脉冲磁致振荡凝固均质化技术及装备

 脉冲磁致振荡(PMO)是上海大学先进凝固技术中心历经18年开发成功的凝固组织细化和均质化技术，是冶金领域近年来取得的一项原创性技术突破。目前，PMO技术与装备已成功实现商业化应用。研究和应用表明，PMO可以显著细化凝固组织并提高等轴晶占比，在连铸坯凝固组织细化和均质化方面效果显著、稳定，且对连铸工艺适应性强。简述了其技术原理、装备组成及其对连铸坯凝固均质化的作用效果。目前，适用于大断面铸坯的PMO凝固均质化技术正在研发阶段，今后该技术有望在各种连铸机型上运用。     

金属凝固过程与细晶技术

为了实现连续铸造金属材料的均质化,研究了以数值模拟得到的凝固过程函数曲线方程控制自行设计单向凝固炉中凝固过程的工艺参数选择原则;并基于以自行设计并制备的脉冲电磁振荡凝固实验装置获得的大量实验结果及对其进行的理论分析,揭示了脉冲电磁振荡细化金属凝固组织的机理,提出了脉冲磁致振荡凝固细晶化技术.     

凝固过程中的脉冲磁场和脉冲磁力研究

用数值模拟方法研究了凝固样品中的脉冲磁感应强度和电磁力的分布特性。模拟结果表明，脉冲磁场凝固条件下，脉冲磁感就强度和电磁力分布具有轴向、径向分布的不对称性和不均匀性。这种分布特性有助于了解脉冲磁场对金属凝固组织的细化作用。     

氧化物冶金与脉冲磁场参数影响钢组织的研究

 脉冲磁场处理与氧化物冶金技术是细化组织、提升材料性能的两种常用方法,将其有机结合可进一步优化钢铁材料的性能。利用自主研制的高频感应线圈加热炉与脉冲磁场发生装置将脉冲磁场非接触式地施加在钛处理低碳钢的凝固过程中,利用金相显微镜、多重分形软件与维氏硬度仪研究了不同脉冲磁场参数对凝固组织的影响。结果表明,脉冲磁场感应强度为135~190 mT、磁场作用时间为5~10 min时,试样的金相组织最细小均匀,原始奥氏体晶粒得到明显细化,原始奥氏体晶粒面积由15.79 mm2下降到1.25 mm2,试样的硬度值由118.1HV提升到165.4HV,此参数下的脉冲磁场对凝固组织的细化程度最佳。     

铝合金表面脉冲电磁场对半连续铸造晶粒的细化

采用一种新型熔体表面脉冲电磁技术对7A04铝合金半连续铸造凝固组织细化处理,分析脉冲电磁场对凝固组织及性能的影响.引入势能的观点,探讨脉冲磁能作用下的晶体形核动力学及初生晶核运动形式.结果表明,经表面脉冲电磁场处理后,凝固组织由晶粒尺寸粗大的玫瑰结构转变为细小且圆整的球状结构,铸锭心部及边部晶粒尺寸分别下降22.7%和14.2%,强度、塑性均有提高.动力学分析认为,脉冲电磁能降低体系形核所需的临界吉布斯自由能是增加形核率的重要原因,同时可导致初生α-Al运动的势能增加,促使初生α-Al颗粒优先到达稳定位置.      

低压脉冲磁场凝固硅钢的热变形组织和行为

为了细化硅钢铸锭组织，提高其加工性能，研究了低压脉冲磁场对硅钢凝固组织的影响规律，并对比了不同凝固组织的热变形行为和组织。研究结果表明，在硅钢的凝固过程中施加脉冲磁场，传统的柱状晶组织转变为等轴晶组织。随着磁场频率的增加，等轴晶比率(等轴晶面积/整体面积)先增加后降低，等轴晶晶粒尺寸先减小后增大。随着励磁电压增加，等轴晶比率增加，等轴晶晶粒尺寸不断增加，获得最佳脉冲磁场工艺参数，磁场频率为5 Hz,励磁电压为200 V。在该脉冲磁场工艺参数下，硅钢凝固组织中的等轴晶尺寸细化至无磁场作用时的1/2,等轴晶比率提高至100%。对无脉冲磁场的柱状晶组织和施加脉冲磁场(磁场频率为5 Hz,励磁电压为200 V)的等轴晶组织，进行了热压缩试验(温度为950℃,应变速率为0.01 s^-1和0.1 s^-1)和热轧试验(初轧温度1 100℃)。通过微观组织结果发现，无脉冲磁场作用的柱状晶凝固组织在热压缩过程中发生少量动态再结晶，组织中存在难以消除的变形晶粒，影响后续加工性能。脉冲磁场作用下等轴晶凝固组织在相同热压缩条件下获得细小均匀的再结晶组织，平均晶粒尺寸...     

