球形模胀形法和双板试验法建立FLC的比较

用两种不同的双拉成形技术获取成形极限曲线。其一是允许在平板面内自由变形;其二是由半球形凸模胀形,限制板料的变形,前者不受摩擦及模具曲率的影响,有效地显示了三种不同形式的极限应变行为,后者与实际生产更符合,产生更大的根限应变。     

车门外板用钢冲压开裂分析与仿真工艺优化

针对车门外板用钢由DC04替换成DC01引起的冲压起皱及开裂问题，分析了两种材料力学性能和成形极限的差异，并利用成形仿真软件开展了适用于鞍钢产品的车门外板成形工艺研究。结果表明，塑性应变比r值和双拉变形区极限应变的降低会导致材料的成形能力变差，使材料与模具的匹配设计难度大幅增加。仿真模型预测准确，且通过灵活设计拉延筋及阻力系数，有效控制了冲压缺陷，显著提高产品的订货量及应用稳定性。     

非线性组合硬化条件下镁合金板料变形回弹预测

变形回弹作为金属板料成形的主要缺陷之一,如何提高变应变路径条件下的回弹预测精度一直是研究者们面临的难题.本文针对镁合金变形特点,提出了同时考虑同向硬化、动态硬化和屈服圆畸变的本构模型.以0.8 mm厚AZ31B镁合金板料为研究对象,施加不同预拉伸后进行弯曲变形试验,观察了不同预变形对回弹规律的影响.同时结合有限元分析ABAQUS-Explicit (Vumat)和ABAQUS-Implicit (Umat)对板料的变形及回弹过程进行模拟仿真,对比试验与模拟结果,验证动态硬化对于镁合金板料变形回弹的重要影响.      

成形极限图左半部薄板的集中性失稳与极限应变——一种新的集中性失稳模型与极限应变计算方法

本文根据实验研究结果针对成形极限图左半部(拉——压区)提出了一种新的板料拉伸失稳物理模型、集中性失稳准则以及相应的极限应变理论计算方法。与现有的其它理论和模型相比,用本模型计算的结果能更好地吻合材料的成形极限实验曲线。本模型只涉及板料的基本成形性能参数,可方便地应用于工程实际。     

成形极限图的测试、应用和可信度分析

成形极限图通常是通过钢模胀形试验测得，实际测量的成形极限图与ASAME自动应变测试分析系统模拟计算的成形极限曲线吻合较好。应用成形极限图分析冲压零件成形的安全裕度和进行选材预测时，对以平面应变和胀形为主的成形零件具有较高的可信度，而对以深拉延变形为主的成形零件可信度不高。     

轧制差厚板成形极限场

摘要：轧制差厚板板料厚度和材料性能分布不均匀,准确预测差厚板零件的成形极限变得十分困难。首先将差厚板的过渡区离散成若干等厚板的组合,对每一个厚度下的板料进行不同路径的凸模胀形数值模拟,拟合出不同厚度板料的成形极限曲线,对厚度进行插值获取差厚板的成形极限场。最后,采用成形极限场对差厚板筒形件的成形极限进行预测。研究结果表明：采用数值模拟、拟合以及插值的方法得到的成形极限场,可以较为准确地预测实际差厚零件的成形极限。     

基于插值多项式的成形极限图数据处理方法

为了获取准确的金属板材成形极限曲线,采用成形极限实验数据路径选择和分区域多项式拟合的方法处理实验数据。数据路径曲线根据最小主应变划分为左侧曲线和右侧曲线,采用多项式法对曲线进行拟合计算,然后合并形成成形极限曲线。计算结果表明,采用该方法可以快速准确地生成需要的成形极限曲线。     

商用车侧围外板冲压开裂应变分析

针对国内某商用车侧围外板DC06在冲压过程中出现了批量开裂的问题,通过对其力学性能和成形极限曲线进行测试,结合应变云图与成形极限图,采用网格应变分析的方法,确定了板料冲压开裂的原因。从冲压模具和冲压材料两方面入手,提出优化方案,做出相应改进措施,消除开裂缺陷。     

