Theoretical prediction of forming limit diagram of AZ31 magnesium alloy sheet at warm temperatures

A theoretical prediction on forming limit diagram（FLD） of AZ31 magnesium alloy sheet was developed at warm temperatures based on the M-K theory. Two different yield criteria of von Mises and Hill＇48 were applied in this model. Mechanical properties of AZ31 magnesium alloy used in the prediction were obtained by uniaxial tensile tests and the Fields-Backofen equation was incorporated in the analysis. In addition, experimental FLDs of AZ31 were acquired by conducting rigid die swell test at different temperatures to verify the prediction. It is demonstrated from a comparison between the predicted and the experimental FLDs at 473 K and 523 K that the predicted results are influenced by the type of yield criterion used in the calculation, especially at lower temperatures. Furthermore, a better agreement between the predicted results and experimental data for AZ31 magnesium alloy sheet at warm temperatures was obtained when Hill＇48 yield criterion was applied.

Forming limit of textured AZ31B magnesium alloy sheet at different temperatures

Repeated unidirectional bending （RUB） was carried out to improve the texture of commercial AZ31B magnesium alloy sheets. All specimens were prepared in the rolling direction. The forming limit diagrams （FLDs） of AZ31B magnesium alloy sheet were determined experimentally by conducting stretch-forming tests at room temperature, 100, 200 and 300 ℃ Compared with the as-received sheet, the lowest limited strain of AZ31B magnesium alloy sheet with tilted texture in the FLD increased by 79% at room temperature and 104% at 100 ℃. The texture also affected the extension of the forming limit curves （FLC） in the FLD. However, the FLCs of two kinds of sheets almost overlapped at temperature above 200 ℃. It can be concluded that the reduction of （0002） texture intensity is effective to the improvement of formability not only at room temperature but also at low-and-medium temperature. The effect of texture on FLDs becomes weak with increasing temperature.

Prediction of fracture limits of Ni-Cr based alloy under warm forming condition using ductile damage models and numerical method

The stretch forming and the deep-drawing processes were carried out at 300 and 673 K to determine the safe forming and fracture limits of IN625 alloy.The experimentally obtained strain-based fracture forming limit diagram(FFLD)was transformed into a stress-based(σ-FFLD)and effective plastic strain(EPS)vs triaxiality(η)plot to remove the excess dependency of fracture limits over the strains.For the prediction of fracture limits,seven different damage models were calibrated.The Oh model displayed the best ability to predict the fracture locus with the least absolute error.Though the experimentally obtained fracture limits have only been used for the numerical analysis,none of the considered damage models predicted the fracture strains over the entire considered range of stress triaxiality(0.33<η<0.66).The deep drawing process window helped to determine wrinkling,safe and fracture zones while drawing the cylindrical cups under different temperature and lubricating conditions.Further,the highest drawing ratio of 2 was achieved at 673 K under the lubricating condition.All the numerically predicted results of both stretch forming and deep drawing processes using the Hill 1948 anisotropic yielding function were found to be good within the acceptable range of error.

Viscous warm pressure bulging process of AZ31B magnesium alloy with different ellipticity dies

Based on the bulging principle of different ellipticity dies, the methyl vinyl silicone rubber with excellent thermal stability and heat transfer performance was chosen as the viscous medium. The finite element analysis and experiments of viscous warm pressure bulging （VWPB） of AZ31B magnesium alloy were conducted to analyze the influence of different ellipticity dies on the formability of AZ31B magnesium alloy. At the same time, based on the grid strain rule, the forming limit diagram （FLD） of VWPB of AZ31B magnesium alloy was obtained through measuring the strain of bulging specimens. The results showed that at the temperature range of viscous medium thermal stability, the viscous medium can fit the geometry variation of sheet and generate non-uniform pressure field, and as the die ellipticity increases, the difference value of non-uniform pressure reduces. Meanwhile, according to the FLD, the relationship between part complexity and ultimate deformation was investigated.

