Cutting Edge Process: Downward Directional Solidification
Directional solidification is a type of solidification that occurs within castings. In other words, it occurs in such a manner that liquid feed metal is always available for that portion that is just solidifying. Downward directional solidification is a novel process that encompasses heat transfer, fluid flow, and solute transport mechanisms. These in turn affect the development of the microstructure and morphology of the phases. Effects on the direction of heat extraction, either downwards or upwards, during solidification of metallic alloys were rarely studied. This is a new approach to solidification. Burden and Hunts were two scientists who carried out experiments with non-metallic alloys. They used NH4Cl/H2O in order to investigate the effects of downward and upward directional solidification on the dendritic growth of primary branches. These scientists have observed through their results that small differences in the solidification thermal parameters do not affect the primary dendrite arm spacing significantly. Instead, the primary dendritic arm spacing is more influenced by the fluid flow generated due to solute rejection. Directional solidification can also be used as a purification process. Most impurities are more soluble in the liquid phase than in the solid phase during solidification. As a result, these impurities will be pushed by the solidification. This causes the finished casting product to have a lower concentration of impurities than the feedstock material. The last solidified metal, however, will be filled with impurities. Thus, that last piece can be recycled 13.
This figure above is an in-house designed directional solidification device that comprises of a thermal system and a transmission system. The thermal system is composed of 1) a controllable electric resistance furnace and 2) a protective/cooling part which consists of a water-cooled copper chill rod and gas-cooled nozzles. The gas-cooled nozzles are in close proximity to the baffle. The transmission system has a withdrawal rate setting that is adjustable. The rate ranges from 0.0003 to 0.33 cm/s. Pure Al (99.99%) is used as a heating medium.
The cooling rates at the positions 5.5 cm and 12.5 cm from the chill end in the rods were measured and recorded into the table below. The changing rates of both positions are also recorded in the table. In the samples solidified by using the downward directional solidification (DWDS) process, the cooling rates at the two locations were 8 times (for 5.5 cm) and 13.5 times (for 12.5 cm), respectively, greater than those in the Bridgman samples.
The morphologies of the primary dendrite at the two locations of the rods solidified by using the DWDS and the Bridgman processes are shown in figure above. Morphologies of ? phase on the transversal sections are located 5.5 cm and 12.5 cm from the chill end. (a) and (c) are the DWDS process. (b) and (d) are the Bridgman process 14.
In the table above, the measured average primary dendrite arm spacings (PDAS) corresponding to the locations and processes are recorded. From the experimental results, one can observe that the primary dendrites are refined better in the DWDS process in than comapred to the Bridgman process. The primary dendrite arm spacings for DWDS process are around 2.3 times smaller than those in the Bridgman process. In addition, the variation in the primary dendrite arm spacing is 1.35 ?m/mm for the DWDS process. On the other hand, this value is 1.92 ?m/mm for the Bridgman process. As a result, it can be concluded that the DWDS process has greater ability to retain the stability of the cooling rates as the distance from the chill end increases when compared to the Bridgeman process.
This figure above shows the morphologies of ? phase in the longitudinal sections which are located 5.5 cm and 12.5 cm from the chill end. (a) and (c) are the DWDS process. (b) and (d) are the Bridgman process. It shows the typical micrographs of the secondary dendrite at various distances from the chill end.
The graph above is a typical secondary dendrite arm spacings at the distances of 5.5 cm and 12.5 cm from the chill end. It presents the measured secondary dendrite arm spacings. This figure indicate that the closer to the chill surface, the finer the secondary dendrite are in comparison to the distant locations. The secondary dendrites solidified by using the DWDS process are significantly more refined when compared to Bridgman process in those locations 14.
Downward directional solidification is transfer of heat through conduction. When the liquid metal gets solidified, heat is being transferred. It is not transferred through convection because there are no material medium in DWDS process. It is not radiation because DWDS process does not deal with electromagnetic waves. The only option is conduction which make sense since none of the particles in the body is displaced after the heat transfer is completed.