Microstructure influenced by electromagnetic stirring
"The solidification of metallic and non-metallic melts leads to the formation of crystallites (grains), ... The crystal growth largely determines the structure and the properties of the cast materials."  The microstructure is determined by the chemical composition of the alloy and the solidification conditions. The application of electromagnetic fields allows a contactless influence on solidification conditions, such as cooling rate, temperature gradient, temperature fluctuations and concentration gradient. On the one hand this is achieved by introducing an additional volumetric heating (inductive heat coupling), on the other hand a volumetric force (Lorentz force) can generated both an forced convection as well as an damping of an existing convection.
The focus of this field of work is on the one hand to gain knowledge about how flow influences the structure and on the other hand how one can create tailor-made microstructures by a specific control of the convective transport processes in the melt. The aim is to optimize the microstructure in an application-specific manner while reducing the energy requirement or saving on aggregates, such as chemical grain refiner in aluminum alloys. 
In order to understand the influence of flow on solidification structure, a sufficient knowledge of the flow conditions is a prerequisite. For this purpose, velocity measurements are carried out by means of Ultrasonic Doppler Velocity (UDV) method in melts. In order to reduce the experimental effort and improve the technical measurement accessibility investigations in a cold melt are carried out. For this purpose, the eutectic alloy of Ga, In and Sn is very suitable because it is non-toxic and the liquidus temperature is with 10.5 ° C below room temperature. An experimental setup for flow measurement is shown in Figure 1. Based on these measurements, the magnetic fields can be adjusted to produce a tailored flow in the melt. The measurement results are transferred to technical alloy systems by dimensionless number.
For the solidification experiments, PbSn and AlSi alloys are used. Various magnet systems and molds (examples in Figure 1) allow the realization of a large variety of parameters with regard to the solidification conditions.
For the qualitative and quantitative microstructure analysis, the solidified cylindrical samples are cut axially and prepared over a large area. In addition, mechanical characteristics can be determined.
|Figure 1:Principle experimental opportunities: Plexiglas vessel filled with GaInSn eutectic alloy for isothermal fluid flow investigations (left), stainless steel mould for solidification experiments (middle) and combined magnetic systems (right)|
In general, a flow leads to a better mixing in the liquid phase. In most engineering alloys, the solubility in the liquid and solid phases is different. As a result, during the phase transition, a concentration gradient builds up before the solidification front. A flow causes a homogenization of the temperature and concentration field in the volume. This results in a larger temperature and concentration gradient in front of solidification front. If the flow is turbulent, additional fluctuations occur in the concentration and temperature field.
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|Video 1: Solidification of an hypereutectic PbSn alloy under the influence of an rotating magnetic field|
On the left in Figure 2, the macrostructure is visible on the probe surface of a Sn - 15 wt.% Pb alloy. In the first phase, the metal solidifies without convection. This is realized by cooling from below. In this composition initially crystallizes out a Sn solid solution. The lead-rich solute is heavier, resulting in stable density stratification. The Sn solid solution grow up against the temperature gradient, creating a structure with vertical columnar grains. After approximately 25% of the sample has solidified, a rotating flow was generated with a magnetic field.
Due to the horizontal boundary layers, this rotating flow generates a secondary meridional flow in the form of two toroidal vortices (right in Figure 2). The resulting helical flow structure leads to the preferred growth of the columnar grains in the direction of the new temperature or concentration gradient. This takes place because the transported tin-rich melt to the solidification front by the flow and the local increasing of undercooling. Warmer fluid flows down the sidewalls and then along the solidification front into the interior of the sample. On this way it cools down and enrich by lead which is segregate at the solidification on the dendrite tips and in the upper part of the mushy zone. The radial transport of the lead-rich melt leads to the formation of macroscopic segregations along the axis. In the upper half of the sample, equiaxial growth occurs with a sharp transition (CET). 
|Figure 2: Solidification of an SnPb alloy with an vertical temperature gradient. An rotational convection has been generated after the first quarter was solid: solidified body (left), frontal section through the solidified body (middle), eutectic content in the frontal section (right)|
Advanced stirring methods have been developed at HZDR where the use of time-modulated magnetic fields significantly minimizes or even prevents the formation the macrosegregation. At the same time, the stirring action is strong enough to achieve grain refinement. These by means of electromagnetic stirring grain-refined structure offer an increase in strength. In addition, the scatter of mechanical properties in the sample is reduced. In addition to the use of alternating magnetic fields, further methods to obtain grain refined structures are proposed in the literature. It has shown that the application of strong current pulses leads to grain refining. The associated mechanism of action has been discussed controversial for long time. Experiments at HZDR in the recent past have been show that intensive forced convection in the melt is also generated with this method. These forced convection is the reason of grain refinement by means of current pulses. By combination with a permanent magnetic field, the flow intensity can be further increased. Another promising field of application of magnetic DC fields is the flow control during mold filling in casting processes. By settle down the inflowing melt and stabilizing the free andurface, flow-induced casting defects, like the emergence of pores, can be dramatically reduced.
