板材冲压成形的晶体塑性有限元模拟

将率相关晶体塑性本构理论引入Mindlin曲壳单元模型与动力显式有限元法,采用切线系数法计算剪切应变率,根据晶体取向正态分布规律在单元积分点处分配晶体取向,按各晶体取向体积分数的加权平均计算多晶体应力,开发晶体塑性动力显式有限元程序,实现板材冲压成形过程模拟和晶体取向演化预示.以主要初始织构为铜织构和S织构的轧制铝板为对象,对方盒件冲压成形过程及织构演化进行数值模拟,计算结果与试验结果呈现出较好的一致性.通过晶体弹塑性有限元法不仅可以预示板材宏观成形构形变化,而且能够预测板材织构的演化情况.模拟结果显示在方盒件冲压成形过程中,铜织构和S织构为不稳定取向,变形后逐渐转到其他取向.     

FCC多晶体的黄铜型织构演化模拟

高层错能FCC材料塑性变形主要靠晶体滑移，而对于低层错能材料，除晶体滑移之外，在维持正常塑性流动方面，变形孪晶也起重要作用。建立了可解释滑移和孪晶导致塑性变形的率相关本构模型，并在研究开发的有限元程序中予以实现，通过弹性／晶体粘塑性分析计算，对面心立方金属在滑移与孪晶作用下的织构演化过程进行了计算机模拟。计算表明，在平面应变压缩下，黄铜晶体织构的演化是滑移和孪晶导致的晶格转动的结果，孪晶是造成低层错能FCC晶体中黄铜型织构的重要原因。     

金属塑性成形的晶体塑性学有限元模拟研究进展

综述了金属塑性成形过程中晶体塑性有限元模拟的理论背景和应用方面的研究进展,同时总结了国内研究者该领域的研究现状,指出了晶体塑性有限元模拟所要解决的问题及研究重点。晶体塑性理论起源于20世纪20 年代,包括单晶塑性本构理论和多晶塑性本构理论,能够深刻揭示材料变形的规律。与此同时,开始于30多年前的有限元法也已经日益成为求解材料成形理论公式的有效工具。晶体塑性有限元法作为一个强大的模拟工具将二者有机地结合在一起,已经广泛地用于模拟材料的微观结构和各种力学响应,越来越被材料界和力学界的研究者所重视;然而,无论是在理论方面还是应用方面晶体塑性有限元法都还不尽完善。未来晶体塑性有限元模拟的理论研究重点是建立系统的理论架构用于预测由滑移和孪晶引起塑性变形材料的各种力学响应,应用研究重点是运用各种模型模拟其他与织构相关的性能或参数。晶体塑性有限元模拟不仅能够深化人们对材料成形规律的理解, 而且可以不断推进晶体塑性理论的发展。     

基于多尺度晶体塑性有限元的GCr15轴承钢拉-压不对称性研究

将应力三轴度引入晶体塑性本构模型,从宏、细观尺度阐明了造成GCr15轴承钢拉-压不对称性的原因.首先,拓展晶体塑性本构模型,在滑移系中引入应力三轴度对屈服准则和流变准则的影响.其次,采用了基于代表性体元(RVE)的多尺度晶体塑性有限元仿真模拟,对GCr15轴承钢的准静态单向拉伸和压缩试验进行了数值实现.研究结果表明,由于多晶体晶粒间的晶向差异,在晶界处会产生应力集中现象,且在应力三轴度的影响下,该应力集中现象出现了显著的拉-压不对称性.故得出在以位错滑移为主的金属塑性变形中,应力三轴度对细观晶体织构的影响是造成拉-压不对称性的重要因素.     