热作模具钢H13热处理技术研究

高温均质化、超细化处理是优质H13模块生产中的关键热处理技术。高温均质化处理可基本消除钢中的共晶碳化物，显著改善成分偏析；超细化处理可使H13组织得到细化，进一步提高性能均匀性。五钢新流程生产的P级H13(SWPH13)等向性明显改善，冲击韧性、热疲劳抗力显著提高。     

镍基高温合金均质化冶炼研究进展

高温合金的冶金质量与材料的综合使用性能密切相关,均质化冶炼已经成为改善合金成分、组织和第二相分布均匀性,提高合金力学性能的重要途径.在总结前人研究的基础上,综述了镍基高温合金均质化冶炼的影响因素和改善合金均质化水平的有效措施,重点讨论了凝固偏析、元素烧损和均匀化处理对合金均质化的影响,以期为提高镍基高温合金的均质化冶炼...     

基于多重分形研究脉冲磁场周期对凝固组织的影响

为了研究脉冲磁场对凝固组织的影响,采用自发研制的脉冲磁场发生装置将脉冲磁场非接触式地施加到钛处理低碳钢的凝固过程中,利用金相显微镜和实验室设计的多重分形软件研究了不同周期的脉冲磁场对凝固组织的影响。结果表明,该钢种的凝固组织具有分形特征;随着脉冲磁场周期的延长,Δα和Δf(α)呈现为先减小后增大的规律,在脉冲磁场周期为0.5 s时,组织最为均匀。由此可以推断脉冲磁场周期对凝固组织的影响存在一个最佳值。     

DSM11合金定向凝固工艺的研究

研究了定向凝固速度对DSM11合金组织结构和性能的影响.结果表明,在获得柱状晶生长的条件下,随凝固速度提高,枝晶间距细化,合金中的γ'相、碳化物和共晶组织细而均匀;当凝固速度为11mm/min时,出现严重的枝晶分叉和不连续现象.凝固速度变化对合金高温持久性能影响显著.凝固速度为7 mm/min时,合金持久性能最高,对应枝晶间距和共晶含量的低谷,伴随着细小枝晶和均匀的显微组织.     

PMO凝固均质化技术在20CrMnTi齿轮钢上的应用

 为了解决20CrMnTi齿轮钢在生产过程中铸坯柱状晶发达及铸坯元素分布不均匀等缺陷,运用上海大学研发并已在连铸生产中取得理想效果的脉冲磁致振荡(pulsed magneto oscillation,简称PMO)凝固均质化技术对连铸20CrMnTi齿轮钢矩形坯进行均质化处理。结果表明,经PMO处理的20CrMnTi齿轮钢铸坯中心等轴晶率增加1倍左右,中心碳偏析指数从1.22降为1.04。金属原位分析仪元素面扫描检测发现,PMO处理铸坯的各元素分布更加均匀,连铸坯宏观偏析得到有效控制。该技术为今后提高连铸坯内部质量提供了一种新方法。     

Enhancement of coupled growth of off-eutectic alloys by “stop-and-go” technique

The beneficial effect of solutal (gravity induced) or other convective segregation on promoting plane front growth in directionally solidified off-eutectic alloys has been investigated. Specifically, it has been found that halting directional solidification for periods of the order an hour frequently results in plane front growth after resumption of solidification in alloys which originally grow dendritically. This is attributed to liquid homogenization during the stop period. This homogenization produces a liquid with composition within the coupled growth region. The process has been analyzed and conditions under which this process is feasible are related to the alloy composition and phase diagram characteristics as well as the growth parameters, thermal gradient, and solidification rate. The process is illustrated by and the analysis compared with selected experiments in the Co-CoAl eutectic. S.S. FRYDMAN, formerly Postdoctoral Research Associate, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI     

