比例/非比例加载路径下5754O汽车用铝合金板各向异性变形行为的精确解析

随着越来越高的汽车轻量化需求,铝合金板在现代汽车工业中的应用越来越广。在不同加载路径下,包括比例和非比例加载,5754O铝合金板在塑性成形过程中具有复杂的各向异性规律。试验表明5754O铝合金板的各向异性规律随变形量的增加会发生改变,因此在常参数屈服准则理论框架下,基于传统的单一曲线假设难以对5754O铝合金板在整个塑性变形过程中的各向异性行为进行精确描述。鉴于上述问题,并同时考虑到大变形过程中材料变形的稳定性,对Yld2000-2d屈服准则进行改进。基于改进的Yld2000-2d屈服准则和单一曲线假设推导不同方向的单向拉伸应力应变曲线,并与试验结果进行了对比。结果表明,与原始的Yld2000-2d屈服准则不同,基于改进的Yld2000-2d屈服准则,传统的单一曲线假设仍然适用于5754O铝合金板各向异性问题。给出不同强化方式在比例加载路径下的统一性和非比例加载路径下的分散性证明。基于改进的Yld2000-2d屈服准则和等向强化和混合两种强化方式,推导非比例加载路径下板料的应力应变曲线。基于试验结果,验证了推导的理论曲线的精度。实现了5754O铝合金板在比例和非比例加载路径下变形行为的精确描述,为其工业应用提供了重要的理论支撑。     

参数求解方法对屈服准则的各向异性行为描述能力的影响

详细分析基于应力各向异性和变形各向异性两种求解Hill48屈服准则参数的方法。在给出两种各向异性参数求解表达式的基础上,具体分析Hill48屈服准则本身的局限性。以5754O铝合金板为研究对象,进行不同方向的单向拉伸试验。采用两种各向异性参数求解方法,基于Hill48屈服准则推导不同方向拉伸过程中的理论应力-应变曲线和拉伸过程中的变形规律。通过对比理论与试验结果具体分析参数求解方法对屈服准则精度的影响。基于两种参数求解方法,进行5754O铝合金板拉深试验的有限元模拟,讨论不同求解方法对凸耳现象的描述精度。得出结论：当对应力各向异性为主的问题进行分析时,应采用应力各向异性法求解;当对变形各向异性为主的问题进行分析时,则应采用变形各向异性法求解。研究结果对屈服准则在板料成形方面的合理应用具有重要的参考价值。     

基于循环拉-压试验的混合硬化模型参数确定及模型应用

材料弹塑性本构模型是影响有限元模拟精度的最重要因素,混合硬化本构模型能较准确表现材料塑性变形过程真实硬化特征,而本构模型中材料特性相关参数是否准确直接影响到有限元模拟的精度。基于Hill48各向异性屈服准则,结合Voce各向同性硬化模型和Armstrong-Frederic非线性随动硬化模型,建立一个考虑材料各向异性和Bauschinger效应的混合硬化弹塑性本构模型。通过循环拉伸-压缩试验,获得DC54D+ZF镀锌板的循环变形应力-应变曲线,并利用通用全局优化算法,根据单向应力状态混合硬化本构方程,准确地确定了混合硬化模型中的材料特性参数。最后,使用ABAQUS有限元软件对板材循环拉伸-压缩问题和板材过拉深筋问题进行该本构模型的适用性分析,验证了所建立的各向异性混合硬化材料本构模型的可靠性和精确性。循环拉伸-压缩试验是直接准确地获得本构模型材料参数的有效方法。     

各向异性非线性随动硬化本构模型的建立及应用

基于平面应力假设,采用HILL48各向异性屈服准则和A-F非线性随动硬化模型及塑性流动法则建立一种各向异性非线性随动硬化本构模型。运用向后欧拉回映算法,通过Fortran语言编写UMAT子程序,将该本构模型嵌入到ABAQUS软件中。以板料经过拉深筋的循环加载问题为研究对象,利用开发的本构模型、ABAQUS软件中的各向同性屈服及随动硬化模型对板料经过拉深筋的变形过程进行数值模拟分析,得到切向应力-应变循环变化曲线。对比试验结果,开发的弹塑性本构模型的计算精度更高,验证了所建本构模型的有效性,该模型可以应用于板料反向加载变形行为研究。     

