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.     

Nonclassical constitutive model involving void evolution of casting magnesium alloy

The void evolution equation and the elastoplastic constitutive model of casting magnesium alloy were investigated. The void evolution equation consists of the void growth and the void nucleation equations. The void growth equation was obtained based on the continuous supposition of the material matrix, and the void nucleation equation was derived by assuming that the void nucleation follows a normal distribution. A softening function related to the void evolution was given. After the softening function was embedded to a nonclassical elastoplastic constitutive equation, a constitutive model involving void evolution was obtained. The numerical algorithm and the finite element procedure related to the constitutive model were developed and applied to the analysis of the distributions of the stress and the porosity of the notched cylindrical specimens of casting magnesium alloy ZL305. The computed results show satisfactory agreement with the 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.     

含孔洞的Al2Cu单轴拉伸的分子动力学模拟

建立含孔洞的Al₂Cu分子动力学模拟模型,采用嵌入原子法模拟Al₂Cu模型在常温、恒定工程应变速率的拉伸环境下孔洞大小、数量及孔洞分布对Al₂Cu力学性能的影响。研究结果表明：孔洞的出现使模型内部出现了自由表面并在孔洞内边缘产生了应力集中,从而大大降低材料的抗拉强度以及变形能力;孔洞增大,Al₂Cu的塑性和抗拉强度均明显下降;不同孔洞数量对应的应力应变曲线在弹性变形阶段基本重合,孔洞增多,Al₂Cu的塑性以及抗拉强度都有不同程度的下降;改变孔洞分布,孔洞连线方向与拉伸方向的夹角越小,Al₂Cu表现出越强的塑性和抗拉强度。     

基于随机介质理论的复合材料孔隙二维形貌几何仿真

含孔隙复合材料具有非均质性和各向异性。根据随机介质理论,采取统计学方法,将孔隙看作是小尺度上的随机扰动,叠加于由基体构成的大尺度背景介质平均特性之上,并利用空间平稳随机过程以及自相关长度、自相关长度比、粗糙度因子及扰动标准差等参数加以描述,建立复合材料随机孔隙模型。该模型采取极值搜索法将连续随机介质改造为适合于描述含孔隙缺陷的离散介质,并依据不同孔隙率试样的孔隙形貌特征统计数据对其进行优化。针对孔隙率为0.03%~4.62%的碳纤维增强树脂基复合材料（CFRP）进行了模拟,结果表明：利用该模型能够得到具有与真实孔隙几何相似性良好的随机性孔隙形貌。     

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.     

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.     

单晶合金中孔洞对蠕变行为的影响

通过对有/无缺陷单晶镍基合金蠕变性能测试、组织形貌观察及采用有限元对近孔洞区域的应力场分析,研究了组织缺陷对单晶合金蠕变行为及组织演化的影响。结果表明:组织缺陷可明显降低单晶镍基合金的塑性和蠕变寿命。在高温蠕变期间,近孔洞区域的应力等值线具有碟形分布特征,并沿与施加应力轴成45°角方向有较大值,该应力分布特征可使合金中γ′相转变成与施加应力轴成45°角的筏状结构,并使圆形孔洞沿应力轴方向伸长成椭圆状。蠕变期间,在合金圆形孔洞缺陷的上、下区域具有较小的应力值,而在圆形孔洞的两侧极点处具有最大应力值,随蠕变时间延长,应力值增大,促使裂纹在该处萌生,并沿垂直于应力轴方向扩展是降低合金蠕变寿命的主要原因。     

Estimating the hotmix asphalt air voids from ground penetrating radar

Traditionally, in-place air voids are obtained based on field cores. Coring is a destructive and time-consuming process. This study presents a high speed Non-Destructive Testing (NDT) technique with Ground Penetrating Radar (GPR) to characterize the in-place air voids. A total of 92 cores were retrieved from three field projects to establish relationship between the air voids and the measured dielectric by GPR. A statistical model was developed to express the air void value as a function of dielectric and other variables. Contour air void maps could also be produced for the entire pavement sections. The results from the underlying studies have been used as the basis for the repair strategy selections.     

