Ti-6Al-4V钛合金热成形极限图及其应用

成形极限是板材成形中十分重要的性能指标和工艺参数,反映了变形过程中板材在塑性失稳状态前的最大变形程度,是进行模具设计和工艺设计的主要依据。目前,国内外学者对室温条件下的板材成形极限做了诸多研究,而热态成形极限由于受温度、润滑、氧化以及应变量测量方法等诸多影响,相关研究相对较少。     

铝合金薄壁矩形管绕弯成形起皱极限的预测

失稳起皱是铝合金薄壁矩形波导管绕弯成形过程中的主要缺陷之一,严重制约着薄壁矩形管绕弯成形极限的提高.笔者采用有限元数值模拟结合正交回归分析的方法,建立了薄壁矩形管绕弯成形起皱波纹度回归预测模型,并通过实验验证了该模型的可靠性;在此基础上推导出了基于失稳起皱成形极限的解析模型.研究获得了芯头个数、防皱块与管坯间隙及芯模与管坯间隙对起皱极限的影响规律,并获得了铝合金薄壁矩形管绕弯成形起皱极限图.该研究为提高实际生产中薄壁矩形管绕弯成形质量提供了依据和指导.     

新型汽车用高强度钢的应用现状与发展趋势

对先进的高强度钢及其在汽车工业中的应用进行了评述,详细介绍了几种高强度钢,如烘烤硬化钢、双相钢、相变诱发塑性钢等的原理、成分、性能特点以及目前的应用现状与开发进展状况.简述了高强度钢板在成形中所遇到的延伸凸缘性和弯曲成形性有所降低、回弹和弯曲开裂现象增多等问题,指出应把高强度钢板研究与对应的成形应用技术研究结合起来.还简述了国内外在高强度钢板上的研究方向,即在进一步提高强度、降低车重的同时,综合考虑安全、成形等因素,通过更多的新的强化机制来提高强韧性.     

日本JFE钢铁公司最近取得的部分具有代表性的研发成果

正建筑机械用超高强度、高韧性厚板"JFEHYDll00LE"的开发,将在线热处理(HOP)工艺与在线超速冷却技术相结合,使生产的厚板屈服强度达1 100MPa以上,抗拉强度在1 180 MPa以上,保证-40℃低温韧性,同时具有优良的耐延迟断裂性;汽车用新型多相组织高强度热轧钢板的开发,延伸率比传统同级别钢板提高了40%;1 180 MPa级高强热轧钢板NANO HITENTM的开发,该钢板比公司现有780     

汽车用高强度钢板及镀锌板的现状和发展

综述了国内外汽车用高强度钢板、超细晶粒钢板、镀锌钢板的研究、应用和最新进展,并分别对热轧和冷轧高强度钢板、超细晶粒钢板的工艺机理、组织性能、成形技术、焊接技术、成形件的韧性和强度试验以及评价方法、在汽车上的应用等进行了系统的分析.     

汽车外面板用高强度钢板

利用Nb(C、N)的微细析出物和晶粒细化机理,开发出了具有优良成形性能的汽车外面板用SFG(超细晶)高强度钢.钢中的Nb析出物呈现出目前薄板中所没有的特殊析出形态,并且在晶界附近,有形成PFZ(无析出物区)析出物缺乏层的趋势.由于PFZ的存在,虽然晶粒细小,但也显示出了低屈服强度.另外,r-值也比传统钢高,由于添加了B,使钢板显示出优良的抗二次加工脆性.模拟实际的冲压工艺,采用汽车前挡板的金属冲模进行冲压试验,其结果表明,该开发钢比传统钢具有更宽的成形范围.实践证明,该钢具有优良的成形性能.     

铍板材深冲压成形异形件探讨

冲压成形是金属板材加工异形件的最基本方式。金属材质不同,冲压成形异形件的难易程度不同,主要影响因素是冲压成形性能和冲压成形极限。文章结合铍板材的冲压成形性能和冲压成形极限,对铍板材冲压成形异形件的难易性作了探讨。     

基于M-K理论的6016铝合金成形极限曲线预测

近年来,为实现汽车车身轻量化,大量的铝合金材料被用于汽车车身制造,由于6016铝合金具有良好的烘烤性能,被大量使用.但是传统的冷成形技术并不能成形复杂零件,因此热冲压-冷模具淬火成形技术被用到铝合金的成形过程中,板材成形领域中一个重要的性能指标是成形极限.本论文使用理论预测和试验两种方法对6016铝合金成形极限曲线进行了研究.首先,建立了考虑应变强化和应变速率强化的Fields-Bachofen本构方程,并将此本构方程引入到成形极限理论推导过程中;然后,基于M-K凹槽理论,对6016铝合金成形极限曲线进行了理论预测,并且采用Nakazima试验方法对预测结果进行了验证.结果显示,随着初始厚度不均度的增加,预测曲线向纵坐标的正方向移动;通过实验值和预测值的对比发现M-K凹槽理论对成形极限曲线的预测是可行的、准确的.      

