Morphological instabilities and alignment of lamellar eutectics during directional solidification under a strong magnetic field

The effects of a strong magnetic field on Al–Al₂Cu and Pb–Sn lamellar eutectics during directional solidification have been investigated experimentally. The results show that the application of a strong magnetic field caused “tilting and oscillatory” morphological instabilities and deformation of the eutectic lamellae. Moreover, it was found that the Al–Al₂Cu eutectic grain became aligned under a strong magnetic field and that with an increase in the magnetic field intensity this alignment was gradually enhanced. Further, the stresses caused by the magnetization force and the thermoelectric magnetic force during directional solidification under a strong magnetic field were analyzed and it was found that they are likely responsible for the “tilting and oscillatory” morphological instabilities and deformation. This is experimental evidence that the stresses imposed on a solid are capable of inducing the morphological instabilities of lamellar eutectics. The magnetic crystalline anisotropy of the Al₂Cu phases and the growth relationship between the primary Al₂Cu phase and the eutectic phases was investigated and it was found that the Al₂Cu phase had a remarkable magnetic crystalline anisotropy which determined the growth of the Al–Al₂Cu eutectic grain. Thus, alignment of the Al–Al₂Cu eutectic grain under a strong magnetic field may be attributed to the magnetic crystalline anisotropy of the Al₂Cu phase. Based on the growth behaviour of Al–Al₂Cu and Pb–Sn lamellar eutectics under a strong magnetic field, an alignment model of lamellar eutectics during directional solidification under a strong magnetic field is proposed.     

Influence of a high magnetic field on columnar dendrite growth during directional solidification

The influence of a high magnetic field on the growth of MnBi, α-Al and Al₃Ni dendrites in directionally solidified Bi–Mn, Al–Cu and Al–Ni alloys have been investigated. Results indicate that the magnetic field changes the dendrite growth significantly. Indeed, the magnetic field aligns the primary dendrite arm and the effect is different for different dendrites. For the MnBi dendrite, an axial high magnetic field enhanced the growth of the primary dendrite arm along the solidification direction; however, for the α-Al and Al₃Ni dendrites, the magnetic field caused the primary dendrite arm to deviate from the solidification direction. At a lower growth speed, a high magnetic field is capable of causing the occurrence of the columnar-to-equiaxed transition (CET). Moreover, it has also been observed that a high magnetic field affects the growth of the high-order (i.e., secondary and tertiary) dendrite arms of the α-Al dendrite at a higher growth speed; as a consequence, the field enhances the branching of the dendrite and the formation of the (1 1 1)-twin planes. The above results may be attributed to the alignment of the primary dendrite arm under a high magnetic field and the effect of a high magnetic field on crystalline anisotropy during directional solidification.     

Morphological instability of cell and dendrite during directional solidification under a high magnetic field

The effects of a high magnetic field on the cellular and dendritic morphology in the Al–Cu alloy during directional solidification have been investigated, and results show that morphological instability of cell and dendrite has occurred. Indeed, at lower growth speeds, a high magnetic field of 10 T caused the cell and dendrite to twist and deflect from the solidification direction. Regular tilted structure forms at moderate growth speeds and the secondary dendritic arm in the upstream direction is more developed than the one in the downstream direction. In the case where the primary trunk has not deflected from the solidification direction, the field has caused the side-branching and the tip-splitting of the cell. These experimental results may be attributed to the thermoelectric magnetic force in the solid cell and dendrite and the change of the surface chemical potential and surface tension of the cellular and dendritic tip.     

行波磁场对定向凝固Pb-Sn合金枝晶生长的影响

在1g的重力加速度条件下,研究熔体对流对向上生长的定向凝固Pb-33%Sn合金枝晶生长行为的影响。熔体对流由行波磁场进行调制。当行波磁场方向由向上转变为向下时,一次枝晶间距逐渐增大,一次枝晶间距的分布更加紧凑,且峰值趋于降低。分析表明：行波磁场对熔体对流的调制作用与改变重力加速度的效果类似,当抽拉速率为50μm/s,行波磁场强度为1mT时,在向上和向下的行波磁场作用下有效重力加速度分别为3.07g和0.22g。     

Dendrite growth directions and morphology in the directional solidification of anisotropic materials

This study has experimentally determined the growth direction and the morphology of dendrites in a thin sample of succinonitrile alloy in configurations where the heat flow direction differs from the preferred crystalline orientations. A large data set has been obtained over a range of growth velocity, dendrite spacing, and misorientation angle between the two directions. Data analysis has provided evidence of an internal symmetry from which the expression of the orientational response of dendrites to the growth conditions has been identified. This has been complemented by the identification of a new dendrite scale, more relevant than the dendrite spacing to the present issue. Altogether, these results provide new insights on the growth direction and the morphology of dendrites that could be applied to more practical configurations.     