Formability of AZ31 Mg alloy sheets within medium temperatures

The stretching tests of the commercial AZ31 Mg alloy were conducted at 130 ℃, 170 ℃, 210 ℃, at the forming speeds of 10 mm/min and 50 mm/min, respectively. The formability of AZ31 sheets at high temperature was evaluated by forming limit diagrams (FLD). The fracture morphologies were analyzed using a scanning electron microscope. The results show that the FLD of AZ31 Mg alloy is affected by the forming temperature, in another word, the formability increases with the increasing of the forming temperature. That may be because the non-basal slip system starts to move by thermal activation at high forming temperature. It is also demonstrated that the formability of the AZ31 Mg alloy is on the decline with the increasing of the forming speed. The slipping performs thoroughly to release the stress during the deformation if the forming speed decreases. In addition, the higher the forming temperature is, the more obvious the effect of the forming speed is. The forming temperature is the main dominating factor on the formability of AZ31 Mg alloy.

The Formablity of AZ31B Magnesium Alloy Sheet

The forming limit diagrams （ FLD ） of AZ31B magnesium alloy sheet were tested by means of the electro etching grid method based on the forming experiment of magnesium alloy sheet carried out with a BCS- 30D sheet forming testing machine and the strain testing analysis made with an advanced ASAME automatic strain measuring system. Experiments show that, at room temperature, the mechanical properties and deep drawing peorformance of AZ31B cold-rolled magnesium alloy sheet were so poor that it failed to test the forming limit diagrams without an ideal forming and processing capacity, while the hot-rolled magnesium alloy sheet was of a little better plasticity and forming peorformance after testing its forming limit diagrams. It can be concluded that the testing of the forming limit curves （ FLC ） offers the theoretical foundation for the drawing of the deep drawing and forming process of magnesium alloy sheet.

基于损伤耦合粘塑性模型的AZ31B镁合金温变形行为研究

为了准确描述AZ31B镁合金温变形中材料硬化、损伤和断裂行为,建立了基于位错密度的粘塑性模型与GTN损伤模型耦合的本构模型。在不同温度和应变率下对试样进行温拉伸试验,获得合金温变形下的应力-应变曲线。基于材料高温变形机理,以位错密度为内变量,建立粘塑性模型来描述材料的硬化行为,并采用智能算法来识别模型的材料参数。为描述材料温变形过程中的损伤演变,将GTN损伤模型与已建立的粘塑性模型进行耦合,建立温拉伸仿真有限元模型,利用Kriging代理模型结合智能算法反求损伤模型参数。最后,利用已建立的损伤耦合粘塑性模型分析温拉伸过程中的损伤演化和预测温胀形过程的破裂行为,并与试验结果进行对比。结果表明,该本构模型可以准确地预测AZ31B温变形过程中的损伤和断裂。

基于翻孔试验的AZ31B镁合金薄板温成形特点研究

以1.6 mm厚的AZ31B镁合金板料为研究对象,利用Deform-3D软件探究了温成形翻孔工艺的成形特点,分析了成形温度、冲压速度和压边间隙对制件成形过程中等效应力、成形载荷及厚度变化的影响规律和机理,在此分析结果基础上提出了翻孔优化工艺,并通过对比优化工艺下仿真试验和物理试验中制件的变形情况,验证了模拟仿真的正确性。结果表明,扩孔阶段制件凸缘区材料应力波动幅度最大,达到141.29 MPa,减薄和增厚最严重的区域分别是伸长区底部和凸缘区,分别达到20.00%和3.13%,温度与材料的等效应力峰值及应力分布不均匀性呈负相关,较低的冲压速度可以给材料变形提供充足的缓冲时间,有利于降低等效应力峰值和增厚率,压边间隙主要影响材料厚度变化情况而对材料主变形区等效应力的影响较小,仿真试验与物理试验制件的终态材料厚度分布趋势一致,凸模峰值载荷误差为0.94 kN,仿真试验结果与物理试验结果具有良好的一致性。

