金属凝固：基于同步辐射原位成像技术的应用研究

全面深入研究金属凝固过程中组织和缺陷的演变规律是优化材料成分与制备工艺,提高材料综合性能的关键。作为一种通用型、高精尖、无损定量研究方法,同步辐射X射线因其具有高通量、高时空分辨率等特点,在金属凝固过程的研究中得到了广泛应用。综述了国内外基于同步辐射成像技术开展金属凝固过程的研究进展,涉及金属凝固原位研究用装置建设与技术开发,以及针对各类金属材料如铝合金、镁合金、高温合金等的凝固研究进展,重点关注凝固过程的组织演变、外加物理场(如电场、磁场、超声场)作用下的金属凝固行为,并阐述了同步辐射原位成像技术在凝固过程数值模型研究中起到的重要作用。最后,对未来的研究方向进行了展望。     

金属材料的组织演化机理:基于同步辐射光源的原位研究进展

金属材料是一类最重要的承力结构材料,其通常在复杂的恶劣环境下服役.为获得优良的材料性能,金属材料的制造工艺和流程往往较为复杂,并且包含众多工序,深入研究并掌握制备过程中和复杂服役环境下的金属材料组织与缺陷的演化机理是提高材料性能和确保材料安全服役的重要基础.同步辐射光源具备的高时空分辨率为研究金属材料组织结构微观动力学...     

基于原位观察的Cu-Ni-Si合金凝固过程及机理研究

利用共聚焦激光扫描显微镜(CLSM)原位观察薄带连铸Cu-3.2Ni-0.75Si合金在不同冷却速率下的凝固过程,定量分析了结晶温度和凝固组织二次枝晶间距依赖于冷却速率的函数关系.结果表明:冷却速率的提高可以显著降低合金的结晶温度,减小合金凝固组织的二次枝晶间距,当冷却速率由0.1?℃/s提高至100?℃/s时,结晶温度由1096?℃降低至890?℃,二次枝晶间距由77.5?μm减小至23.5?μm,结晶温度和二次枝晶间距依赖于冷却速率的函数关系式分别为TL→S=194×(-v/14)+895和λ2=60×v-0.11.薄带连铸的亚快速凝固特点可以显著细化合金凝固组织,抑制Ni、Si元素的显微偏析,其原因在于冷却速率的提高加大了合金熔体结晶时的初始过冷度.     

定向凝固Ni—20Al—30Fe合金变形的TEM原位观察

通过对定向凝固Ｎｉ－２０Ａｌ－３０Ｆｅ合金进行的ＴＥＭ原位拉伸观察得出，塑性γ相向脆性β相的形变形传递是促进Ｂ２结构β相进行塑性变形的内在机制。首次在该合金中观察到变形的相界弯折，相内的位错来自于这些相界弯折。两相间的变形传递并不限定在Ｋ－Ｓ关系中的平行晶面进行，位错同位界的作用过程，存在相界对位错吸收以及发射的现象。     

镍基高温合金U720Li的熔化和凝固过程原位观察

本文采用高温激光共聚焦显微镜（HT-CLSM）和差热分析（DTA）研究了U720Li镍基高温合金的熔化和凝固行为。HT-CLSM熔化过程原位观察表明,升温至1122 ℃时在部分(γ + γ′)共晶颗粒前沿发生了初熔,但温度超过1173 ℃时该熔池才开始迅速扩大;升温至1195 ℃时枝晶干处开始出现离散的斑点状熔池,且该熔池随温度的升高缓慢扩大;升温至1235 ℃时(γ + γ′)共晶开始熔化且随温度的升高该熔池向枝晶干迅速扩展,最终在1333 ℃枝晶完全熔化。HT-CLSM凝固过程原位观察表明,降温至1315 ℃时熔体开始形核凝固,并在1180 ℃左右凝固结束;固相转化速率随温度的降低先缓慢增加,再迅速增加到最大值,然后迅速降低并在凝固后期基本为零。DTA分析表明,铸锭中γ′相开始发生溶解的温度约为1047 ℃,铸锭完全熔化的温度约为1362 ℃。将HT-CLSM原位观察结果与本文及我们前期的DTA分析结果进行对比,发现前者的测试结果较后者低30 ℃左右。     

铝合金焊缝疲劳开裂的原位同步辐射X射线成像

基于第三代高精度同步辐射X射线三维成像技术,开展表面经抛光的铝合金光纤激光-脉冲电弧复合焊接头疲劳裂纹萌生和扩展的原位可视化研究.结果表明,疲劳裂纹均优先从样品(亚或次)表面或角(亚)表面上的焊接气孔处萌生和扩展,并呈现典型的I型表面半椭圆或四分之一椭圆(或圆形)裂纹形貌.一旦裂纹萌生,裂纹在表面的扩展速率低于内部,且存在局部加速和停滞现象,萌生源的特征长度越大,则裂纹萌生和扩展寿命越短,裂纹形貌为高度非线性的空间特征.此外基于经典Newman表面半椭圆形裂纹形貌预测结果与试验基本一致.     

