Microgravity Solidification Technology

By Dr Thoguluva Raghavan Vijayaram PhD
Senior Lecturer, Department of Manufacturing Process and System,
Faculty of Manufacturing Engineering, UTeM, Universiti Teknikal Malaysia Melaka, Ayer Keroh, 75450 Melaka Malaysia. Email: thoguluva@utem.edu.my

Microgravity research in casting metallurgical engineering has grown in importance. Materials processing in space have been studied both theoretically and experimentally for over a quarter of a century. In the beginning, we naively spoke of zero gravity, elimination of convection, growth of perfect crystals, and eventual manufacturing in space. Currently focus has shifted to the use of a microgravity environment, where buoyancy driven convection and sedimentation need to be eliminated to validate physical models, as compared to space manufacturing. One of the research areas that have emerged as the forerunner in such studies is solidification science and technology. This is an obvious choice since solidification processing is an important step in most of the fabrication techniques where the starting material stock has to be cast as ingots. Solidification related transformations are significantly affected by the gravity level. This article provides the basic principles and helps us to understand the influence of microgravity on solidification processing of metals, and alloys.

The dreams of manufacturing perfect crystals using zero gravity began in the 1960’s during the Apollo era. Talk of manufacturing other substances continued through the 1970’s and much of the 1980’s. The space environment was not magic and the materials were not sufficiently better to warrant the costs. The value added did not exceed the additional cost. On the other hand, an immense amount was learned about gravitational effects on materials process, both through the results of space experiments and through related ground-based research. In fact, many results were anticipated and some await full explanation.

This knowledge has proven to be extremely useful in improvement and innovation of materials processing on earth.The study of the different states of matter and their interactions in microgravity is an exciting opportunity to expand the frontiers of science, engineering, and technology. Gravitational attraction is a fundamental and basic property of matter that exists throughout the known universe. A microgravity environment is one in which the apparent weight of a system is small compared to its actual weight due to gravity. In practice, the microgravity environments used by the researchers range from about 1% of the earth’s gravitational acceleration to better than one part in a million. The acceleration experienced by an object in a microgravity environment would be one millionth of that experienced at the earth’s surface.

The principal objective of microgravity materials science research is to gain a better understanding of how gravity driven phenomena affect the solidification and crystal growth of materials. Buoyancy driven convection, sedimentation, and hydrostatic pressure can create defects and irregularities in the internal structure of materials, which in turn alter their properties. Materials science and engineering research in microgravity leads to a better understanding of how materials are formed and how the properties of materials are influenced by their formation. Researchers are particularly interested in increasing their fundamental knowledge of the physics and chemistry of phase changes. This knowledge is applied to designing better process control strategies and production facilities in the laboratories on earth. In addition, microgravity experimentation will eventually enable the production of limited quantities of high quality materials and of materials that exhibit unique properties for use as benchmarks.

Microgravity researchers are more interested in studying various methods of crystallization, including solidification, crystallization from solution, and crystal growth from the vapor. These processes involve liquids, which are the materials that are most influenced by gravitational effects. Examining these methods of transforming liquids into a solid in microgravity gives researchers insight into other influential processes at work in the crystallization process. Metals and alloys constitute an important category of engineered materials and these include structural materials, composite materials, electrical conducting materials, and magnetic materials. Research in this field is concerned primarily with advancing the understanding of metals, and alloys processing so that structure and, ultimately properties, can be controlled as the materials are originally formed. By removing the influence of gravity, metallurgists can more closely observe influential processes in structure formation that occurs during solidification. The properties of metals and alloys are linked to their crystalline structure and chemical structure.

One aspect of the solidification of metals and alloys that influences their microstructures is the shape of the boundary, or interface, which exists between a liquid and a solid in a solidifying material. The development of these different interface shapes and the transition from one shape to another is controlled by the morphological stability, particularly the shape stability, of the interface. This stability is influenced by many factors. Gravity plays an important role in a number of them. In particular, buoyancy driven convection can influence the stability and thus the shape of the solidifying interface. Data obtained about the conditions under which certain types of solidification boundaries appear can help to explain the formation of the crystalline structure of a material.

Another area of research interest in metals, and alloys in microgravity is in the area of multiphase solidification. Certain materials, which are known as eutectics, and monotectics, transform from a single phase liquid to substances of more than one phase when they are solidified. When these materials are processed on Earth, the resultant substances have a structure that was influenced by gravity either due to buoyancy driven convection or sedimentation. But, when processed in microgravity, theory predicts that the end product should consist of an evenly dispersed multiphase structure.

Directional solidification in microgravity has often led to cast ingots that grew with little or no contact with the ampoule wall. When this occurred, crystallographic perfection was greatly improved by several orders of magnitude. Many directional solidification experiments have been performed already in orbiting space craft under microgravity. While some of these experiments yielded ingots whose surfaces differed little from those grown on earth, others were vastly different. In vertical directional solidification on earth, the surface of the resulting ingot replicates the surface of the ampoule or crucible from which it was grown, except for the presence of gas bubbles, and voids along the wall. After correcting for thermal contraction, it is the same diameter as its growth container. In contrast, ingots solidified in microgravity frequently have smaller diameters with surface features strikingly different from those of their containers. However, proper design of experimental setup and details is mandatory for the success of the microgravity experiments.

About the author
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy, and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

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