铝合金管弯曲成形表面粗化宏细观耦合建模仿真

铝合金管弯曲成形中的表面粗化缺陷严重影响了其表面质量和使役性能,迫切需要探明该成形过程中的表面粗化形成规律以避免此类缺陷的产生。管材弯曲成形宏观有限元模型无法准确描述细观尺度不均匀变形导致的表面粗化行为,而细观力学模型又难以应用到具有复杂加载边界条件的实际成形过程分析。将基于Voronoi图和晶体塑性理论的代表性体积单元（Representative volume element,RVE）细观模型引入宏观有限元模型,建立了铝管弯曲三维宏细观耦合模型,模拟研究5052-O铝管在弯曲过程中的表面粗化特征,系统分析了细观材料参数对弯曲过程表面粗化行为的影响规律。结果表明,晶粒尺寸越小,晶粒内部与晶界附近的应变差值越小,其变形越均匀,能获得越小的表面粗糙度;对于Cube织构和随机织构混合的模型,随着Cube织构含量的增多,弯曲变形后表面粗糙度呈下降趋势;织构分布与加载方式耦合影响表面粗化行为。     

再结晶后面心立方金属的断裂模式及有限元分析

为了研究再结晶对面心立方(FCC)金属断裂模式的影响,以纯铝为对象采用电子背散射衍射、拉伸试验和扫描电镜分析等研究了纯铝拉伸断裂的模式,并结合3D晶体有限元模拟了变形过程中纯铝中孔洞的长大行为。结果表明:由于晶粒间不均匀变形使得大角度晶界上的等效塑性应变大于单个晶粒中的等效塑性应变,孔洞易沿着大角度晶界长大,使得再结晶后FCC结构的纯铝易发生沿晶断裂。     

硬化模型对多晶变形织构预测的影响

基于变形梯度乘法分解 ,推导了延性单晶材料的弹性变形梯度演化方程 ,利用晶体弹粘塑性本构关系的和Taylor多晶假设 ,以面心立方晶体铝、铜作为模型材料比较分析了两种饱和型硬化模型对有限变形下多晶变形织构形成过程的影响     

晶体塑性模型在板材成形计算机模拟中的应用

阐述晶体塑性理论的基本原理，包括单晶体塑性本构关系，多晶体塑性模型的构造，介绍它在金属成形模拟中的应用概况。在速率相关的晶性模型的引入滑移系活动与否的判断准则，忽略不活动滑移系的贡献，从而在保持计算精度的时间时，大大减少了本构关系的计算时间。采用动力显式有限元方法和多晶体塑性模型模拟铝板的ＬＤＨ试验过程，考察板材的织构对其成形性能的影响。     

金属塑性成形过程再生核质点无网格方法数值模拟

应用微可压缩材料的流动法则,采用再生核函数无网格方法,自行开发了求解方棒压缩、圆棒压缩、反向挤压和轧制等金属塑性成形过程应用程序。应用再生核质点无网格方法计算得到纯铝和铝合金材料金属塑性成形过程的速度场和应力场解析结果,并与自行开发的I-Form有限元程序得到的计算结果以及试验数据进行了分析比较,结果符合良好。再生核质点无网格方法具有求解金属大变形特点,解决了有限元法中的网格重划问题,为复杂金属变形分析提供了良好的研究手段。     

一种估计管材硬化模型参数的方法

管材力学性能参数的准确性是影响管材塑性成形有限元数值模拟质量的关键因素之一。单向拉伸试验的试件取自滚弯和焊接等制管工序之前的平板坯料 ,所测应力—应变关系无法真实描述管材的塑性变形行为。单向拉伸试验也不能精确反映管材在实际塑性成形中所处的复杂应力状态。基于各向同性硬化假设 ,本文提出了一种轴压胀形、单向压缩试验和数据拟合技术相结合的估计管材硬化模型参数的方法。有限元数值模拟结果显示 ,由这种方法所估计出的管材硬化模型参数是相当准确的。     

Crystal Plasticity Finite Modelling of 3D Surface Asperity Flattening in Uniaxial Planar Compression

Rate-dependent crystal plasticity constitutive model has been employed into finite element software ABAQUS to simulate surface asperity flattening in uniaxial planar compression. Measured textures and surface roughness are introduced into the 3D surface roughness model. The calculated results show a good agreement with the experimental results. With an increase of reduction, the surface asperity flattening tends to increase, and Goss texture {011} 〈100〉 and brass component {110} 〈112〉 become stronger, whilst the cubic texture {001} 〈100〉 becomes weaker. If the reduction reaches 40%, Schmid in-grain shear band appears and the strain localisation starts. The evolution of surface feature (roughness) shows the obvious sensitivity on the orientation {111} of near-top surface.     

