Research for the Optimal Structure

Magnetic Fields Optimise Metal Alloys

November 03, 2008

To see how liquids can be made to flow, without being directly heated or touched, you only have to watch a raw egg explode in a microwave oven. Electromagnetic forces can even melt metal at hotter than 1,000 °C. In the Magnetohydrodynamics study group at the Research Centre Dresden-Rossendorf (FZD) these complex interactions between electrically conductive liquids and magnetic fields are used to control the flow and solidification processes of liquid metal alloys. The aim is to create optimised production processes for foundries. To analyse the microstructure of the metal, the scientists use an automated system consisting of a high-end microscope and Power Mosaic image recording software that scans large surfaces of the samples in high-resolution single frames and combines these as a precise overall image for quantitative analysis.

Magnetic fields for optimised technologies in material processing

The automobile and aerospace industries in particular use components made of special alloys which have to satisfy constantly growing quality specifications. There is a demand for thinner, i.e. lighter castings. Sometimes they have to be more complex or larger at the same time and still have to withstand increasing loads. The stability and load bearing capacity of an alloy mainly depends on its microstructure. Magnetic field flow during the solidification of metal alloys determines the transport of heat and material in the melt and thus the nucleation and grain growth. An ideal, i.e. uniform fine-grain microstructure therefore requires controlled solidification of the casting.

The German Community of Research Scientists (DFG ) has set up a collaborative research centre called "Electromagnetic Flow Control in Metallurgy, Crystal Growth and Electrochemistry" at the Technical University of Dresden, the Research Centre in Dresden-Rossendorf, the Leibniz Institute of Solid State and Material Research in Dresden and the TU Bergakademie Freiberg to study specially tailored magnetic fields for optimised technologies in material processing. The application potential is tremendous. Nearly all industrial metals are obtained from metal melts. The advantages of electromagnetic stirring in terms of controllability and absence of contact are also being utilised here to research the influence of flow structures on the solidification process of metal alloys.

Fig. 1: Diagram of the experiment set-up

Optimal non-contact stirring

The scientists at the FZD are conducting a subproject together with the TU Dresden in which they perform solidification experiments using lead-tin and aluminium-silicon alloys under the influence of rotating magnetic fields (RMF) (Figure 1 shows a diagram of the apparatus). The aim is to obtain materials with a fine-grained isotropic structure with almost spherical crystals, also called globulites. Normally, the morphology of many alloys is dominated by columnar dendrites. As materials that solidify in the form of globulites exhibit significantly better mechanical characteristics, the growth of dendrites can be prevented by means of magnetically driven flows in the melt bath. On the basis of already wellinvestigated RMF flows, the scientists at the FZD examined the complex physical phenomena during the controlled solidification process to be able to elaborate an optimal stirring strategy for foundry applications [4–6].

Detailed insights into the structure of RMF-induced flow during solidification are given by numerical simulations carried out in a sub-project at the Institute of Aerospace Engineering of the TU Dresden. The basic findings obtained from the analysis of the flow structures during the solidification process and their influence on the heat and mass transfer in the melt will be directly used in another sub-project at the Foundry Institute of the TU Bergakademie Freiberg and transferred to real castings for aluminium and magnesium alloys.

Fig. 2: Light microscope image of an Sn-15M.%Pb alloy directionally solidified without a magnetic field

Modulated magnetic fields

The experiments showed that, given constant cooling conditions, the proportion of the globulitic structure volume depends directly on the type and intensity of the electromagnetically driven flow and can be controlled by a defined setting of the magnetic field parameters. For instance, a distinct grain refinement was detected in the microstructure after electromagnetic stirring (Figures 2 and 3). At the same time, however, undesirable flow-specific segregations were observed. The Dresden scientists are therefore looking for a specific flow pattern that leads to a fine-grained globulitic structure, but not to any segregation of the phase components.

Results from the numerical simulations show that a controlled modulation of the magnetic field amplitude may generate a suitable flow pattern that significantly reduces the degree of segregation. This was taken as the basis for developing concepts for optimising the time functions for the magnetic field parameters amplitude and frequency which are examined in the solidification experiments. This approach has already brought the first clear success [1, 2].

To obtain a better understanding of correlations between the flow field ahead of the solidification front and the features of the solidification structure, the ultrasound Doppler technique was further developed at the FZD for applications in metal melts. Using this technique, flow speeds can be measured during solidification in the liquid phase for the first time [7].