原位观察镍基高温合金CMSX-4缩松形成过程及机理研究

采用共聚焦激光扫描显微镜(CLSM)对镍基单晶高温合金CMSX-4的凝固行为进行实时观察.并利用扫描电镜(SEM)和能谱仪(EDS)对凝固样品的微观形貌和元素偏析特性进行研究.结果 表明,在合金凝固过程中观察到缩松和碳化物的形成.缩松具有3种形态:方形缩松,五角星形缩松和角形缩松.并讨论了3种缩松的形成机理,缩松的形成...     

原位TiB_2/Al复合材料制备及凝固过程研究

利用原位法制备了TiB2/Al复合材料。根据X-射线衍射图探讨了增强相形成机制,测定了材料凝固动态曲线并分析了影响凝固的3个主要因素,采用扫描电镜研究了材料的拉伸性能。结果表明,当B2O3与TiO2的摩尔比为1.0时,反应式为:3TiO2+10Al+3B2O3→5Al2O3+3TiB2。影响材料凝固的3个主要因素是颗粒在母相中的分布,B2O3与TiO2的摩尔比以及反应温度。材料的强化主要是TiB2颗粒的弥散强化(即奥罗万机制)和TiB2为形核核心作用所导致的晶粒细化,材料的断裂属于TiB2颗粒被拔出而形成的微孔聚集型韧性断裂。     

凝固过程研究与凝固技术的进展及其在材料加工中的应用（上）

在对凝固过程研究的对象与方法作了简要归纳之后,从凝固过程研究的定量化与凝固组织控制两方面的发展,说明了凝固过程研究的新进展及其在铸件和铸锭质量控制方面的应用。进而介绍了近年来国际上出现的几种先进的凝固技术及由此发展起来的一些新的材料加工方法。     

基于耗散结构理论的金属凝固研究进展

总结了以往金属凝固研究理论中存在的问题和不足,为进一步揭示金属凝固的本质,耗散结构理论被应用到金属凝固研究中。详细阐述了耗散结构的基本理论,进一步分析了金属凝固完全具备了耗散结构形成的一切条件;概述了目前耗散结构理论在金属凝固研究中的初步发展情况;指出了目前耗散结构理论在金属凝固研究中存在的主要问题及发展方向。     

Quantitatively comparing phase-field modeling with direct real time observation by synchrotron X-ray radiography of the initial transient during directional solidification of an Al-Cu alloy

The initial transient during directional solidification of an Al-4 wt.% Cu alloy was simulated by a quantitative phase-field model solved with the adaptive finite element method. The simulated solidification process was compared with the related analytical theory and in situ and real time observations by means of X-ray radiography at the European Synchrotron Radiation Facility. The simulated velocity of the planar interface and solute profile ahead of the solidification front in the liquid are close to the predictions of the Warren-Langer model of the initial planar solidification transient, but in fair quantitative agreement with experimental results only at early stages of planar solidification. After the accelerated flat interface lost its stability a transition to cellular solidification was initiated. The initial cell spacing predicted by the phase-field simulation agreed well with the experimental observations in the region where the cell growth direction was perpendicular to the fluid flow, whereas a discrepancy was obvious in the corners where the fluid flow was parallel to growth. An analytical relation describing the wavelength of the initial solid-liquid interface corrugations under diffusion-limited transport is screened out by comparing the phase-field simulation data with expressions based upon the Mullins-Sekerka linear stability analysis theory or derived for primary spacing. The gravity-driven natural convection in the experiment resulted in misfits between the phase-field predictions and the experimental observations in the late stage of planar solidification, onset and development of morphological instability.     

基于同步辐射和中子衍射分析的金属增材制造关键共性问题研究进展

金属增材制造技术具有“微区超常冶金”和“激冷快速凝固”的本征属性。实现动态监测和控制增材制造过程中缺陷、应力和组织的演化,是该领域国际前沿研究的难点和热点。本文从航空、航天及汽车等领域中金属增材制造的关键共性问题入手,阐述了同步辐射和中子衍射2种方法在金属增材制造中的冶金动力学及内部缺陷原位分析,液态金属超常凝固,微观组织及相变过程,内应力的形成及演化行为的研究进展。最后,阐述了研究不足,并展望了金属增材制造的未来发展。     

金属/氧化物界面形成机理原位研究

金属/氧化物界面是控制反应机理和宏观性质的关键区域.原位表征手段可以实时、连续、动态地对金属/氧化物界面形成过程进行研究,其观察和分析结果对理解界面形成过程及其应用有重大学术意义和工程价值.本文综述国内外原位表征金属/氧化物界面的研究工作,主要关注原位透射电子显微镜、原位同步辐射技术、原位中子技术等先进表征分析金属/氧...     

