脉冲磁致振荡下纯铝凝固磁场与流场分布的数值模拟

脉冲磁致振荡可以细化金属晶粒,为了研究其作用机理,采用ANSYS有限元模拟软件对脉冲磁致振荡下纯铝凝固磁场与流场分布进行了数值模拟.模拟结果显示,由于电磁趋肤效应,线圈中脉冲电流只在熔体表面感应产生电磁力,且随时间出现指向内部的压力与指向外部的拉力交替变化,沿径向有指向熔体顶部与底部交替变化.交变电磁力可以振荡熔体表面率先析出的晶核而使之游离,增大熔体形核率.同时感应电磁力迫使熔体产生流动,会利于晶核的均匀分布与温度场、浓度场的均匀化.     

Simulation of the Influence of Pulsed Magnetic Field on the Superalloy Melt with the Solid-Liquid Interface in Directional Solidification

The effect of the pulsed magnetic field on the grain refinement of superalloy K4169 has been studied in directional solidification. In the presence of the solid-liquid interface condition, the distributions of the electromagnetic force, flow field, temperature field, and Joule heat in front of the solid-liquid interface in directional solidification with the pulsed magnetic field are simulated. The calculation results show that the largest electromagnetic force in the melt appears near the solid-liquid interface, and the electromagnetic force is distributed in a gradient. There are intensive electromagnetic vibrations in front of the solid-liquid interface. The forced melt convection is mainly concentrated in front of the solid-liquid interface, accompanied by a larger flow velocity. The simulation results indicate that the grain refinement is attributed to that the electromagnetic vibration and forced convection increase the nucleation rate and the probability of dendrite fragments survival, for making dendrite easily fragmented, homogenizing the melt temperature, and increasing the undercooling in front of the solid-liquid interface.     

磁场类型对金属凝固机理的影响研究综述*

利用磁场辅助制备的合金综合性能优异,广泛应用在工业生产、交通运输、航空航天等领域。不同磁场参数环境下合金硬度、耐磨性等服役性能有所差异,作用机理复杂多变。对新工艺驱动下不同磁场对金属凝固过程的作用规律进行总结, 弥补目前磁场辅助金属表面加工方法的研究短板,对金属表面工程发展有重大意义。归纳科研人员在不同磁场环境对金属表面加工的研究探索,分析对比金属材料在不同类型磁场环境下的晶核形核和生长过程差异,总结金属凝固过程在不同磁场下的变化规律,如晶界形貌改善、形核率提高、晶粒细化等。从晶粒微观形貌和合金宏观性能表现两方面出发,分析磁场作用下熔体内部传热传质变化,揭示稳恒磁场、脉冲磁场和交变磁场对金属凝固影响的作用机理,讨论不同参数的磁场对熔体作用效果差异,如磁场对熔池内部流动扰动、熔体内带电粒子受到的洛伦兹力等。综上,晶粒细化、合金性能提高是磁场作用下熔池传热传质变化和磁场作用力的综合体现。综合研究对比稳恒磁场、脉冲磁场和交变磁场对金属凝固的作用特点和作用机理,综述金属凝固领域当前热点问题,有助于统一磁场环境下金属凝固机理的争论,填补磁场环境下金属表面加工工艺的空白,对推进高性能金属表面制备研究有借鉴意义。     

AN IN SITU SURFACE COMPOSITE AND GRADIENT MATERIALOF Al-Si ALLOY PRODUCED BY ELECTROMAGNETIC FORCE

1. IntroductionAt present, the main processes fOr producing surface composite and gradient materialduring solidiflcation are casting infiltrationl1,2], electromagnetic stirring to separate eutec-tic process of the eutectic alloy[3l4], electromagnetic ..nt.iiug.l casting[5--7] and centrifugalcasting[s]. These processes, however, are complicated and require complex equipment;therefore, they are limited in application. The aim of this research is to develop a newmethod for production of in situ s…     

电磁离心凝固过程熔体流动和传热的有限元数值模拟

利用电磁流体动力学有关理论,建立了电磁离心凝固过程熔体流动与热量传输的耦合分析模型,采用有限元法对电磁离心凝固过程进行了分析,得出了瞬态速度场和温度场。分析结果表明,电磁离心凝固过程中,正是由于电磁力引起的受迫对流运动,对金属熔体起电磁搅拌作用并使晶粒得到细化,从而提高铸件质量。     

电磁搅拌作用下铝合金凝固组织的数值模拟

建立了有限元(Finite Element)和元胞自动机法(Cellular Automaton)相结合的宏微观耦合的CA-FE模型,实现了电磁搅拌作用下Al-5％Cu合金凝固组织的数值模拟.该模型利用ANSYS软件计算电磁力,利用有限差分法计算宏观流场和温度场.在微观计算中,采用基于高斯分布的连续形核模型,并采用KGT生长模型计算枝晶尖端生长速率.模拟和实验结果表明,施加电磁搅拌后,铝合金的温度场均匀,冷却速率加快,利于组织细化.     

