纵向静磁场对定向凝固GCr15轴承钢柱状晶向等轴晶转变的影响

进行了外加纵向静磁场下GCr15轴承钢的定向凝固实验,考察了纵向静磁场对试样凝固过程中柱状枝晶向等轴枝晶转变(columnar to equiaxed transition,CET)的影响。结果表明,在温度梯度(104 K/cm)和抽拉速率(20μm/s)一定时,随着磁场强度的增加(0~5 T),试样棒边缘柱状枝晶的生长逐渐地遭到破坏,从而发生不同程度的CET;当磁场强度和温度梯度分别为4 T和104 K/cm时,在较低抽拉速率(5μm/s)下,试样的凝固组织发生了完全CET;在试样发生完全CET后,其合金元素分布趋于均匀。结合数值模拟,可将这些现象归结为纵向静磁场与热电流相互作用产生的热电磁力对枝晶和熔体的作用所致。     

纵向静磁场对单晶高温合金DD483组织及蠕变性能的影响

在静磁场下进行高温合金DD483的定向凝固实验,研究了不同磁场强度对单晶镍基高温合金DD483凝固组织枝晶形貌、合金元素偏析系数、析出相和蠕变性能的影响.结果表明:纵向静磁场的施加使高温合金DD483单晶生长性受到破坏,强的磁场使得枝晶定向生长形态受到破坏,形成“雀斑组织”,对一次枝晶间距影响不大.施加磁场使得该单晶合金中合金元素的偏析降低;也使单晶高温合金DD483凝固态组织中γ’析出尺寸降低、碳化物和共晶组织尺寸和含量显著减小.同时磁场对该合金的单晶性和枝晶的定向生长行为的破坏使得合金的蠕变性能降低.     

试样尺寸对定向凝固Al-4%Cu合金固/液界面特征的影响

采用定向凝固方法研究不同试样尺寸对Al-4%Cu合金凝固固/液界面特征的影响.结果表明:当凝固速率v=1 μm/s时,小尺寸试样的平界面更加平直;当v=5 μm/s时,随着试样尺寸的增大,界面形态分别为浅胞-深胞一初始枝晶,同时,试样边缘的组织比中心的组织更不稳定;在相同凝固速率下,小尺寸试样的温度梯度较大,促使界面稳定性提高;试样尺寸的增大引起径向温差增大,促进溶质流动,使试样边缘产生溶质富集,从而使平界面弯曲;由于试样中心排出的溶质大部分流向界面前沿糊状区的液相中,而枝晶糊状区的液相比胞晶的多,因而形成的枝晶界面弯曲程度比胞晶的小.     

包晶Nd-Fe-B合金定向凝固与形核有关的组织选择

对Nd_(14)Fe_(79)B_7合金进行了定向凝固试验与形核有关的凝固组织形成研究.发现凝固速率为200和500μm/s试样中局部区域有粗大等轴Fe枝晶形成;并且在凝固速率500 μm/s的试样中存在α-Fe枝晶与Nd_2Fe_(14)B枝晶交替生长的带状组织.分析表明,当凝固速率为200μm/s时,定向生长的Fe枝晶界面前沿液相中的温度分布相对于γ-Fe平衡液相线是过冷的;当凝固速率为500μm/s时,Fe枝晶生长界面前沿相对于γ-Fe和Nd_2Fe_(14)B平衡液相线均会产生过冷.理论分析可以解释定向凝固试样中等轴晶和枝晶带状组织的形成.     

横向静磁场对激光熔化增材制造Al-12%Si合金凝固组织的影响

利用送粉式激光熔化3D打印工艺,研究了外加横向稳恒磁场对3D打印Al-12%Si合金构件凝固组织的影响。结果表明,在有/无横向稳恒磁场下,激光熔化单道薄壁试样的宏观凝固组织未发生明显改变,其主要以白亮带(以α-Al相为主)和灰暗区(以Al-Si共晶相为主)为基本单元叠加构成。而微观组织分析表明,无磁场时灰暗区内的初生α-Al相呈柱状枝晶形态,施加了0.35 T横向稳恒磁场后,试样灰暗区内的初生α-Al相全部转变为等轴枝晶形态,且枝晶臂发达。基于热电磁力及其Hartman无量纲数(用于表征稳恒磁场对金属熔体流动抑制作用的参数)估算分析表明,0.35 T稳恒磁场下,作用于初生α-Al枝晶上的热电磁力可达10~5N/m~3量级;Hartman数远大于10,表明激光熔化微小金属熔池中强烈的Marigoni以及热溶质对流一定程度上被抑制。分析认为,稳恒磁场下凝固组织灰暗区内α-Al相柱状枝晶向等轴枝晶的转变是固相中热电磁力(约10~5N/m~3)对枝晶的破碎作用导致,而等轴枝晶发达的枝晶臂则是横向稳恒磁场抑制熔体流动的结果。     

