【作者】 刘岩；

【导师】 王忠金；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2011， 博士

【摘要】 随着高强度、低塑性轻质合金在现代工业中的应用越来越广泛,如何提这类材料的成形性成为板材加工领域的研究热点之一。粘性介质压力成形(Viscous Pressure Forming, VPF)是一种板材软模成形新工艺,采用高粘度、可流动的半固态软模(称为粘性介质)作为传力介质,能够提高成形件的形状尺寸精度、表面质量和壁厚分布均匀性,在金属精密塑性成形中具有良好的应用前景。深入研究粘性介质对板材变形的影响规律,对于粘性介质性能的合理利用和成形方案的优化具有重要的指导意义。本文通过粘性介质压力胀形(Viscous Pressure Bulge, VPB)过程的光测试验分析、理论分析和有限元分析手段,系统地研究了粘性介质对板材稳定变形和失稳发展过程的影响规律,得到粘性介质切向粘着应力对试件材料流动、形状、应力应变分布以及变形薄弱区失稳发展过程的影响规律,解释了粘性介质有助于提高VPF板材变形均匀性和成形极限的原因。根据本文的工作不仅可以加深对粘性介质与板材相互作用过程的理解,而且可以扩展粘性介质压力成形的应用范围,对于实际生产具有重要的指导意义。首先,设计了粘着拉伸试验和光塑性测量方法相结合的试验方案,分析切向粘着应力作用下板材单向拉伸变形行为,并与传统单向拉伸试验进行比较分析。获得了聚碳酸酯试件的等差线条纹图,根据条纹分布对粘着拉伸试件平面内的应变分布进行全场定性分析和定量计算,并且分析了粘着拉伸试件应力状态,得到应力应变关系。结果表明切向粘着应力可以使试件发生较大的塑性变形,粘着拉伸试件的变形符合传统单向拉伸试件变形规律。面分布的切向粘着应力可提高试件应力和应变分布的连续性和稳定性,有利于延缓应变集中的发生。利用投影散斑数字相关(Projected Speckle Digital Correlation, PSDC)方法的连续测量特征,设计粘性介质压力胀形过程试件三维形状的投影散斑测量装置,分析不同粘度粘性介质和不同加载速度条件下试件形状的变化规律,确定切向粘着应力对胀形试件应力和应变分布的影响。相对于液压胀形试件,粘性介质胀形试件半径较大处趋于扁球状,试件顶端区域趋于长椭球状。切向粘着应力随介质粘度和加载速度增加而增大,促进半径较大区域的变形,阻碍试件中心的变形,降低应变梯度,使应变分布趋于均匀。试件顶点的流动应力随介质粘度减小和加载速度增加而增大。利用电子散斑干涉方法(Electronic Speckle Pattern Interferometry, ESPI)连续测量了胀形过程的应变速率,根据应变速率分布特点分析试件的应变集中现象,研究了分散性失稳和集中性失稳发展过程。通过散斑条纹图实时观察了缺陷发展引起分散性失稳、凹槽形成和扩展、引起集中性失稳直至试件破裂的失稳演化全过程,确定了分散性失稳和集中性失稳极限应变,研究粘性介质对胀形试件失稳发展过程的影响。试验结果表明粘性介质提高胀形试件成形极限的主要原因是粘性介质能明显延缓板材缺陷出现和发展,阻碍失稳区扩展,提高分散性失稳极限应变,从而提高了集中性失稳极限应变。其次,在形状变化与失稳过程试验研究结果的基础上,建立了有限元模型对粘性介质胀形失稳过程进行分析。采用带有损伤效应的塑性模型作为板材材料模型,通过有限元方法得到胀形试件缺陷出现和发展、分散性失稳、集中性失稳和破裂等各失稳阶段失稳区内的变形发展过程,失稳区位置、形状和发展过程与试验结果比较一致。有限元分析模型可为粘性介质压力成形试件的成形极限预测和成形方案的优化提供指导。最后,建立了加入切向粘着应力的粘性介质胀形理论分析模型,研究切向粘着应力对胀形试件应力状态和形状、应变分布等力学行为的影响规律。理论分析结果与试验结果相互验证,解释了粘性介质使试件应变分布趋于均匀的试验现象,并预测了粘性介质对试件变形危险点位置的影响规律,为切向粘着应力的合理应用提供了理论指导。更多还原

