Overview on constitutive modeling for hydroforming with the existence of through-thickness normal stress

The hydroforming process is used widely across many industrial fields. High applied pressure during hydroforming makes it necessary to consider the influence of the through-thickness normal stress, while traditional approaches based upon a plane-stress assumption are not appropriate in such cases. Reliable constitutive models that consider the through-thickness normal stress are summarized in this paper, which focuses on the state of the art in the following several aspects: determine the flow stress curve with proper experimental methods and choose the measurement and computational methods to minimize the error as much as possible; select the proper three-dimensional anisotropic yield criterion for the specific material; Define the forming limit model and construct corresponding experimental verification method. The review of existing work has revealed several gaps in current knowledge of the hydroforming process accounting for the through-thickness normal stress. Conclusions are drawn concerning some critical issues and potential future developments in hydroforming modeling.     

Efects of Die Corner Radius and Temperature on the Formability of AA7075-T6 Alloy

This study focused on the formability of aluminium alloy（7075-T6） sheets through hydroforming route. Formability of these sheets was tested using a warm forming setup at three diferent temperatures and four diferent die corner radii. Forming limit diagrams（FLD） were generated by measuring the grids of the sheet formed. The results show that the forming limit of AA7075-T6 can be significantly improved when the blank was heated to 140–250℃. It was also observed that as the temperature increases above 140℃, dome height began to decrease. Also the results indicated that both the die corner radius and temperature have a significant efect on the stress-strain curve and warm forming of AA7075-T6 sheets. Thus, with the temperature increased from room temperature（RT） to 140℃, the flow stress decreased and the strain increased, hence, the formability is enhanced. However, further increase in temperature causes decreases the flow stress and strain. Similar changes of the flow curve were seen in die corner radius. Decreasing the die corner radius decreases the flow stress and increase the strain. Moreover, an equation was obtained by establishing correlations between the experimental parameters and their results. In this way, it became possible to make predictions.     

The application of a ductile fracture criterion to the prediction of the forming limit of sheet metals

To predict accurately the forming limit in sheet metal forming, the combination of FE simulation with tension tests is adopted in this paper to determine the material constants p and C in a ductile fracture criterion (DFC), which is advanced by the author. Forming limits of bore-expanding, hemispherical punch bulging and deep drawing (cylindrical, square-cup parts) are predicted by means of the DFC. Comparison of the results predicted by the DFC with experimental values shows that the precision of forming limit predicted by material constants obtained by the combination method is more accurate than that predicted by material constants obtained by the tension method, and that the critical punch stoke and the fracture initiation position in forming processes above mentioned are predicted accurately by the DFC.     

Anisotropic mechanical behavior and formability criterion for zinc sheets

The mechanical behavior of zinc has been studied and linked to the formability of sheets. An anisotropic elastic–viscoplastic behavior law has been developed to take into account the anisotropy of the material. Anisotropy is induced by crystallographic and morphological textures, and possibly by the spatial distribution of intermetallics. The temperature dependence is introduced through a Zener–Hollomon type term. The resulting anisotropic formability of sheets implies a new approach by adapting the forming limit diagram with a stress based criterion. This approach is confronted and validated by considering the industrial forming of a head clip.     

Fundamental studies on the incremental sheet metal forming technique

The idea of incremental forming technique has been investigated for production of sheet metal components. With this technique, the forming limit curve (FLC) appears in a different pattern, revealing an enhanced formability, compared to conventional forming techniques. In the present study, the formability of an aluminum sheet under various forming conditions was assessed and difficult-to-form shapes were produced with the technique. By utilizing knowledge and experience obtained during the present study, it became possible to produce some free surfaces.     

Experimental evaluation of strains in the tension–compression using a new tool geometry, X-Die

Sheet-metal forming involves a complex distribution of strains throughout the part. The strains occur due to tension, compression and a mix of both. A geometry has been developed, the X-Die, in order to gain insight into the strain behavior of different materials. The X-Die enables strain paths far into the tension–compression region, thus creating the possibility to extend the experimental base both for definition and for further extrapolation of the forming limit curve (FLC) in the tension–compression region, as well as to evaluate FE-simulation results for the same region.

The experimental results show that the strain signature is impacted by material quality. In qualities such as extra high strength steel (EHSS) and aluminum the strains do not reach as far into the tension–compression region as the strains do in e.g. mild steel. This is due to failure in plane strain tension. Strain paths in materials such as mild steel and high strength steel (HSS) reach far into the tension–compression region before failure. Use of the X-Die provides possibilities to reach farther into the tension–compression region compared with traditional test methods for creating a forming limit diagram (FLD).

Use of the X-Die yields well-defined strain signatures. These clearly defined strain signatures are favorable for comparison with numerical simulations, especially for strain signatures in the tension–compression region.

Furthermore, the experiments using the X-Die indicate that a possible additional forming limit curve, which intersects the original forming limit curve (shear failure), exists so far into the tension–compression region that it is not applicable.

Even though the experiments indicate compression strains >100% (material DX56D), the experiments show potential for an experimentally determined extrapolation of the FLC up to 75% compression strain. The results of the experiments indicate that the X-Die geometry is suitable as a supplementary tool in identifying the strain behavior of different materials far into the tension–compression region and is also a good tool for verification of numerical results in the tension–compression region.     

高应变率成形极限图测试与仿真研究进展

高应变率成形是一种在极短时间内释放高能量而使金属变形的成形方法,研究表明高应变率能显著提高多种金属材料的成形极限能力.通过高速杯突实验绘制成形极限曲线,可反映金属在高应变率下的成形能力,从而指导冲压设计.文中介绍了目前常用的几种高速杯突实验设备,包含气压、液压和电磁等驱动方式,并对高应变率下的成形极限图进行了分析和总结...     