5A06-O铝镁合金板材差温拉深凸耳行为研究

采用单向热拉伸试验测定了5A06-O铝镁合金在温热条件下(150~275℃)的流变应力曲线及厚向异性系数,通过扫描电镜对不同温度下单向拉伸试样颈缩断裂处的断口进行形貌观察分析。通过差温拉深试验研究了5A06-O铝镁合金在温热条件下的凸耳形成及演变规律。结果表明,拉深件的凸耳出现在与轧制方向成45°方向处,且平均凸耳率随着成形温度的升高先降低后增大,在250℃差温拉深条件下平均凸耳率最小,这与平面各向异性系数∆r有密切关系;在150、200、275℃,平均凸耳率随拉深比的增大呈现先增大后减小的趋势,而在250℃平均凸耳率随着拉深比的增大而增大;拉深件45°方向的减薄率与增厚率明显低于0°及90°方向,其原因在于该方向有着最大的塑性应变比r;250℃成形的零件三个方向均有着最低的减薄率与增厚率,壁厚分布最为均匀,其原因在于该温度有着最大的平均塑性应变比r。成形速度对5A06-O铝镁合金的成形性能有着显著影响,在室温及温热条件宜采用较快的拉深速度,本试验中5mm/s为最佳,但随着拉深速度的增加,平均凸耳率呈现上升趋势。

Numerical and Experimental Investigation of Electromagnetic Free Bulging of AZ31 Magnesium Alloy Sheet

The electromagnetic free bulging experiment and the coupled-field numerical simulation of the EMF process were carried out to determine the possibility to improve the formability of magnesium alloy AZ31 sheets(1 mm thick)at room temperature.Formability data were examined using strain measurements.The numerical simulation for the electromagnetic sheet free bulging is performed by means of ANSYS FEA software.Compared with quasi-static FLD results,increase in the major and minor principal strains of approximately 87%and 39%were achieved,respectively.The effects of various process parameters on electromagnetic bulging of AZ31 magnesium alloy sheets were evaluated. Three-dimensional(3D)FE model is established to predict the electromagnetic bulging of the sheet.The simulation results agreed well with experimental observations.

镁合金板材温热电磁复合成形试验研究

由于节能、环保的需要,轻量化已成为现代结构设计的主流趋势,作为最轻的金属结构材料,镁合金受到强烈的关注。但由于其室温塑性较差,现有产品主要采用压铸法制造。由于镁合金板材在200～400℃时表现出良好的塑性,因此温热成形成为一种很好的方法;电磁成形是一种高能高速率成形工艺(应变速率大于103～104 s–1),能显著提高金属板材的成形性能,并且不需要润滑。充分利用温成形与电磁成形的复合优势来提高镁合金板材的成形性能,研究电压、电容、温度等工艺参数对镁合金板材温热电磁胀形高度的影响规律,结果表明:在一定的范围内提高电压或增加电容,胀形高度得到提高;增加电压比增加电容能更有效地提高胀形高度;在相同的放电能量下,成形温度提高时,板材胀形高度先降低(θ<150℃)然后增加(150℃<θ<230℃);随着成形温度增加,极限胀形高度增加,但所需的放电能量增加。

镁合金板材温热变形机理及温热成形技术

镁合金温热成形工艺具有较好的应用前景,是实现轻量化的重要途径,但镁合金温热成形机理需要进一步深入研究。通过电子背散射衍射(Electron back scatter diffraction,EBSD)原位跟踪观测方法,针对100～230℃范围,对轧制镁合金板材在单向压缩和单向拉伸变形时的变形机理进行系统研究和定量分析。分析镁合金板材在不同条件下的力学性能、织构转变特点、孪晶与滑移系启动规律,揭示不同变形条件下镁合金板材的塑性变形机理。研究结果表明,镁合金板材在变形过程的力学性能变化、织构演化和晶粒取向变化在很大程度上取决于孪晶参与变形的比例。镁合金板材在170℃具有较高的塑性成形能力,该温度下的大量锥面滑移系启动有利于协调轧板在厚度方向的变形。根据已获得镁合金板材变形机理,为镁合金板件冲压成形工艺提出建议。提出镁合金板件温热成形工艺,开发若干典型镁合金板件产品。

