超声振动对7055铝合金组织及力学性能的影响

为改善7055铝合金的凝固组织,在合金凝固过程中施加超声振动.采用光学显微镜观察和室温压缩试验等手段研究了超声振动对7055铝合金凝固组织及力学性能的影响,并对超声细化机理进行了探讨.结果表明:未施加超声时,7055合金中形成粗大枝晶组织;施加超声振动后,可获得细小均匀的非枝晶组织.由于超声振动的空化和声流效应,凝固前对熔体施加超声,产生大量微泡核心,这些微泡核心保留下来而成为形核核心,使凝固组织得到细化;凝固过程中导入超声,枝晶出现断裂,导致了细小均匀组织的形成.此外,超声振动的7055合金铸锭显示了较好的室温压缩性能,压缩屈服强度、抗压强度及其对应的工程应变分别为165、729.2 MPa和34.8%.     

强磁场下Cu-50%Ag合金定向凝固过程中的组织及固液界面形貌演变

目的 研究强磁场下Cu-50%(质量分数)Ag合金定向凝固过程中的组织演变、固液界面形貌变化及溶质迁移行为,分析强磁场对金属凝固过程的作用机制,为强磁场下的金属材料制备提供理论借鉴和指导。方法 在不同的凝固速率与磁场条件下进行定向凝固和淬火实验,对合金的定向凝固组织、糊状区与固液界面形貌以及溶质分布行为进行考察。结果 强磁场破坏了凝固组织的定向生长,使凝固组织转变为枝晶与等轴晶共存的形貌;强磁场诱发了熔体对流,减少了糊状区中溶质的含量;强磁场改变了固液界面处的溶质分布和固液界面形貌,破坏了固液界面的稳定性。结论 强磁场通过洛伦兹力和热电磁力的共同作用,诱发了糊状区内液相的纵向环流,改变了固液界面及糊状区中的组织形貌与元素分布。     

液固相变中的界面形态选择

在凝固过程中，液固界面形态经历了从平面向胞状进而树枝晶，然后又向针状胞直至绝对稳定平界面的演泽过程。本文以理论分析与实验结果为基础，讨论了液固 界面稳定性与界面形态的关系和生长速率的影响。阐明亚快凝固段生长形成的超细柱晶组织特性及其应用前景。     

温度梯度对Ti-45Al合金定向凝固组织演化影响的数值模拟

采用基于溶质扩散控制模型结合CA方法对Ti-45Al合金定向凝固过程中温度梯度对显微组织演化影响进行了数值模拟.模拟结果表明,随着温度梯度增加,凝固形态经历了树枝晶→胞/枝混合结构→粗胞晶→细胞晶→超细胞晶的转变,而一次胞/枝晶臂间距随着凝固组织变化不断减小.此外,温度梯度很高时,固液界面处溶质富集程度很小,界面处出现溶质截流.     

凝固速度对奥氏体不锈钢定向凝固组织及其固液界面稳定性的影响

本文研究了凝固速率对1Cr18Ni9Ti不锈钢定向凝固组织及其固液界面稳定性转变规律的影响.结果表明,在某特一定的温度梯度下,随着凝固速度的增加,定向凝固的固液界面由平面转变为胞状晶,再转变为树枝晶.研究发现,随着凝固速率的增大,定向凝固组织枝晶形貌逐渐细化,枝晶间距减小.     

镁合金铸造成形过程宏观/微观模拟

通过研究镁合金压铸过程中界面热,采用热传导反算法确定压铸过程的界面换热系数,研究镁合金压铸过程中工艺参数及凝固过程对铸件界面换热系数的影响规律,建立镁合金压铸过程界面换热边界条件的处理模型,以实现镁合金压铸过程中凝固过程的准确预测。通过实验研究镁合金压铸过程中凝固组织,建立了镁合金压铸过程中形核模型。采用CA方法,建立了镁合金枝晶生长模型,以实现镁合金凝固组织的预测。采用相场方法研究了镁合金枝晶生长形貌。     

轨道参数对轮轨耦合系统固有频率的影响

轮轨系统固有振动特性对车轮失圆和钢轨波磨的形成和发展具有重要影响。建立普通短轨枕整体道床轨道有限元模型和簧下质量-轨道耦合系统有限元模型,分析扣件刚度、地基刚度、簧下质量及轨枕间距对轨道和耦合系统固有频率的影响。结果表明：轨道一阶垂向弯曲频率随扣件刚度的增大而增大,地基刚度对轨道一阶垂向弯曲频率的影响较小;耦合系统一阶垂向弯曲频率（P2共振频率）随扣件刚度的增大而增大,随簧下质量的增大而减小;P2共振频率随地基刚度的增大而增大,当地基刚度大于300 MPa/m,地基刚度的变化对P2共振频率影响较小;轨道一阶垂向弯曲频率和P2共振频率随轨枕间距的增大而减小;轨枕间距随机变化可降低Pinned-Pinned共振响应峰值。通过现场力锤敲击与车辆轨道振动测试结果对模型进行验证,仿真结果与现场测试结果基本一致。     