脉冲磁场下铝合金定向凝固磁场分布数值模拟

采用ANSYS 8.0有限元分析软件对铝合金脉冲磁场定向凝固磁场分布作了数值模拟,得到了凝固过程中的磁场分布状态.结果表明:在试样中心附近磁感应强度较强,而在端部较弱.模拟结果和实验测得结果相符合.通过数值模拟,不仅可以初步了解凝固过程中的磁场分布状态,而且有助于进一步研究凝固组织细化的原因.     

横向稳恒磁场对M2高速钢定向凝固过程V元素含量分布的影响

通过数值模拟探讨了M2高速钢定向凝固过程中无外加磁场以及外加0.5T横向稳恒磁场对V含量分布的影响,并建立数值模型,将模拟结果与试验结果进行对比,两者结果相符。数值模拟分析结果显示:当不加磁场时,坩埚中对流由自然对流占主导,并不会发生溶质的单向传输;当外加0.5T横向磁场时,坩埚中对流由单向的热电磁流主导,单向运输的溶质会富集于试样一侧。     

纵向磁场作用下DZ417G高温合金的枝晶生长行为

研究纵向磁场对高温合金DZ417G定向凝固显微组织的影响。结果表明:在较低生长速率下,磁场能显著影响高温合金柱状枝晶的生长;低磁场(0.1T)能使枝晶生长规则化,生长方向逐渐统一并平行于磁场方向,一次枝晶臂间距减小;高磁场(2T)破坏枝晶生长,枝晶发生断裂,逐渐出现一些云状组织;随着生长速率的增大,磁场的影响逐渐减弱。并从磁场诱发热电磁对流和熔体流动影响枝晶生长的角度对实验结果进行分析。     

Al-Cu合金定向凝固枝晶尖端开裂及间距调整机制

采用定向凝固技术研究A1-4%Cu(质量分数)合金凝固界而形态和胞/枝晶间距调整.结果表明,在ν=30um/s低速时,枝晶通过竞争淘汰调整一次枝品间距;在ν=300um/s高速时,枝晶通过尖端开裂而不是以往的三次枝生长调整间距.尖端半径与枝品间距相互影响,在稳定枝晶间距范围内随枝晶间距的增加而增大.采用LMK理论进行分析,得到枝晶尖端开裂是尖端溶质浓度变化引起尖端半径发生变化.通过扩散模型建立了枝晶问距与凝固界面成分之间的关系,得出枝晶通过自动调节间距大小来降低成分过冷作用.     

稳恒磁场对多晶硅定向凝固结晶阶段流速场的影响

采用有限元Comsol 4.3a软件对多晶硅定向凝固结晶阶段3个时期（前期、中期、后期）坩埚内硅熔体的流速场进行数值模拟,并与不同磁场强度下的硅熔体流速场进行了对比分析。结果表明,硅熔体的平均流速随着结晶时间的延长而减小,从初期的38 μm/s减小到后期的16 μm/s,下降了57.89%。施加磁场后,硅熔体的平均流速随着磁场强度的增大而减小：结晶初期、中期和后期,硅熔体的平均流速分别减小了42.11%、58.59%和45.16%。结晶阶段3个时期分别施加磁场强度为0.6、0.4、0.2 T时,磁场对硅熔体的对流抑制作用最为明显。     

Grain refinement of Sn-Pb alloy under a novel combined pulsed magnetic field during solidification

The combined pulsed magnetic field (C-PMF) obtained by simultaneously imposing pulsed and static magnetic field during solidification has been proposed to refine the solidification structure. Compared to the imposition of a single pulsed magnetic field, a more refined structure can be observed under C-PMF. The key factors to affect grain refinement under C-PMF consisted of the vibration frequency characterized by the static magnetic field, pulsed discharge voltage, and the vibration frequency characterized by the pulsed discharge frequency. The microstructure revealed that the grain size decreased with the increasing static magnetic field. The pulsed discharge voltage had an optimum value for obtaining fine grains. Furthermore, when the pulsed discharge frequency was equal to the intrinsic frequency of the liquid metal in a filled cylindrical vessel, resonance vibration occurred in the liquid surface, and grain refinement was promoted.     

