大型车车身覆盖件冲压成形特征分析及选材研究

为满足大型车身覆盖件冲压成形要求并实现钢板与零件的对路供应,必须在深入解析零件成形特征的基础上,确定高应变部位的成形敏感材料参数.在材料力学性能测试和成形极限分析的基础上,对侧围外板的冲压成形特征和成形敏感参数进行了分析.结果表明:侧围外板零件高应变部位的变形方式为胀形-深拉延变形,n值和r值为影响侧围外板零件成形的主要材料敏感参数.     

硬铝合金半球形零件的渐进成形工艺研究

目的 研究冲压胀形工艺与渐进成形工艺成形半球形零件的区别。方法 使用两种热处理状态的硬铝合金,AA2024-O和AA2024-T4,分别用冲压胀形工艺和渐进成形工艺成形半球形零件。结果 相比较渐进成形零件而言,使用冲压胀形工艺得出的半球形零件的壁厚相对均匀,变形程度可以达到更大;在相同的试验条件下,AA2024-O的成形性能远高于AA2024-T4的成形性能;在本实验所研究的参数范围内,下压量越小,成形高度越大,对AA2024-O进给速率越快,成形高度越大;而对AA2024-T4进给速率越慢,成形高度越大;对于胀形零件,材料在胀形过程中处于双向拉伸应变状态,而渐进成形零件在成形过程中处于平面应变状态;胀形零件的最大应变和最大减薄处是半球的中心,而渐进成形零件的最大应变和最大减薄处是半球的边缘。结论 胀形零件的危险截面在半球的中心,渐进成形零件的危险截面在半球的边缘。     

坐标网分析技术与成形极限图的应用

用网格自动应变测试分析系统ASAME测试了11炉批冷轧薄板的成形极限，对实际测量的成形极限与模拟计算的成形极限进行了比较。对冲压的摩托车油箱进行应变分析，根据成形极限图（FLD）确定冲压零件的安全裕度，评价材料的适用性。     

汽车钢板胀形区成形极限图的预测与验证

通过对胀形区成形极限图的理论分析和经验修正,建立了胀形区成形极限图的计算公式,经试验验证,在不同钢种、不同退火工艺、不同镀锌方式、不同表面处理方式下,按此公式计算的结果都能很好地符合试验结果.     

国产轿车用冷轧薄钢板的成形性能评价

就评价国产轿车用冷轧薄钢板成形性力学性能参数做了详细的分析;并就轿车零件冲压成形的特点,将材料的FLD图和冲压件应变图合成为零件应变极限图,从而对零件的冲压安全裕度进行评价,用于指导国产轿车用冷轧薄钢板的冲压生产.     

AZ31 镁合金板材气体胀形的模拟及成形极限的预测

建立了300℃下AZ31镁合金板材气体胀形实验方法的有限元模型,并对板材胀形过程进行了仿真分析。基于板材应变历史分析,以二阶主应变转折点作为判别准则,预测了板材成形极限应变。通过模拟结果与实验结果的对比,分析解释了不同尺寸试样的变形情况。     

基于Oyane韧性断裂准则的板料成形极限预测

本文基于Oyane韧性断裂准则,结合数值模拟方法,预测板料不同应变状态下的极限应变.准则中的材料参数通过单向拉伸和平面应变拉伸试验确定.在模拟胀形试验获得每一时间步应力、应变值的基础上,应用韧性断裂准则预测板料的成形极限.模拟结果表明用韧性断裂准则和数值模拟相结合的方法能成功获得板料的成形极限图.     

铜铝复层板成形极限图研究

为了评价铜铝复层板成形性能,采用Hosford高阶屈服准则和M-K理论推导复层板的成形极限应变表达式,通过理论计算得到铜铝复层板的成形极限图,利用胀形试验分析热处理工艺及接触方式铜铝复层板成形极限的影响,采用扫描电子显微镜对界面与断口形貌进行观察分析.研究结果表明:理论计算结果与试验结果吻合较好,铜在外侧复层板成形极限高于铝在外侧时的成形极限,热处理工艺对提高复层板的成形极限具有重要的作用,复层板两侧金属发生断裂的同时,界面产生部分脱离.     

