The production of metals and their alloys in usable solid form begins, with a few exceptions (high-melting-point metals, e.g., W, Ta, Nb, etc.), with the production of melts due to extraction from ores and their solidification. The process during solidification is significantly more complicated than plastic forming or heat treatment because both the solid and melt phases can move during solidification. One of the reasons for this movement is the difference in density in the melt due to the difference in temperature and concentration. Another reason is the specific volume difference between the solid and melt phases, while the third reason is the melt flow resulting from filling the moulds with melt (e.g., continuous steel casting, aluminium pressure casting, etc.). In some solidification technologies, melt flow is deliberately created (e.g., centrifugal casting, magnetic stirring of melt in continuous steel casting).
In order to understand the processes of solidification, there are two options that assume each other and build on each other: the modelling (simulation) of processes and experiments. The first model describing the microsegregation in solid solutions was the well-known Scheil equation (Bemerkungen zur Schichtkristallbildung, Z. Metallkd., 1942, 34, p 70-72), which is still used many times today. Subsequently, several models and analytical equations were created to describe the various subprocesses (e.g., nucleation, growth, micro and macro segregation, columnar equiaxed transition, etc.). With these models and the analytical mathematical simulations based on them, many subprocesses could be interpreted and explained. However, analytical solutions could not consider the parameters changing during solidification (temperature, concentration, density, thermal conductivity) or only with significant simplifications. A particular difficulty was the simulation of the movement of the phases (melt flow, movement of fragmenting dendrite particles, etc.). A breakthrough was using various numerical methods (Finite Difference, Cell Automata and especially Phase Field), which can consider all changing parameters, and it was made possible by the explosive development of computational technology.
The test of the pudding is in the eating, says a well-known English proverb. So, the results obtained by simulation should be verified by experiments under precisely known conditions. We have three options:
- The results of simulations based on diffusion models that do not consider phase movements by experiments under weightlessness conditions (space station, rocket experiments, drop tower, etc.),
- Simulations considering melt flow at the acceleration of 1g using high-precision ground experiments,
- Simulations considering flows generated by accelerations greater than 1g in devices enabling forced current (centrifuges, mixing by magnetic induction).
In this book, we present the results of experiments carried out in recent years in solidification in the joint Materials Science Research Group of the Hungarian Research Network (HUN REN) and the University of Miskolc (UM). Some research group members have been engaged in solidification research since 1980, when the first and so far, only Hungarian experiments in space materials science (BEALUCA, 1980). Since 2000, they have been involved in the international ESA projects MIcrostructure Formation in CASTing of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions (MICAST) and Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL) for space materials science. For ground-based (1g) comparative experiments, a complex solidification laboratory was built for several unique solidification experiments. To investigate the effect of accelerations greater than 1g, they built inductors to create the Rotation Magnetic Field (RMF) and the Travelling Magnetic Field (TMF). The melt flow due to magnetic induction was studied at 75% Ga25% In melt. The effect of melt flow on the micro and mesostructure of the solidified test for aluminium alloys was investigated.
Dear reader, we hope the methods and results presented in this book will be interesting and valuable in your further research.
