Micro-Gravity CD-rom

Experiment

Experiment

A.028

Experiment

Unidirectional solidification of cast iron.

Primary investigator

Luyendijk, T.

Co-investigator

Nieswaag, H; Alsem, W.H.M.

Contact point

Technical University Delft, Department of Material Science

Category

Material Sciences

Keywords

Directional solidification
Cast iron
Segregation of sulphur
Space processing

Abstract

Eutectic cast iron with 3.9%C-1.5%Si-0.01%P-0.025%S was solidified unidirectionally in the IHF with gradient device at four different growth rates between 0.1 and 1.0 mm/min. The aim was to study the graphite growth and the sulphur transport in the liquid during solidification under u-g conditions. From the sulphur concentration profile it can be concluded that a stronger convection occurred in space. Transients in the sulphur concentration at changing solidification rate proved, however, that a narrow diffusive boundary layer must be present. From the u-g experiment it appeared that the graphite flakes had been not better aligned than at 1 g. The flight specimen has been separated into two parts in a more pronounced way than observed under terrestial conditions. Cavities were found in the u-g sample at the position where the solid-liquid interface stopped after melting. The plasma-sprayed alumina skin was rigid enough to keep the liquid specimen in its original shape, although a number of cracks could be observed.

Keywords: Directional solidification, cast iron, segregation of sulphur, space processing.

Objectives

The aim was to study the graphite growth and the sulphur transport in the liquid during solidificationunder microgravity conditions.

Reference

ESA SP-222 pg. 79 - 85

Experiment procedure

In a number of ground-experiments carried out at DFVLR this experimental set-up has been approved to be reliable. The life-time of the thermocouples was sufficiently long. Undesirable reactions between skin and cast iron did not occur.

Results

Experiment ES-325, processed in space during FSLP-mission, showed a number of interesting features. One of them is a strong tendency to separation in the liquid column of cast iron, which divided the specimen into two parts. Besides a piling-up of cavities at the melting interface has been observed. Opposite to the observation in the ground-experiment a significant convection appeared in the liquid under micro-gravity conditions, although a small diffusion boundary layer remains present. The surface-active element sulphur, which is less soluble in the solid than in the liquid, seems to be responsible to that convection. Due to this convection the sulphur content just in front of the interface increases gradually during directional solidification. Since sulphur coarsens the graphite structure (Ref. 1), the refining tendency of an increasing growth rate will be opposed. Except the change from 0.1 to 0.4 mm/min, the other transitions in growth rate are not well reflected in the structure due to a higher sulphur content.

From a technological point of view experiment ES-325 has proved, that the skin technology is a good method for obtaining shape stability of the specimen. Due to the experimental conditions in vacuum, account must be given to possible reactions between carbon and ceramic oxydes of the skin. Another disadvantage of working in vacuum is the tendency of evaporation of the metal and alloying elements, especially for materials with a high melting temperature.

A more general problem which seems to appear during space-processing of liquid metals is the unexpected cavity-formation due to the change in specific volume during the solid-liquid transformation. More research should be done on the behaviour of liquid metals in relation to the interaction with the container wall. To control the cavity-formation a directional melting procedure should be developed depending on the shape of the specimen. Mathematical modelling will be helpful in these studies.

ACKNOWLEDGEMENTS
The authors appreciate very much the financial support given by the section "Wetenschapsbeleid" of the State Department " Onderwijs en Wetenschappen". The stimulating discussions with Dr. H. Sprenger are greatfully acknowledged.

Experiment