横向磁场对镍基高温合金定向凝固组织的影响

研究了横向磁场对镍基高温合金DZ417G定向凝固显微组织的影响. 在较低生长速率条件下, 磁场显著影响合金的枝晶生长和宏观偏析. 施加磁场后一次枝晶间距减小并在沿磁场方向试样的左侧出现了“斑状”偏析. 随着生长速率的增加, 磁场的影响减弱. 从磁场在合金熔体中诱发热电磁对流, 并影响枝晶生长的角度对实验结果进行了分析.     

强磁场对Cu-25Ag合金凝固、拉拔组织及导电性能的影响

在有无磁场条件下进行Cu-25Ag(%,质量分数,下同)合金凝固实验,并对铸锭进行冷拉拔处理,系统的研究强磁场对Cu-25Ag合金凝固组织、拉拔组织以及复合材料电导率的影响。发现有无磁场条件下合金凝固组织和拉拔组织都有所不同。无磁场条件下初生Cu一次枝晶较长,以柱状枝晶方式生长,在试样顶部,枝晶生长方向沿弧形径向;在试样中部,生长方向与试样轴向夹角约45°;试样下部,生长方向与试样轴向夹角约90°。另外,共晶组织壁厚较薄,两相分布不均匀,片层间距较大。强磁场条件下初生Cu一次枝晶变短,以胞状枝晶方式生长,在试样顶部,枝晶生长方向沿弧形径向,试样中部和下部,枝晶生长方向与试样轴向夹角约90°。共晶组织壁厚较大,两相分布比较均匀,片层间距较小。冷拉拔后,共晶网状结构被拉长、变细,形成纤维结构,无磁场条件试样中共晶纤维厚度和间距较小,强磁场试样中共晶纤维厚度和间距较大。随着纤维组织厚度不断减小,试样的电导率降低,并且相同变形量下有无磁场条件的试样电导率有所差别。对强磁场下合金凝固组织及拉拔组织影响机理进行了探讨,并分析了纤维组织对复合材料电导率的影响机制。     

稳恒强磁场对Cu-15Ni-8Sn合金凝固组织、微观偏析及显微硬度的影响EI北大核心CSCD

研究稳恒强磁场对Cu-15Ni-8Sn合金凝固过程中微观组织形貌、枝晶主干处成分偏析及显微硬度的影响规律。结果表明:对比无磁场条件,2 T强磁场对Cu-15Ni-8Sn合金试样微观组织与枝晶主干处微观偏析的影响并不大。但当磁感应强度提高至4~6 T时,试样枝晶数量明显减少,尺寸显著粗化;且枝晶主干处Sn元素含量明显下降,Ni元素含量则明显升高。此外,强磁场的施加能显著提高Cu-15Ni-8Sn合金枝晶主干的显微硬度,对比无磁场条件,施加6 T强磁场时合金中枝晶主干处显微硬度上升74.4%。强磁场对Cu-Ni-Sn合金微观偏析及显微硬度的影响主要与磁场在合金凝固过程中对Sn、Ni等元素扩散的影响有关。     

直流电磁场对Al-7%Si亚共晶合金组织结构的影响

研究在直流电磁场下凝固的Al-7%Si合金晶内Si元素含量及凝固组织的变化.结果表明,与未施加磁场的试样相比,整个凝固过程或凝固后期施加直流电磁场,Al-7%Si合金晶内Si含量增加,在整个凝固过程都施加直流电磁场,初生α-Al枝晶较长,随着开始施加直流电磁场的温度降低,初生α-Al枝晶越来越短.     