考虑成形方向的航空铝合金修正本构模型的构建

为了能够准确地反映材料成形方向对其动态力学性能的影响,利用电子万能试验机及分离式霍普金森压杆（SHPB）装置,对航空铝合金7050-T7451板材沿不同成形方向（法向ND,横向TD,轧向RD）取样,并进行准静态加载试验和动态冲击剪切试验。结果表明：成形方向是影响材料准静态和动态力学性能的重要因素之一,在动态冲击剪切过程中,铝合金7050-T7451表现出一定的应变率敏感性和正应变率强化效应。基于材料的成形方向影响规律,构建包含应变率敏感函数项的修正的Johnson-Cook本构模型,并对比验证修正模型与试验数据的结果,证明了修正的、包含应变率函数项的材料本构模型更适用于描述不同成形方向下的材料动态力学性能,该模型能够为建立精确可靠的各向异性材料仿真模型提供数据支持。     

机织复合材料各向异性超弹性本构模型

基于纤维增强连续介质力学理论,提出一种超弹性本构模型来描述机织复合材料织物在成形过程中由于大变形所引起的非线性各向异性力学行为。在这一模型中,应变能函数被分解成两部分:一部分代表由于经纱和纬纱各自拉伸所产生的拉伸应变能;另一部分代表由于经纱和纬纱之间角度变化而产生的剪切应变能。本构模型中所需的材料参数通过对机织复合材料织物的单向拉伸以及偏拉试验数据的分析与拟合来求得。利用对一平纹机织复合材料的单曲率半球形冲压成形试验进行有限元模拟来验证模型。模拟结果与试验结果对比表明所提出的超弹性本构模型能够很好地表征机织复合材料在大变形下的非线性各向异性力学行为。这一本构模型具有简单实用、材料参数容易确定的优点。该模型对于机织复合材料成形的数值模拟与成形工艺优化设计有着重要的意义。     

双相不锈钢各组相循环变形行为的纳米压痕试验和有限元表征方法研究

对双相不锈钢的奥氏体相和铁素体相，分别开展了不同加载模式（接触载荷和压入位移）和不同加载波形下的单向、循环纳米压痕试验，对比分析了两相的基本力学性能和压痕循环变形行为的演化规律。基于压痕试验结果和修正ABDEL-KARIM-OHNO非线性随动硬化准则的弹塑性本构模型，提出一套双相不锈钢奥氏体相和铁素体相的塑性和循环塑性行为的本构模型参数表征方法。通过对微结构代表性体积单元整体拉伸和循环变形行为进行模拟，并与宏观试验结果对比，验证了参数表征方法的合理性。研究结果表明，铁素体相的强度、硬度和抗棘轮变形的能力均高于奥氏体相，两相之间通过晶界产生一定的交互作用；在接触载荷控制的循环加载条件下，奥氏体相与铁素体相均产生明显的压痕棘轮现象，且载荷水平越高压痕棘轮变形程度越大；所发展的本构模型参数表征方法可为研究多相材料各组相、小体积材料的循环变形行为提供借鉴和参考。     

不同成形工艺对7050铝合金壁板试验件拉伸性能的影响研究

为探究采用数控成形、机加+轧压、机加+轧压+去应力退火三种不同成形工艺将7050-T7451铝合金板材呈10°弯曲成形情况下对拉伸性能的影响，通过静力拉伸试验研究了三种情况下7050铝合金板材在沿轧制方向(0°)和垂直于轧制方向(90°)的力学性能差异，并进行了金相观察。试验结果表明，数控成形下得到的型材屈服强度最高，90°方向的流体应力略低于0°方向的流体应力，其最大破坏载荷、抗拉强度和断后伸长率未表现出明显的差异性。三种不同成形工艺下的7050铝合金板材对其晶粒尺寸的影响较小。     

面向高性能塑性成形的多尺度建模仿真研究进展

塑性成形是利用材料的塑性产生变形从而形成一定形状并获得一定性能的工艺方法,是零部件制造不可或缺的重要途径。随着我国航空航天等领域高端装备技术的发展,其关键零部件的高精度高性能要求,迫切需要发展可计算、可设计的高性能塑性成形理论与技术,多尺度建模仿真是实现该需求的关键手段。然而,塑性成形涉及材料非线性、几何非线性和工艺非线性等复杂问题,以及力热加载、特种能场加载、组织性能演化等复杂过程,给多尺度建模仿真提出了挑战。从分析高性能塑性成形的科学问题出发,通过梳理材料多尺度变形响应特征,讨论了塑性成形多尺度建模面临的挑战。给出了近年来多尺度耦合建模仿真方面的新进展,及其在塑性成形过程多尺度响应机理分析及面向高性能的成形工艺设计中的应用,并对其发展趋势进行了展望。     