成形极限图的测试、应用和可信度分析

成形极限图也称成形极限曲线,常用FLD或FLC表示.成形极限图是判断和评定金属薄板成形性的最为简便和直观的方法,是对板材成形性能的一种定量描述,是解决板材冲压问题的一个非常有效的工具,同时也是对冲压工艺成败性的一种判断曲线.相对于通常使用的基本成形性能指标(σs、σb、δ10)及杯突值而言,成形极限图可以较好地反映材料的极限变形能力,定量衡量钢板冲压成形性能的好坏.对具体冲压零件成形后的应变进行测试和分析,得到零件表面应变分布情况,将应变分布点放在该材料的成形极限图中,有助于科学评估板材对零件的实际成形效果,通过分析零件变形大小与成形极限的关系,可以确定零件冲压成形的危险部位、材料使用是否合理.     

应变比对TRIP钢残余奥氏体转变量的影响

以冷轧TRIP钢为研究对象,通过试验建立成形极限图(FLD),采用金相显微镜、扫描电镜对原始状态和变形后的显微组织进行观察,并利用X射线衍射测定了经历不同应变比试样破裂区域的残余奥氏体含量。试验结果表明:在平面应变状态下极限应变值(FLD0)为0.397。随着应变比的增加,应变路径由单轴拉伸至平面应变,再到双向拉伸,残余奥氏体转变量逐渐增加。与双相钢相比,TRIP钢较高的FLD0值是由于TRIP效应的存在,变形过程中,缩颈区域较宽。     

Effects of strain path changes on the formability of sheet metals

The effects of a change in strain path on the deformation characteristics of aluminum-killed steel and 2036-T4 aluminum sheets have been studied. These sheets were pre-strained various amounts in balanced biaxial tension and the resulting uniaxial proper-ties and forming limits for other loading paths were determined. In comparison to uni-axial prestrain the steel was found to suffer a more rapid loss in uniform strain upon the strain path change from biaxial to uniaxial. In contrast, the uniform strain in aluminum does not drop as rapidly after the same change. In keeping with this behavior, the form-ing limit diagram of steel is found to decrease with prestrain at a much faster rate than that of aluminum. Such effects can be explained in terms of the transition flow behavior of the metals occurring upon the path change. Thus, the path change produces strain soften-ing and premature failure in steel, while causing additional strain hardening and consequent flow stabilization in aluminum. AMIT K. GHOSH, formerly with General Motors Research Laboratories     

Anisotropic strain localization in tensile prestrained sheet steel

Multiple forming operations are often needed to stamp complex shapes out of sheet metal. Large changes in strain path can occur from such operations. This study examined the effects of a particular strain path change, tensile-tensile, on the mechanical properties of an aluminum-killed steel. Large tensile specimens were prestrained various amounts in one direction followed by machining smaller tensile specimens at 0, 45, and/or 90 deg to the prestrain direction. The smaller samples were then pulled to failure. For samples pulled in the same direction as the prestrain, the residual strength and ductility were equivalent to those obtained from an interrupted tensile test. In contrast, both the 45 and the 90 deg prestrained specimens showed a larger than expected flow stress and an abrupt change in the nature of the residual ductility at prestrains of 7.5 pct and larger. At 7.5 pct prestrain, the uniform strain, as measured by the maximum-load point on the load vs elongation tensile curve, decreased abruptly. The decrease was accompanied by a corresponding increase in the post-uniform strain. This unusual behavior is explained in terms of a rapid increase in strain-hardening with strain.     

