Experimental Observations of Quasi-Static-Dynamic Formability in Biaxially Strained AA5052-O

To establish the efficacy of electromagnetically assisted sheet metal stamping (EMAS), a series of combined hydraulic bulging and electromagnetic forming (EMF) experiments are presented to evaluate the biaxial quasi-static-dynamic formability of an aluminum alloy (AA5052-O) sheet material. Data on formability are plotted in principal strain space and show an enhanced biaxial formability beyond the corresponding experimental results from conventional forming limit diagram. The plastic strains produced by the combined process are a little larger than or at least similar with those obtained in the fully dynamic EMF process. In addition, the biaxial forming limits of aluminum sheets undergoing both very low and high quasi-static prestraining are almost similar in quasi-static-dynamic bulging process. Limit formability seems to depend largely on the high-velocity loading condition as dictated by EMF. It appears that in quasi-static-dynamic forming, quasi-static loading is not of primary importance to the material’s formability. Based on these observations, one may be able to develop forming operations that take advantage of this formability improvement of quasi-static-dynamic deformation. Also, this could enable the use of a quasi-static preform fairly close to the quasi-static material limits for the design of an EMAS process.     

有色金属板材若干温热加工成形技术的发展

介绍了有色金属材料加工先进新技术国内外发展和应用概况,包括近年来关于镁合金、钛合金、铝合金等典型有色金属材料领域出现的先进塑性成形技术,尤其温热加工成形技术。在镁合金板材冲压成形领域,介绍了镁合金板材温热冲压成形、差温冲压成形、温热液压成形和热冲锻成形以及镁合金型材温热拉弯成形等新工艺技术,为镁合金板材在汽车、电子、机车车辆等领域的应用奠定了技术基础。钛合金板材零件的热应力成形、热胀形成形、激光弯曲成形、高温蠕变成形技术都得到了发展和应用。随着铝合金的进一步应用和发展,一些低塑性难成形高强铝合金的用量在增加,应用领域在扩展。因此铝合金的温热液压成形、冲锻成形都有所发展。     

Evaluation of the plastic yield locus for embossed sheet using biaxial tensile tests

3D-structured (embossed) aluminium sheets have been used as heat insulation materials in automotive exhaust parts because the embossments on the sheets increase the surface area and reinforce the stiffness of exhaust components. Unlike the press-forming process for flat (non-embossed) sheets, however, that for embossed aluminium sheets is constrained by many restrictions given the distinct mechanical properties and geometric 3D shape of the latter. In designing sheet-stamping tools, manufacturers have recently used CAE technologies based on finite element analysis. Guaranteeing the effectiveness of CAE technologies necessitates information about the plastic yield criterion, which is determined primarily by performing a biaxial tensile test on cruciform-shaped specimens. We measured the yield locus of an embossed aluminium 3004-P sheet by using the camera vision method instead of strain gauge measurement because of the difficulty in attaching a strain gauge to the central region of the aluminium body. The measured yield locus of the studied sheet shows that its yield stress in equi-biaxial stress is smaller than the flat sheet yield locus measured by the strain gauge method. The shape of the yield locus of the embossed aluminium sheet also adequately corresponds with Logan-Hosford anisotropic yield function.     

Subsequent yield loci of 57540 aluminum alloy sheet

Complex loading paths were realized with cruciform specimens and biaxial loading testing machine. Experimental method for determining the subsequent yield locus of sheet metal was established. With this method, the subsequent yield loci of 5754O aluminum alloy sheet were obtained under complex loading paths. Theoretical subsequent yield loci based on Yld2000-2d yield criterion and three kinds of hardening modes were calculated and compared with the experimental results. The results show that the theoretical subsequent yield loci based on mixed hardening mode describe the experimental subsequent yield loci well, whereas isotropic hardening mode, which is widely used in sheet metal forming fields, predicts values larger than the experimental results. Kinematic hardening mode predicts values smaller than the experimental results and its errors are the largest.     