La对Fe-36Ni低膨胀合金凝固组织的影响

研究了La对Fe-36Ni低膨胀合金凝固组织的影响.结果表明,加La处理后,合金的凝固组织得到显著改善,随着合金中La含量的增加,凝固组织的等轴晶比例增加,等轴晶尺寸减小,树枝晶变细变短,一次枝晶间距减小,二次枝晶间距基本不变.经La处理后,Fe-36Ni低膨胀合金中形成了大量高熔点的La2O2S颗粒,尺寸约为4μm.错配度计算表明,Fe-36Ni低膨胀合金的低指数面与La2O2S的低指数面有5.42％的较低错配度,因此La2O2S可以作为有效的非均质形核的核心,提高形核率,细化合金的凝固组织.     

激光金属成形定向凝固显微组织及成分偏析研究

利用高温合金Rene95粉末在镍基高温合金基材上进行激光多层涂覆。研究熔覆层中凝固显微组织的生长特性。基于对柱状晶向等轴晶转化理论的分析，证实通过控制工艺参数组合，可获得具有良好取向的单道多层、多道搭接多层定向凝固涂层和圆环的定向凝固试样，涂层内部的定向凝固柱状枝晶组织细密，枝晶一次间距为5－30μm，二次臂很小或者完全退化。涂层内无明显的成分偏析现象。     

Dendrite fragmentation and columnar-to-equiaxed transition during directional solidification at lower growth speed under a strong magnetic field

The effects of strong magnetic fields on the columnar-to-equiaxed transition (CET) have been investigated experimentally. Six alloys have been directionally solidified at low growth speeds (1–10 μm s^?1) under magnetic fields up to 10 T. Experimental results show that the application of a strong magnetic field causes a dendrite fragmentation and then the CET. The thermoelectric magnetic force acting on cells/dendrites and equiaxed grains in the mushy zone has been studied numerically. Numerical results reveal that the value of the thermoelectric magnetic force increases as the magnetic field intensity and the temperature gradient increase. A torque is created on cells/dendrites and equiaxed grains. This torque breaks cells/dendrites and drives the rotation of equiaxed grains. The rotation of equiaxed grains in the mushy zone will further destroy cells/dendrites. Thus, with the increase of the magnetic field intensity and the temperature gradient, the volume fraction of equiaxed grains in front of columnar dendrites increases. When the magnetic field intensity and the temperature gradient reach a critical value, the growth of columnar dendrites is blocked and the CET then occurs. The present work may initiate a new method of inducing the CET via an applied strong magnetic field during directional solidification.     

螺旋磁场对Pb-Sn合金成分偏析的影响

测试分析新型电磁搅拌器内螺旋磁场和旋转磁场的磁感应强度、分布和作用规律,研究螺旋磁场对Pb-80%Sn过共晶合金凝固组织影响的作用机理,并与无磁场和旋转磁场条件下合金凝固组织的形貌特征及成分分布进行对比分析。结果表明:螺旋磁场相比于旋转磁场可以在铸锭内部更大区域内形成均匀搅拌,更易于破碎和细化枝晶组织,既能促进椭球或球状晶的生成,又能更好地改善宏观偏析;在频率一定的情况下,初生相晶粒尺寸随着励磁电流的增大而减小;当励磁电流为125A时,旋转磁场和螺旋磁场细化晶粒的效果最好;继续增大电流,晶粒产生粗化;螺旋磁场可基本消除成分偏析,并在较小励磁电流(100A)下达到采用旋转磁场(125A)时的最佳搅拌效果。     

Improving the carbon macrosegregation in high-carbon steel by an electric current pulse

Thus far, the relationship between the macrosegregation and refinement of solidification structures in metals is not clearly understood. In this paper, the effect of an electric current pulse (ECP) on the refinement of the solidification structure as well as carbon macrosegregation in high-carbon steel was investigated. The experimental results revealed that if central porosity exists in the solidification structure, the carbon macrosegregation in high-carbon steel cannot be improved by applying an ECP, although increased equiaxed dendrites and a reduced primary dendritic arm in a solidification structure were obtained after an ingot was exposed to an ECP. In contrast, after central porosity was eliminated, carbon macrosegregation was improved through the refinement of the solidification structure in high-carbon steel through the use of an ECP.     

