磁脉冲成形板坯动态结构失稳力学分析

为揭示磁脉冲成形过程中板坯的塑性失稳机制,基于板坯磁脉冲拉伸成形工艺试验,结合板坯在脉冲电磁力作用下的变形特点,对板坯在动态加载条件下的结构失稳行为进行了力学解析。结果表明,脉冲电磁力作用的板坯动态变形过程中惯性力的影响显著。惯性力作用下,板坯变形依次经历脉冲力加载和惯性运动两个典型阶段,其动态塑性变形符合初期塑性变形伴随着动能的吸收、后期塑性变形产生于动能的耗散模式。惯性力的存在使得板坯对自身的结构失稳具有自我抑制作用,有助于分散变形,提高板坯的塑性。     

筒形件磁脉冲辅助冲压成形数值模拟

针对磁脉冲辅助冲压成形过程,提出了基于ANSYS平台的有限元分析程式,通过编制用户子程序,采用重启动分析法,建立了冲压预成形过程显式求解和瞬态磁脉冲成形"松散"耦合分析之间的动态连接,并用于5052-O铝合金圆筒形拉深件磁脉冲辅助冲压成形过程的有限元仿真研究。结果表明,所建立的有限元分析方案,能实现圆筒形件磁脉冲辅助冲压成形连续变形过程的模拟,板坯变形信息体现了准静态冲压变形和高速率磁脉冲成形之间的耦合。有限元模拟结果与实验吻合较好。     

铝合金薄壁柱壳电磁胀形塑性失稳实验研究

利用高速相机记录电磁驱动6063-T6铝合金薄壁柱壳膨胀变形过程,结合二维数字图像相关法分析试样表面应变场演化,对金属柱壳电磁膨胀塑性失稳过程开展研究,探讨金属高速成形极限及影响因素。实验结果表明:6063-T6铝合金薄壁柱壳电磁高速膨胀成形开始为均匀变形,随变形发展试样表面应变场出现局域化集中并沿45°或135°方向失稳扩展,裂纹沿局域化集中带发展导致碎裂;电磁膨胀局域化过程中分散性失稳时的平均应变与准静态拉伸失稳应变相比并没有明显变化,而局域化带形成时的应变随加载率提高显著增大,分析认为薄壁板电磁高速胀形成形极限提高的"增塑性"主要与惯性效应与结构效应相关。     

汽车用6005A-T6铝合金动态性能研究

通过对6005A-T6铝合金进行准静态拉伸试验和动态拉伸试验,研究了应变速率对6005A-T6铝合金准静态和动态力学性能及断裂行为的影响。6005A-T6铝合金的强度随着应变速率提高而增大,应变速率200/s拉伸的抗拉强度、屈服强度分别较准静态拉伸提升30MPa、25MPa,其中以准静态到应变速率10/s的过程中,材料的抗拉强度、屈服强度上升最为明显;6005A-T6铝合金塑性随着应变速率的增大而逐渐增大,当应变速率达到200/s时塑性反而下降。在高速拉伸变形状态下,位错密度的增加和滑移带的增多是导致高速状态下强度及延伸率提高的主要原因;当应变速率达到200/s时由于拉伸速率过快,晶粒来不及进行大量变形是断后延伸率反而降低的主要原因。     

电磁力体积力效应对AA5052板材动态成形性的影响

为揭示板材磁脉冲成形的增塑机制,通过设计有/无驱动片作用下的磁脉冲成形工艺方案,对比体积力和面力加载条件下的AA5052板材的平面应变成形极限问题,研究了电磁力体积力效应对材料动态成形性的影响。结果表明,板坯磁脉冲动态成形时,脉冲电磁力体积力的作用形式有助于分散变形,提高板坯的整体塑性水平。随着变形速率的增加,体积力的作用效果更加显著,板坯极限变形能力提高。     