Prestrain and Aging Treatment to Improve the Mechanical Properties of AA6022 Aluminum Alloy Laser Weldment

This study aimed to improve the mechanical properties of aluminum alloy sheet laser weldments. Different tensile prestrains, which simulate sheet forming, were applied to a base metal and its Nd:YAG laser weldment, respectively; then, some samples were subjected to a paint-bake-cycling (PBC) process and some samples were subjected to an artificial aging treatment of 175 °C/1000 min. The tensile test and Vickers hardness test, followed by transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), and electron-probe microanalysis (EPMA), were applied to examine the microstructures and compositions. The results showed that tensile prestrain and heat-treatment conditions affect the mechanical properties in relation to the different strengthening mechanisms involved. Compared to traditional PBC processes of auto-body panel production, a weldment applied with an artificial aging treatment at 175 °C/1000 min could significantly improve the yield strength and hardness through its precipitation strengthening mechanism. This operation process also reduces the property mismatch between the base metal and weldment. Although the elongation ratio of the weldment is typically inferior to the base metal, the sheet forming process (as prestrain) can be operated on the weldment successfully, while leaving some elongation capability for auto-body panel. Therefore, the proposed aging treatment for the weldment provided in this article is a promising technique.     

The influence of strain path on subsequent mechanical properties—Orthogonal tensile paths

Several aluminum alloys have been subjected to two stage tensile straining, an initial prestrain followed by a subsequent tensile strain at 90 deg to the initial direction. In AA1100-0 and AA3003-0 the prestrain produces dislocation tangling and diffuse cell walls resulting in an enhanced flow stress and decrease in ductility when the material, is subsequently strained in the orthogonal direction. In a fine grained experimental Al−Fe−Ni alloy the prestrain is accompanied by a very low accumulation of dislocations and in this case the flow stress is reduced and ductility enhanced in subsequent orthogonal straining. The commercial alloys AA2036-T4 and AA5182-0 are unaffected by the two stage tensile strain path. The results are considered in terms of the forming limit curve and it is also shown that the behavior is consistent with the concept of an “alien” dislocation distribution being generated during the prestrain.     

金属薄板面内压剪变形的损伤断裂行为

相变诱导塑性钢（TRansformation induced plasticity, TRIP）作为常用的先进高强钢在汽车等交通工具的轻量化方面有广泛的应用前景。而对于其复杂零件的成形过程,韧性断裂是不可忽视的问题之一。本文针对现有实验装置不易诱发薄板承受面内压剪时断裂失效,从而无法研究板料负应力三轴度区间断裂行为的问题,以高强钢TRIP800薄板为研究对象,设计了可在单向试验机完成压剪实验的试样和夹具。通过调整夹具旋转角度和试样装夹位置可以实现同一种试样在广泛的负应力三轴度范围内进行压剪断裂分析。基于ABAQUS/Explicit平台建立了三个典型加载方向20°、30°和45°对应的压剪过程有限元模型,分析表明:三种情况的试样局部变形区域的应力三轴度都小于0且断裂点的应力三轴度低至?0.485,验证了设计的装置可实现负应力三轴度区间的断裂失效分析,同时基于MMC断裂准则分析了不同应力状态的初始损伤情况及损伤扩展路径。      

Experimental and Numerical Failure Criteria for Sheet Metal Forming

For sheet metal forming, often the forming limit diagram (FLD) is used as failure criterion as it can be derived easily in experiments. It is based on the assumption that localization of strain in the sheet plane is responsible for crack initiation, but application of FLD is limited to linear strain paths. Hence, only forming processes with approximately the same deformation history as the experiments carried out for FLD determination should be evaluated by this criterion. Forming limit stress diagrams (FLSD) do not exhibit such strict limitations. They are based on the assumption that principal stresses in the sheet plane are responsible for crack initiation. As these stresses are usually calculated by FE analysis using elastic plastic material laws, strain hardening is considered. Two‐step forming tests as application examples prove the FLSD to be adequate for evaluation of non‐linear forming processes with alternating forming directions. Nevertheless, FLSD are derived in extensive investigations which makes them unattractive for most industrial applications. Furthermore, both FLD and FLSD do not consider the physical background of ductile crack initiation which is provoked by an interaction of local stress triaxiality and equivalent plastic strain. Hence, a reliable failure criterion should concentrate on these two parameters. The Gurson‐Tvergaard‐Needleman‐ (GTN‐) damage model can predict crack initiation during sheet metal forming. Application of the GTN model to 2 step forming tests with the bake hardening steel H220BD+Z showed good agreement to experimental results although a sensitivity of the model to mesh size and stress triaxiality is observed.     