国产轿车用冷轧薄钢板的成形性能评价

本文就评价国产轿车用冷轧薄钢板成形性力学性能参数做了详细的分析;并就轿车零件冲压成形的特点,将材料的FLD图和冲压件应变图合成为零件应变极限图,从而对零件的冲压安全裕度进行评价,用于指导国产轿车用冷轧薄钢板的冲压生产.同时就冷轧薄板的表面粗糙度、表面微观形貌及表面润滑条件等因素对钢板成形性的影响作了探讨,认为润滑有利于金属薄板在冲压成形过程中的流动,可使整个零件的应变分布趋于均匀,从而提高零件的冲压成形能力,降低冲压件的废品率.     

航空用GH625镍基高温合金带材的成形极限研究

目的 通过理论预测及胀形实验建立GH625高温合金的成形极限曲线,并结合仿真手段揭示其成形性能.方法 首先,通过基本力学性能测试获取不同方向下GH625材料的基本力学参数;然后,基于颈缩理论和M-K理论模型预测GH625材料的成形极限曲线;其次,通过胀形实验建立相应的成形极限图,并与理论结果进行对比;最后,结合有限元方法进一步研究GH625材料的成形特性.结果 准确获得了GH625高温合金的塑性应变比r值、应变硬化指数n值等参数;通过理论模型及胀形实验分别获得了相应的成形极限曲线,基于颈缩理论的集中性失稳预测结果与实验结果吻合较好;建立了可靠的有限元模型,进一步分析了摩擦因数及球头直径对GH625材料成形性能的影响规律.结论 建立了准确的GH625材料成形极限曲线的理论预测模型,并通过半球胀形实验验证了理论结果的可靠性,数值仿真结果发现,较小的摩擦因数或者冲头直径有利于改善GH625材料在胀形实验中的失效位置.     

Local bifurcation and instability theory applied to formability analysis

The design and optimization of both sheet metal formed parts and processes are nowadays carried out virtually making use of numerical tools by finite element analysis. Such virtual try-out approach contributes with significant savings in terms of money, time and effort in the design, production and process set-up of deep drawn parts. The analysis of either forming success operation or surface defects, in each of the development phases, is generally performed by means of the material’s forming limit diagram (FLD), since it allows to define a safe region that reduces the probability of: (i) necking; (ii) wrinkling and (iii) large deformation occurrence. However, the FLD represented in the strain space is known to present some disadvantages. To overcome this problem, Ito and Goya proposed a local bifurcation criterion that defines the critical state for a local bifurcation to set in as a function of the stress level to work-hardening rate ratio, leading to a FLD represented in the stress space. This suggests that the FLD obtained is completely objective in the sense that it is completely independent of the strain or stress history paths (Ito et al. 2000). In this work the Ito and Goya model is used to evaluate formability, as well as fracture mode and direction on the deep drawing of a square cup. Since the analysis is performed based on the stress state, it is also possible to determine an instability factor that “measures” the degree of acceleration by current stress for the local bifurcation mode towards fracture. The selected example highlights the potential use of the criterion which, once combined with the finite element analysis, can undeniably improve the mechanical design of forming processes.     

Fatigue strength analysis of welded joints in closed steel sections in rail vehicles

A great need exists for practicability and reliability analyses of the various dimensioning concepts in railway vehicle production, as currently parts are commonly sized according to the nominal stress concept. Although Finite Element Analysis is used for the dimensioning of parts according to the nominal stress concept, no use is made of the locally resolved information from these calculation methods. Concepts based on local stress are highly applicable, as the Finite Element Analysis allows detailed modelling of critical areas of parts.     

PIV experimental investigation of entrance configuration on flow maldistribution in plate-fin heat exchanger

Flow characteristics of flow field in the entrance of plate-fin heat exchanger have been investigated by means of particle image velocimetry (PIV). The velocity fields were measured using the two-frame cross-correlation technique. A series of velocity vector and streamline graphs of different cross-sections are achieved in the experiment. The experimental results indicate that performance of fluid maldistribution in conventional entrance configuration is very serious, while the improved entrance configuration with punched baffle can effectively improve the performance of fluid flow distribution in the entrance. Based on the analysis of the fluid flow maldistribution, a baffle with small holes is recommended to install in the entrance configuration in order to improve the performance of flow distribution. When the punched baffle is proper in length, the small holes is distributed in staggered arrangement, and the punched ratio gradually increases from central axis to the boundary along with the baffle length, the performance of flow distribution in plate-fin heat exchanger is effectively improved by the optimum design of the entrance configuration. The flow maldistribution parameter S in plate-fin heat exchanger has been reduced from 1.21 to 0.209 and the ratio of the maximum velocity to the minimum θ is reduced from 23.2 to 1.76 by installing the punched baffle. The results validate that PIV is well suitable to investigate complex flow pattern and the conclusion of this paper is of great significance in the optimum design of plate-fin heat exchanger.     

