Multistage Fatigue Modeling of Cast A356-T6 and A380-F Aluminum Alloys

This article presents a microstructure-based multistage fatigue (MSF) model extended from the model developed by McDowell et al.[1,2] to an A380-F aluminum alloy to consider microstructure-property relations of descending order, signifying deleterious effects of defects/discontinuities: (1) pores or oxides greater than 100 μm, (2) pores or oxides greater than 50 μm near the free surface, (3) a high porosity region with an area greater than 200 μm, and (4) oxide film of an area greater than 10,000 μm². These microconstituents, inclusions, or discontinuities represent different casting features that may dominate fatigue life at stages of fatigue damage evolutions. The incubation life is estimated using a modified Coffin–Mansion law at the microscale based on the microplasticity at the discontinuity. The microstructurally small crack (MSC) and physically small crack (PSC) growth was modeled using the crack tip displacement as the driving force, which is affected by the porosity and dendrite cell size (DCS). When the fatigue damage evolves to several DCSs, cracks behave as long cracks with growth subject to the effective stress intensity factor in linear elastic fracture mechanics. Based on an understanding of the microstructures of A380-F and A356-T6 aluminum alloys, an engineering treatment of the MSF model was introduced for A380-F aluminum alloys by tailoring a few model parameters based on the mechanical properties of the alloy. The MSF model is used to predict the upper and lower bounds of the experimental fatigue strain life and stress life of the two cast aluminum alloys. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.

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铝合金点焊过程中铜铝合金化的有限元分析

为了研究铜铝合金化反应的机理,本文建立了一种基于铜和铝扩散反应进行的有限元模型。由于热传导方程和扩散方程在表达形式上的相似性,所以用模拟热传导的方式来模拟铝在铜电极中的单向扩散过程,以温度分布表示扩散浓度的分布情况。研究表明：若只考虑铝在铜中的单向扩散,将试验中的数据按比例进行换算,这样计算出来的试验结果与模拟结果比较相近。     

摩托车铝合金车轮扭转疲劳有限元分析

使用三维建模软件UG和有限元分析软件ANSYS建立某款摩托车铝合金车轮的扭转疲劳试验有限元分析模型。结合A356铝合金的材料属性,通过ANSYS软件模拟分析车轮在试验条件下的应力应变分布规律,找出车轮结构中最大危险点的应力值。采用名义应力法对摩托车车轮扭转疲劳寿命进行预测,并与台架试验结果进行比较,验证了分析模型的正确性。     

Deformation Behavior Estimation of Aluminum Foam by X-ray CT Image-based Finite Element Analysis

Aluminum foam is a lightweight material owing to the existence of a large number of internal pores. The compressive properties and deformation behavior of aluminum foam are considered to be directly affected by the shape and distribution of these pores. In this study, we performed image-based finite element (FE) analyses of aluminum foam using X-ray computed tomography (CT) images and investigated the possibility of predicting its deformation behavior by comparing the results of FE analyses with those of actual compressive tests. We found that it was possible to create an analytic model reflecting the three-dimensional (3D) pore structure using image-based modeling based on X-ray CT images. The stress distribution obtained from image-based FE analysis correctly indicates the layer where deformation first occurs as observed in actual compressive tests. Also, by calculating the mean stress of each plane perpendicular to the direction of compression based on the stress distribution obtained from image-based FE analysis, it was found that deformation begins in the layer containing the plane with maximum stress. It was thus possible to estimate the layer where deformation begins during the compression of aluminum foam.     

As-Cast Residual Stresses in an Aluminum Alloy AA6063 Billet: Neutron Diffraction Measurements and Finite Element Modeling

The presence of thermally induced residual stresses, created during the industrial direct chill (DC) casting process of aluminum alloys, can cause both significant safety concerns and the formation of defects during downstream processing. Although numerical models have been previously developed to compute these residual stresses, most of the computations have been validated only against measured surface distortions. Recently, the variation in residual elastic strains in the steady-state regime of casting has been measured as a function of radial position using neutron diffraction (ND) in an AA6063 grain-refined cylindrical billet. In the present study, these measurements are used to show that a well-designed thermomechanical finite element (FE) process model can reproduce relatively well the experimental results. A sensitivity analysis is then carried out to determine the relative effect of the various mechanical parameters when computing the as-cast residual stresses in a cylindrical billet. Two model parameters have been investigated: the temperature when the alloy starts to thermally contract and the plasticity behavior. It is shown that the mechanical properties at low temperatures have a much larger influence on the residual stresses than those at high temperatures.     

5t工频有芯感应熔铝炉熔炼铸造铝合金的实践

采用5 t工频有芯感应熔铝炉,利用电解铝液直接熔炼铸造铝合金(A356),劳动生产率高,环境污染小,合金化学成分均匀(同一熔次硅的级差只有0.07个百分点),产品质量稳定,能耗低(交流电耗为365.15kW·h/t,比原有的同类型1 t熔铝炉低135 kW·h/t).2台5 t工频有芯感应炉的生产能力可达10 000t/a以上.     

