Solidification Processing of Metallic Materials in Static Magnetic Field: A Review

Abstract

The application of a static magnetic field (SMF) to solidification processing has emerged as an advanced strategy for efficiently regulating the macro/micro structures and the mechanical performance of metallic materials. The SMF effects have been proved to be positive in various processes of metal solidification. Firstly, this review briefly introduces two basic magnetic effects, i.e., magnetohydrodynamic effects and magnetization effects, which play crucial roles in regulating metal solidification. Further, the state of the art of solidification processing in the SMF, including undercooling and nucleation, interface energy, grain coarsening and refinement, segregation and porosity, are comprehensively summarized. Finally, the perspective future of taking advantage of the SMF for regulating metal solidification is presented.

1. Introduction

It is an extremely important topic to regulate the solidification processing of metallic materials, as it determines the micro/macro structures and, thus, the mechanical properties of the as-cast products [1,2]. Over the past century, a lot of effort has been dedicated to theoretical and experimental studies in the field of solidification and many milestone techniques and methods have been developed to obtain the required physico-chemical/mechanical properties of metallic materials [3,4]. As the solidification theories and processing methods are well-developed, the conventional routines with respect to regulating the solidification parameters, such as the cooling rate, undercooling and the temperature gradient, have entered the bottleneck stage [5]. From the point of view of the development of science and technology, it is never an outdated theme to explore new techniques and methods of solidification processing. Additionally, the developed and flourished world economy provides strong demand for high-quality and high-performance as-cast products, which also motivates the renovation of solidification processing methods.

In past decades, the emergence of effective strategies using external fields, such as magnetic field, electric field and ultrasonic field, has become a potential trend in the field of metal solidification processes, which can effectively regulate nucleation and growth and further modify solidification structures [6,7]. Owing to contactless interaction and various magnetic effects, the application of the magnetic field to solidification processing has attracted intensive attention [8,9,10,11,12,13]. The magnetic field is implemented in two types, i.e., the time-varying and static magnetic fields. In this review, we emphasize the state of the art of solidification processing in the static magnetic field (SMF). The numerous studies confirm that applying the SMF to metal solidification processes is a promising and novel strategy to modify nucleation [14], undercooling [15], interface free energy [16], grain coarsening [17] and refinement [18], segregation [19], and porosity [20]. Although the magnetic field has exerted significant effects on metal solidification processes, its applicability needs further development and the related physical mechanisms deserve further elucidation.

Although the effects of the SMF on solidification processing were reviewed with respect to orientation, melt flow, solidification structure, etc. [6], progress has been made in recent years and, thus, an updated review is warranted. This review firstly briefly introduce two magnetic effects, i.e., magnetohydrodynamic and magnetization effects. Further, recent reports regarding the SMF-regulated metal solidification processes are summarized, including undercooling, interface free energy, grain coarsening and refinement, segregation and porosity. In addition, in-situ and 3D imaging strategies have been developed to accelerate scientific discovery of the SMF-regulated solidification processing. Finally, the perspective future on the potential and challenge of SMF-regulated metal solidification processes is presented.

2. Principles of SMF-Regulated Metal Solidification

In principle, the SMF exerts an effect on metal solidification through two mechanisms: (1) a magnetohydrodynamic (MHD) effect resulting from the Lorentz force; and (2) magnetization effect originating from the magnetic force.

2.1. Magnetohydrodynamic Effect

It is well-known that an SMF changes the melt flow, e.g., electromagnetic braking [21] and thermoelectromagnetic convection (TEMC) [6,22,23]. The former is used to suppress fluid flow and widely applied to industrial production such as continuous casting. The latter induces interdendritic forced convection and further modifies solidification structures. In principle, the above-mentioned magnetohydrodynamic effects originate from the interaction between the SMF and the electric current.

Ohm’s law relates the electric current density to the generalized electric field (E) by

$$j=\sigma E+\sigma u\times B-\sigma S\nabla T$$

(1)

where σ is the electric conductivity, u is the velocity, $\nabla$T is the temperature gradient and S is the thermo-electric power of the medium. The term $\sigma E$ is the current directly generated by the applied electric field. The term $\sigma$u × B is known as the induced current, owing to fluid flow. The term $\sigma$S∇T is the internal electric current near the interface owing to the thermoelectric effect.