先进高强度汽车用钢板研究进展与技术应用现状

综述了第一、二、三代先进高强度汽车用钢的理论研究与生产现状,并具体介绍了辊压成形、液压成形、热成形、百足成形和链模成形等几种先进高强钢应用技术的最新进展。先进高强度用钢成分与性能体系趋于完备,强塑积从10GPa·%到60GPa·%钢种的研发与应用极大地满足了汽车轻量化选材与用材的需求,可以预见兼具高强度与高塑性的第三代先进高强度汽车板是未来发展的方向,将得到越来越广泛地应用。先进高强度用钢仍有很多与生产相关的技术需要突破,应用技术的开发也需要加大力度。     

国内外汽车板的现状 需求和发展趋势

论述了目前国内外汽车板的质量性能发展状况,分析了汽车结构对板材的要求,钢所面临的挑战及在汽车轻量化中的进展,简述了超深冲钢板的超低碳、洁净化控制,新型高强度钢板和汽车用热镀锌钢板的最新进展,同时指出必须重视高强度钢板等新型汽车板的使用技术研究和开发。     

Numerical analysis of the formability of an aluminum 2024 alloy sheet and its laminates with steel sheets

A criterion for ductile fracture is applied to the formability prediction of an aluminum 2024 alloy sheet and its laminated composite sheets. Axisymmetric deep-drawing processes of the 2024 sheet and the laminates clad by mild steel sheets are simulated by the finite-element method. From the calculated distributions and histories of stress and strain, the fracture initiation site and the forming limit are predicted by means of the ductile fracture criterion. The predictions so obtained are compared with experimental observations. The results show that the fracture initiation in the 2024 sheet with no appearance of necking is successfully predicted by the present numerical approach. Furthermore, it is found that the formability of the 2024 sheet is improved by sandwiching it with the mild steel sheets.     

Fracture mechanism maps in stress space

Diagrams can be constructed in stress space which show, for metals and alloys, the competition between the processes which lead to fracture. These include yield, necking, void nucleation, ductile fracture, brittle grain-boundary fracture, cleavage, shear fracture, and plastic rupture. Simplified diagrams are constructed for E.T.P. copper, α-brass, two steels and an aluminium alloy. The diagrams show how the fracture mechanism changes with stress state and help rationalize a number of apparently conflicting observations. They have application in predicting the behaviour of metals under complex stress states.     

Comparative Study on Failure Prediction in Warm Forming Processes of Mg Alloy Sheet by the FEM and Ductile Fracture Criteria

An important concern in metal forming is whether the desired deformation can be accomplished without any failure of the material, even at elevated temperatures. This paper describes the utilization of ductile fracture criteria in conjunction with the finite element (FE) method for predicting the onset of fracture in warm metal working processes of magnesium alloy sheets. The uniaxial tensile tests of AZ31 alloy sheets with a thickness of 3 mm and FE simulations were performed to calculate the critical damage values for three kinds of ductile fracture criteria. The critical damage values for each criterion were expressed as the function of strain rate at various temperatures. In order to find out the best criterion for failure prediction, Erichsen cupping tests under isothermal conditions were carried out at various temperatures and punch velocities. Based on the plastic deformation histories obtained from FE analysis of the Erichsen cupping tests and the critical damage value curves, the initiation time and location of fracture were predicted under bi-axial tensile conditions. As a result, Cockcroft–Latham’s criterion showed good agreement with the experiments.     

Finite element analysis of formability of a few kinds of special steel sheets

Deep drawing processes of various special steel sheets are simulated by the rigid-plastic finite element method. To predict the forming limit a criterion for ductile fracture is applied. From the histories of stress and strain in each element calculated by the finite element simulation, the fracture initiation site and the critical stroke are predicted by means of the ductile fracture criterion. The predictions so obtained are compared with experimental observations. The results show that the finite element analysis combined with the ductile fracture criterion is useful to predict the forming limit in a wide range of sheet steels.     

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.     