偏晶合金凝固过程及凝固组织控制方法研究进展

偏晶合金的应用十分广泛,其中许多具有重要的工业应用背景,但该类合金凝固时首先发生液-液相变,极易形成偏析严重乃至两相分离的凝固组织,因此其凝固过程研究与工业制备技术研发受到了严重限制。近年来,偏晶合金凝固理论受到材料科学领域的高度重视,为了深入研究偏晶合金凝固组织形成过程及演变规律,材料科学工作者分别在空间微重力条件下和地面重力场内对该类合金开展了大量凝固实验,探索了偏晶合金的常规凝固行为、快速凝固行为、定向凝固行为和连续凝固行为等,考察了微重力场、电场、磁场、离心力场、超声场等外场以及微合金化元素对偏晶合金凝固过程的影响,并结合实验对偏晶合金凝固组织形成过程开展了深入的建模与模拟研究,阐明了偏晶合金凝固机理及关键影响因素。综述了有关偏晶合金凝固研究的进展,介绍了本研究团队相关研究工作,并展望了未来偏晶合金凝固研究的方向。     

籽晶法生长高温合金单晶凝固界面研究

研究了籽晶法生长单晶高温合金中固液界面的演变规律。实验结果表明：在籽晶法生长高温合金单晶的起始过程中,凝固系统经非稳态自组织过程达到稳态生长,凝固界面经历了由平界面—胞状界面—枝晶界面的转化过程,胞枝晶间距根据凝固参数及初始形态自行调整,其调整模式主要有竞争淘汰、尖端开裂和高次分枝三种。在生长起始端存在着籽晶与单晶的融合区,融合区的成分取决于籽晶成分和单晶成分。     

Ti-34Al基复合材料颗粒相组成及分布

采用XD法制备Ti-34Al基复合材料，XRD、OM和EPM的分析结果表明：复合材料的基体由Ti3Al和TiAl双相组成，增哟相由单相TiC和具有Ti3AlC包覆层的TiC两种结构的颗粒组成，单相TiC来自于凝固过程中的共晶反应，而具有Ti3AlC包覆层的TiC则是由于不完成包晶反应的结晶，单相TiC颗料沿晶界均匀分布的特征表明在较快的凝固条件下，初生相β-Ti以树枝晶方式生长，树枝晶凝固过程中颗粒枝晶凝固界面排斥在枝晶间。固液界面结构，冷却速度和颗粒大小影响颗粒与固液界面的相互作用。     

Application of vibrational parameters to locate the solid/liquid interface during solidification and melting of metals

A method to locate the solid/liquid interface with vibrational parameters during solidification is proposed for the first time. The sufficient difference in resistance to shear stresses between liquid and solid phases of metals and alloys permits the application of vibrational parameters to locate the interface in real time and in a situation during solidification. Based on the solidification theory, continuum mechanics, vibrational modal analysis and sensitivity analysis, the mechanical model and the dynamic equations of a typical Bridgman solidifying system have been established, the sensitivity of eigenvalues of the Bridgman system to the location of the solid/liquid interface has been derived, and the formulae concerned calculated. The experimental results are in good agreement with the computed ones.     

机械振动对SHS 陶瓷内衬复合管显微结构和力学性能的影响

通过在重力分离SHS 法制备陶瓷内衬复合管过程中施以不同振幅与振频的单自由度上下往复式机械振动, 研究了机械振动对SHS 铝热燃烧、陶瓷凝固及复合管组织性能的影响。研究发现, 机械振动并相应提高振频可以提高燃烧温度、燃烧速率和反应转化率, 促进Al₂O₃-Fe 液相重力分离和陶瓷致密过程, 并使陶瓷层的凝固组织发生改变; 性能测试结果表明, 机械振动并相应提高振频可以提高复合管的各项力学性能指标和内衬陶瓷层的表面质量。     

纵-弯复合系统的振动特性研究

本文对一种常用的纵－弯复合振动系统进行了研究，该系统由一个弯曲振动工具头及一个纵振动驱动系统组成，文中对复合振动系统的振动特性进行了理论及实验研究，推出了系统的共振及反共振频率方程，还导出了复合振动系统的机械输入阻抗，并对系统的共振频率随长度的变化规律进行了探讨，实验表明，复合振动系统共振频率的测试值与计算值符合较好，系统的共振频率随振动系统几何尺寸的变化而变化，并且，弯曲振动系统的尺寸变化对频率     

机械振动对SHS陶瓷内衬复合管显微结构和力学性能的影响

通过在重力分离SHS法制备陶瓷内衬复合管过程中施以不同振幅与振频的单自由度上下往复式机械振动 ,研究了机械振动对SHS铝热燃烧、陶瓷凝固及复合管组织性能的影响 .研究发现 ,机械振动并相应提高振频可以提高燃烧温度、燃烧速率和反应转化率 ,促进Al2 O3 Fe液相重力分离和陶瓷致密过程 ,并使陶瓷层的凝固组织发生改变 ;性能测试结果表明机械振动并相应提高振频可以提高复合管的各项力学性能指标和内衬陶瓷层的表面质量 .     

