TA4纯钛带材各向异性屈服行为表征与研究

目的 基于复杂加载状态试验和先进屈服准则,实现考虑塑性演化的TA4纯钛在复杂加载状态下塑性各向异性行为的精确表征。方法 通过0°、45°、90°方向的单拉试验和复杂加载比例的十字形试件双向拉伸试验,获得TA4纯钛的基本力学性能参数和拉伸屈服轨迹,采用不同的屈服准则对试验屈服轨迹进行预测,并通过变r值的屈服准则预测其屈服轨迹的塑性演变规律。结果 在小变形范围内,Yld2000?2d屈服准则对TA4屈服轨迹的预测精度最高;塑性变形过程中,呈线性增大趋势的r值与TA4纯钛的屈服轨迹演变现象直接相关。结论 试验与理论屈服轨迹的对比表明,Yld2000?2d屈服准则可以实现TA4纯钛初始屈服行为的精确表征。TA4纯钛带材的r值随塑性变形呈线性增大趋势,考虑塑性演化的Barlat89屈服准则预测的TA4屈服轨迹外凸性更显著。在TA4纯钛带材冲压成形过程的有限元分析、模具设计和工艺优化中,仅考虑初始屈服轨迹时,可采用Yld2000?2d屈服准则;当各向异性特征存在较强的塑性演化相关性时,可采用形式相对简单的Barlat89屈服准则。     

钴基高温合金GH188板材的屈服行为研究

目的 研究钴基高温合金GH188板材在不同应力状态下的屈服行为,并确定适用的屈服准则,为工程应用提供科学指导。方法 通过开展不同方向的单向拉伸试验得到基本力学性能参数,以轧制方向为参考方向,利用其真实应力-应变曲线标定Ludwik本构模型参数;根据国际标准设计十字形试件,利用ABAQUS软件对其等双拉加载进行有限元仿真,然后基于设计的试样对GH188高温合金板料进行不同载荷比例下的双向拉伸试验,并获取屈服点;结合以上数据,采用Mises、Hill48、Barlat89和Yld2000-2d 4种屈服准则对屈服轨迹、r值和应力值进行预测。结果 等双拉过程中,十字形试件中心区应力分布均匀且无切应力,表明试样的设计是合理有效的;4种屈服准则对试验屈服轨迹的平均预测误差相差不大,为(2±0.5)%,但整体而言,Hill48-σ的预测误差最大,Hill48-r和Barlat89屈服准则呈现相似的预测能力,误差约为20%,Yld2000-2d屈服准则预测精度最高,综合误差仅有2.41%,远远小于其他3种屈服准则的预测误差。结论 钴基高温合金GH188板材具有明显的各向异性,Yld2000-2d屈服准则可以精确地描述该材料在复杂应力状态下的屈服行为。     

正交异性板料拉深件快速展开与成形模拟

针对板件初始毛坯形状计算以及正交异性板的快速有限元分析问题,依据拉深件的UG模型,导出了零件的三角网格数据,并采用几何映射方法得到了用于快速有限元分析的拉深件初始展开毛坯.在此基础上,基于理想变形假设以及Hill’48正交异性屈服准则,给出了用于拉深件成形过程快速分析的一步法数学公式和有限元表达,并在Unigraphics系统中进行了有限元分析的后置处理.此外,对TC1钛板筒型拉深件毛坯初始形状进行了优化,并对成形中拉深件厚向应变分布进行了分析,得到了满意结果.     

控制压边力改善铝合金板成形性能的研究

以5052H32铝合金板为研究对象,以有限元数值模拟和基于计算机控制多点变压边力液压压力机的实验为手段,研究随位置变化的压边力对铝合金板成形性能的影响.研究结果表明:合理地控制随位置变化的变压边力可以显著提高铝合金板的成形性能,增加盒形件的拉深深度(最大拉深高度提高约12%);随位置变化的压边力对铝合金成形性能改进的主要原因是减少破裂危险区域壁厚减薄和应变的峰值;采用Barlat 96屈服准则描述铝合金的屈服行为具有较高精度,数值模拟和实验的偏差较小.     