8090Al—Li合金焊缝金属形核机制

８０９０Ａｌ—Ｌｉ合金焊缝凝固组织中除了柱状枝晶和等轴枝晶外，还有一种极细小的等轴非枝晶，它主要分布在熔合区边缘，有时也能呈带状出现在焊缝中部．等轴枝晶与柱状枝晶的结晶核心是平衡态四方Ａｌ_３Ｚｒ粒子，而等轴非枝晶的结晶核心是亚稳态立方Ａｌ_３Ｚｒ粒子．     

Porosity formation in columnar region of unidirectional solidified Al–4˙5Cu casting

Abstract

In this research, a vertical unidirectional solidification experiment was performed with Al–4˙5 wt-%Cu alloy. The microstructure of specimens obtained from the casting were analysed, which showed that the casting had columnar, columnar to equiaxed transition and equiaxed regions along upward direction. Porosity fractions in the specimens were measured quantitatively. It was found that there exists a decreasing distribution of porosity in columnar region from bottom chill and porosity with a high content in the specimen next to the chill. The general model coupling the Darcy's law with continuous equation was applied to the casting, however, the simulation results did not well agree with the experiment. Furthermore, hydrogen macrosegregation was incorporated into the model to predict the porosity formation, and effect of the macrosegregation was analysed and discussed. The results have shown that hydrogen macrosegregation has an important effect on porosity formation.     

The impact of turbulent flow on the solidification of metal alloys driven by a rotating magnetic field

Abstract

The formation of Taylor-Görtler vortices inside a melt, driven by a rotating magnetic field, is an intrinsic phenomenon occurring at supercritical Taylor numbers. In this work we numerically study their impact on the macrosegregation and the shape of the mushy zone during unidirectional solidification of Al–7wt-%Si alloy. The weakly turbulent flow was modelled by means of direct numerical simulations in an axisymmetric approach, where the transient heat and mass transfer were simulated by means of a standard mixture model. Both types of solidification, columnar and equiaxed, were considered by the application of both a permeability and a hybrid model to treat the fluid flow in the mushy zone. Our results demonstrate that in the case of columnar solidification the Taylor-Görtler vortices cause both a wavy shape of the mushy zone and segregations in the form of a fir tree with a distinct accumulation of silicon along the axis of the cylinder.     

Grain Refinement During Directionally Solidifying GCr18Mo Steel at Low Pulling Speeds Under an Axial Static Magnetic Field

The present work investigates how axial static magnetic field affects the solidification structure and the solute distribution in directionally solidified GCr18Mo steel. Experimental results show that grain refinement and the columnar to equiaxed transition is enhanced with the increases in the magnetic field intensity(B) and temperature gradient(G) and the decrease in the growth speed. This phenomenon is simultaneously accompanied by more uniformly distributed alloying elements.The corresponding numerical simulations verify a thermoelectric(TE) magnetic convection pattern in the mushy zone due to the interaction between the magnetic field and TE current. The TE magnetic convection in the liquid should be responsible for the motion of dendrite fragments. The TE magnetic force acting on the dendrite is one of the driving forces trigging fragmentation.     

连铸空心管坯内置行波磁场对凝固组织的影响

利用低熔点Sn-3．5％Pb合金进行了空心管坯内置行波磁场的静态模拟实验研究。结果表明：管坯内置行波磁场可以引起金属熔体内部纵截面上的大环流，减小了熔体凝固前沿的温度梯度，降低了金属熔体的冷却速率，抑制了熔体的枝晶生长，从而促进了晶粒的球化，改善了管坯的凝固组织。     

An idea to treat the dendritic morphology in mixed columnar–equiaxed solidification

Abstract

To treat mixed columnar–equiaxed solidification with dendritic morphology, five phase regions have been distinguished: extradendritic melt, interdendritic melt and solid dendrites in equiaxed grains, interdendritic melt and solid dendrites in columnar arrays of dendrites. These five phases are quantified by their volume fractions, and characterized by different volume-averaged solute concentrations. The equiaxed grains and columnar dendrites are confined by their envelopes, whose shapes are described by morphological parameters. The evolution of the envelopes is derived based on recent growth theories: the growth of primary columnar dendrite tips by the Kurz–Giovanola–Trivedi (KGT) model, the growth of secondary dendrite tips in radial direction of columnar trunk and the equiaxed dendrite tips by the Lipton–Glicksman–Kurz (LGK) model. The solidification of the interdendritic melt is governed by diffusion in the interdendritic melt region. Preliminary modelling results on a benchmark casting (Al–4·7wt-% Cu) show the potentials of the model.