Numerical simulation of sheet metal stamping by using ODF data

An elasto-polycrystalline plastic finite element method is developed to simulate the deformation behavior and corresponding crystallographic texture evolution of sheet metals in stamping processes. A rate-independent crystal plasticity model and a degenerated shell element are introduced into the explicit finite element formulation. A successive integration method by which the shear rates on the slip systems can be calculated without prior determination of active/non-active state of the slip systems, as generally done in rate-dependent models, is employed to calculate the plastic strain rate of a crystal during the deformation. Representative crystal orientations are extracted from the orientation distribution function (ODF) data and assigned to the integration points of elements according to the distributional characteristic of crystal orientations. The macroscopic stress of a polycrystalline aggregate is evaluated by a volume-averaged response of the representative crystals. Two drawing processes are simulated, one with a cylindrical punch and the other with a square punch. Good agreements between the calculated results and experimental ones are obtained.     

工艺路径对纯铝粉末材料多道次等径角挤扭变形的影响

结合实际变形过程,采用DEFORM-3D对不同变形工艺路径下纯铝粉末材料等径角挤扭(ECAPT)多道次变形过程进行了三维数值模拟。有限元模拟结果表明:A和C工艺路径可在较小的挤压载荷下实现有效的应变累积和均匀的变形分布,是两种较为理想的变形路径;BA和BC两种工艺路径虽然能够使变形材料累积的等效应变量大大增加,但却会造成试样变形分布的严重不均匀。电子背散射衍射(EBSD)实验结果表明,A路径下纯铝粉末材料经4道次等径角挤扭变形后,可获得组织均匀、性能优良的块体超细晶铝,材料平均晶粒尺寸约为0.8μm,抗压强度高达123.3MPa,验证了前述模拟结果的可靠性。     

Static recrystallization simulations starting from predicted deformation microstructure by coupling multi-phase-field method and finite element method based on crystal plasticity

We have developed a numerical model of recrystallization taking the inhomogeneities of the plastic deformation of a polycrystalline metal into account. Here, the plastic deformation of the polycrystalline metal is simulated by the finite element method based on crystal plasticity theory and the microstructure evolution during recrystallization is simulated by the multi-phase-field method. In primary recrystallization simulations, nucleation is the most difficult problem. In the present model, the deformation microstructure is predicted from the results of a crystal plasticity finite element simulation, and spontaneous nucleation is achieved through abnormal grain growth that is enabled by introducing the misorientation dependences of grain boundary energy and mobility. As a result of simulations under three different compression strains, it is confirmed that primary recrystallization simulations depending on the amount of deformation and taking the inhomogeneities of the plastic deformation of a polycrystalline metal into consideration can be successfully performed by employing the proposed model.     

射频器件超细引线键合工艺及性能研究

作为有源相控阵雷达的关键组成部分,T/R （Transmitter and receiver）组件的尺寸与性能决定着装备的重量和功能。引线键合是T/R组件中常用的互连技术之一,随着组件集成度的提高势必也要开发相应的高密度引线键合技术,这使得键合线的尺寸越来越小,而超细的引线会使焊点力学性能降低,造成可靠性下降等问题。采用超声热压楔形键合的方法实现了的超细金丝与金焊盘的连接,并对工艺进行优化。结果表明,随键合压力、键合时间和超声功率的增大,键合后引线形变量逐渐增大,而键合后金丝的拉力先增加后减小,且工艺参数对金带形变量的影响小于金丝;由于第二焊点作用力过大会导致引线形变量较大、最大拉力小于第一焊点,需增加题焊点数量;最后,通过正交试验方法获得了金线和金带的最佳键合工艺参数,实现了超细尺寸引线的键合,对T/R组件的小型化具有重要意义。     