仪表指针裁搭边冲模设计

通过对仪表指针的工艺特点进行分析,为了保证工件的成形质量,提出了裁搭边的冲压方案,介绍了裁搭边排样图的特点,并设计了该零件排样图.论述了仪表指针裁搭边冲模的结构设计要点和工作过程,最后介绍了模具的装配过程,实现了该零件冲裁加工的顺利进行.     

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.     

Experimental strain-hardening behaviors and associated computational models with anisotropic sheet metals

Several anisotropic hardening models with simple loading conditions are proposed, including exponential hardening model, linear hardening model and multi-linear hardening model (which also suits for using the experimental data directly). Three special hardening curves, in the 0°, 45° and 90° angles, respectively, measured against the rolling direction, are the special cases of the proposed hardening models. These models make sense to be applied for simulating forming processes of the planar anisotropic hardening sheet metals, in particular for determining the springback and FLD of formed parts. In order to describe this view further, the constitutive relations of stress and strain, based on the plastic potential flow rule with isotropic hardening and kinematic hardening assumptions, have been discussed briefly. And then, the corresponding results of constitutive relations dependent on the proposed hardening models of materials will be represented, respectively.     

Experimental and theoretical investigation of forming limit diagram for Ti-6Al-4 V alloy at warm condition

Forming limit diagram (FLD) is an important performance index to describe the maximum limit of principal strains that can be sustained by sheet metals till to the onset of localized necking. It is useful tool to access the forming severity of a drawing or stamping processes. In the present work, FLD has been determined experimentally for Ti-6Al-4 V alloy at 400 °C by conducting a hemispherical dome test with specimens of different widths. Additionally, theoretical FLDs have been determined using Marciniak Kuczynski (M-K) model. Various yield criteria namely: Von Mises, Hill 1948, Hill 1993 and Cazacu Barlat in combination with different hardening models viz., Hollomon power law (HPL), Johnson-Cook (JC), modified Johnson-Cook (m-JC), modified Arrhenius (m-Arr.), modified Zerilli–Armstrong (m-ZA) have been used in M-K analysis for theoretical FLD prediction. The material properties required for determination of yield criteria and hardening models constants have been calculated using uniaxial tensile tests. The predicted theoretical FLDs results are compared with experimental FLD. It can be observed that influence of yield criterion in M-K analysis for theoretical FLD prediction is predominant than the hardening model. Based on the results; it is observed that the theoretical FLD using Cazacu Barlat and Hill 1993 yield criteria with m-Arr. hardening model has a very good agreement with experimental FLD.     

针刺C/C复合材料面内拉伸强度预测

为研究针刺碳纤维增强碳基体复合材料(针刺C/C复合材料)面内拉伸强度与渐进损伤,建立了针刺C/C复合材料代表性体积单元有限元模型。模型包含无纬布层、网胎层、针刺纤维束、界面4类子区域,并考虑了孔隙的影响。采用基于应变的破坏准则及指数型损伤演化规律研究无纬布层及针刺纤维束损伤,采用弹塑性本构研究网胎层损伤,采用内聚力牵引分离定律和二次应力破坏准则分析界面损伤。通过两步法计算了孔隙对材料性能的折减效果,并得到上述4个子区域的力学性能,通过ABAQUS UMAT预测了材料的面内拉伸应力-应变曲线及各子区域损伤起始、演化与失效过程,非线性趋势及拉伸强度数值与试验数值吻合较好,验证了该模型有效性。      

塑性应变比r值的测定和应用

塑性应变比r值已作为评价汽车专用薄板成形性的一个重要参数。通过对试样尺寸及不同变形量的测试和r值对冲压件开裂率影响的研究分析，说明r值对汽车专用薄板的冲压生产有重大的指导意义。