In the SMF, the Lorentz force F_EM is expressed as

$$F_{EM}=j\times B$$

(2)

Assuming that the fluid is incompressible, the fluid flow is governed by Navier–Stokes equation.

$$\begin{gathered} \rho \frac{\partial u}{\partial t}+\rho \left(u·\nabla u\right)=-\nabla p+j\times B+\rho \nu {\nabla }^{2}u \\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}=-\nabla p+\sigma E\times B+\sigma \left(u\times B\right)\times B-\sigma S\nabla T\times B+\rho \nu {\nabla }^{2}u \end{gathered}$$

(3)

This combines with the mass conservation equation:

$$\nabla ·u=0$$

(4)

where $\rho$ is the density of the fluid, p is the pressure and $\nu$ is the kinematic viscosity of the fluid.

From Equation (3), one can obtain that the term $\sigma \left(u\times B\right)\times B$ is the braking force, whereas $\sigma S\nabla T\times B$ is a driving force for the fluid flow. During dendrite growth in the SMF, the contributions of two effects to the melt flow vary when increasing the SMF [6].

Additionally, the thermoelectromagnetic force (TEMF), which stems from the internal electric current acting on the dendrite ($F_{TE}^{\prime }$), can be written as

$$F_{TE}^{\prime }=\frac{{\sigma }_L{\sigma }_S{f}_L}{{\sigma }_L{f}_L+{\sigma }_S{f}_S} \left(S_S-S_L\right)\nabla TB$$

(5)

where the subscripts S and L denote the liquid and solid, respectively. The TEMF leads to the formation of numerous dislocations in the dendrites and even to fragmenting the dendrite, which have been observed in previous studies [6].

In summary, the thermoelectromagnetic effects during dendrite solidification are schematically described in Figure 1 [24].

2.2. Magnetization Effect

Since every substance has its own magnetism, scientists hope to use magnetism to control metal solidification processes. The magnetism can be characterized using magnetic susceptibility, which is defined using the following equation.

$$M=\chi H$$

(6)

where χ is the volume magnetic susceptibility, M is the magnetic magnetization, and H is the magnetic field strength. The magnetic susceptibility closely relates to some magnetic effects, such as the magnetic force and orientation.

When the metal is placed in the magnetic field gradient, it is subjected to the magnetic force (F_m), which is written as:

$$F_m={\mu }_0\left(M·\nabla \right)H=\frac{\chi }{{\mu }_0}\left(B·\nabla \right)B$$

(7)

The magnetic force can be used to separate minerals, to levitate the substances, and to control flow.

Additionally, the magnetization energy of the material is induced by the SMF and it is defined as

$$E_m=-\underset{0}{\overset{H}{{\displaystyle \int }}}{\mu }_{0}MdH$$

(8)

The magnetization energy may modify the phase boundary, kinetics of phase transformation, orientation and microstructures [25]. The orientation is schematically described in Figure 2.

3. SMF-Regulated Metal Solidification

As mentioned above, there have been excellent reviews regarding solidification processing of metallic materials, as referred to in the literature [5,6,7,8,10,12,13,26]. The objective of this section is to summarize the research findings in recent years.

3.1. SMF Effects

3.1.1. SMF-Induced Undercooling

The degree of undercooling is an important parameter affecting metal solidification. A large undercooling can refine grains, reduce segregation and realize non-equilibrium effects [14,15]. It was found that the SMF changed the undercooling of different metal melts and some possible explanations were proposed, such as the delayed formation of the nucleation-catalyzed oxides [27] and the increase in solid/liquid interfacial tension due to the magnetic dipolar interaction [28]. Nevertheless, these explanations are not convincing.

Typically, a recent work regarding the change in the undercooling of Sn, Al, Zn and three Al-Cu alloy melts in different SMFs was performed by Rui Guo and his co-workers [29,30,31] (Figure 3). They proposed that the change in undercooling is related to the increase in the liquid/solid interfacial free energy and to the reduction in migration rates of atoms in the liquid phase in the SMFs. This mechanism is schematically illustrated in Figure 3. When the movement direction of liquid atoms is not parallel to the SMF, these atoms will spirally move and the probability of a collision with other atoms will increase under the influence of Lorenz force. This leads to an increase in the transferring time of the atom to nucleus, which, in turn, enhances the undercooling.

Wang et al. systematically investigated the effect of the SMF on the degree of undercooling [32,33]. It was found that the undercooling for homogeneous nucleation is irrelevant to the SMF intensity, but influenced undercooling for heterogeneous nucleation, as shown in Figure 4. It was thought that the phenomena can be interpreted using magnetohydrodynamic effects. Interestingly, they found that the highest temperature after recalescence is altered by the SMF and the gradient SMF depressed the mean undercooling.