基于单胞模型镍基单晶高温合金低周疲劳寿命的预测

基于镍基单晶高温合金的微观尺度结构特征,建立了γ/γ′双相单胞有限元模型,并基于能量耗散理论,引入统一表征非对称载荷循环特性和拉/扭多轴效应的k参量,以循环塑性应变能作为损伤参量,建立了镍基单晶高温合金低周疲劳寿命预测模型;分别利用宏观有限元模型和γ/γ′双相单胞微观有限元模型对[001]取向的DD3、PWA1480和CMSX-2镍基单晶高温合金的单轴、多轴低周疲劳试验过程进行数值模拟。结果表明:微观单胞有限元模型反映了试验合金的微观结构特性,其预测精度明显优于宏观有限元模型的;上述3种合金几乎所有的试验数据分别落在1.3,2.0和2.0倍偏差分布带内。     

Multi-scale parallel finite element analyses of LDH sheet formability tests based on crystallographic homogenization method

A multi-scale parallel finite element (FE) procedure based on the crystallographic homogenization method was applied to the LDH sheet formability test analysis. For the multi-scale structure, two scales are considered. One is a microscopic polycrystal structure and the other is a macroscopic elastic plastic continuum. The analysis code can predict the formability of sheet metal in macro-scale, simultaneously the crystal texture and hardening evolutions in the micro-scale (Nakamachi E et al. Int J Plasticity 2007;23:450-8). Since huge computation time is required for the nonlinear dynamic multi-scale FE analysis, parallel computing technique based on domain partitioning of FE model for macro-continuum is introduced into the multi-scale code using the message passing interface (MPI) library and PC cluster (Kuramae H et al. In: Proceedings of the eighth international conference on computational plasticity, Part 1, 2005. p. 622-5). The explicit time stepping solution scheme in the nonlinear multi-scale FE dynamic problem is well-suited for parallel computing on distributed memory environment such as PC cluster because solving simultaneous equation is not required. We measured crystal morphologies of four automotive sheet metals, aluminum alloy sheet metals A6022-T43 and A5182-O, an asymmetrically rolled aluminum alloy sheet metal A6022-ASR, and mild steel HC220YD, by using the scanning electron microscope (SEM) with electron back scattered diffraction (EBSD) analyses, and defined a three-dimensional representative volume element (RVE) of micro polycrystal structure, which satisfy the periodicity condition of crystal orientation distribution. We evaluate not only macroscopic formability of the automotive sheet metals by the multi-scale LDH test analysis, but also microcrystalline texture evolution during plastic deformation. Furthermore, a relationship between the macroscopic formability and the microcrystal texture evolution was discussed through looking at multi-scale FE results. It is concluded that the mild steel HC220YD was the highest formability than the aluminum alloy sheet metals because of remaining and generating the γ-fiber texture, such as {1 1 1}〈1 1 0〉-{1 1 1}〈1 1 2〉 orientations, during plastic deformation.     

Densification behavior of aluminum alloy powder under cold compaction

Densification behavior of aluminum alloy (Al6061) powder was investigated under cold compaction. Experimental data were obtained under triaxial compression with various loading conditions. A special form of the Cap model was proposed from experimental data of Al6061 powder under triaxial compression. The proposed yield function and several other yield functions in the literature were implemented into a finite element program (ABAQUS) to compare with experimental data for densification behavior of Al6061 powder under cold isostatic pressing and die compaction. The agreement between finite element calculations from the proposed yield function and experimental data is very good under cold isostatic pressing and die compaction.