Current research topics are the effect of electromagnetic microstructural influence on the compressibility of wrought aluminum alloys and the influence of flow on the precipitation morphology of intermetallic phases.
 Schatt, W. und Worch, H.: Werkstoffwissenschaften, 8. Auflage, Deutscher Verlag für Grundstoffindustrie Stuttgart, 1996
 Metan, V.; Eigenfeld, K.; Räbiger, D.; Leonhardt, M.; Eckert, S.: Grain size control in Al-Si Alloys by grain refinement and electromagnetic stirring, Journal of Alloys and Compounds 487(2009)1-2, 163-172
 Willers, B.; Eckert, S.; Nikrityuk, Petr A.; Räbiger, D.; Dong, J.; Eckert, K.; Gerbeth, G.: Efficient melt stirring using pulse sequences of a rotating magnetic field: II Application during solidification of Al-Si alloys, Metallurgical and Materials Transactions B 39(2008)2, 304-316
 Willers, B.; Eckert, S.; Michel, U.; Haase, I.; Zouhar G.: The columnar-to-equiaxed transition in Pb-Sn alloys affected by electromagnetically driven convection, Materials Science and Engineering A 402, pp.55-65, 2005
 Räbiger, D.; Willers, B.; Eckert, S.: Flow control during solidification of AlSi-alloys by means of tailored AC magnetic fields and the impact on the mechanical properties, Materials Science Forum 790-791(2014), 384-389
 Räbiger, D.; Zhang, Y.; Galindo V.; Franke S.; Willers B.; Eckert S.: On the relevance of melt convection to grain refinement in Al-Si alloys solidified under the impact of electric currents, Acta Materialia 79 (2014), 327-338
 Eckert, S.; Galindo, V.; Gerbeth, G.; Witke, W.; Buchenau, D.; Gerke-Cantow, R.; Nicolai, H.-P.; Steinrücken, U. Strömungskontrolle bei Formfüllung mittels Magnetfeldern, Giesserei 92(2005)5, 26-31
further publications about this topic
 Zhang, Y.; Räbiger, D.; Willers, B.; Eckert, S.:
The effect of pulsed electrical currents on the formation of macrosegregation in solidifying Al - Si hypoeutectic phases
, International Journal of Cast Metals Research 30(2017)1, 13-19
 Räbiger, D.; Zhang, Y.; Galindo, V.; Franke, S.; Willers, B.; Eckert, S.: Experimental study on directional solidification of Al-Si alloys under the influence of electric currents, IOP Conference Series: Materials Science and Engineering 143(2016)1, 012021
 Kaya, H.; Çadirli, E.; Gündüz, M.; Räbiger, D.; Eckert, S.: Depencency of Structure, Mechanical and Electrical Properties on Rotating Magnetic Field in the Bi-Sn-Ag Ternary Eutectic Alloy , International Journal of Materials Research 107(2016)4, 362-371
 Zhang, Y.; Räbiger, D.; Eckert, S.: Solidification of pure aluminium affected by a pulsed electrical field and electromagnetic stirring, Journal of Materials Science 51(2016)4, 2153-2159
 Cadirli, E.; Kaya, H.; Räbiger, D.; Eckert, S.; Gündüz, M.: Effect of rotating magnetic field on the microstructures and physical properties of Al-Cu-Co ternary eutectic alloy, Journal of Alloys and Compounds 647(2015), 471-480
 Eckert, S.; Nikrityuk, P. A.; Räbiger, D.; Willers, B.; Eckert, K.: Anwendung zeitmodulierter Magnetfelder zur Strömungskontrolle während der gerichteten Erstarrung metallischer Legierungen, Berg- und hüttenmännische Monatshefte 154(2009)3, 117-120
 Eckert, S.; Nikrityuk, Petr A.; Räbiger, D.; Eckert, K.; Gerbeth, G.: Efficient melt stirring using pulse sequences of a rotating magnetic field: I - Flow field in a liquid metal column, Metallurgical and Materials Transactions B 39(2007), 374-386