一种改进的Lemaitre断裂准则及其在弯曲成形裂纹预测中的应用

针对弯曲成形中的裂纹缺陷,应用Lemaitre韧性断裂准则,同时考虑应力三轴度,最大主应力比,以及塑性应变对损伤的影响,对Lemaitre准则进行改进,有效预测了弯曲成形中金属板料的成形极限。以7075-T6铝合金为研究对象,模拟得到该合金板材的裂纹产生条件,获取改进的Lemaitre准则材料参数,确定破裂阈值。对6 mm厚7075-T6铝合金板材进行三点弯曲实验,并观察其金相组织。对其产生裂纹时的压下量与断裂准则所得的理论压下量进行比较,验证了改进后断裂准则对裂纹预测的准确性。通过对比产生裂纹时的压下量,结果表明,改进后的Lemaitre断裂准则所得理论压下量为9.7 mm,与模拟和实验结果一致,证明改进后的Lemaitre准则对弯曲成形裂纹预测具有一定的准确性。     

一种建立板料成形极限应力图的新方法

基于塑性理论建立了比例加载条件下双向拉伸应力应变关系,结合Swift分散性失稳准则,提出了一种建立板料成形极限应力图的方法。分别应用Hill 48和Hosford屈服准则以及单向拉伸性能参数,建立了铝合金板(r<1)和薄钢板(r>1)两种材料的成形极限应力图(FLSD),分析表明,不同的屈服准则的选取对于成形极限应力曲线有不同的影响,对于不同类型的材料屈服准则的影响程度也不同。与由通常的成形极限图(FLD)转换所得到的成形极限应力图(FLSD)进行了对比分析,结果表明,所提出的方法计算过程更为简便,并能较为准确地建立成形极限应力图,可以作为复杂加载路径下的成形极限破裂判据。     

高温合金超薄带材屈服轨迹的晶粒尺度依赖性研究

为了研究高温合金超薄带材屈服轨迹的晶粒尺度依赖性,通过单向拉伸试验和不同加载比例的双向拉伸试验获得了不同厚度和晶粒尺寸的高温合金超薄带材的实验屈服轨迹。结果表明,随着高温合金带材晶粒尺寸的增大,屈服轨迹整体向内收缩,且形状由椭圆形向方形转变,证明了高温合金带材屈服轨迹存在晶粒尺度依赖性。同时,评估了4种典型宏观屈服准则Mises、Hill48、Barlat89和Yld2000-2d预测不同厚度与晶粒尺寸高温合金带材屈服轨迹的能力,发现Yld2000-2d屈服准则预测精度最高,可以较为精确地描述不同厚度与晶粒尺寸高温合金超薄带材的屈服行为。     

Prediction of forming limit curve for pure titanium sheet

Commercially pure titanium (CP Ti) has been actively used in the plate heat exchanger due to its light weight, high specific strength, and excellent corrosion resistance. However, researches for the plastic deformation characteristics and press formability of the CP Ti sheet are not much in comparison with automotive steels and aluminum alloys. The mechanical properties and hardening behavior evaluated in stress–strain relation of the CP Ti sheet are clarified in relation with press formability. The flow curve denoting true stress–true strain relation for CP Ti sheet is fitted well by the Kim–Tuan hardening equation rather than Voce and Swift models. The forming limit curve (FLC) of CP Ti sheet as a criterion for press formability was experimentally evaluated by punch stretching test and analytically predicted via Hora's modified maximum force criterion. The predicted FLC by adopting Kim–Tuan hardening model and appropriate yield function shows good correlation with the experimental results of punch stretching test.     

The effects of texture on formability of aluminum alloy sheets

The effects of typical texture components observed in rolled aluminum alloy sheets (i.e., copper, brass, S, cube and Goss texture components) on forming limit strains are numerically studied, utilizing the Marciniak–Kuczyński-type approach and a generalized Taylor-type polycrystal model. It is shown that only the cube texture component yields forming limits much higher than those for a random texture in a biaxial stretch range, while the other four texture components tend to yield lower forming limits. Furthermore, it is found that when the orthotropic axes of the cube texture material are inclined at 45° relative to the major stretch direction, abnormally high forming limits are obtained for near plane strain forming paths.     