AZ31镁合金无缝管材冷轧成形过程研究

镁合金在室温下塑性较差,导致其在轧制过程中组织分布不均而引起应力差异,因此从镁合金材料特性角度出发,分析成形过程中力学特征及组织演变过程就显得至关重要。基于此,本研究通过实验建立AZ31镁合金分段本构模型,构建包含晶粒拓扑技术的元胞自动机模型。借助二次开发技术,将上述本构模型、元胞自动机与有限元软件结合起来进行仿真,获得包含应力、应变、晶粒大小和分布规律等预测结果,对控制皮尔格轧制AZ31镁合金成形工艺以实现对成形和性能的协调控制具有一定意义,并通过实验对模拟结果进行验证。     

陶瓷间约束金属薄层中孔洞间的屏蔽效应及破坏机理

通过体胞分析方法,对处于陶瓷间约束金属薄层中和陶瓷／金属界面上的孔洞间的相互作用进行了大变形弹塑性有限元分析。计算结果表明：（１）较软的约束层中的孔洞对界面的长大具有屏蔽作用,当约束层中孔洞与界面上孔洞的大小相差不大时,约束金属薄层中的孔洞对界面上的孔洞具有较强的屏蔽作用,破坏一般起始于约束层中;当支中孔洞较界面孔洞小得多时,它对界面上孔洞的屏蔽作用减弱,界面上的孔洞长大较快,破坏一般起始于陶瓷／     

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。     

采用反重力渗流法制备开孔泡沫铝(英文)

采用一套新颖的反重力渗流装置制备开孔泡沫铝。研究表明,采用反重力渗流法所制备的泡沫铝,表现出优良的力学性能且几乎没有渗流不足与渗流过度的缺陷;泡沫铝中的空隙度对其力学性能的影响很大,泡沫铝的屈服应力与平台应力均随孔隙度的降低而升高;升高预热温度与保压压强可有效降低泡沫铝中的空隙度。     

Implementation of VPSC polycrystal model into rigid plastic finite element method and its application to Erichsen test of Mg alloy

A methodology on the multiscale simulation of metal forming processes is presented, which fully integrates the visco-plastic self-consistent (VPSC) polycrystal model into rigid plastic finite element method (FEM). To accurately predict the material behavior of a magnesium alloy from the microstructural level, the VPSC crystal plasticity model was used as a constitutive equation in this methodology. An optimization program VPSC-GA was developed in order to calculate the hardening parameters for each slip and twin mode of a single crystal from a couple of simple tension/compression tests. The existing constitutive equation for rigid plastic FEM is modified using the deviatoric stress components and the derivatives of them with respect to strain rate components. The stiffness matrix and the load vector were derived based on a new approach and implemented into DEFORM^TM-3D via a user subroutine which handles stiffness matrix in elemental level. An application to the Erichsen tests of magnesium alloys was done and the stretch formability of two different Mg alloy sheets was analyzed using the results of both experiment and simulation.     

FCC晶体中晶体取向对孔洞长大和聚合的影响

通过编制率相关有限元用户子程序，采用包含一个和两个球形孔洞的单胞探求了FCC晶体中晶体取向对孔洞长大和聚合的影响。计算结果表明：晶体取向对孔洞长大的影响较大，孔洞的形状和长大方向与晶体取向密切相关：由于变形不均匀，孔洞在晶界处产生尖角，易形成裂纹。由于约束较少，孔洞周围和两孔洞间的区域塑性变形较大，晶体的转动和滑移主要集中在孔洞周围以及两孔洞间的区域。     