退火对5052铝合金搅拌摩擦加工性能的影响

为了寻求低能耗的铝合金强化工艺,分别对5052-H34和5052-O铝合金进行了搅拌摩擦加工(FSP),利用OM、TEM和拉伸研究了搅拌摩擦加工对铝合金显微组织和拉伸性能的影响。结果表明:退火处理和搅拌摩擦加工均可降低5052-H34合金的位错密度;5052-H34铝合金经搅拌摩擦加工后伸长率提高了111.8%,但搅拌摩擦加工对5052-O铝合金的力学性能影响不明显;搅拌摩擦加工引起的动态再结晶有利于获得细小的等轴晶,从而导致晶界面积和动态应变效应增加。因此,FSP试样拉伸曲线的应力降较明显。     

汽车用6005A-T6铝合金动态力学性能研究

通过对6005A-T6铝合金进行准静态拉伸试验和动态拉伸试验,研究了应变速率对6005A-T6铝合金准静态和动态力学性能及断裂行为的影响。结果表明:6005A-T6铝合金的强度随着应变速率提高而增大,当拉伸应变速率达到200s-1时,抗拉强度、屈服强度分别较准静态拉伸提高30MPa、25MPa,其中以准静态到应变速率10 s-1的过程中,材料的抗拉强度、屈服强度上升最为明显;试样应变速率与流变应力的关系符合Johnson-Cook本构模型,其拟合得到的本构方程为σ=(220.56+298.85ε~(0.506))(1+0.0209ln■)。6005A-T6铝合金塑性随着应变速率的增大而逐渐增大,当应变速率达到200s-1时塑性反而下降。在高速拉伸变形状态下,位错密度的增加和滑移带的增多是导致高速状态下强度及延伸率提高的主要原因;当应变速率达到200s-1时由于拉伸速率过快,晶粒来不及进行大量变形是断后延伸率反而降低的主要原因。     

基于冲击屈曲能量准则的磁脉冲缩径塑性失稳判据

磁脉冲成形是利用磁压力对金属坯料进行塑性加工的高能、高速率加工方法,与常规成形工艺相比有诸多优点,为高强度、难成形材料开辟了新的成形加工途径。而瞬时近似正弦衰减的脉冲磁压力和不均匀起皱变形,使圆管磁脉冲缩径压缩失稳判据研究非常棘手。文章选择王仁提出的冲击屈曲能量准则进行此塑性屈曲判据研究。首先,应用该能量准则确定冲击载荷作用下圆管产生塑性屈曲的临界冲击能量;然后,通过使临界冲击能量与磁压力脉冲第一波对工件变形和运动做功相等,建立圆管产生径向塑性压缩失稳的临界判据。研究表明,产生径向屈曲塑性变形的临界放电电压随径厚比增加而提高,其试验值与判据确定的理论值吻合较好。应用该判据能够预测使圆管产生失稳屈曲的临界放电电压(能量),使均匀径向塑性变形的定量分析成为可能。     

铝合金筒形件磁脉冲辅助冲压成形试验研究

针对铝合金筒形件传统拉深成形中由于成形性差容易出现拉裂问题,采用拉深预成形和磁脉冲辅助成形相结合的方法对5052-O铝合金板材进行筒形件成形性试验研究,探索磁脉冲辅助冲压成形工艺提高材料成形性的可能性。并研究应用磁脉冲成形减小预成形筒形件圆角半径的工艺可行性。结果表明:与普通冲压相比,磁脉冲辅助冲压成形能够提高材料的成形性,且提高放电电压和增加放电次数能增强圆角的再变形能力,圆角变形更加均匀。普通拉深筒形件减薄最严重部位出现在筒壁和圆角相接处,而磁脉冲辅助冲压成形筒形件有两个减薄严重部位:侧壁与圆角相接处和筒底与圆角相接处。     

超细晶Al-Cu合金的力学性能与点腐蚀行为

对2A80铝合金进行多向锻造加时效处理。研究具有高强度和塑性的超细晶Al-Cu合金的显微组织和点腐蚀行为之间的关系。结果表明,多向锻造加时效处理使2A80铝合金的组织显著细化且晶粒中弥散分布着超细的第二相微粒,位错变形和抵抗位错滑移变形的能力得到增强,从而使试样的强度和塑性得到提高。第二相微粒的析出有助于增强抗点腐蚀性能。     