镍基合金圆柱形热模拟试样热变形后鼓形的有限元研究

通过abaqus有限元分析软件,对镍基合金圆柱形热模拟试样在加热和保温后的单向压缩变形过程进行了模拟。研究结果表明,由于镍基合金材料导热系数较小,在快速变形条件下,试样在单向压缩过程中产生的变形热不能充分扩散,从而导致变形后试样内部局部温度升高。通过与热模拟试样变形后鼓形的实测值进行比较,结果均表明,变形后试样的鼓形随着应变速率的提高而增大。     

FGH95粉末高温合金蠕变裂纹扩展有限元数值模拟研究

对FGH95粉末高温合金标准CT试样在700℃下的蠕变裂纹扩展过程进行了有限元数值模拟研究.FGH95合金假设为弹性-蠕变体,蠕变变形采用Norton模型描述.蠕变裂纹扩展模拟时考虑了两种裂纹扩展速率,分别为3.25×10-2mm/h(快速裂纹扩展)和6.5×10-4mm/h(慢速裂纹扩展).数值模拟结果表明:FGH95合金在700℃下发生蠕变裂纹扩展时,弹性变形引起的标准CT试样加载线位移Vc在总位移中起主导作用,蠕变变形引起的加载线位移Vc很小,加载线位移率比值(·Vc)/(·V)远小于1,蠕变变形被限制在临近裂纹扩展路径的细长条带状区域内,裂纹尖端没有发生大范围蠕变变形.上述结果说明FGH95合金在700℃下为蠕变脆性材料,应力强度因子K可作为FGH95合金高温疲劳裂纹扩展的有效驱动力参数.     

Modeling of Microporosity Size Distribution in Aluminum Alloy A356

Porosity is one of the most common defects to degrade the mechanical properties of aluminum alloys. Prediction of pore size, therefore, is critical to optimize the quality of castings. Moreover, to the design engineer, knowledge of the inherent pore population in a casting is essential to avoid potential fatigue failure of the component. In this work, the size distribution of the porosity was modeled based on the assumptions that the hydrogen pores are nucleated heterogeneously and that the nucleation site distribution is a Gaussian function of hydrogen supersaturation in the melt. The pore growth is simulated as a hydrogen-diffusion-controlled process, which is driven by the hydrogen concentration gradient at the pore liquid interface. Directionally solidified A356 (Al-7Si-0.3Mg) alloy castings were used to evaluate the predictive capability of the proposed model. The cast pore volume fraction and size distributions were measured using X-ray microtomography (XMT). Comparison of the experimental and simulation results showed that good agreement could be obtained in terms of both porosity fraction and size distribution. The model can effectively evaluate the effect of hydrogen content, heterogeneous pore nucleation population, cooling conditions, and degassing time on microporosity formation.     

Discrete Element Modeling of Contact Creep and Aging in Sand

In this study, aging in dry, clean sand induced by contact creep is investigated through numerical simulations using the discrete element method. Simulation results demonstrate that contact creep initiates the redistribution of contact forces. Although contact creep produces a very small decrease in porosity (approximately 1.7%), a significant change in the contact force distribution is produced in the aged sample. The contact forces ultimately become more uniform in both magnitude and spatial distribution. This homogenization of contact forces leads to more stable force chains and therefore produces an increase in the small-strain stiffness, early strength, and dilatancy in the aged sample. Such increases are not found in the sample prepared to the same porosity as the aged sample but without aging. This is because, in generating this sample, the contact creep is not allowed and therefore its associated contact force distribution is less homogenized compared with that in the aged sample.     

Quantitative Atomic Force Microscopy Characterization and Crystal Plasticity Finite Element Modeling of Heterogeneous Deformation in Commercial Purity Titanium

Using a four-point bend sample of commercial purity titanium deformed to a surface strain around 1.5 pct, the active dislocation slip and twin systems in a microstructural patch of about 15 grains were quantitatively analyzed by a technique combining atomic force microscopy (AFM), backscattered electron (BSE) imaging, and electron backscattered diffraction (EBSD). Local shear distribution maps derived from z-displacement data measured by AFM were directly compared to results of a crystal plasticity finite element (CPFE) simulation that incorporates a phenomenological model of the deformation processes to evaluate the ability of the CPFE model to match the experimental observations. The CPFE model successfully predicted most types of active dislocation slip systems within the grains at correct magnitudes, but the spatial distribution of strains within grains differed between the measurements and the simulation.     