基于最大应变速率失稳准则预测铝合金板成形极限曲线

本文提出了一种结合有限元模拟预测金属板材成形极限曲线(FLC)的失稳准则-最大应变速率失稳准则。该准则考察试样所有单元体的整个变形历史,通过厚向应变及厚向应变速率随时间的变化来判定颈缩时刻和颈缩位置,可以应用于变形过程中存在应变路径变化的情况。对比AA3003板材预测与实验结果发现,本方法所预测的FLC整体上与实测结果符合较好,优于传统M-K理论的计算结果。     

Time dependent determination of forming limit diagrams

The forming limit diagram (FLD) is a convenient tool for classification of sheet metals’ formability in the finite element analysis as well as in the press shop. The FLD indicates the maximum strain values which can be applied on a material without failure as a function of the strain condition. In contrast to the standardized evaluation method described in the standard ISO 12004-2 a new time dependent analysis method is presented. Using a regression analysis the onset of necking can be detected automatically independent of the strain state. Results will be presented and discussed in contrast to the existing standard procedure.     

半固态流变成形数值模拟研究现状及展望

合金流变成形数值模拟可有效预测半固态成形中充型和凝固过程,对压力场、速度场、固相分布、充填速度、充填温度,以及成形过程的缺陷等进行分析,对工艺、设计相关的方案优化提供相关的决策帮助。综述了近年来半固态流变成形数值模拟技术的理论基础,以及国内外流变成形合金的研究进展和未来发展方向。     

工艺参数对板料成形性能的影响

传统上,人们对于工艺参数对板料成形性能的影响的认识只是定性的。利用板料成形的计算机数值模拟技术,可以定量地考察各种工艺参数对板料成形性能的影响。     

A new method of determining forming limit diagram for sheet materials by gas blow forming

A new methodology is proposed to obtain forming limit diagrams (FLDs) of sheet materials using gas blow forming process at elevated temperatures. Tension–tension side of the forming FLD is achieved by using circular as well as elliptical dies of different aspect ratios. To achieve tension–compression side of FLD un-bonded bi-layer specimen with slots are utilized. The widths between the slots are varied to achieve different strain paths. A correlation is established between the hemispherical punch-based tests and GBF tests of samples with slots to achieve different strain paths. FLDs for automotive AZ31 magnesium sheet at 300 °C and 400 °C in two different orientations are determined. Increase in forming limits of AZ31 with increase in temperature is observed.     

稳压器下封头成形工艺的研究

介绍了一重集团公司生产的工程稳压器下封头锻件的研制,利用成形分析软件,对稳压器下封头板坯尺寸参数进行了优化,提出了整体封头的成形工艺,使封头各方面的指标达到了技术条件要求,从而验证了这种封头成形新工艺是可行的。     

Dieless incremental micro-forming of miniature shell objects of aluminum foils

Single-point incremental forming of thin aluminum foils, which did not require any die and any backing plate, was developed for fabricating customized parts of thin shell miniature objects. Thin aluminum foils were deformed incrementally using a round-tip single point tool on a desktop type of milling machine controlled with a personal computer. It was found that an aluminum foil 12 μm thick was much smaller in forming limit than an aluminum foil 50 μm thick when a single point tool with a tip radius of 0.5 mm was not rotated, but its forming limit was improved greatly by rotating the tool up to 20,000 rpm. Measured forming forces revealed that the tool rotation decreases the in-plane forces by approximately 50%. Hydrodynamic lubrication at the interface between a rotating tool and aluminum foil must have had a direct and favorable influence on the forming process, while increasing the forming limit of the thinner foil. It was also found that the optimum stepwise axial feed was found to be around the value of foil thickness. Then, incremental forming of arrays of dots 0.1 mm in diameter, miniature pyramids, a miniature car and miniature letters was performed successfully under the optimized conditions at arbitrary positions of 12-μm-thick foils.     

行李箱内板零件冲压成形分析

在材料力学性能测试和成形极限分析的基础上,采用网格应变分析技术研究了行李箱内板零件冲压过程危险区域的金属流动规律,评价了两种钢板的成形效果。结果表明：行李箱内板危险部位的变形方式为胀形一深拉延变形,r值和n值较高的冷轧薄板具有较高的成形极限,相应地其冲压成形安全裕度也较大。     

硼酸镁晶须增强镁基复合材料挤压过程数值模拟

对不同挤压工艺下Mg2B2O5w/AZ63B棒材的热挤压过程进行了有限元模拟,分析了热挤压过程中挤压温度（250、300、350℃）、挤压速度（1、4mm/s）和挤压比（6.25、14.00、20.25）对Mg2B2O5w/AZ63B复合材料挤压过程中等效应力的影响。模拟结果表明,温度对等效应力的影响最为显著,当挤压温度由250℃升至350℃时,合金的最大等效应力由185MPa降低到138MPa;当温度与挤压比恒定时,挤压速度从1mm/s增大到4mm/s时,最大等效应力值从184MPa降低到167 MPa;随着挤压比的增大,坯料在挤压筒内的等效应力逐渐增大,挤压坯料在挤压模具锥角处受到强烈的挤压变形和剪切变形,晶粒得到细化,使得等效应力的分布更加均匀。     

Design and use of a novel sample design for formability testing in pure shear

Existing tests for assessing the formability of sheet metal samples can create strain ratios in the range from uniaxial to biaxial stretching, −1/2 ≤ ?₂/?₁ ≤ 1. A novel sample design is proposed, with shaped cut-outs in a rectangular sheet specimen, intended to produce with strain ratios in the range, −1 ≤ ?₂/?₁ ≤ 1/2 in a small zone, while the sample is uniaxially extended. The strain ratio is controlled by changing the geometry of the cut-outs.