Research on formability of 5052 aluminum alloy sheet in a quasi-static–dynamic tensile process

In order to establish the efficacy of electromagnetically assisted sheet metal stamping (EMAS), the formability of 5052 aluminum alloy sheet in a quasi-static–dynamic tensile process is experimentally investigated using a combined quasi-static tension and the pulsed electromagnetic forming (EMF) method. Data on the formability of aluminum alloy 5052-O employing this combined loading method is compared with data for traditional quasi-static tensile tests. Results show that the formability of aluminum alloy sheet undergoing a quasi-static–dynamic tensile process is dramatically increased beyond that exhibited in quasi-static tensile tests, and a little higher than or at least similar with that obtained in the fully dynamic EMF process. The forming limits of aluminum samples with both low and high pre-strain levels are almost similar in quasi-static–dynamic tensile process, which makes it possible stretching the sheet to a higher quasi-static pre-strain level without weakening its total quasi-static–dynamic formability. This would enable the use of a quasi-static pre-form fairly close to the quasi-static material limits for design of an EMAS process in manufacturing large aluminum alloy shell parts.     

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.     

Experimental Observations of 5A02 Aluminum Alloy in Electromagnetically Assisted Tube Hydroforming

To establish the efficiency of electromagnetically assisted tube hydroforming, a typical experimental test for hydroforming, i.e., hydrobulging, was carried out on a 5A02 tube blank by using a combined quasi-static axial feeding and pulsed electromagnetic hydrobulging method. Data on the formability of an aluminum alloy 5A02 tube employing this combined loading method is compared with data for traditional quasi-static tests. The results show that the formability of aluminum alloy undergoing a quasi-static–dynamic process is dramatically increased beyond that exhibited in quasi-static or fully dynamic tests. The ultimate expansion ratio of an aluminum alloy tube undergoing a pulsed electromagnetic hydrobulging process is greatly increased beyond that exhibited in quasi-static hydrobulging tests. Both the expansion ratio and the effective strain exhibited in electromagnetically assisted tube hydroforming tests are about four and two times of that in quasi-static and fully dynamic hydrobulging tests, respectively. The forming limits of aluminum samples with both low and high prestrain levels are almost similar in the electromagnetically assisted tube hydroforming process, which makes it possible to stretch the aluminum alloy to a higher quasi-static prestrain level without weakening its total quasi-static–dynamic formability.     

Improvement of the drawability based on the surface friction stir process of AA5052-H32 automotive sheets

Based on the imperative social demand for lighter vehicles, lightweight materials such as aluminum alloys are expected to replace conventional steels in many automotive applications. In automotive parts manufacturing, most of the components produced in conventional stamping operations are geometrically complex as the blanks are subjected to both stretching and drawing deformations. However, aluminum alloys have intrinsic drawbacks, such as the inferior formability of these materials, although the effects of the weight reduction in terms of performance are highly promising. In an effort to improve the formability of aluminum alloy sheets, the surface friction stir process is proposed in this study. This process locally modifies the surface of automotive aluminum alloy sheets via stirring and advancing on the surface of the sheet, similar to the Friction Stir Welding (FSW) process that utilizes a probe without a pin. When the surface of the sheet is modified locally by stirring, dynamic recrystallization due to the severe shear deformation along with heat resulting from the friction occur due to changes in the micro-structure and mechanical properties in the stirred zone, while the dislocation density and grain size refinement are curtailed. In this work, the drawability performance of AA5052-H32 sheets (thickness 1.5 mm) that were welded using the surface friction stir process was experimentally and numerically investigated in cylindrical cup drawing tests. When applied to AA5052-H32 automotive sheets, the surface friction stir process improved the drawability of the entire aluminum alloy sheet. For numerical simulations, the non-quadratic anisotropic yield function Yld2000-2d was employed along with isotropic hardening, while the formability was evaluated by utilizing theoretical forming limit diagrams (FLD) based on Hill's bifurcation and M-K theories.     