镦粗过程中摩擦对空洞演变行为的影响

采用有限元模拟方法,模拟计算了摩擦因子为0. 3,0. 5和0. 7时,锻件不同位置空洞的闭合情况。分析了不同摩擦因子下,锻件不同位置的空洞高度的变化规律,结果发现:锻件内部空洞比边缘的空洞更容易闭合,并且随着摩擦因子的增大,内部空洞闭合速度加快,边缘空洞闭合速度减慢。分析摩擦因子、静水应力、等效应变、空洞闭合速度的关系发现:在锻件的内部位置,随摩擦因子增大,静水应力绝对值增大、等效应变增大、空洞闭合速度增大;在锻件的边缘位置,随着摩擦因子增大,等效应变减小、空洞闭合速度减小。摩擦因子对锻件边缘位置空洞闭合情况和等效应变的影响大于对锻件内部位置空洞的影响。     

镁合金熔模精密铸造技术研究现状

概述了镁合金材料的基本特性及性能、镁合金的分类,介绍了镁合金的成形工艺研究现状,着重对镁合金熔模精密铸造技术进行了论述,对其存在的问题进行了讨论,并展望了其发展前景。     

A model for shrinkage of a spherical void in the center of a grain: Influence of lattice diffusion

A model is presented to describe a spherical void shrinkage at the center of a quasi-spherical grain dominated by lattice self-diffusion. The model is based on the difference in chemical potential between the spherical void surface and the grain boundary interface. The quantitative calculations for pure iron predicted that only small, micron-sized spherical voids could be wholly healed within hours at high temperature. The spherical void shrinkage process can be greatly promoted with an increase in temperature, which depends strongly on crystal lattices, particularly the initial radius of the spherical void and the grain size. The time to eliminate a spherical void with an identical radius within grains is close to that for grain boundaries, while different shrinkage processes were undergone, at fixed temperatures, and related to spherical void size, void spacing, and the grain size.     

Characterization and Modeling Study on Interfacial Heat Transfer Behavior and Solidified Microstructure of Die Cast Magnesium AlloysEI北大核心CSCD

Magnesium alloys are widely used in various fields because of their outstanding properties. High-pressure die casting (HPDC) is one of the primary manufacturing methods of magnesium alloys. During the HPDC process, the solidification manner of casting is highly dependent on the heat transfer behavior at metal-die interface, which directly affects the solidified microstructure evolution, defect distribution and mechanical properties of the cast products. As common solidified microstructures of die cast magnesium alloys, the externally solidified crystals (ESCs), divorced eutectics and primary dendrites have important influences on the final performance of castings. Therefore, investigations on the interfacial heat transfer behavior and the solidified microstructures of magnesium alloys have considerable significance on the optimization of die-casting process and the prediction of casting quality. In this paper, recent research progress on theoretical simulation and experimental characterization of the heat transfer behaviors and the solidified microstructures of die cast magnesium alloys was systematically presented. The contents include: (1) A boundary-condition model developed based on the interfacial heat transfer coefficients (IHTCs), which could precisely simulate the boundary condition at the metal-die interface during solidification process. Accordingly, the IHTCs can be divided into four stages, namely the initial increasing stage, the high value maintaining stage, the fast decreasing stage and the low value maintaining stage. (2) A numerical model developed to simulate and predict the flow patterns of the externally solidified crystals (ESCs) in the shot sleeve during mold filling process, together with discussion on the influence of the ESCs distribution on the defect bands of die cast magnesium alloys. (3) Nucleation and growth models of the primary alpha-Mg phases developed by considering the ESCs in the shot sleeve. (4) Nucleation and growth models of the divorced eutectic phase, which can be used to simulate the microstructure evolution of die cast magnesium alloys. (5) The 3D morphology and orientation selection of magnesium alloy dendrite. It was found that magnesium alloy dendrite exhibits an eighteen-primary branch pattern in 3D, with six growing along < 11(2)over bar0 > in the basal plane and the other twelve along < 11(2)over bar3 > in non-basal planes. Accordingly, an anisotropy growth function was developed and coupled into the phase field model to achieve the 3D simulation of magnesium alloy dendrite.