5042 铝合金板复合拉深工艺下的制耳规律

基于ABAQUS有限元仿真软件,对5042铝合金板在复合拉深工艺下的制耳规律进行了三维数值模拟。通过分析复合拉深工艺中板料成形后的筒高,及厚度、应变的分布规律,探讨了5042铝合金板在这种复合工艺下的成形和制耳规律,揭示了相关塑性变形的机理。     

汽车车身板用6A16铝合金拉深成形金属流动和微观组织相关性研究

由于能源和环境问题的日益严峻,汽车轻量化的研究和发展成为热点,其中6000系铝合金在汽车车身板上的应用受到广泛关注。本研究基于ABAQUS/Explicit有限元方法,对预时效并室温停放一周后的6A16铝合金板材在适宜条件下的拉深成形过程进行模拟,并通过试验进行验证。研究结果表明:Barlat91屈服准则适用于模拟6A16铝合金的拉深成形。在6A16铝合金的拉深成形过程中,铝合金板材顶部增厚,易产生褶皱;筒壁随着筒高的减小而减薄;圆角处最薄,易破裂;底部形变量少。经560℃/30 min的固溶处理+140℃/5min预时效处理后的6A16铝合金板材(T4P态)晶粒取向以Cube取向为主,随着拉深成形的进行,Cube取向晶粒逐渐消失,并在轧制方向和垂直轧制方向四个位置产生制耳。在本试验研究范围内,随着筒壁高度的增加,板材晶粒尺寸变大,小角度晶界比例增大,Mg_2Si和富Si相尺寸变小。     

各向异性板料屈服轨迹的研究

针对建立的十字形双向拉伸试验系统,利用有限元模拟优化得到的十字形试件,采用载荷控制方式对SPEN钢板和2024-O铝合金板进行了不同加载路径下的双向拉伸试验,得到了不同硬化阶段下的实验屈服轨迹,并与现有屈服准则Hill48、Hill79、Hill90、Hill93、Gotoh、Hosford、Barlat-Lian以及Mises的理论屈服轨迹进行了对比.结果表明:对于SPEN钢板,Hosford各向异性屈服准则得到的理论屈服轨迹与实验屈服轨迹符合得最好,其次是Mises屈服准则,Hill48屈服准则最差;对2024-O铝合金板,Barlat89、Hosford屈服轨迹与实验屈服轨迹符合得最好,Mises屈服准则最差.     

屈服准则对不锈钢弯管变形预测精度影响

通过XRD衍射及不同方向单向拉伸试验验证,经过多道次拉拔生产出的304奥氏体不锈钢管材存在有明显的各向异性现象.采用Mises各向同性、Hill1948和Barlat1991各向异性屈服准则对不锈钢弯管过程进行有限元模拟,分析弯曲后管材内外侧壁厚分布、弯曲角度及管坯截面椭圆度的变化规律,通过模拟与实验结果对比发现,当实验数据较少时,采用Hill1948各向异性屈服准则,能够很好预测304奥氏体不锈钢管材弯曲成形过程,而Barlat1991各向异性屈服准则中的一些参数经过近似后,对成形行为的预测精度明显降低.     

非线性路径下板料拉深成形过程中破裂预测

在优选模型参数和简化孔洞形核规律的基础上,采用Gurson-Tvergaard (GT) 多孔材料本构模型分析圆筒件拉深过程;根据金属成形工艺特点,综合考虑拉伸型和剪切型2种不同韧性断裂机制,提出一个统一的韧性断裂准则形式.对于未经过预变形和经过预变形的圆筒件拉深试验和数值模拟进行了比较,结果表明:相对于成形极限图,新的韧性断裂准则可以更加准确地预测非线性路径下圆筒件的拉深破裂.     