脉冲磁场强度对GH99镍基合金力学和微动磨损性能的影响

为研究不同磁场强度对镍基合金力学性能和耐磨性能的影响规律,在脉冲强磁场设备上对GH99镍基合金试样进行脉冲磁处理。通过观察显微结构,分析了GH99镍基合金的磨损机理和强化机制。结果表明：外加脉冲磁场可改善材料位错分布,减小试样表面残余应力的分散性;在磁场强度为10 T时残余压应力达到最大值（-223.45 MPa）,且此时材料拉伸断口的特征主要表现为韧性断裂,脉冲磁场处理合金产生亚结构位错胞有助于发挥细晶强化作用;在0~15 T范围内,随磁场强度增大,材料表面显微硬度和耐磨性能呈现先增强后减弱的规律,在脉冲磁场作用下合金材料内部的位错发生增殖致使位错密度增大,产生类似加工硬化现象,但磁场强度过大会导致位错塞积从而造成晶胞点阵畸变严重,出现材料性能恶化。     

Effects of process parameters on numerical control bending process for large diameter thin-walled aluminum alloy tubes

Numerical control(NC) bending experiments with different process parameters were carried out for 5052O aluminum alloy tubes with outer diameter of 70 mm, wall thickness of 1.5 mm, and centerline bending radius of 105 mm. And the effects of process parameters on tube wall thinning and cross section distortion were investigated. Meanwhile, acceptable bending of the 5052O aluminum tubes was accomplished based on the above experiments. The results show that the effects of process parameters on bending process for large diameter thin-walled aluminum alloy tubes are similar to those for small diameter thin-walled tubes, but the forming quality of the large diameter thin-walled aluminum alloy tubes is much more sensitive to the process parameters and thus it is more difficult to form.     

Microstructural Evolutions of AA7055 Aluminum Alloy Under Dynamic and Quasi-static Compressions

The microstructural evolution of AA7055 aluminum alloy under dynamic impact loading with the strain rate of 1.3 × 10^4 s^-1 controlled by a split Hopkinson pressure bar was investigated, and compared with that under quasi-static mechanical loading in compression with strain rate of 1.0 × 10^-3 s^-1. The quasi-static-compressed sample exhibited equiaxed dislocation cells, which were different from the elongated and incomplete dislocation cells for the alloy undergoing dynamic compression. The high strain-rate compression also induced the formation of localized shear bands in which the recrystallizations characterized as fine equiaxed grains were observed. The microstructural evolutions under both quasi-static and dynamic compressions are rationalized in terms of the dislocation cell model combined with the dislocation kinetics, in addition to the adiabatic temperature rise in shear bands at high strain rate.     

2024-T4铝合金搅拌摩擦焊搅拌区位错及组织性能

采用金相、显微硬度及透射分析方法对2024铝合金搅拌摩擦焊接头搅拌微区的组织性能和位错分布特征进行试验分析,基于位错分析深入了解各微区位错形成与接头组织结构与硬度变化之间的关系。研究表明,WNZ位错主要分布在晶粒内部,大部分是以位错缠结的形式存在,并伴随有大量的沉淀强化析出相Cu_2Mg;而在TMAZ区,大量的位错是以位错塞积的形式存在于晶界或晶粒内部;HAZ区域的位错多以位错塞积的形式存在于晶界附近,并伴随一些典型的Al Cu_3析出相。WNZ和TMAZ区中并未随晶粒细化而造成位错数量和类型的减少,这与FSW特殊的动态回复和动态再结晶过程有关,此外位错分布特征与接头微区硬度分布特征基本吻合。     

热处理参数对5052铝合金板材力学性能的影响

以5052-H32铝合金冷轧板为研究对象,借助于单向拉伸试验、硬度测试对不同热处理参数条件下的力学性能进行研究,分析不同热处理工艺参数对5052铝合金力学性能参数的影响机制,结果表明:对5052-H32铝合金进行再次热处理时,可以显著地降低其变形抗力,提高塑性变形能力;在热处理过程中的加热温度起主要作用,在合理的热处理温度和保温时间条件下,冷却方式对5052铝合金的强度、硬度的影响不大。在此基础上,借助Johnson-Cook模型建立了再次热处理后5052铝合金板材的本构模型。