玻璃与铝阳极键合中残余应力有限元分析

叙述了玻璃与铝阳极键合试件冷却过程的应力有限元分析方法,获得了键合件从450℃冷却到室温后,键合件内的应力分布信息。     

Effect of Welding Parameters on Tensile Properties and Fatigue Behavior of Friction Stir Welded 2014-T6 Aluminum Alloy

The present work describes the effect of welding parameters on the tensile properties and fatigue behaviour of 2014-T6 aluminum alloy joints produced by friction stir welding (FSW). Characterization of the samples has been carried out by means of microstructure, microhardness, tensile properties and fatigue behaviors. The hardness in the softened weld region decreases with decreasing the welding speed. Irrespective of the tool rotation speeds, the best tensile and fatigue properties were obtained in the joints with the welding speed of 80 mm/min. The joint welded with a rotating speed of 1520 rpm at 80 mm/min has given a highest tensile and fatigue properties. The fatigue behaviors of the joints are almost consistent with the tensile properties, especially elongations. Higher ductility in FSW joints made the material less sensitive to fatigue. The location of tensile fractures of the joints is dependent on the welding parameters. On the other hand, the fatigue fracture locations change depending on the welding parameters and stress range. In addition, a considerable correlation could not be established in between heat indexes and mechanical properties of FSW 2014-T6 joints under the investigated welding parameters.     

冷轧5E06和5E83铝合金板的疲劳断裂行为研究

测试并对比了掺杂微量稀土Er中高Mg铝合金5E83和5E06冷轧板的疲劳性能,运用XRD、扫描电镜(SEM)以及能谱(EDS)等对合金组织与疲劳断口进行表征,探讨了Er与Mg元素对Al-Mg合金疲劳断裂行为的影响.结果表明:合金中Mg固溶量的增加可提高合金疲劳强度并减缓裂纹扩展速率.合金中Er含量超过其固溶度后,其含量的增加也可减缓裂纹的扩展,这归结为粗大析出物Al3Er能够使裂纹偏折并发生闭合从而有效阻碍裂纹的扩展.     

滴定法联合快速测定镍硼合金中的硼和铝

试样用过氧化钠熔融,用热水浸取熔融物,在乙醇存在下,加热煮沸驱除过氧化氢,定容后,过滤铁、镍、锰等沉淀与硼、铝分离,移取部分滤液分别测定硼、铝量。测定方法简便、快速,测定结果令人满意,硼的相对标准偏差≤0.74%,铝的相对标准偏差≤2.89%。     

20辊森吉米尔轧机板形调控性能研究

运用通用有限元软件ANSYS建立了具有多种辊间接触状态的20辊森吉米尔轧机辊系三维弹性变形的有限元模型,合理解决了对辊间接触压扁变形的建模,实现了对此较复杂辊系变形的正确求解.以国内某厂森吉米尔轧机为对象,结合现场实际数据,成功实现了运用通用有限元软件ANSYS求解20辊森吉米尔轧机的辊系三维弹性变形,进而分析了森吉米尔轧机的板形调控性能.     

A Numerical Method for Microstructure Generation of a Binary Aluminum Alloy and Study of Its Mechanical Properties Using the Finite Element Method

A numerical method for the generation of the microstructure of a binary aluminum copper alloy is presented. This method is based on the repeated addition of some basic grain shapes into a representative volume element. Depending of the orientation of adjacent grains, different type of grain boundaries can be formed. The primary and secondary phases are distinguishable in our model and have distinct properties, reflecting the heterogeneous nature of the microstructure. The digital microstructure was then transformed into a finite element model. Using the finite element software ABAQUS, the stress distribution inside our heterogeneous material model has been studied and its mechanical properties have been found. That also makes possible to study and to visualize the cracks generated during the loading of the material where the local stress was sufficiently high. As a result of these analyses, the elastic modulus of such a heterogeneous domain and the effect of crack formation on ductility were evaluated.     

Corrosion Behavior of Top and Bottom Surfaces for Single-Side and Double-Side Friction Stir Welded 7085-T7651 Aluminum Alloy Thick Plate Joints

Thick plate joints of 7085-T7451 aluminum alloy were obtained through both single-side and double-side friction stir welding (SS or DS-FSW). The chloride ions effects on the corrosion behavior of the top and bottom surfaces of the joints were examined by cyclic potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). Results show that the corrosion susceptibility was suppressed significantly in the weld nugget zone, while the base material and heat-affected zone were prone to be corrosion attacked. For the SS-FSWed joint, the top surface showed a higher corrosion resistance than that of the bottom surface, but the larger corrosive heterogeneity was observed between the top and bottom surfaces compared with the two welds of DS-FSWed joint, which was confirmed by the morphology of corrosion attack. A deep insight on the microstructure of the joints indicates that the intermetallic particles played a key role in the corrosion behavior of the FSWed AA7085 aluminum alloy joints in chloride solution.