Solidification Processing of Metallic Alloys Under External Fields

The micro- and macrostructure of cast metal is very important from the point of view of casting and downstream processing performance, as it determines the quality of the casting and mechanical properties of as-cast and deformed products. Fine grain structure means uniform distribution of grain size in the billet (ingot) cross-section, elimination of columnar and feathery grains, lesser macrosegregation, uniform and improved mechanical properties in the semisolid and solid states, decreased propensity to hot and cold cracks, etc. Intermetallics and other excess inclusions (oxides, carbides, nitrides, borides, etc.) should also be fine and evenly distributed in the cast matrix. These inclusions are usually intrinsic to the metallic material and result from its composition or contamination. Sometimes, however, the foreign inclusions may be intentionally added to form a composite material or for the purpose of grain refining (acting as substrates).

Since the 1990s, tens of 3^rd generation synchrotron X-ray facilities have been built around the world and made available for research in almost all scientific disciplines. The high brilliance, high coherence and tunable energy of synchrotron X-rays allow researchers in physical and biological science to probe many new dynamic processes in spatial and temporal resolution that are not possible before. This chapter firstly gives a brief review of the advances of X-ray science and the fundamental laws governing the interactions of X-rays with matter, and then focuses on a critical review and discussion of state of the art real-time and in situ studies of the solidification processes using synchrotron X-rays. The emphasis is on new scientific insights and discoveries in solidification science of metallic alloys, which are enabled by synchrotron X-rays based advanced real-time characterization techniques and the future challenges in this fast advancing research field. Although the knowledge, techniques, and practice described in Chap. 2 are generally in the context of solidification processes, they are applicable to many other materials syntheses and manufacturing processes. Readers are referred to the monographs or books published in the field of synchrotron science for broader topics, knowledge, and practice.

Magnetohydrodynamics (MHD) is the scientific field devoted to studying the interaction between electric or magnetic fields and fluid flow in electrically conducting liquids. MHD phenomena appear in a wide range of metal processing applications, either as an unintended consequence of the presence of electric currents during processing or by the deliberate application of external magnetic fields or currents to a process. The allied field of Electromagnetic Processing of Materials (EPM) has many applications, ranging from melting, flow control, melt cleaning, stirring, arc processes (welding, AM, vacuum arc remelting) and electrolytic processes, including recently the field of liquid metal batteries for renewable energy storage. In terms of modeling and simulation, EPM remains a difficult task to master, since in addition to the usual flow, heat transfer, and solidification, one has to address the coupled electric and magnetic fields, often in a dynamic fashion. In this chapter, we divide applications in terms of applied current type, i.e. DC (aluminum electrolysis, vacuum arc remelting) or AC (EM levitation, induction crucibles, PV silicon kerf recycling and the contactless ultrasonic vibration of melts).

As the key parts of Electromagnetic Processing of Materials (EPM), electromagnetic stirring and low-frequency electromagnetic vibration processes have been developed to cause melt motion or vibration. In this chapter, we will firstly give a brief description of the physical principles of electromagnetic melt processing and solidification. Though partially repeating Chap. 3, it is still useful to recall them here. Next, the effects of electromagnetic stirring with a single set of induction coil and low-frequency electromagnetic vibration on heat/mass transfer and solidification structures are introduced. The last portion of this chapter deals with the casting technologies based on electromagnetic stirring and low-frequency electromagnetic vibration.

The application of ultrasound to the processing of liquids and slurries has a long history. This chapter considers the main mechanisms of ultrasonic processing of metallic alloys as well as principal applications of this technology to processing of liquid metals, casting of alloys and manufacturing of new materials. Some theoretical background is given as well, The text is illustrated with historical and new results including those obtained with most advanced techniques such as high-temperature cavitometry, high-speed in-situ observations and X-ray synchrotron imaging.

Recently, Direct Current (DC) magnetic field processing of materials has found widespread applications in metallurgy, especially in metals and semiconductor industries. The main goal is to control the behavior of melts during solidification so as to improve process performance and achieve better quality products. DC magnetic fields are effective in introducing some special magnetohydrodynamic effects, e.g., flow damping, which are commonly used in continuous casting of steels or crystal growth control. In parallel, the development of super conducting technology, which is able to produce high magnetic fields in a large space, has open many new possibilities in control of the processing of materials in solid and liquid state. The novelty comes from the creation of magnetization forces on non-magnetic or feeble magnetic materials due to high magnetic fields. Morerover, it has been realized quite recently that the thermo-electric phenomena under high DC magnetic field can produce strong electromagnetic forces in solid and liquid metals, leading to a phenomenon called Thermo-Electric-Magnetic Convection (TEMC). The forces are able to generate significant liquid motion especially when temperature gradients are present, and therefore strongly influence the solidification of metallic alloys. This chapter reviews the major progresses and applications related to the uses of strong/intense DC magnetic fields in processing of materials (mainly metallic alloys) in solidification processes. In the first section, we review the underlying principles in magnetohydrodynamics and magnetic effects. In the second section, we discuss the phenomena induced by DC magnetic fields in materials processing. We deal in particular with flow damping effects on liquid metals, and control of structure of materials during solidification, including texturing, phase separation and thermoelectric effect. Finally we give two examples of successful industrial applications.

Research and applications of electromagnetic processing of materials began in the 1930s. In those early days, electromagnetic induction was the main technique and it was used for heating or melting metal alloys, stirring melt or controlling melt flow against gravity. In the 1970s, researchers and engineers in materials and metallurgical sector started to adopt the established theories and knowledge in the field of magnetohydrodynamics to interpret the similar phenomena and solve the problems found in the metallurgical processes where electromagnetic fields are present. This marked the birth of “Electromagnetic Metallurgy” and later named as “Electromagnetic Processing of Materials” by Shigeo Asai in 1989. Since then, eight series of international Symposiums have been dedicated to the developments in this field, and the ninth Symposium (EPM 2018) will be held in Awaji Island, Hyogo, Japan on 14–18 Oct 2018 [http://​www.​epm2018.​org/​]. This Chapter primarily describes the historical and recent research and technological developments on pulse external fields with a focus on their applications to the solidification processes. Pulse field methods are relatively new compared to other types of electromagnetic processing methods. Two major types of pulse fields, i.e. pulse electric currents and pulse electromagnetic fields, are described with a critical review on the most recent developments and future directions.

Using melt superheating as a means to control the structure and properties of metallic alloys has been studied extensively and demonstrated some promising results, though the industrial implementation is limited due to the required high energy for melt heating and holding. The physical mechanisms behind this technology can be divided into two major groups: (1) achieving homogeneous metallic melt with the resultant high undercooling upon solidification and (2) formation of heterogeneous substrates either by formation or transformation of insoluble impurities. In this chapter, we first discuss the structure of melts and its changes with temperature during high-temperature holding. Although mostly of academic interest, these studies demonstrate the complexity of temperature influence on the molten and solidifying melt. Some examples on the effects of the initial melt condition on the solidification microstructures are given as well. After that we consider some practical implications of the changes in insoluble impurities with temperature on the microstructure formed during solidification in some metallic alloys.