3.1.2. Additional Interfacial Tension Induced by SMF

The interface tension influences the nucleation, growth and morphology [1]. In light of the importance of interface phenomena, some attempts have been made to explore the change in the interfacial tension of inorganic liquids in an SMF [34,35]. Recently, investigation into the interfacial properties of metallic systems in an SMF have been presented [36,37,38,39,40,41].

Li et al. investigated the change in the wettability of a solid by a liquid metal in the SMF [16]. They applied a sessile drop technique to systematically observe the effect of an SMF on the wettability of copper and silica substrates by a Ga-In-Sn liquid alloy, respectively (Figure 5). It was found that the contact angles on both substrates increased with increasing the SMF intensity. The reduced wettability can be attributed to the change in liquid surface tension under the action of an SMF. They further developed an equation to predict the variation in the contact angle with the SMF intensity, as follows

$$\frac{1+\cos \theta }{1+\cos {\theta }_m}=1+\frac{1}{3}BL\sqrt{\frac{\sigma }{\eta }}$$

(9)

where θ and θ_m are the contact angles with and without an SMF; and B, L, σ and η are the SMF intensity, characteristic length, electric conductivity and viscosity, respectively. The above equation can reasonably predict the wettability as a function of the SMF intensity, as shown in Figure 5.

More recently, Huang et al. performed experiments of measurement solid/liquid interfacial free energy of Al-Cu alloy system under SMF [42]. They improved the grain boundary groove method to measure the interfacial free energy. It was found that the interfacial free energy between the solid α-Al and liquid Al-Cu system increased by 31.5%, while this value of the solid CuAl₂-liquid Al-Cu system was found to decrease by 46.17%. The microstructure showed that the CuAl₂ phase nucleated on the surface of α-Al phase and, thus, the high Cu concentration in the liquid phase was formed. The entire liquid Cu-rich phase has diamagnetism due to the magnetic susceptibilities of the atoms in the liquid phase (Figure 6). It follows that the SMF promotes the formation of magnetic dipoles on the solid–liquid interface and, thus, resulted in the change in the surface tension.

3.1.3. Dendrite Coarsening in SMF

The dendritic growth kinetics of crystals is one of the important issues in various processes of metal solidification. Gao et al. measured the dendritic growth velocities of pure nickel in the SMF [43,44]. Dendrite coarsening, as an aspect of dendritic growth, affects the final secondary dendritic arm spacing (SDAS) and, thus, plays a crucial role in microsegregation [45,46,47,48,49,50,51,52]. Some investigations imply that the SMF possibly modifies the coarsening of dendrites [53,54,55,56]. Nevertheless, there has been no work on coarsening kinetic under the SMF.

He et al. investigated dendrite coarsening in the Al 4.5 wt% Cu alloy in the SMF by the quenching technique [17]. The experimental results showed that the SDAS increased in the SMF (Figure 7), which is attributed to the TEM convection as well as the change in solid/liquid interfacial tension. The coarsening of SDAS in the SMF led to a long diffusion path across the dendrite arm and enlarged the microsegregation level.

3.1.4. SMF-Induced Grain Refinement

The grain refinement is beneficial for improving the mechanical performance [57,58,59,60]. Conventionally, the adding of the grain refiner or increasing the cooling rate are the preferential methods for grain refinement [61,62,63]. Nevertheless, those methods have some limitations, e.g., the grain refiner either increases the content of impurities or the effective grain refiner lacks a given alloy. The cooling rate is hardly changed in a large ingot. It was found that the SMF provides a potential method for grain refinement. The grain refinement has been observed in the solidified Al-base alloys, Pb–Sn alloys and Ni base superalloys under an SMF [18,64,65,66]. Recently, our group took advantage of the TEM effect to refine grains in directionally solidified GCr18Mo steel (Figure 8) [67]. We found that the grain refinement was enhanced with the increase in the SMF intensity and in temperature gradient but the refinement was weakened with a decrease in the growth speed. The enhancement of the grain refinement in the SMF can be rationalized by the TEM convection in the melt and the TEM force acting on the dendrite during directional solidification.

Meanwhile, the SMF can reduce the microsegregation level in the directionally solidifying GCr18Mo steel. Figure 9 presents that the fluctuation range of the Cr content at the position of 20 mm from the solid/liquid interface decreases as the SMF intensity increases.