Forming limits of sandwich sheet materials

Failure of sandwich sheet materials by tensile instability and localized necking was studied by performing punch-forming experiments on stainless steel clad aluminum. By using narrow blanks and no lubrication, lateral contraction was possible, and failures could be produced in the drawing area of the forming limit diagram. For this deformation regime, diffuse instability led to localized necking. As in monolithic materials, the development of the localized neck in stainless steel clad aluminum determined the forming limit, and predictions of the strain levels for the onset of local instability correlated well with the observed forming limit strains. By preventing lateral contraction, failures in stretching were produced. The forming limit strains in this case depended on the strains at the onset of diffuse instability in much the same manner as is observed for monolithic materials. The strains at the onset of diffuse instability were predicted using a generalized rule of mixtures, and agreement between measured values and values predicted from component properties was good when the strain-path dependence of the instability strain for the individual components was taken into account. The diffuse necking process in stretching of stainless steel clad aluminum led to local thinning when deformations involved small degrees of biaxiallity. On the other hand, nonuniform through thickness straining of the component layers in specimens strained close to balanced biaxial stretching appeared to control the localization process and gave rise to forming limit strains lower than expected from observations of punch formed monolithic sheet materials. For all deformation modes, localized flow culminated in delamination and fracture. S. L. SEMIATIN, formerly graduate student, Department of Metallurgy and Materials Science, Carnegie-Mellon University     

Enhancement of Process Capabilities and Numerical Prediction of Geometric Profiles and Global Springback in Incrementally Formed AA 1050 Sheets

In single point incremental forming (SPIF) process, parts suffer from dimensional inaccuracy and limited formability, mainly, due to occurrence of springback and abrupt fracture, respectively. Orientation imaging microscopy of the original AA1050 sheet revealed dislocated and distorted microstructure and texture in comparison to the same sheet preheated at different temperatures. The objective of this work is to investigate formability and geometrical accuracy due to variations in microstructure and texture of the sheet and to propose a methodology, which can predict the geometric profiles and springback effect at different preheating temperatures. The work reports enhanced formability and geometrical accuracy of parts formed by SPIF, owing to the reformation of grain structure due to preheating. However, preheating at higher temperatures, i.e., 330 and 500 °C deteriorated the surface quality, as homogenization of grain orientation led to orange peel effect. The proposed methodology, based on reverse engineering and numerical formulation, is capable of predicting two-dimensional cone and pyramid profiles as well as global spring back values associated with different preheating temperatures. The results predicted by proposed method were validated by experiments and could be implemented to enhance the accuracy of SPIF process.     

一个基于孔洞演化机制的韧性断裂预测模型

在韧性断裂中微观孔洞演化机制的基础上,提出了一个基于孔洞演化机制的非耦合型韧性断裂预测模型.模型充分考虑了两种典型的孔洞演化机制:孔洞的长大机制和孔洞的拉长扭转机制.该模型引入了三个具有不同物理意义的材料参数:材料对不同孔洞演化机制的敏感度、应力状态敏感度系数和材料的损伤阈值,并使用等效塑性应变增量表征其对韧性损伤累积过程的驱动作用.为了使模型可以更好地反映三维应力状态对材料韧性断裂性能的影响,将该模型从主应力空间转换到由应力三轴度、罗德参数和临界断裂应变构成的三维空间,得到了由模型确定的三维韧性断裂曲面,并研究了相关参数对三维韧性断裂曲面及平面应力二维韧性断裂曲线的影响.利用5083-O铝合金、TRIP690钢和Docol 600DL双相钢三个典型的轻质高强板材的韧性断裂数据验证了该模型对不同材料和不同应力状态的适用性和准确性.      

The Morphology of Tensile Failure in Tantalum

The deformation, crack nucleation, coalescence, and rupture process of pure tantalum (99.9 pct) were studied under room temperature quasistatic loading using several in situ and ex-situ techniques including optical metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission-electron microscopy (TEM). The fracture surface of tantalum forms a ridge-and-valley morphology that is distinct from conventional notions of ductile dimple microvoid coalescence, and also distinct from spall damage formed during dynamic shock conditions. Failure proceeds by void nucleation at a dislocation cell wall or in subgrain interiors. Coalescence appears to involve a two-stage damage progression: first individual voids coalesce along the tensile axis forming diamond-shaped multivoid cavities; then cavities link-up by intercavity necking. Final rupture occurs when the intercavity necks thin to ~100-nm films and fail by crystallographic cleavage. This final tearing process was observed using in situ TEM tensile deformation of a thin tantalum film. The detailed microstructural and morphological observations of the current study can be used to guide the development of improved models for tearing of ductile metals.