激光快速熔凝Fe基、Ni基合金的细化形态

采用脉冲激光对纯 Fe、纯 Ni、20钢和 Ni 基合金进行了快速熔凝处理。结果表明,在纯 Fe和纯 Ni 熔凝区内,生成了一种外延生长的晶粒组织;在20钢和 Ni 基合金熔凝区内,生成了外延生长晶粒和极细的枝晶组织。在金属与合金的熔区中均形成了精细的的亚晶粒。激光快速熔凝大大地细化了校晶和亚晶,却没有得到明显细化的晶粒。     

Solidification parameters of the solid-liquid interface in crystal growth in response to vibration

The influence of vibration on the solidification parameters of the solid-liquid interface was investigated by applying a stable longitudinal sinusoidal vibration with different orders of resonant frequencies to the solidification system during crystal growth. Experimental results including the temperature profiles, temperature gradients of the liquid in front of the solid-liquid interface and the growth rates of the solid-liquid interface, are presented and analysed.     

Vibration analysis of functionally graded carbon nanotube-reinforced composite shell structures

This article deals with the vibration analysis of functionally graded carbon nanotube-reinforced composite (FG-CNTRC) shell structures. The material properties of an FG-CNTRC shell are graded smoothly through the thickness direction of the shell according to uniform distribution and some other functionally graded (FG) distributions (such as FG-X, FG-V, FG-O and FG-\({\Lambda}\)) of the volume fraction of the carbon nanotube (CNT), and the effective material properties are estimated by employing the extended rule of mixture. An eight-noded shell element considering transverse shear effect according to Mindlin’s hypothesis has been employed for the finite element modelling and analysis of the composite shell structures. The formulation of the shell midsurface in an arbitrary curvilinear coordinate system based on the tensorial notation is also presented. The Rayleigh damping model has been implemented in order to study the effects of carbon nanotubes (CNTs) on the damping capacity of such shell structures. Different types of shell panels have been analyzed in order to study the impulse and frequency responses. The influences of CNT volume fraction, CNT distribution, geometry of the shell and material distributions on the dynamic behavior of FG-CNTRC shell structures have also been presented and discussed. Various types of FG-CNTRC shell structures (such as spherical, ellipsoidal, doubly curved and cylindrical) have been analyzed and discussed in order to compare studies in terms of settling time, first resonant frequency and absolute amplitude corresponding to first resonant frequency based on the impulse and frequency responses, and the effects of CNTs on vibration responses of such shell structures are also presented. The results show that the CNT distribution and volume fraction of CNT have a significant effect on vibration and damping characteristics of the structure.     

Effect of solidification conditions on deformation behaviour of semi-solid Sn-Pb alloys

The compressive behaviour of Sn-Pb alloys is studied with materials of conventionally dendritic structure and non-dendritic (rheocast) structure obtained through mechanical stirring during solidification. The alloys are found to deform similarly in the fully solid state but their behaviour becomes very different in the semi-solid range depending on the solidification mode. The holding time in the semi-solid state before compression also affects the mechanical properties: the influence of these two parameters is discussed in terms of the initial structure of the alloy and its evolution. Advantages of using semi-solid materials in metal forming processes are also presented.     

SCN-Cam共晶合金的定向凝固研究

采用实时观测装置和定向凝固系统研究了SCN-Cam(Succinonitrile-wt%Camphor,wt%为质量分数)模型合金的凝固过程.实验结果表明,SCN-23.6%Cam共晶合金在常规条件下形成规则的共晶组织,共晶间距随界面推移速度的增大而减小;加入超声振动时,共晶合金生长出初生相;SCN-21%Cam亚共晶...     

Evaluation of microstructure of A356 aluminum alloy casting prepared under vibratory conditions during the solidification

The objective of this investigation was to evaluate the effect of vibrations (during solidification) on the metallurgical properties of A356 aluminum casting. Mechanical vibrations were applied to A356 aluminum alloy through set up. A356 melt has been subjected to mechanical vibration with the frequency range from 0 to 400 Hz with constant amplitude 5 µm. Grain refinement was obtained through mold vibration. Metallurgical properties were examined through optical microstructure, tensile fracture scanning electron microscope (SEM) and SEM image of test specimens prepared under different conditions of solidification. Results indicate that mold vibration effectively modified the microstructure of A356 casting and it has uniform and smaller grain size with fibrous silicon particle than nonvibrated casting. Grain refinement results increase in mechanical properties with increase in frequency of vibration of mold during the solidification. SEM micrograph of tensile fracture surface was carried out to study the influence of microstructure on fracture mode. SEM image of tensile fractured surface shows transgranular cleavage facets due to fracture of primary silicon particles. Fractures are brittle in nature so observation indicates low ductility and brittle fracture.