不同硬化模型对铝合金板冲压成形模拟结果的影响

研究不同塑性变形硬化模型对汽车5182-O铝合金板材冲压成形模拟结果的影响。采用材料单向拉伸试验得到应力应变关系曲线,基于Hollomom、Krupskowsky与Power方程对曲线进行拟合,建立材料室温下塑性变形硬化模型,对厚度为1.5 mm和0.85 mm的5182板材进行冲压试验和有限元模拟分析,对比分析冲压试验与模拟结果。试验与模拟结果显示,当板料厚度为1.5 mm时,板料冲压试验的成形力最大为42.95 kN,板料拉深深度为30.58 mm,基于Power方程计算得到的最大成形力为41.5kN与试验结果比较接近,Hollomom方程计算得到的拉深深度为30.546 mm,板材成形厚度分布与试验结果比较接近;当板料厚度为0.85 mm时,板料冲压试验的成形力最大为34.47kN,板料拉深深度为33.792 mm,基于Power方程计算得到的最大成形力为34.27 kN与试验结果比较接近,Hollomom方程计算得到的拉深深度为33.636 mm,板材成形厚度分布与试验结果比较接近。基于三种硬化模型铝合金冲压成形过程的计算模拟分析结果,并通过与试验对比得到不同硬化模型对铝合金板材冲压成形计算模拟的影响,进一步为汽车铝合金覆盖件在成形工艺的研究分析提供理论指导。     

Earing predictions for strongly textured aluminum sheets

Metallic alloy sheets develop crystallographic texture and plastic anisotropy during rolling. Deep drawing of a cylindrical cup from a rolled sheet is one of the typical forming operations where the effect of this anisotropy is most evident. Generally, in the finite element analyses of this process, the evolution of anisotropy during forming is neglected. In this paper, results of an experimental program carried out to quantify the anisotropy of aluminum alloy AA5042-H2 are reported. In addition to tensile tests along seven directions in the plane of the sheet, cup-drawing tests were conducted. It was observed that the material displays eight ears. The effects of the evolution in anisotropy and the directionality in hardening on the predictions of the earing profile for this material are investigated using a new methodology that incorporates multiple hardening curves corresponding to uniaxial tension along several orientations with respect to the rolling direction, and to biaxial tension. Yielding is described using the anisotropic yield function Yld2000-2D [1] and a form of CPB06ex2 yield function [2], which is tailored for metals with no tension–compression asymmetry. It is shown that even if distortional hardening is neglected, the latter yield function predicts a cup with eight ears as was observed experimentally. Consideration of distortional only leads to improved accuracy in prediction of the non-uniformity of the cup height profile.     

Formability evaluation of friction stir welded 6111-T4 sheet with respect to joining material direction

In order to evaluate the formability of friction stir welded (FSW) automotive TWB (tailor-welded blank) sheets with respect to base material direction, the aluminum alloy 6111-T4 sheet was joined with three different types of combination: RD||RD, TD||RD, TD||TD (Here, RD and TD mean the rolling direction and transverse direction, respectively). Formability performance was experimentally and numerically studied in three applications including the simple tension tests, hemisphere dome stretching and cylindrical cup drawing tests. For numerical simulations, the non-quadratic orthogonal anisotropic yield function, Yld2004-18p and the isotropic hardening law were implemented into the material constitutive model. As for the failure criterion, the forming limit diagram (FLD) was utilized to determine the failure strain.     

Comparison of the work hardening of metallic sheets in bending-unbending and simple shear

Constitutive equations for sheet metals are important input data for the numerical simulation of forming processes. Their identification usually comes from uniaxial tensile tests at several orientations to the rolling direction and from equibiaxial tests. To characterize the kinematic part of the work hardening, strain path reversals are needed and different mechanical tests have been developed, such as tension-compression and simple shear. The aim of this work is to investigate the influence of the database used for identification, by using either bending-unbending or simple shear for strain path reversal. This study is performed both on an aluminum alloy and a TRIP steel. The presented results show that, for a limited strain range, the experiments are consistent.     

利用激光局部硬化效应提高2B06铝合金板材拉深成形性研究

针对2B06铝合金复杂零件成形困难问题,提出了利用激光热处理局部硬化提高板材成形性的思路。在通过激光热处理试验研究了铝合金板的激光硬化效应的基础上,采用数值模拟计算了铝合金板激光热处理过程中激光光斑路径和其周边热影响区域的峰值温度场,并通过实际测温验证了其准确性。提出并构建了耦合性能梯度的差性坯料模型,对激光局部硬化的杯形件拉深成形性进行了模拟和试验研究。结果表明,激光扫描方式可以形成稳定的梯度温度场并对周边非加热区影响较小,且可以通过多道次扫描方式设计不同宽度范围的大梯度差性板材坯料。力学性能试验表明激光热处理可以有效地提高铝合金的局部加工硬化能力,这种效应可以有效抑制杯形件拉深时圆角大变形区的减薄,从而提升了板材的拉深性能。在上述基础上,将激光局部热处理用于2B06铝合金航空复杂口框零件的成形,通过设计激光处理路径和参数,获得合理的局部硬化区域,可有效地避免在加强筋处出现过度减薄导致的破裂,大大提高复杂零件的成形性。     

