在热压缩过程中微观组织演变为基础,结合元胞自动机模型(CA),建立了镁合金变形过程中再结晶晶粒尺寸模型和动态再结晶百分数模型。通过对铸态AZ31镁合金在不同变形条件下的热压缩实验,推导出镁合金的位错密度模型、临界位错密度模型、形核率模型和晶粒长大模型。结合元胞自动机具体演变规则,建立元胞自动机模型,并利用应力应变曲线及晶粒大小验证元胞自动机的模拟结果,验证该模型的准确性,结合实验数据和JMAK理论,推导出再结晶晶粒尺寸模型和动态再结晶百分数模型。借助DEFORM-3D分析软件得到镁合金在变形过程中,晶粒尺寸分布的变化情况以及动态再结晶百分数分布的变化情况。     

一种新型亚稳β钛合金热变形的本构模型(英文)

基于新型亚稳β钛合金Ti2448在温度1023～1123K、应变速率63～0.001s-1下的等温热压缩流动应力曲线特征,构建能够完整描述该合金流动应力与应变、应变速率、变形温度之间关系的本构模型。在此过程中,通过基于统一黏塑形理论改进双曲正弦函数,构建合金在高应变速率(≥1s-1)下发生动态回复(DRV)的模型;通过对标准的Avrami方程进行简化,表征了Ti2448在低应变率(1s-1)下发生的动态再结晶(DRX)软化机制。最终通过应用全局优化求解非线性方程的新方法确定模型中的相关参数。根据所建模型得到的预测曲线和实验曲线吻合得较好,能够有效预测Ti2448在热变形过程中的流动应力,为构建亚稳β钛合金热变形本构模型提供一种有效的方法。     

考虑固溶及时效处理的镁合金铸件微观组织模拟及力学性能预测

基于改进元胞自动机(CA)模型,综合考虑铸造、固溶处理和时效处理过程中的微观组织转变,建立了镁合金铸件微观组织演化模型;在分析Mg-Al系镁合金第二相析出过程和强化机理的基础上,建立了镁合金铸件力学性能模型;针对镁合金汽车轮毂,采用建立的模型,模拟预测了铸件关键部位的微观组织演化和力学性能.结果表明,铸态和固溶处理条件下屈服强度的预测值与实际测量平均值吻合较好,而时效处理状态下的预测值与实测平均值有一定差别,抗拉强度的模拟预测值与实际测量的平均值吻合较好     

AZ31B镁合金中厚板轧制热力耦合场数学模型

采用Gleeble-1500D热力模拟试验机对铸态AZ31B镁合金圆柱试样进行了宽范围变形条件下的热压缩试验,拟合热压缩试验数据,针对镁合金应变软化特性建立了一种新的热力本构模型;依托于Deform-3D对镁板的实际热轧过程进行了热力仿真分析,依据轧制理论假设、宏观连续介质力学以及热力学原理,采用数学解析的方法建立了镁板热轧制区域中的应变、应变速率值分布模型以及三维温度场、应力场数学模型。研究结果表明:新建的热力本构模型预测精度较高,平均相对误差为5.1%;建立的轧制变形区域中的应变、应变速率值分布模型,温度场数学模型以及热力耦合场数学模型不仅形式简单易于为生产利用,更能精确表征中厚规格镁板热轧制过程中的热-力耦合变形机制。     

不同工艺状态下AZ31B镁合金热压缩变形行为研究

镁合金在热加工过程中的变形机制复杂,且容易受到材料初始工艺状态和变形条件影响,因此呈现出不同的应力应变关系。采用铸态和变形态的AZ31B作为研究对象,通过Gleeble-1500获取坯料的应力应变曲线随温度和应变率的变化关系,基于Arrhenius双曲正弦型函数构建两种不同工艺状态下镁合金的本构模型,分析初始加工状态对镁合金应力应变关系及变形机制的影响。实验结果表明：当应变速率大于0.1s_^-1,变形态镁合金在低温下由于变形织构及大量孪生产生而出现45°剪切断裂;在高温和低应变速率下两种工艺状态的镁合金变形机制相同,应力应变曲线基本相似;变形态镁合金的硬化指数n及变形激活能Q相比铸态镁合金更低。     

ZL201合金半固态成形本构模型

对采用近液相线铸造法制备的ZL201合金半固态坯料进行了热模拟压缩试验。根据试验获得的ZL201合金不同温度与应变速率下的应力-应变曲线,采用多元回归线性方法建立了能够表征ZL201合金半固态变形行为的本构方程。同时,根据半固态浆料的表观粘度与热模拟压缩试验中的应变速率等参数间的关系式,对表观粘度和剪切速率之间的关系进行了研究。利用仿真软件Anycasting对ZL201合金半固态触变压铸成形过程进行了数值模拟,研究了内浇道厚度对ZL201合金半固态触变压铸过程的影响,并预测了铸件缺陷。结果表明,当内浇道厚度为3mm时,半固态金属浆料充型过程最为理想。     

Ti-22Al-26Nb合金热变形本构方程建立及软化行为研究

作为最具潜力的航空航天高温结构材料,Ti2AlNb基合金具有高的比强度和良好的高温蠕变性能。本文对热轧态Ti-22Al-26Nb合金高温变形中的力学行为和再结晶行为进行研究,建立其高温本构关系模型,对其中呈现出的动态再结晶多应力峰值曲线特征（以1000℃,0.1s-1为例）进行拟合分析。结果表明：基于双曲正弦函数建立Ti-22Al-26Nb合金的高温本构关系模型的精度较高,最大误差为2.6%,可以很好地描述合金在高温变形时各热力学参数之间高度非线性的复杂关系,由修正的Avrami方程预测得知再结晶体积分数与应变呈现典型的再结晶动力学增长趋势,揭示了该合金高温变形过程中复杂的软化行为。