屈服准则和硬化模型对5052铝板回弹仿真的影响

为选择适合于5052铝合金回弹仿真的材料模型,对LS-Dyna软件中4个材料模型MAT_36、MAT_122、MAT_125和MAT_226所采用的屈服准则和硬化模型进行了分析,采用这4个模型对5052铝板U形件的回弹进行了仿真,对回弹过程中圆角区的应力释放进行了讨论.同时,进行了U形件的回弹试验,并与仿真结果进行了比较.结果表明,4个材料模型中,基于Yoshida-Uemori随动硬化模型和Barlat’89屈服准则的材料模型MAT_226具有最好的回弹预测精度,由各向同性硬化模型和Hill’48屈服准则组合的材料模型MAT_122的回弹预测结果与试验结果的偏差最大.硬化模型对回弹预测精度的影响大于屈服准则的影响.     

Analytical and finite element simulation studies on earing profile of Ti-6Al-4V deep drawn cups at elevated temperatures

Rolled sheet metal alloys exhibit plastic anisotropy, which leads to the formation of ears during the deep drawing process. An analytical function proposed by Yoon et al. (Int J Plast 27(8):1165–1184, 2011) predicts earing profile based on yield stress and r value directionalities for circular cup drawing. In this study, this analytical approach is applied for a deep drawing of Ti-6Al-4V at elevated temperatures up to 400 °C. Three yield criteria namely, Hill 1948, Barlat 1989 and Barlat Yld2000-2d are used to obtain the directionality inputs for the analytical formula. The analytical model is validated using experimental results and FE simulations and is found to be closely matched while requiring very less CPU time. FE simulation has been also conducted with various yield functions. Barlat Yld2000-2d is considered to be the most suitable yield criterion for very accurate earing prediction in deep drawing of Ti-6Al-4V as the inputs for both the analytical and FEM models.     

Detailed experimental and numerical analysis of a cylindrical cup deep drawing: Pros and cons of using solid-shell elements

The Swift test was originally proposed as a formability test to reproduce the conditions observed in deep drawing operations. This test consists on forming a cylindrical cup from a circular blank, using a flat bottom cylindrical punch and has been extensively studied using both analytical and numerical methods. This test can also be combined with the Demeri test, which consists in cutting a ring from the wall of a cylindrical cup, in order to open it afterwards to measure the springback. This combination allows their use as benchmark test, in order to improve the knowledge concerning the numerical simulation models, through the comparison between experimental and numerical results. The focus of this study is the experimental and numerical analyses of the Swift cup test, followed by the Demeri test, performed with an AA5754-O alloy at room temperature. In this context, a detailed analysis of the punch force evolution, the thickness evolution along the cup wall, the earing profile, the strain paths and their evolution and the ring opening is performed. The numerical simulation is performed using the finite element code ABAQUS, with solid and solid-shell elements, in order to compare the computational efficiency of these type of elements. The results show that the solid-shell element is more cost-effective than the solid, presenting global accurate predictions, excepted for the thinning zones. Both the von Mises and the Hill48 yield criteria predict the strain distributions in the final cup quite accurately. However, improved knowledge concerning the stress states is still required, because the Hill48 criterion showed difficulties in the correct prediction of the springback, whatever the type of finite element adopted.     

Experimental and numerical investigation of anisotropic yield criteria for warm deep drawing of Ti–6Al–4V alloy

Accuracy of the finite element simulation of sheet metal forming is significantly dependent on the correctness of input properties and appropriate selection of material models. In this work, anisotropic yield criteria namely, Hill 1948, Barlat 1989, Barlat 1996, Barlat 2000 and Cazacu Barlat have been implemented for Ti–6Al–4V alloy at 400 °C. Material constants required for the yield criteria have been determined and deformation behavior in deep drawing process has been analyzed in finite element software. Also, deep drawing experiments on Ti–6Al–4V alloy have been performed at 400 °C to validate finite element simulation results. Further, comparison of yield criteria based on thickness distribution, earing profile, complexity in material parameter identification and computational time has shown Cazacu-Barlat to be well suited for deep drawing of Ti–6Al–4V alloy.     

