Project A4: Magnetic flow control in solidification

PI: Sven Eckert (HZDR)
Partners: HZDR, TUD, RWTH

1. Scientific case for the project

1.1 Background

The use of lightweight materials such as aluminium or magnesium alloys is one of the fastest growing trends in the automotive industry. Substitution of steel by light alloys implies a huge reduction in weight, which in turn has a positive effect on the energy efficiency and C02 emissions of vehicles. At the same time the lightweight materials have to meet exceptional requirements with respect to their mechanical properties in order to ensure the passenger safety during a crash. Grain refinement in lightweight casting products is an important issue in this context offering a number of advantages in foundry operations and in subsequent thermal and mechanical treatment such as a better feeding, reduced chemical segregation, porosity and hot tearing and improved strength characteristics. The inoculation of the melt by grain refining particles makes alloys pourable but may produce undesired particle agglomerates, local defects or impurities. Floating or sedimentation may cause an imbalanced distribution of inoculants leading to an inhomogeneous microstructure.
Forcing a melt flow during solidification by electromagnetic stirring has been shown as another successful way to achieve grain-refined microstructures in non-refined alloys [1,2,5]. Melt agitation at the solidification front promotes dendrite fragmentation by local remelting due to changes of the local temperature and/or concentration. Fluid flow is also important for a transport of the fragments from the mushy zone into the region ahead of the solidification front where they act as origin for equiaxed grains [3]. There are two main reasons why magnetic fields are attractive for industrial technologies: a) they exert a completely contactless influence on the liquid metal, and b) the flow intensity can be elegantly controlled by electric quantities such as the electric current or the frequency. Almost every conceivable flow configuration can be realized by a well-considered tailoring of the electromagnetic forces. This offers a considerable potential to adjust an optimal microstructure within the solidification process. A combined application of inoculation and electromagnetic stirring may improve the efficiency of grain refiners because the melt stirring acts as an effective tool to counteract the sedimentation of the grain-refining particles [6].
Rotating or travelling magnetic fields have already been used in metals casting providing beneficial effects, e.g. a distinct grain refinement or the promotion of a transition from a columnar to an equiaxed dendritic growth (CET). However, melt convection may also produce segregation freckles on the macro-scale especially in solute-rich alloys showing different equilibrium concentrations of solute in the mixed crystal and the surrounding melt [4,7]. Therefore, we follow a new approach using time-modulated AC magnetic fields to control the heat and mass transfer in the vicinity of the solidification front. First studies have shown that this novel stirring method can provide an equiaxed microstructure without chemical segregation. Furthermore, similar stirring intensities, compared to conventional electromagnetic stirring, can be obtained at a reduced input of energy [8]. However, the success of the technique requires a precise tuning of the magnetic field parameters with respect to the material properties, geometry of the casting and the cooling rate.

1.2 Most important goals of the planned work

The main goal of the project is to improve the understanding of the complex interrelation between melt flow and solidification process optionally in presence of a grain refiner. To approach this goal, both model experiments and accompanying numerical simulations shall be performed. The investigation of the flow field resulting from the application of time-modulated AC magnetic fields forms an important basis. The respective flow measurements will be carried out both under isothermal conditions and during solidification. The numerical simulations focus on the impact of pulsed temperature and concentration fields on the grain growth and dendrite fragmentation. X-ray visualizations of the growth process performed under the action of a pulsating melt flow will deliver experimental data for a validation of the numerical predictions. The knowledge gained offers the basis to elaborate suitable methods for an efficient flow control during solidification. An optimal process control with the objective of providing a homogeneous microstructure with reduced grain size has to be achieved by a proper adjustment of both the electromagnetic fields and the type and amount of grain refining particles. The final goal is the development of technologies which deliver casting products with superior mechanical properties, thereby avoiding any additional processing in form of an energy-intensive heat treatment.