Characterization of yielding behavior of sheet metal under biaxial stress condition at elevated temperatures

The enhancement of material modeling in fields of sheet metal forming is essential for finite element-based designing of processes and dimensioning of parts. Since new lightweight materials, e.g. aluminum and magnesium wrought alloys show anisotropic and temperature-dependent forming behavior adequate testing methods and evaluation strategies have to be developed to obtain the reliable material data. For that purpose an experimental setup has been designed at the Chair of Manufacturing Technology which enables biaxial tensile testing of sheet metal at elevated temperatures. In this paper, the setup is introduced and the obtained results, i.e. experimentally determined yield loci and subsequent yield loci as a function of temperature are given for the well-known aluminum alloy AA6016 as well as the magnesium alloy AZ31.     

汽车覆盖件用6016铝合金板高应变速率敏感性试验研究

汽车碰撞(被动)安全性常用仿真分析方法来预测,高应变速率下材料性能参数的准确性关系到汽车碰撞仿真分析的精度。针对国内外的4种汽车覆盖件用6016铝合金板,进行了不同应变速率(5~500 s-1范围内)的高速拉伸试验,利用Johnson-Cook模型进行数据处理,系统研究了4种汽车覆盖件用6016铝合金板的力学性能(屈服强度、抗拉强度及断裂伸长率)和耐碰撞性能的应变速率敏感性。研究结果表明,4种6016铝合金板的屈服强度和抗拉强度具有较低的速率敏感性,其断裂伸长率和耐碰撞性能具有较高的速率敏感性。     

Influence of initial texture on formability of AZ31B magnesium alloy sheets at different temperatures

Repeated unidirectional bending (RUB) process was carried out to improve the texture of AZ31B magnesium alloy sheets. Influence of initial texture on formability of AZ31B magnesium alloy sheets at different temperatures was investigated. Compared with the as-received sheets, the limiting drawing ratio of the RUB processed sheets increased to 1.3 at room temperature, 1.5 at 50 °C and 1.7 at 100 °C, respectively. The improvement of the press formability at lower temperatures can be attributed to the texture modification, which led to a smaller Lankford value and a larger strain hardening exponent. However, the press formability of the sheet with a weakened basal texture has no advantage at higher temperature. This is due to much smaller r-value that results in severe thinning in thickness direction during the stamping process which is unfavorable to forming. Anyhow it is likely that the texture control has more effect on the press formability at lower temperature.     

基于韧性断裂准则的铝合金板材成形极限预测

为了准确地预测铝合金板材成形极限，将韧性断裂准则引入到数值模拟中。在数值模拟获得的应力应变值基础上，采用简单拉伸试验和数值模拟相结合的方法确定了韧性断裂准则中的材料常数，并应用该韧性断裂准则预测了铝合金LYl2(M)的圆筒件拉深和半球形凸模胀形的成形极限。预测结果与实验值吻合较好，该韧性断裂准则能够预测铝合金板材成形极限。     

变压边力对铝合金板冲压成形的影响研究

通过数值模拟和实验,研究了变压边力对铝合金板成形过程中应变路径的变化规律,基于MK法研究应变路径的变化对铝合金板成形极限(FLC)的影响,通过比较不同随时间变化的压边力对铝合金成形极限的影响,揭示变压边力改善铝合金板冲压成形性能的机理。实验和数值模拟结果表明,变压边力改善铝合金板成形性能的主要原因是变压边力对应变路径的影响,对杯形拉深来说,渐增型随时间变化的变压边力可以改善铝合金板的成形性能。     

Prediction of forming limit in deep drawing of Fe/Al laminated composite sheets using ductile fracture criterion

Axisymmetric deep drawing processes of laminates composed of mild steel and various aluminium alloy sheets are simulated by FEM. From the calculated stress and strain histories of elements in each layer, the fracture initiation site and the forming limit are predicted by using the ductile fracture criterion. The predictions so obtained are compared with experimental observations. The results exhibit that various types of fracture initiations in deep drawing of the laminated composite sheets are successfully predicted. Furthermore it is found that the drawability is improved by setting the mild steel sheet on the punch side for the case of aluminium alloy sheet with comparatively high ductility, and by sandwiching the aluminium alloy sheet with the mild steel sheets for the case of low ductility.     