Refinement mechanism of low voltage pulsed magnetic field on solidification structure of silicon steel

The grain refinement of a silicon steel solidified with a low voltage pulsed magnetic field (LVPMF) was investigated by experiments and modeling. The experiment results show that complete fine equiaxed grains are acquired by applying the LVPMF. The effects of process parameters, such as the melt cooling rate and superheating on the solidification structure, were also studied. Complete fine equiaxed grains can be obtained under a larger range of cooling rate or superheating when the LVPMF is applied. The magnetic force and the melt flow during solidification were modeled and simulated to reveal the grain refinement mechanism. The simulation results show that the LVPMF has a double role in electromagnetic convection and electromagnetic vibration on the alloy melt. We propose the refining mechanism: The melt vibration and convection can promote nucleus multiplication, which contributes to heterogeneous nucleation enhancement and leads to a high nucleation rate and grain refinement.     

Effect of wide-spectrum pulsed magnetic field on grain refinement of pure aluminium

A wide-spectrum pulsed magnetic field(WSPMF) was obtained by adjusting the number of current pulses and the pulse interval between adjacent pulses. The effect of WSPMF on the grain refinement of pure aluminium was studied. The distribution of electromagnetic force and flow field in the melt under the WSPMF was simulated to reveal the grain refining mechanism. Results show that the grain refinement is attributed to the combined effect of the melt flow and oscillation under a WSPMF. When the pulse...     

Improving the Solidified Structure by Optimization of Coil Configuration in Pulsed Magneto-Oscillation

Using both numerical and experimental methods, we studied the effect of coil configuration of pulsed magneto-oscillation (PMO) on distribution of electromagnetic field, flow field and solidification structure with the same pulse current parameters in Al ingots. We designed and constructed three types of coils: surface pulsed magneto-oscillation, hot-top pulsed magneto-oscillation (HPMO) and combined pulsed magneto-oscillation (CPMO). PMO treatment refined the solidification structure in all the ingots. The configuration of the PMO, however, introduced differences in magnetic field intensity, electromagnetic force, Joule heat, flow field, equiaxed grain zone, grain size and growth direction of columnar grains. The largest equiaxed grain zone was found in CPMO treated ingot, and the smallest grain size was found in both HPMO and CPMO treated ingots. Numerical simulation indicated that difference in electromagnetic field and flow field resulted in differences in solidification structure. HPMO is more advantageous over others for large ingot production.     

熔体处理及电磁搅拌对AZ91D合金组织的影响

利用金相显微分析等手段，研究了熔体处理及电磁搅拌对AZ91D合金组织的影响。结果表明，C粉对AZ91D合金晶粒具有明显的细化作用，优于C2Cl6和Al—10Sr间合金，无水FeCl3、Al-5Ti—B测效果甚微。熔体处理与电磁搅拌复合作用使AZ91D合金晶粒转化为均匀细小的类球状半固态组织，作用优于单纯的电磁搅拌。     

电磁搅拌对激光熔凝TA15钛合金熔池凝固研究

为了研究电磁搅拌作用对激光熔凝熔池凝固过程的影响,采用有限体积法对施加磁场前后激光单道动态熔凝TA15钛合金过程进行三维磁-热耦合数值模拟。研究了磁场对激光熔池流场、熔凝单道及其周边基材温度分布、固液界面处温度梯度和凝固速度的影响,并采用试验手段对模拟结果进行了验证。模拟结果表明:电磁搅拌作用使激光熔池最大流速增加了约20%,对流加剧促进了熔池热交换作用,使其最高温度下降,固液分界面处温度梯度大幅降低,凝固速度小幅增大,从而有利于熔池顶部组织发生柱状晶-等轴晶转变(CET)。试验结果显示施加磁场后熔凝层顶部有等轴晶组织生成,且随着远离磁场中心,电磁力增大,等轴晶区有扩大趋势。试验结果和模拟结果一致性较好。     

电磁振荡对铝合金半连铸坯微观组织及溶质元素晶内含量的影响

研究了电磁振荡作用下7075铝合金半连铸坯微观组织及溶质元素的晶内分布，对其细化、非枝晶组织形成及溶质元素晶内含量的影响机制进行了探讨。认为在电磁振荡作用下，熔体中结晶核心增加，游离晶粒的枝晶生长方式得到抑制是形成均匀细小的近球形和蔷薇形非枝晶组织的原因，并从电磁场改变7075铝合金凝固过程中的溶质分配系数、结晶间隔、液穴内部温度场、流动场以及微观组织形貌等方面出发，分析了电磁振荡对溶质元素晶内含量的影响。     

Simulation of electromagnetic-flow fields in Mg melt under pulsed magnetic field

The effects of a pulsed magnetic field on the solidified microstructure of pure Mg were investigated.The results show that microstructure of pure Mg is considerably refined via columnar-to-equiaxed growth under the pulsed magnetic field and the average grain size is refined to 260μm under the optimal processing conditions.A mathematical model was built to describe the interaction of the electromagnetic-flow fields during solidification with ANSYS software.The pulsed electric circuit was first solved and the...     