2. Existing competencies and infrastructure

The present project benefits from the collaboration between HZDR (Dr. S. Eckert), Access e.V./RWTH (Dr. G. Zimmermann) and TUD (Dr. K. Eckert). The HZDR contributes its potential and experiences to perform model experiments equipped with powerful measuring techniques allowing for in-situ investigations of fluid flow, temperature and concentration field. Several magnetic field systems (MULTIMAG, PERM, KOMMA) are available to provide generic combinations of different kinds of AC and DC magnetic fields at variable field strength and frequency which can be flexibly modulated and tailored by special power supplies. Previous investigations were carried out to characterize the impact of electromagnetic stirring on the microstructure [5]. Modulated AC magnetic fields have been proposed to overcome the handicaps of conventional rotary stirring with the specific goal to generate a vigorous stirring in the bulk without considerable deformations of the free surface [8]. The ultrasound Doppler velocimetry (UDV) will be used for measuring the velocity fields in solidifying metal alloys [9]. Furthermore, X-ray radioscopy can be applied to visualize solidification process at scales down to 5 m. The local composition of the alloy can be determined from the relative brightness in the X-ray images acquired whereas the local flow field is accessible by means of the Optical Flow approach [10].
Access e.V. is an independent research centre associated with the Technical University of Aachen (RWTH). The expertise of Access covers materials research of multi-component and multi-phase alloys together with optimization of solidification processes. Experimental results are used for numerical modeling of the casting process and the microstructure formation. Access operates various gradient furnaces for special applications like liquid metal cooling for high temperature gradient growth, quenching and decanting units for solid-liquid interface investigations, or rotating magnetic fields to induce forced melt flow [11-15]. A large variety of metals like Al-based alloys, titanium aluminides [16] or magnetic shape memory alloys [17] are used to study different kinds of microstructure formation, ranging from single crystal growth to the formation of dendritic, eutectic or peritectic morphologies. For quantitative analysis of solidified micro- and macrostructures Access is equipped with all necessary devices for metallic sample preparation at high quality level, and with experienced technical staff. Different types of light microscopes as well as electron scanning microscopes (SEM) with attached element analysis (EDX) and orientation dependent diagnostic (EBSD) are available. An ion milling system and a Focused Ion Beam Microscope (FIB) allows for 3D analysis of metallic structures in high resolution.
The TU Dresden (Chair of Magnetofluiddynamics) has a long lasting experience in modelling directional solidification processes on various length scales in the presence of AC and DC magnetic fields. While a former focus was on simulating the heat and mass transport in the entire mold using a finite-volume formulation of the transport equations for momentum, concentration and enthalpy [7,18], recent activities are devoted to understand the microstructure formation under transient conditions by means of phase-field simulations employing a finite-element formulation with adaptive meshes. Three-dimensional, fully parallelized simulations can be performed supported by the High-Performance Computing center of the TUD. For both oscillating velocity and temperature fields, as they are characteristic for modulated AC magnetic fields, a resonant growth of the dendrite side-arm structure has been shown in [19]. Highly regular side-arm structures are considered to be prone to fragmentation events for which a novel mechanism has been analysed in [20].

3. Resource planning and Budget Justification

Within the project an experienced scientist is needed at HZDR, who has to contribute for the design of the experimental setup and the appropriate installation of the measuring techniques. The solidification experiments at Access (RWTH) and the numerical simulations at TUD will be conducted by one PhD student each.

Links: Close relations exist to the projects C1, C2, C3, C4 and the YIG.