Finite element method for sheet forming based on an anisotropic strain-rate potential and the convected coordinate system

A variational formulation and the associated finite element (FE) equations have been derived for general three-dimensional deformation of a planar anisotropic rigid-plastic sheet metal which obeys the strain-rate potential proposed by [11.]. By using the natural convected coordinate system, the effect of geometric change and the rotation of planar anisotropic axes were efficiently considered. In order to check the validity of the present formulation, a cylindrical cup deep drawing test was modeled for a 2008-T4 aluminum alloy sheet sample. Eating simulations were performed and planar anisotropic material properties were experimentally determined. Even though quantitative agreement was not fully achieved, reasonably good agreement was found between the FE simulation and the experiment in thickness strain distribution and caring. No numerical difficulty due to planar anisotropy was encountered, and the computational procedure was found to be very stable, requiring only moderate computational time. The results have shown that the present formulation for planar anisotropic deformation can provide a good basis for the analysis of sheet metal forming processes for planar anisotropic materials, especially for aluminum alloy sheets.     

Constitutive law for AZ31B Mg alloy sheets and finite element simulation for three-point bending

The constitutive law to describe the anisotropic and asymmetric mechanical behavior of AZ31B magnesium (Mg) alloy sheets at room temperature has been developed here for the plane stress condition, based on the orthotropic yield criterion proposed by Cazacu O, Plunkett B, Barlat F. [Orthotropic yield criterion for hexagonal closed packed metals. International Journal of Plasticity 2006;22:1171–94] and different isotropic hardening laws for tension and compression. Experimental procedures to obtain the material parameters of the yield surface and the hardening laws have been discussed for the AZ31B Mg alloy sheet. For verification purposes, finite element simulation results based on the developed constitutive laws have been compared with experimental results for a three-point bending test.     

Experimental and numerical study of stamp hydroforming of sheet metals

The objectives of this research was to experimentally and numerically study the stamp hydroforming process as a means for shaping aluminum alloy sheets. In stamp hydroforming, one or both surfaces of the sheet metal are supported with a pressurized viscous fluid to assist with the stamping of the part thereby eliminating the need for a female die. The pressurized fluid serves several purposes: (1) supports the sheet metal from the start to the end of the forming process, thus yielding a better formed part, (2) delays the onset of material failure and (3) reduces wrinkle formation. This paper focuses on the experimental and numerical results of the stamp hydroforming process utilizing a fluid pressure applied to one surface of the sheet metal. The effects of applying a constant, varying and localized pressure to the surface of 3003-H14-aluminum sheet alloy were evaluated. Experiments demonstrated draw depths improvements up to 31% before the material failed. A failure prediction analysis by Hsu was also carried out to predict an optimal fluid pressure path for the varying fluid pressure case. The commercial finite element analysis code Ls-Dyna3D was used to numerically simulate the stamp hydroforming process. Both isotropic and anisotropic material models were used and their predictions compared against the experimental results. The numerical simulations utilizing Barlat's anisotropic yield function accurately predicted the location of the material failure and the wrinkling characteristics of the aluminum sheet.     

Identification of sheet metal plastic anisotropy using heterogeneous biaxial tensile tests

The plastic behavior of anisotropic steel and aluminum sheets is identified by combining the results of classical uniaxial tensile tests and heterogeneous biaxial tensile tests on non-standard cruciform specimens specifically designed for obtaining a high sensitivity of strain fields to material anisotropy. The strain fields are measured on the surface of the specimens by means of an image correlation method. The 8-parameter anisotropic yield function proposed by Ferron et al. [1] is adopted for identification. On the one hand, the results of uniaxial tensile tests are analyzed to determine the strain-hardening parameters and yield function parameters related to transverse strain-anisotropy (angular variation of the anisotropy coefficient R) and stress-anisotropy (angular variation of the yield stress σ). On the other hand, strain fields measured in the biaxial tests are used as input data in an optimization procedure that consists of fitting simulated fields with experimental ones in order to determine the material parameters describing the shape of the yield surface in the biaxial stretching range. The identified yield function is validated using experimental data issued from biaxial tests that were not considered during the optimization process.