An Experiment on Magnetic Pulse Uniaxial Tension of AZ31 Magnesium Alloy Sheet at Room Temperature

Formability of magnesium alloys at room temperature or slightly elevated temperatures is low, and exhibiting poor resistance to strain localization and failure. However, it is possible to improve the formability of AZ31 magnesium alloy sheet at high strain rate such as by electromagnetic forming (or magnetic pulse forming). In this study, experimental investigation of uniaxial tension of AZ31 sheet by magnetic pulse forming at room temperature was presented. The approximate rectangular flat spiral coil was employed to carry out the experiments. The specimens used in the tension test by magnetic pulse forming were same as the quasi-static uniaxial test. The samples were placed close to the outside of coil where an approximately homogenous magnetic field distribution prevailed. The experimental results indicate that the total elongation of AZ31 sheet improves about 37% compared with the quasi-static case. Non-uniform deformation occurs in the specimen. The maximum strain takes place on the area C, where is plotted on the specimen. The major and minor principal strains at most increase by approximately 112 and 96% under 5.12 kJ energy. The experimental results obtained in this study provide the fundamentals for the investigation of high speed forming of AZ31 magnesium alloy sheets.     

Experimental determination of yield loci for magnesium alloy AZ31 under biaxial tensile stress conditions at elevated temperatures

The finite element analysis (FEA) has become one of the most relevant and most important tools in fields of sheet metal forming for designing processes and dimensioning parts. However, reliability and quality of the numerical results strongly depend on the whole FE-model and especially on the modeling of the material behavior, which shows wide impact on calculated stresses and strains of sheet metal parts. Therefore, the experimental determination of characteristic material data concerning anisotropic and temperature-effects is essential. In this paper the influence of temperature on the yielding and the hardening behavior of the magnesium sheet metal alloy AZ31 are investigated for different uniaxial and biaxial stress conditions. For that purpose an experimental setup has been developed at the Chair of Manufacturing Technology (LFT) which enables biaxial tensile testing of sheet metal. Yield loci of AZ31 are determined as a function of temperature and they are based on solely measurement data of the forming process itself.     

Deformation characteristics and cavitation during multiaxial blow forming in superplastic 8090 alloy

This work examined the effect of multiaxial stress on deformation characteristics of a superplastic aluminum alloy 8090 by deforming the sheet into a die with a cylindrical cavity. Several interrupted tests were performed to bulge the sheets to various depths for different strain rates, the formed parts were utilized to evaluate the deformation status, thickness distribution, local strain states, and cavitation. It was found that evolution of cavity volume fraction with forming time could be related to the thinning behavior of the deformed sheet during forming. Decrease in cavity volume fraction at the central region was observed in the later stage of forming as the thickness of the deformed sheet remained constant for all test forming rates.     

Flow characteristics of aluminum coated boron steel in hot press forming

The flow characteristics of aluminum coated boron steel in hot press forming were investigated. Furthermore, the effects of aluminum coated layer on press forming were analyzed during deep drawing. The results show that aluminum coated boron steel exhibits a high sensitivity on temperature and strain rate. Aluminum coating layer appears in surface flaking in a temperature range of 700–800 °C, but smooth surface is formed above 900 °C.     

Characterization of Microstructure,Mechanical Properties and Formability of Cryorolled AA5083 Alloy Sheets

In this work, microstructure, mechanical properties and formability of cryorolled and annealed AA5083 alloy sheets have been characterized and a comparison has been made with cold rolled and annealed sheets. Five-millimeter-thick sheets of this alloy were cryorolled in multiple passes to a final thickness of 1 mm (80% reduction with a true strain of 1.6). Effect of annealing time and temperature on hardness has been studied, and it has been found that a short annealing at 275 °C for 15 min after cryorolling would yield a good combination of strength and ductility. Microstructural investigations showed that the cryorolled and short annealed samples possess bimodal grain structure which is responsible for better mechanical properties than cold rolled sheets. From the experimentally determined forming limit diagrams, the limit strains of cryorolled sheets have been found to be almost equal to conventional cold rolled and annealed sheets in all modes of deformation. No major differences have been found in strain distribution also. This work clearly demonstrates that cryorolling of AA5083 alloy sheets followed by a short annealing with bimodal grain structure can be used for sheet metal forming applications with higher strength and toughness than conventional sheets without any reduction in formability.