[1] C. Vives, 1990, Hydrodynamic, thermal and crystallographical effects of an electromagnetically driven rotating flow in solidifying aluminium alloys. Int. J. Heat Mass Transfer, Vol. 33, 2585-2598.
[2] J.K. Roplekar, J.A. Dantzig, 2001, A study of solidification with a rotating magnetic field. Int. J. Cast Metals Res., Vol. 14, 79-95.
[3] A. Hellawell, S. Liu, S.Z. Lu, 1997, Dendrite Fragmentation and the Effects of Fluid Flow in Castings. JOM, Vol. 49, 18-20.
[4] M. Medina, Y. Terrail, F. Durand, Y. Fautrelle, 2004, Channel Segregation during Solidification and the Effects of an Alternating Traveling Magnetic Field, Met. Mater. Trans. B35, 743-755.
[5] B. Willers, S. Eckert, U. Michel, I. Haase, G. Zouhar, 2005, The columnar-to-equiaxed transition in Pb-Sn alloys affected by electromagnetically driven convection. Mater. Sci. Eng. A402, 55-65.
[6] V. Metan, K. Eigenfeld, D. Räbiger, M. Leonhardt, S. Eckert, 2009, Grain size control in Al-Si Alloys by grain refinement and electromagnetic stirring. J. Alloys Compounds, Vol. 487, 163-172.
[7] P.A. Nikrityuk, K. Eckert, R. Grundmann, 2006, A numerical study of unidirectional solidification of a binary metal alloy under influence of a rotating magnetic field, Int. J. Heat and Mass Transfer 49, 1501–1515.
[8] S. Eckert, P.A. Nikrityuk, D. Räbiger, K. Eckert, G. Gerbeth, 2007, Efficient melt stirring using pulse sequences of a rotating magnetic field. Metall. Mater. Trans. B, Vol. 39B, 374-386.
[9] S. Eckert, B. Willers, G. Gerbeth, 2005, Measurements of the Bulk Velocity during Solidification of Metallic Alloys. Metall. Mater. Trans. A, Vol. 36A, 267-270.
[10] S. Boden, S. Eckert, B. Willers, G. Gerbeth, 2008, X-Ray Radioscopic Visualization of the Solutal Convection during Solidification of a Ga-30wt%In Alloy. Metall. Mater. Trans. 39A, 613- 623.
[11] G. Zimmermann, A. Weiss, Z. Mbaya, 2005, Effect of forced melt flow on microstructure evolution in AlSi7Mg0,6 alloy during directional solidification. Mater. Sci. Eng. A, Vol. 413-414, 236-242.
[12] L. Sturz, A. Drevermann, C. Pickmann, G. Zimmermann, 2005, Influence of grain refinement on the columnar-to-equiaxed transition in binary AlSi alloys. Mater. Sci. Eng. A 413-414, 379-383.
[13] G. Zimmermann, L. Sturz, 2007, Microstructure formation in AlSi7Mg alloys directionally solidified in a rotating magnetic field. Steel Res. International, Vol. 78, 379-385.
[14] G. Zimmermann, L. Sturz, M. Walterfang, J. Dagner, 2009, Effect of melt flow on dendritic growth in AlSi7-based alloy during directional solidification, Int. J. Cast Metals Research 22, 335-338.
[15] G. Zimmermann, V.T. Vitusevych, L. Sturz, 2010, Microstructure Formation in AlSi6Cu4 Alloy with Forced Melt Flow induced by a Rotating Magnetic Field, Materials Science Forum 649, 249-254.
[16] U. Hecht, V. Witusiewicz, A. Drevermann, J. Zollinger, 2008, Grain refinement by low boron additions in niobium-rich TiAl-based alloys. Intermetallics, Vol. 16, 969-978.
[17] L. Sturz, A. Drevermann, U. Hecht, E. Pagounis, M. Laufenberg, 2010, Production and characterization of large single crystals made of ferromagnetic shape memory alloys Ni-Mn-Ga. Physics Procedia, Vol. 10, 81–86.
[18] P.A. Nikrityuk, K. Eckert, S. Eckert, 2009, The impact of turbulent flow on solidification of metal alloys driven by a rotating magnetic field, Int. J. Cast Metals Res., Vol. 22, 236-239.
[19]H. Neumann-Heyme, K. Eckert, S. Odenbach, 2011, Free dendrite growth under modulated flow in pure substances: two-dimensional phase-field simulations. IOP Conf. Ser.: Mater. Sci. Eng., Vol. 27, 012045.
[20] S. Ananiev, P.A. Nikrityuk, K. Eckert, 2009, On the role of mechanical stresses in the fragmentation of dendrite arms, Acta Materialia, Vol. 57, 657-665.