金属板材成形性能的力学分析

根据金属材料拉伸试验所获得的强度、塑性指标、成形性能如Rp0.2、Rm、A、n、r等,并结合成形极限图(FLD)试验实测的金属板材拉胀成形性能,介绍了一整套用于评价金属板材塑性成形性能的实验室分析技术.文章以两种厚薄规格的冷轧板材为例,根据拉伸应变硬化指数动态跟踪技术实测的n值随应变量变化的趋势,揭示了上述两种材料拉伸性能与成形性能异同特点的力学本质.     

Localized Necking of Sheet at Negative Minor Strains

Localized necking in sheet metal has been examined for strain paths between uniaxial tension and plane strain (i.e., the negative minor strain region of a forming limit diagram). The behavior of sheet with preexisting imperfections has been analyzed and is contrasted to that free of imperfections. In particular, it is shown that the size and orientation of an imperfection is critical in determining whether or not localized necking is initiated along the imperfection. The influence of strain hardening, strain rate hardening, and plastic anisotropy on localized necking of an imperfect sheet is also examined. One of the most significant conclusions obtained from present analysis and from a reexamination of Hill’s theory is the prediction of a critical thickness strain criterion for the onset of localized necking at negative minor strains, regardless of whether or not an imperfection is present. The critical thickness strain criterion is observed in Ti alloys, Al alloys, steels, and brass. Formerly with the Department of Metallurgical Engineering, Michigan Technological University.     

Stretch forming and fracture of strongly textured Ti alloy sheets

The influence of plastic anisotropy and R -value on the stretch forming and fracture behavior of strongly textured Ti-6A1-4V and Ti-5Al-2.5Sn sheets has been examined utilizing sheet specimens with a wide range of R -values but with similar work hardening and strain-rate sensitivity characteristics. The results indicate that a high R -value and difficult through-thickness slip enhance the forming limit as well as fracture strains when the minor strain in the plane of the sheet is negative, this effect being most pronounced at uniaxial tension. At plane strain, the R -value has little or no influence on the limit or fracture strain. A direct determination of the effect of R -value on the biaxial stretch forming characteristics of Ti-6-4 sheet is precluded by the intervention of fracture prior to localized necking when the minor strain is positive. The influence of plastic anisotropy on both the localized necking and the fracture behavior can be generally understood in terms of the difficulty of attaining critical thickness strains as through-thickness slip becomes more difficult.     

Induction Heat Treatment of Sheet‐Bulk Metal‐Formed Parts Assisted by Water–Air Spray Cooling

In order to produce components with massive secondary functional elements from sheet metal bulk forming operations, termed sheet‐bulk metal forming, can be applied. Owing to high, three‐dimensional stress and strain states present during sheet‐bulk metal forming, ductile damage occurs in the form of micro‐voids. Depending on the material flow properties, tensile residual stresses can also be present in the components' formed functional elements. During service, the components are subjected to cyclic loading via these functional elements, and tensile residual stresses exert an unfavorable influence on crack initiation and crack growth, and therefore on the fatigue life. Following the forming process, temperature and microstructurally related compressive residual stresses can be induced by local heat treating of the surface. These residual stresses can counteract potential crack initiation on the surface or in the subsurface regions. In the present study, the adjustability of the residual stress state is investigated using a workpiece manufactured by orbital cold‐forming, which possesses an accumulation of material in its edge region. Based on residual stress measurements in the workpiece's edge region using x‐ray diffractometry, it is possible to verify the compressive residual stresses adjusted by varying the cooling conditions.