镁合金板件温热成形技术的几个新进展

介绍了镁合金薄壁板材的温热冲压成形技术、温热液压成形技术和局部热冲锻成形技术。与传统镁合金冲压成形在250℃以上温度不同,温热成形可以在105～170℃之间进行,筒形件温热成形时其极限拉深比可达1.4～2.6,可以满足大多数镁合金薄板件的成形需要。镁合金薄板温热成形工艺在电子器件外壳和汽车薄板件的生产方面具有很好的发展前景,可以取代压铸技术生产新一代优质的镁合金产品。温热液压成形技术适合生产数码相机壳等复杂形面的镁合金板件,有很好的应用前景。局部热冲锻成形技术可以成形局部带有凸台的镁合金薄板件,该技术使镁合金板件具有替代压铸件的技术优势。

铝合金磁脉冲辅助板材冲压技术研究进展

铝合金板材室温下的成形性较差,但在高速变形条件下成形极限大幅提高。为了促进铝合金板材在汽车制造、航空航天及3G领域的应用,需要开发先进的板材成形技术。磁脉冲辅助板材冲压成形技术(EMAS)通过在普通冲压模具中嵌入线圈,能有效控制和改善变形板料的应变分布。针对这些问题,重点介绍和讨论了EMAS技术的国内外研发现状,阐明了开展该技术研究的必要性和重要意义。

轻合金管材热态内压成形性能测试及样件试制

介绍了轻合金热介质内压成形基本原理,研究了温度对镁合金和铝合金管材热态内压成形性能的影响,进行了镁合金变截面件和铝合金变径管成形试验。所建立的热介质内高压成形装置最高加热温度315℃,压力100 MPa,可实现高压热介质的传输和密封、成形件温度和加载曲线控制等。对镁合金和铝合金管材在不同温度下成形性能的测试分析表明,轻合金热成形需综合考虑总延伸率、均匀延伸率和n值等参数随温度的变化,选取合适的成形温度。

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。

镁合金AZ31B板材温成形流变规律及本构模型

通过AZ31B镁合金板材高温拉伸实验,分别讨论了成形温度、应变速率以及各向异性对镁合金流变的影响。实验结果表明,温度越高、应变速率越低,镁合金的塑性越好;取样方向与轧制方向成45°时,由于在此方位孪生取向因子最大,因而该方向的塑性很高,与0°方向塑性基本相同。对变形后的镁合金进行金相组织观察发现,250℃时镁合金显微组织几乎都是由细小的等轴晶粒组成,优于其他温度下的显微组织;结合温度对镁合金流变的影响,确定镁合金的最佳温成形温度为250℃。依据实验数据建立了两种硬化本构模型,即Fields-Backofen模型和指数模型。分别将两种模型预测结果与实验数据对比表明,采用指数模型能更好的预测镁合金温成形流变应力。

AZ31镁合金板材温热电磁成形能力

将电磁成形和温热成形方法相结合,以AZ31镁合金板材为对象,测试了25℃至230℃内AZ31镁合金板材的极限应变,得到了AZ31镁合金板材在温热电磁成形条件下的成形极限图。温热电磁成形条件下的成形与准静态胀形结果对比表明:随温度提高,镁合金板材温热电磁成形的极限得到提高;相比于单一电磁成形和单一温成形,温热电磁成形可获得更大的极限变形,从而有利于提高材料的成形能力。

温成形中铝合金板成形极限图的预测研究

在铝合金板温成形数值仿真中,成形极限图是判断材料颈缩失效和评价温冲压成形能力的基础.提出了一种温成形条件下铝合金板成形极限图的理论预测方法.采用曲线拟合方法建立了Al5083-O铝合金板应变硬化指数、应变率硬化指数随成形温度的变化规律;采用M-K理论模型,结合Logan-Hosford屈服函数计算获得温成形条件下铝合金板的成形极限图.计算结果与实验数据吻合较好,证实了温成形条件下铝板成形极限图的理论预测方法是正确的.

AZ31B镁合金板材冲压性能的实验研究

系统地研究了AZ31B镁合金板材的冲压性能。通过室温和中高温拉伸实验研究了AZ31B镁合金板材的基本冲压性能,如σb、σs、σs/σb、δ、n、r和Δr等性能指标;通过模拟成形实验研究了杯突值、弯曲性能、拉深性能、扩孔性能以及成形极限图FLD等,为制定冲压成形工艺提供参考依据。研究表明,AZ31B镁合金板材的冲压性能室温下较低而中高温则较好。