Research for the Optimal Structure

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.

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.

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].

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.