【Abstract】 The forming application of high strength and lightweight alloys are wider and wider in modern industry. The way to promote the quality of these materials becomes one of the research focuses. Viscous Pressure Forming (VPF) is one of the soft-die forming processes for sheet metal proposed in recent years. The high viscous, flowable and semi-solid soft-die (so-called viscous medium) can promote the dimensional accuracy, surface quality and thickness uniformity of parts. VPF has a good application prospect in precision plastic forming. Research on the influence of viscous medium on sheet metal formability possesses guiding significance for reasonable using of viscous medium and optimization of processing scheme. In this dissertation, optical measuring methods for experimental research, theoretical analysis and Finite Element Analysis (FEA) are combined to analyze the influence of viscous medium on both stable and instable deformation behavior of specimens in Viscous Pressure Bulging (VPB). The influence of tangential adhesive stress on material flow, shape, stress and strain distributions and evolution of weak region of the specimen is obtained. The way how viscous medium promotes uniformity and forming limit of sheet metals is explained. The obtained results would help deeper understanding the interaction of viscous medium and sheet blanks and possess guiding significance for VPF applications.Firstly, the tension of specimens loaded by tangential adhesive stress is researched by viscous adhesive tensile test. Photoplastic method is used for qualitative and quantitative analysis of the specimens’strain distributions. Relation of stress and strain is also obtained. Deformations of viscous adhesive tensile specimens agree well with that of uniaxial tensile specimens. The surface loaded tangential adhesive stress can promote the continuity and stability of deformation. Strain localization of specimen is postponed as a result.Projection speckle digital correlation method is applied in the 3D shape measurement of viscous pressure bulge specimen. The influence of viscous medium and loading speed on specimens’shape, strain and stress distributions during the whole deformation process is analyzed. The tangential adhesive stress increases with viscosity of viscous medium and loading speed. The deformation of VPB specimen is promoted by viscous medium at positions away from the center of the specimen. However, viscus medium would restrain the deformation at the apex of the dome. The strain gradient along the specimen’s radius is decreased. Curvature radius on the apex of dome is measured by PSCM to calculate flow stress. Flow stress increases with lower viscous medium viscosity or larger loading speed.Electronic speckle pattern interferometry is applied in the real-timely strain rate measurement of VPB. Strain localization and diffuse necking and localized necking of VPB specimen are analyzed though strain rate distribution. The evolution of weaken region, including defect emergence, diffuse necking, groove expanding and the onset of localized necking are observed directly and real-timely. The effects of viscous medium properties on the deformation of specimen from stable stage to failure process are discussed. Critical strains on diffuse necking and localized necking are determined with higher accuracy. Experimental results show that visocus medium can distinctly delays the evolution of weaken region and the onset of diffuse necking. The critical strain of diffuse necking is increased. So the forming limit is improved.Secondly, the effect of viscous medium on the instable deformation of VPB specimen is analyzed by finite element method (FEM). Plasticity with damage modle is used as material model to simulate the deformation of weaken region and predict necking. Each instable states including emergence of flaw, diffuse necking, localized necking and broken are simulated successfully by the proposed FEM model. Results such as weaken region’s location, shape, and evolution agree well with the results measured by ESPI. This model indicates a new way for quality prediction and forming method optimization of VPF.A theoretical model of VPB is proposed to consider the effect of tangential adhesive stress on specimens’shape, stress and strain distribution. Variety of specimen’s shape and strain distribution are explained. The location of largest strain is predicted. The results provide theoretical foundation for usage of vicous medium.更多还原