Fig. 3: Light microscope image of an Sn-15M.%Pb alloy directionally solidified under the influence of a rotating magnetic field.

Fig. 4: Photograph of a directionally solidified Al-7M.%Si alloy: Solidification in the lower area without a magnetic field leads to columnar growth. Switching on a temporally modulated magnetic field leads to globulitic grain growth in the upper part of the sample.

Quantitative analysis of microstructures

Macroscopic examination of solidified metal cylinders already shows distinct differences caused by the influence of RMF, or how columnar morphology can be changed into globulitic morphology by temporal or spatial variation of the magnetic field (Figure 4). Microscopic analysis of cross- and longitudinal sections of the 5 cm thick and 6 cm tall sample cylinders enables quantitative analysis of grain size, phase distribution and, in particular, the proportional volume of globulitic structure (Figure 5). To obtain a high-resolution image of the entire surface of the sample, the Rossendorf scientists use the high performance image recording software Leica LAS Power Mosaic combined with a Leica DM6000 M automated research microscope (Figure 6). The polished metal sections are automatically scanned at a rate of about 400 frames per minute and a complete image is produced at the full camera resolution.

Fig. 5: Proportion of volume of the eutectic phase in the longitudinal section of an Al-7M.%Si sample. A temporally modulated magnetic field was switched on during solidification.

Fig. 6: Photograph of the longitudinal section of an Sn-38M.%Pb alloy. A temporally modulated magnetic field was switched on during solidification.

The system generates a mosaic of single images at high speed, high accuracy and focusing in manageable data formats. Special advantages of the product from Leica Microsystems: the optimized offset of the image overlap of 16 pixels between the individual images and the autocalibration function directly at the sample to compensate for the finest inaccuracies of stage movement and optics. The individual image size and the compensation of the camera rotation are automatically set by the system. The focus positions can be interpolated over a random number of reference points depending on the surface topography of the sample. The precise control of the microscope stage is crucial for the entire scan and the image recording speed. A fast digital camera triggered via the stage records the images as soon as the calculated position has been reached, without the stage having to stop. If required, a 3D reconstruction over various focal planes can be carried out using a special xyz control board. In this case, however, several images have to be recorded per image field, which slows down the recording speed.

"Compared to the days when we had to compose the single images by hand, the automated LAS Power Mosaic Software from Leica Microsystems saves us a lot of time and effort. We are now able to obtain quick and efficient quantitative analysis over the whole cross section of the sample", emphasizes Dr. Sven Eckert from the FZD. "Apart from this, we have to document and archive all experiment data at the FZD. Here too, the software provides far easier work routines than the ones we did before. We can manage nearly all the main analysis of the metal microstructure with light microscopic techniques. Further quantification of the results with an electron microscope is only necessary for special findings."

References

  1. Willers B, Eckert S, Nikritjuk P, 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. Metall Mater Trans 39B (2008) 304–316.
  2. Eckert S, Nikrityuk PA, 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. Metall Mater Trans 39B (2008) 374–386.
  3. Eckert S, Gerbeth G, Räbiger D, Willers B, Zhang C: Experimental modelling using low melting point metallic melts: Relevance for metallurgical engineering. Steel Res Int 78 (2007) 419–425.
  4. Eckert S, Willers B, Michel U: Directional solidification of Pb-Sn alloys affected by a rotating magnetic field. Int Foundry Res 58 (2006) 38–46.
  5. Willers B, Eckert S, Nikrityuk PA, Eckert K, Michel U, Zouhar G: Application of a rotating magnetic field during directional solidification of Pb-Sn alloys: Consequences on the CET. Mater Sci Eng A413–414 (2005) 211–216.
  6. Willers B, Eckert S, Michel U, Haase I, Zouhar H: The columnar- to-equiaxed transition in Pb-Sn alloys affected by electromagnetically driven convection. Mater Sci Eng A402 (2005) 55–65.
  7. Eckert S, Willers B, Gerbeth G: Measurements of the Bulk Velocity during solidification of Metallic Alloys. Metall Mater Trans 36A (2005) 267–270.


This article appeared in: Quality Engineering, 10/2008, Konradin Verlag, Ernst-Mey-Str. D-70771 Leinfelden-Echterdingen, Germany, www.qe-online.de

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