Stability of remelting and solidification interfaces of triple-phase region during peritectic reaction at lower speed

Peritectic reaction was studied by directional solidification of Cu-Ge alloys. A larger triple junction region of peritectic reaction was used to analyze the interface stability of the triple junction region during peritectic reaction. Under different growth conditions and compositions, different growth morphologies of triple junction region are presented. For the hypoperitectic Cu-13.5%Ge alloy, as the pulling velocity (v) increases from 2 to 5 μm/s, the morphological instability of the peritectic phase occurs during the peritectic reaction and the remelting interface of the primary phase is relatively stable. However, for the hyperperitectic Cu-15.6%Ge alloy with v=5 μm/s, the nonplanar remelting interface near the trijunction is presented. The morphological stabilities of the solidifying peritectic phase and the remelting primary phase are analyzed in terms of the constitutional undercooling criterion.     

Effect of a high magnetic field on the morphological instability and irregularity of the interface of a binary alloy during directional solidification

The influence of an axial high magnetic field (up to 12 T) on the stability and morphology of the liquid–solid interface of a binary alloy has been investigated experimentally during the directional solidification of the Al–0.85 wt.% Cu and Zn–2.0 wt.% Cu alloys. Experimental results indicate that the high magnetic field caused the breakdown of a planar interface into cellular undulations and the formation of an irregular shape. Specifically, for the Zn–2.0 wt.% Cu peritectic alloy, a wavy band-like structure appears under a high magnetic field. Moreover, the high magnetic field promoted the enrichment of the solute Cu element in the diffusion boundary layer. A theory about the magnetization and solute build-up in the diffusion boundary layer under a high magnetic field for a binary alloy has been proposed. This magnetization and solute build-up could be partly responsible for the breakdown of the planar interface and the formation of the band-like structure in a peritectic alloy. Moreover, the stresses in the solid near the interface under a high magnetic field were analyzed, measured and simulated numerically. It is suggested that they are responsible to the interface irregularity, and are also capable of inducing the interface instability.     

Effect of power parameter and induction coil on magnetic field in cold crucible during continuous melting and directional solidification

Bottomless electromagnetic cold crucible is a new apparatus for continuous melting and directional solidification;however,improving its power efficiency and optimizing the configuration are important for experiment and production.In this study,a 3-D finite element (FE) method based on experimental verification was applied to calculate the magnetic flux density (Bz).The effects of the power parameters and the induction coil on the magnetic field distribution in the cold crucible were investigated.The results show that higher current intensity and lower frequency are beneficial to the increase of Bz at both the segment midpoint and the slit location.The induction coil with racetrack section can induce greater Bz,and a larger gap between the induction coil and the shield ring increases Bz.The mechanism for this effect is also discussed.     

Phase-field simulation of competitive growth of grains in a binary alloy during directional solidification

Taking Al-2%mole-Cu binary alloy as an example, the influence of grain orientation on competitive growth of dendrites under different competitive modes was investigated by using the three-dimensional(3-D) phasefield method. The result of phase-field simulation was verified by applying cold spray and directional remelting. In the simulation process, two competitive modes were designed: in Scheme 1, the monolayer columnar grains in multilayer columnar crystals had different orientations; while in Scheme 2, they had the same orientation. The simulation result showed that in Scheme 1, the growth of the dendrites, whose orientation had a certain included angle with the direction of temperature gradient, was restrained by the growth of other dendrites whose direction was parallel to the direction of temperature gradient. Moreover, the larger the included angle between the grain orientation and temperature gradient, the earlier the cessation of dendrite growth. The secondary dendrites of dendrites whose grain orientation was parallel to the temperature gradient flourished with increasing included angles between the grain orientation and temperature gradient. In Scheme 2, the greater the included angle between grain orientation and temperature gradient, the easier the dendrites whose orientation showed a certain included angle with temperature gradient inserted between those grew parallel to the temperature gradient, and the better the growth condition thereafter. Some growing dendrites after intercalation were deflected to the temperature gradient, and the greater the included angle, the lower the deflection. The morphologies of the competitive growth dendrites obtained through simulation can also be found in metallographs of practical solidification experiments. This implies that the two modes of competitive growth of dendrites characterized in the simulation do exist and frequently appear in practical solidification processes.     

共晶和包晶合金定向凝固过程中共生生长的形态稳定性

近年来对共晶合金共生(耦合)生长形态稳定性的研究取得了一系列重要的成果。这些成果主要包括:片层共晶共生生长在最小过冷度下的超稳定性、片层共晶共生生长的Z字形分叉、熔体流动及非线性动力学对共生生长形态稳定性的影响等。在此基础上人们又开始关注包晶合金定向凝固中的稳态共生生长及其形态稳定性。综述了这些研究成果,并对其发展方向进行了探讨。