Further, we obtained a process window for the columnar to equiaxed transition of GCr18Mo steel in SMFs (Figure 10) [68]. Applying 5 T SMF can form equiaxed grains below the cooling rate of 0.312 K/s under the temperature gradient of 104 K/cm, which may potentially be used to control the columnar to the equiaxed transition of the steel casting (10⁻¹−10⁰ K/s).

3.1.5. SMF-Controlled Segregation

The reduction in the microsegregation level during solidification can shorten the subsequent heat treatment process and also improve the mechanical performance [47,69,70]. Previous experimental studies have shown that the SMF was capable of modifying the extent of microsegregation during alloy solidification [19,71,72,73,74]. However, these studies were mostly limited to qualitative analysis with respect to the effect of the SMF on segregation. He et al. experimentally investigated the microsegregation formation of two Al-Cu alloys directionally solidified in the SMF and theoretically explored the contributions of various processing factors such as undercooling, back diffusion, dendrite coarsening and melt flow to microsegregation using the modified microsegregation model (Figure 11) [75]. It was found that the microsegregation level was alleviated for cellular and dendritic growth while the amount of microsegregation increased for planar growth under the SMF. Considering contributions of the change in various factors in an SMF, the change in the microsegretation level was in agreement with the experiment result.

3.1.6. SMF-Controlled Porosity

The shrinkage porosity due to the sample shrinkage or volume variation at the final stage of solidification plays a negative role in the hot rolling and final performance of the metallic materials [2,57,76]. Adopting effective strategies to eliminate the shrinkage porosity is one of the most attractive research subjects for metallurgists [77,78,79,80]. By imposing a 29 T SMF, Zheng et al. found the morphologies of shrinkages are changed to approximately regular spheres and the average volume of shrinkages become larger, resulting from the suppression of melt convection (Figure 12) [20].

Nevertheless, the TEM convection occurs ahead of solid/liquid interface and accelerates the motion of fluid in the mushy zone. The present authors systematically investigated the effect of TEM convection on the shrinkage porosity at the final stage of the solidification of GCr18Mo steel in the SMF (Figure 13) [81]. It was found that the amount of porosity first decreases and then increases with increasing the SMF intensity. The decrease in porosity is attributed to an adequate development of the TEM convection, which leads to sufficient feeding. This study sheds light on an alternative technique for eliminating the shrinkage porosity at the final stage of alloy solidification in industrial applications.

3.1.7. Crystal Orientation in SMF

Magnetic orientation is one of the important magnetic effects [82,83]. Various alloys, including Zn-Bi alloy [84], Bi-Mn alloy [85], Al-Fe alloy [86], Fe-Tb alloy [87], etc., have been employed to orient in the SMF. Recently, the SMF was used to fabricate the oriented silicon steel, which has excellent magnetic properties such as high magnetic permeability and low core loss. Nevertheless, it is hard to obtain [1 0 0] crystal orientation by conventional methods [88,89]. Liu et al. investigated the influence of a high SMF on the crystal orientation and magnetic properties of the Fe-4.5 wt% Si alloy (Figure 14) [90]. It was found that the crystal orientation of the Fe-4.5 wt% Si alloy turns to the easy magnetization axis [1 0 0] with the increase in magnetic field intensity due to the magnetization force on the Fe-4.5 wt% Si alloy. In addition, the oriented Fe–6.5 wt.% Si–0.05 wt.%B and Fe-1.0 wt.% Si alloy were also obtained through controlling the crystallization process and adjusting the magnetic field intensity [91,92].

3.2. Possible Application of SMF to Rapid Solidification

Additive manufacturing has been gaining increasing attention in recent years [93,94,95,96]. Additive manufacturing has significant features, such as a high temperature gradient and rapid solidification at the micro-sized molten pool [97,98,99]. In this case, appreciable TEM convection most likely appears. Following the idea, Zhao et al. applied an SMF to the laser additive manufacturing in order to achieve the refined grain structure and to enhance mechanical properties without post-treatment and composition changes (Figure 15) [100]. It was found that the SMF of 0.55 T changed the microstructure with twisted prior-β grains (strong <001> orientation to weak <110> orientation) and discontinuous α grain boundaries. The sample without the SMF displayed coarse columnar prior-β grains, whereas the sample with SMF displays refined columnar prior-β grains.