Modeling of solidification microstructures in concentrated solutions and intermetallic systems

Most of the theoretical models for the predictions of solidification microstructure and solute segregation are based on the assumption mat the solute distribution coefficient,k, is independent of temperature. For concentrated alloys and for alloys near intermetallic compounds,k may vary significantly with temperature. A theoretical analysis which shows the necessary modifications in the theoretical models which must be made ifk varies with temperature is developed. It is shown that for phase diagrams with linear liquidus and solidus segments, many of the results derived with constantk can be used if the solute distribution coefficientk is replaced by a modified parameterk* which includesk as well as the derivative ofk with composition. The application of the model to concentrated alloys and to compositions near intermetallic phases is discussed. It is shown that the variation ink with temperature can significantly alter the composition dependence of dendritic microstructural scales and change the solute segregation profiles in solidified alloys.     

Modeling of solidification microstructures in concentrated solutions and intermetallic systems

Most of the theoretical models for the predictions of solidification microstructure and solute segregation are based on the assumption mat the solute distribution coefficient,k, is independent of temperature. For concentrated alloys and for alloys near intermetallic compounds,k may vary significantly with temperature. A theoretical analysis which shows the necessary modifications in the theoretical models which must be made ifk varies with temperature is developed. It is shown that for phase diagrams with linear liquidus and solidus segments, many of the results derived with constantk can be used if the solute distribution coefficientk is replaced by a modified parameterk* which includesk as well as the derivative ofk with composition. The application of the model to concentrated alloys and to compositions near intermetallic phases is discussed. It is shown that the variation ink with temperature can significantly alter the composition dependence of dendritic microstructural scales and change the solute segregation profiles in solidified alloys.     

5%Si高硅奥氏体不锈钢元素偏析及均匀化处理

 高硅奥氏体不锈钢由于高含量硅元素的加入使其具有优异的耐高温腐蚀性能和较低的成本,在制酸行业有着潜在的应用价值。然而,该合金中高含量硅元素的加入会促进凝固过程中溶质再分配,进而造成显著的元素偏析,最终导致合金内部产生枝晶组织和大量的有害相。对铸锭组织进行均匀化处理能够有效消除枝晶与元素偏析,促进析出相回溶和枝晶消融,从而改善材料的热塑性,有效应对热变形开裂问题。因此,采用金相显微镜(OM)、扫描电镜能谱分析(SEM/EDS)、电子探针(EPMA)、JMatPro软件计算等方法,研究了实验室条件下制备的5%Si高硅奥氏体不锈钢铸锭的显微组织和元素分布状态,通过残余偏析指数、扩散动力学计算并结合均匀化处理试验验证,最终确定了5%Si高硅奥氏体不锈钢合理的均匀化处理工艺。结果表明,5%Si高硅奥氏体不锈钢凝固过程中钼元素偏析最为严重,通过残余偏析指数模型计算得到的均匀化动力学方程可用来指导该成分合金的均匀化处理工艺;5%Si高硅奥氏体不锈钢经过1 150 ℃×12 h均匀化处理后,铸锭内枝晶消融,元素偏析基本消除,析出相与铁素体回溶到基体中,合金转变为全奥氏体组织,热塑性得到改善;当加热温度达到1 250 ℃时,合金出现过烧现象,晶界开始熔化。     

Microstructural variations induced by gravity level during directional solidification of near-eutectic iron-carbon type alloys

The effects of gravity on the microstructure of directionally solidified near-eutectic cast irons are studied, using a Bridgman-type automatic directional solidification furnace aboard a NASA KC-135 aircraft which flies parabolic arcs and generates alternating periods of low-g (0.01 to 0.001 g, 30 seconds long) and high-g (1.8 g, 1.5 minutes long). Results show a refinement of the interlamellar spacing of the eutectic during low-g processing of metastable Fe-C eutectic alloys. Low-g processing of stable Fe-C-Si eutectic alloys (lamellar or spheroidal graphite) results in a coarsening of the eutectic grain structure. Secondary dendrite arm spacing of austenite increases in low-g and decreases in high-g. The effectiveness of low-gravity in the removal of buoyancy-driven graphite phase segregation is demonstrated.     