Effects of electromagnetic vibration on the structure and mechanical properties of Al-6%Si alloy during directional solidification

The effects of electromagnetic vibration on the grain refinement in directional solid- ification were investigated. It was found that the electromagnetic vibration applied in the melt not only can refine grains remarkably but also can enhance both tensile strength and ductility values of Al-6%Si alloy. SEM graphs show that coarse dendrite structure was broken up into a somewhat globular structure, and the morphology of eutectic silicon was changed from flaky to fibrous under electromagnetic vibration treatm...     

液态金属电磁离心凝固的力场分析

运用磁流体动力学原理，对在稳恒磁场中高速旋转的液态金属凝固时的受力状态进行了理论解析，结果表明在电磁力场、离心力场和重力场共同作用下，旋转液态金属中受到大小周期变化的径向压力和切向分力，后者起电磁搅拌作用，是细化电磁离心凝固组织的根本原因．     

Mechanism of pulse magneto-oscillation grain refinement on pure Al

Pulse magneto-oscillation (PMO) was developed as a novel technique to refine the solidification structure of pure aluminium.Its grain refining mechanism was proposed.The PMO refinement mechanism is that the nucleus falls off from the mould wall and drifts into the melt under the action of PMO.The solidification structure of Al melt depends on the linear electric current density,and also the discharge and oscillation frequencies.The radial pressure of PMO sound wave is the major factor that contributes to the migration of nucleus into the melt.     

Grain refinement of commercial pure Al treated by Pulsed Magneto-Oscillation on the top surface of melt

Commercial pure Al can be refined by Pulsed Magneto-Oscillation(PMO) treatment applied via a plate induction coil above the top surface of the melt. The proportion of the equiaxed zone area increases with decreasing Height to Diameter(H/D) ratios from 3.5 to1.8 and further to 1.0. Meanwhile, it increases and then decreases with increasing peak current for the three kinds of ingots with H/D ratios of 3.5, 1.8 and 1.0, respectively. However, when the H/D ratio decreases to 0.44, the area proportion of equiaxed zone can reach the maximum value with a lower peak current. FEA software simulation indicates that smaller H/D ratio results in larger current density, electromagnetic force and convection on the top surface of the melt, favoring nucleation and subsequent grain formation. Through evaluating Joule heating effect by PMO, it was found that the proper amount of Joule heating benefits grain refinement. Excessive Joule heating can reduce the size of the equiaxed zone and change the growth morphology of the grains.     

非磁性合金激光焊旋转磁场搅拌机理

详细地研究了激光焊非磁性材料时,熔池液态金属的电磁搅拌机理;探讨了旋转磁场对激光焊接Al-12Si合金显微组织的影响.结果表明,尽管激光束既无磁性又非导体,Al-12Si合金为非磁性材料,但旋转磁场能改变液态Al-12Si合金的运动状态,产生磁搅拌作用,细化晶粒、消除焊接缺陷.对于激光焊,旋转磁场切割焊接熔池中的液态金属,液态金属中产生感生电流,带电的液态金属受到电磁力的作用使熔池中液态金属产生旋转运动.磁场的旋转速率越高,液态金属受到的电磁搅拌作用力越强.     

5056铝合金稳恒磁控激光深熔焊接过程熔池流动与传热行为分析

针对外加稳恒磁场条件下6 mm厚度5056铝合金激光深熔焊接过程,建立了热场—流场—电磁场耦合熔池瞬态动力学数值模型,求解了特定时刻温度场、速度场与电磁场分布,建立了熔池沿不同方向的Peclet数模型,分析了不同磁场感应强度对熔池流动与传热行为的影响.结果表明,稳恒磁场条件下熔池中产生显著的哈特曼效应,表现为液态金属M...     

脉冲磁场对Mg-Gd-Y-Zr合金凝固及力学性能的影响

在Mg-Gd-Y-Zr合金凝固过程中施加不同频率的脉冲磁场,研究脉冲磁场对Mg-Gd-Y-Zr合金凝固的影响.实验结果表明,脉冲磁场使Mg-Gd-Y-Zr合金晶粒细化,在频率为5 Hz条件下获得最佳的晶粒细化效果,平均晶粒尺寸从未加脉冲磁场条件下的65 pm细化到37 pm.脉冲磁场的搅拌导致熔体磁过冷及熔体温度梯度降低是晶粒细化的主要原因.脉冲磁场的施加使抗拉强度和延伸率较常规铸造合金分别提高了4.8%、78.5%.