Apart from additive manufacturing, a number of researchers have reported that the SMF exerted a significant effect on rapid solidification, including pool morphology and microstructure evolution [101,102,103,104]. However, the application of the SMF to advanced manufacturing is still a new field and there are quite a few fundamental scientific questions to be addressed.

3.3. Application of Synchrotron X-ray Imaging to Solidification with SMF

The in-situ synchrotron X-ray imaging technology is a powerful tool for revealing the physical mechanisms of various solidification phenomena [105,106,107]. In consideration of amazing magnetic effects to be clarified, it is of significance to applying the advanced synchrotron X-ray imaging technology to observe dendrite growth in the SMF [108,109,110,111,112].

Applying in-situ synchrotron X-ray imaging, Wang et al. found the motion of detached fragments of dendrite in Al-4 wt.% Cu in a weak SMF [113]. Abou-Khalil et al. took a step further to gain the trajectory and velocities of the equiaxed grains during the solidification of Al-10 wt% Cu alloys, which were in good agreement with the numerical prediction (Figure 16) [114]. Cai et al. employed high-speed synchrotron X-ray tomography and computational simulation to reveal the mechanisms disrupting the columnar dendritic growth and demonstrated that the effect of the MHD can be changed by adjusting the alloy composition, which provides novel methods for optimizing the application of MHD during alloy solidification [115].

A 2D metallography or SEM observation is insufficient to elucidate the effect of the SMF on 3D dendritic morphology and the solid/liquid interface shape, especially the TEM convection ahead of the solid/liquid interface [116]. In-situ synchrotron X-ray imaging technology can give insight into the 3D dendrite morphology. Shuai et al. observed the 3D solid/liquid interface morphology and dendrite growth direction of Al-10 wt.% Zn alloy directionally solidified in the SMF of 5 T using X-ray micro-computed tomography (Figure 17a,b) [117]. In addition, the formation and growth dynamics of intermetallic compounds in a Al-10 wt.% Si-1 wt.% Fe alloy during solidification in the SMF were exhibited using synchrotron X-ray tomography (Figure 17c). This work provides the possibility of regulating the precipitation behavior of Fe-rich intermetallic compounds in Al alloys [118].

4. Conclusions and Perspective

SMF-regulated metal solidification is a novel strategy to improve the solidification structures and mechanical properties of as-cast products. The SMF can generate the electromagnetic force and the magnetic force to modify the solidification processes, such as undercooling and nucleation, interface tension, grain coarsening and refinement, segregation, and porosity. Although much effort has been made to investigate solidification processing in the SMF, there are still a lot of fundamental and practical aspects to be addressed in the future.

(1)

Development of low-cost and large-bore strong magnets

Over the past two decades, research has been very active in the field of materials processing in the SMF and many novel findings have been obtained. However, most studies are still performed in laboratories and almost no findings on the SMF have been applied to industrial products (beside nuclear magnetic resonance and single crystal silicon growth). One of the important limits is the lack of low-cost and large-bore strong magnets. It is necessary to develop low-cost and large-bore strong magnets because magnetic effects would be more remarkable in the high SMF. Additionally, fabricating large-size samples is limited in a small bore magnet. Another reason is that a high-cost magnet limits a wide application of the SMF. Reassuringly, the rapid development of low-cost room-temperature superconducting materials is expected to open up prospects for the application of the SMF in industrial applications.

(2)

Development of in-situ SMF testing devices for high temperature

Thermophysical parameters should be considered carefully since the SMF, especially a high SMF, may change the values of the thermophysical parameters of metallic materials. Hence, it is imperative to exploit an in-situ SMF device to detect the variation in solidification parameters, such as thermoelectric current and melt flow velocity. Meanwhile, the in-situ measurement of the SMF is conducive to revealing the physical nature of solidification phenomena in the SMF. Although a few scientific apparatuses have been developed, such as thermal analysis apparatus and tensiometer, more standard and commercial instruments should be developed to be adaptable to the conditions of high temperatures in the SMF.

(3)

Fundamentals of magnetic effects

Materials processing in the SMF is a new interdisciplinary field. Although many new phenomena have been discovered over the past decades, a lot of fundamental scientific questions in the SMF including nucleation, growth, interfacial phenomena, diffusion, etc., are still to be clarified. Furthermore, more processing methods in combination with the SMF are expected to bring forward the optimization of microstructures and mechanical properties of metallic materials. Hopefully, the SMF will become a general and powerful tool to control solidification processing in practical application in materialists’ and metallurgists’ efforts in the near future.