The equivalent plastic strain-dependent Yld2000-2d yield function and the experimental verification

An equivalent plastic strain-dependent anisotropic material model was developed for 5754O aluminum alloy sheet. In the developed model, the anisotropy coefficients for Barlat’s Yld2000-2d anisotropic yield function were established as a function of the equivalent plastic strain. The developed anisotropic material model was implemented into the commercial FEM code ABAQUS as a user material subroutine (UMAT) for simulations. In order to evaluate the accuracy of the developed material model, biaxial tensile tests were carried out using cruciform specimens and a biaxial loading testing machine. The results show that the developed material model predicts the experimental results better than the other three material models (Yld2000-2d, Mises and Hill48 yield functions). It is also found that the developed material model describes the uniaxial tensile test curves better than Yld2000-2d yield function. The deep drawing test for 5754O aluminum alloy sheet was carried out and was simulated with different material models. The comparison between the experimental and simulation results indicates that the developed material model predicts the earing profile better than other material models. It is concluded that the equivalent plastic strain-dependence of the material coefficients should be considered for the accurate prediction of the anisotropic deformation behavior of materials.     

Comparison of different yield criteria in various deep drawn cups

The aim of the study is to evaluate the performance of three recent yield criteria namely; BBC2008-8p, Yld2003-8p, Hu2003 through the simulation of the hemispherical, cylindrical and square cup drawing processes by comparing the results with the ones obtained by using the von Mises criterion for the isotropic, kinematic and combined hardening and the Hill’48 criterion. For this purpose, two different sheet materials, SS304 stainless steel and DKP6112 steel, and various punch travels were used in the simulations and experimental phases of this study. The BBC2008-8p, Yld2003-8p and Hu2003 models were implemented to the ABAQUS software through the user material subroutine VUMAT. The thickness strain distributions obtained from the simulations were compared with the experimental results to analyze the validity of the three aforementioned criteria. Compared with the other models, the material behavior in deep drawing cases of this study is better predicted with more recent models, namely BBC2008-8p, Yld2003-8p and Hu2003, which include anisotropy parameters found from uniaxial and biaxial tension tests.     

On the influence of kinematic hardening on plastic anisotropy in the context of finite strain plasticity

The paper discusses the application of a newly developed material model for finite anisotropic plasticity to the simulation of earing formation in cylindrical cup drawing. The model incorporates Hill-type plastic anisotropy, nonlinear kinematic and nonlinear isotropic hardening. The constitutive framework is derived in the context of continuum thermodynamics and represents a multiplicative formulation of anisotropic elastoplasticity in the finite strain regime. Plastic anisotropy is described by means of second-order structure tensors which are used as additional tensor-valued arguments in the representation of the yield criterion and the plastic flow rule. The evolution equations are integrated by a form of the exponential map that fullfils plastic incompressibility and preserves the symmetry of the internal variables. The numerical examples investigate the influence of the hardening behaviour on an initially anisotropic yield criterion. In particular, the influence of using the kinematic hardening component of the model in addition to isotropic hardening in the earing simulations is examined. Comparisons with test data for aluminium and steel sheets display a good agreement between the finite element results and the experimental data.     

Numerical and experimental analysis of wrinkling during the cup drawing of an AA5042 aluminium alloy

The recent trend to reduce the thickness of metallic sheets used in forming processes strongly increases the likelihood of the occurrence of wrinkling. Thus, in order to obtain defect-free components, the prediction of this kind of defect becomes extremely important in the tool design and selection of process parameters. In this study, the sheet metal forming process proposed as a benchmark in the Numisheet 2014 conference is selected to analyse the influence of the tool geometry on wrinkling behaviour, as well as the reliability of the developed numerical model. The side-wall wrinkling during the deep drawing process of a cylindrical cup in AA5042 aluminium alloy is investigated through finite element simulation and experimental measurements. The material plastic anisotropy is modelled with an advanced yield criterion beyond the isotropic (von Mises) material behaviour. The results show that the shape of the wrinkles predicted by the numerical model is strongly affected by the finite element mesh used in the blank discretization. The accurate modelling of the plastic anisotropy of the aluminium alloy yields numerical results that are in good agreement with the experiments, particularly the shape and location of the wrinkles. The predicted punch force evolution is strongly influenced by the friction coefficient used in the model. Moreover, the two punch geometries provide drawn cups with different wrinkle waves, mainly differing in amplitude.