Elucidation of the effect of welding speed on melt flows in high-brightness and high-power laser welding of stainless steel on basis of three-dimensional X-ray transmission in situ observation

This research was performed with the objective of clarifying the effect of welding speed on melt flows during melt-run welding of SUS304 stainless steel plates with a 6-kW power laser beam on the basis of three-dimensional X-ray transmission in situ observation. As welding speed increased from 25 to 250 mm/s, three kinds of welds characterized by porosity formation and no defects or underfilling due to spatters were produced. The average and the maximum values of measured melt flow velocity were 3 and 10 times higher than the welding speed, respectively. Two kinds of circulation flows at the inlet or the tip of a keyhole were confirmed to control heat transfer in a molten pool. It was found that the circulation flows were so sensitive to the welding speed that bubbles resulting in porosity or spatters were often formed. According to X-ray observation of the formation of spatters with tungsten carbide (WC) tracers, as the melt flow rose along the keyhole wall, the velocity was accelerated from 0.24–0.54 m/s near the keyhole inlet. Consequently, the melt flows made the convex surface behind the keyhole grow higher, resulting in spattering.     

Relationship between melt flows based on three-dimensional X-ray transmission in situ observation and spatter reduction by angle of incidence and defocussing distance in high-power laser welding of stainless steel

ABSTRACT

The objective of this research is to reveal relationship between melt flows and spatter reduction by angle of incidence and defocusing distance in partial penetration welding of an SUS304 stainless steel plate using a 6-kW power laser beam. In welding speeds from 50 to 250 mm/s, underfilled weld beads with spatters were obtained at more than 150 mm/s. According to the three-dimensional X-ray transmission in situ observation of melt flows at 150 mm/s in welding speed with tungsten carbide (WC) tracers, the melt flows achieved approximately 2.3 m/s in speed and made convex molten-pool surface behind a keyhole inlet grow higher, resulting in spattering over 0.1 mm in diameter. A 2 mm inner defocusing distance or a 20° angle of advance decreased the number of spatter over 0.1 mm in diameter by half or one-third in comparison with that at focal point and 0°. The X-ray transmission images demonstrate that the appropriate defocusing distance and angle of incidence made the speed of the melt flow decrease and the melt flow behind a keyhole inlet circulate, which led to not only suppressing the convex surface but also improving the frequency that the convex surface went back to the molten pool.     

Recent Progress in Metallurgical Bonding Mechanisms at the Liquid/Solid Interface of Dissimilar Metals Investigated via in situ X-ray Imaging Technologies

The liquid/solid(L/S) interface of dissimilar metals is critical to the microstructure, mechanical strength, and structural integrity of interconnects in many important applications such as electronics, automotive, aeronautics, and astronautics, and therefore has drawn increasing research interests. To design preferential microstructure and optimize mechanical properties of the interconnects, it is crucial to understand the formation and growth mechanisms of diversified structures at the L/S interface during interconnecting. In situ synchrotron radiation or tube-generated X-ray radiography and tomography technologies make it possible to observe the evolution of the L/S interface directly and therefore have greatly propelled the research in this field. Here, we review the recent progress in understanding the L/S interface behaviors using advanced in situ X-ray imaging techniques with a particular focus on the following two issues:(1) interface behaviors in the solder joints for microelectronic packaging including the intermetallic compounds(IMCs) during refl ow, Sn dendrites, and IMCs during solidification and refl ow porosities and(2) growth characteristics and morphological transition of IMCs in the interconnect of dissimilar metals at high temperature. Furthermore, the main achievements and future research perspectives in terms of metallurgical bonding mechanisms under complex conditions with improved X-ray sources and detectors are remarked and discussed.     

超声化学原位合成（Al3Zr＋ZrB2）／A356复合材料的力学行为

利用超声化学熔体原位反应技术合成颗粒增强(Al3Zr ZrB2)/A356复合材料,通过SEM原位拉伸实验及其断裂表面研究分析复合材料的断裂行为。结果表明,复合材料的抗拉强度、屈服强度和伸长率分别达到403.61MPa、343.98MPa和8.9%,较未施加超声作用的复合材料分别提高16.09%、12.9%和32.83%;复合材料的室温拉伸断口SEM形貌表现出明显的韧窝断裂特征,为塑性断裂。裂纹的萌生机制主要有基体在滑移过程中的位错作用机制、内生Al3Zr和ZrB2颗粒脱落或破碎形成的空穴成核机制和基体缺陷诱发机制;由于内生增强颗粒微观分布上的不均匀性,当主裂纹扩展前方遇到颗粒密集区时,其扩展方向偏向颗粒贫化区,绕过颗粒密集区,并沿颗粒富集区与贫化区的界面向前扩展、延伸,形成宏观裂纹。     

Quantitative evaluation of reheat cracking susceptibility by in situ observation and measurement using laser confocal microscope*

In the post-weld heat treatment process, the reheat cracking which might occur in the weldments of low-alloy steels has been a serious problem. So, it is considered to be important to predict the possibility of occurrence of reheat cracking in these steels. It is however recognized as a time-consuming procedure to evaluate quantitatively the susceptibility to this type of cracking. In the present study, a new quantitative evaluation method of reheat cracking susceptibility by in situ observation and measurement using a laser confocal microscope has been proposed. Through this new method, the reheat cracking susceptibility of any kind of steels can be evaluated with the same standard. Moreover, because the position of the initial crack can be focused and the critical ductility to initiate the crack is measured by in situ observation, the reheat cracking susceptibility can be evaluated using only one specimen. So the newly developed method can provide efficient quantitative assessment of the reheat cracking sensitivity with high accuracy.