镁合金晶粒异质形核的细化方法

总结了目前镁合金晶粒细化技术中广泛应用的异质形核细化方法,并对其细化机理进行论述。边对边匹配(edge-to-edge matching,简称E2EM)模型与第一性原理计算方法均可以判断镁合金的潜在异质形核核心,是近年来研究异质形核晶粒细化的有效方法。向镁合金中添加溶质元素及中间合金等物质已成为现工业生产中广泛适用的细化技术。为了更好地从微观尺度理解晶粒细化机制,基于密度泛函理论(density functional theory,简称DFT)的第一性原理计算方法不仅能够准确提供界面处原子结合情况,还可以定量预测凝固中异质晶核与初生相之间界面能和黏附功等,为晶粒细化剂的发展提供了理论基础,在镁合金晶粒细化中发挥越来越重要的作用。     

Effect of Alloy Composition on the Dendrite Arm Spacing of Multicomponent Aluminum Alloys

Predictions of secondary dendrite arm spacing (SDAS) for multicomponent aluminum alloys using a dendrite ripening model are compared with experimental observations. For six of the seven alloys studied, the predicted SDAS was within 20 pct of the measured SDAS. It was found that the final SDAS was dependent upon both the solidification time and the solute profile of the solidifying alloys. It is interesting that while the solidification times and the solute segregation during solidification varied significantly over the range of alloys, these two factors largely canceled each other out so that the predicted SDAS did not vary much between the alloys. The experimental and modeling results show that elements causing high constitutional undercooling near the beginning of solidification, e.g., Ti, which reduces the grain size substantially, have little effect on the SDAS. Instead, it was found that elements that strongly partitioned toward the end of solidification were more effective at restricting SDAS coarsening.     

The role of solute in grain refinement of magnesium

The effect of separate solute additions of Al, Zr, Sr, Si, and Ca on grain size of Mg has been investigated. Increasing the Al content in hypoeutectic Mg-Al alloys resulted in a continuous reduction in grain size up to 5 wt pct Al, reaching a relatively constant grain size for higher Al contents (above 5 wt pct). The effect of Sr additions was investigated in both low- and high-Al content magnesium alloys, and it was found that Sr had a significant grain refining effect in low-Al containing alloys but a negligible effect on grain size in Mg-9Al. Additions of Zr, Si, and Ca to pure magnesium resulted in efficient grain refinement. The grain refinement is mainly caused by their growth restriction effects, i.e., constitutional undercooling, during solidification, but the effect of nucleant particles, either introduced with the alloying additions or as secondary phases formed as a result of these additions, may enhance the grain refinement. A brief review of grain refinement of magnesium alloys is included in this article to provide an update on research in this field.     

Measurement and analysis of distribution coefficients in Fe-Ni alloys containing S and/or P: Part I.K Ni andK P

Fe-Ni alloys containing S and/or P were grown by a plane front solidification technique. The compositions of each element of these alloys were measured along the growth direction with an electron probe microanalyzer and were plotted as a function of fraction solidified. The distribution coefficient for each solute element was determined by an analytical procedure from the composition data as a function of fraction solidified. The distribution coefficient of Ni varies from 0.9 to 1.30. The distribution coefficient of P varies from 0.10 to 0.20 in a S-free Fe-Ni-P alloy, and from 0.25 to 5.80 in S-containing Fe-Ni-P alloys. This significant variation in the distribution coefficient of P in Fe-Ni-S-P alloys is attributed to the interactions occurring between S and P in the liquid during solidification. The distribution coefficient of Ni increases with the S and P contents of the liquid while the distribution coefficient of P increases with the S content and decreases with the P content of the liquid. Mathematical expressions have been formulated to relate the distribution coefficients of Ni and P to the S and P contents in the liquid.