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2002 | Buch

Science and Engineering of Casting Solidification

verfasst von: Professor Doru Michael Stefanescu

Verlag: Springer US

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Über dieses Buch

We come to know about the world in two distinctive ways: by direct perception and by application of rational reasoning which, in its highest form, is mathematical thinking. The belief that the underlying order of the world can be expressed in mathematical form lies at the very heart of science. In other words, we only know what we can describe through mathematical models. Casting of metals has evolved first as witchcraft, to gradually become an art, then a technology, and only recently a science. Many of the processes used in metal casting are still empirical in nature, but many others are deep-rooted in mathematics. In whatever form, casting of metals is an activity fundamental to the very existence of our world, as we know it today. Foundry reports indicate that solidification modeling is not only a cost-effective investment but also a major technical asset. It helps foundries move into markets with more complcx and technically demanding work. The ability to predict internal soundness allows foundries to improve quality and deliveries, and provides the information required to make key manufacturing decisions based on accurate cost estimates before pattern construction even begins. The acceptance of computational modeling of solidification by the industry is a direct result of the gigantic strides made by solidification science in the last two decades.

Inhaltsverzeichnis

Frontmatter
1. Length-Scale in Solidification Analysis
Abstract
Cast copper is about 7000 years old. Cast iron, the first man-made composite, is at least 2500 years old. However, as the human species gradually moves from the “iron age” to the age of “engineered materials,” of all metal forming processes, the casting process remains the most direct and shortest route from component design to finished product. This makes casting one of the major manufacturing processes, while casting alloys are some of the most widely used materials. The main reasons for the longevity of the casting process are the wide range of mechanical and physical properties covered by casting alloys, the near-net shape capability of the casting process, the versatility of the process (weight from grams to hundred of tons, casting of any metal that can be melted, intricate shapes that cannot be produced by other manufacturing methods), and the competitive delivery price of the manufactured goods. While castings are “invisible” in many of their applications, since they may be part of complex equipment, they are used in 90% of all manufactured goods.
Doru Michael Stefanescu
2. Thermodynamics of Solidification
Abstract
Thermodynamics is a useful tool for the analysis of solidification. It is used to evaluate alloy phase constitution, the solidification path, basic alloy properties such as partition coefficients, slopes of liquidus and solidus phase boundaries.
Doru Michael Stefanescu
3. Macro-Scale Phenomena — Formation of Macrostructure
Abstract
The problem that we would like to solve is to describe mathematically casting solidification and associated phenomena such as macrosegregation and macroshrinkage. To build a correct model it is first necessary to understand the physics associated with the solidification of castings. At the macro-scale level the casting is a two-phase system comprised of solid and liquid. Nucleation and growth of phases is ignored at this stage.
Doru Michael Stefanescu
4. Macro — Mass Transport
Abstract
The mechanisms of mass transport include species diffusion and momentum transfer (fluid convection). The effects of these mechanisms on casting solidification will be discussed in this chapter.
Doru Michael Stefanescu
5. Macro-Energy Transport
Abstract
There are three forms of energy transport: conduction (diffusive transport), convection (heat transmitted by the mechanical motion of the fluid) and/or radiation (through space). All three are active during solidification of a casting. Energy diffusion and convection occurs within the casting, at the metal/mold interface, and within the mold. Energy is transported by radiation from the mold to its environment, which is typically the air.
Doru Michael Stefanescu
6. Macro-Modeling of Solidification. Numerical Approximation Methods
Abstract
From the analysis of solidification based on the energy transport equation presented in the previous section, it was seen that analytical solutions of this equation are not always available. Significant simplifying assumptions must be used, assumptions that are many times debilitating to the point that the solution is of little engineering interest. Fortunately, with the development of numerical methods and their application to the solution of partial differential equations, the most complicated equations can be solved numerically. Numerical solutions rely on replacing the continuous information contained in the exact solution of the differential equation with discrete values. Discretization equations are derived from the governing differential equation.
Doru Michael Stefanescu
7. Micro-Scale Phenomena and Interface Dynamics
Abstract
Interface dynamics deals with phenomena occurring at scales smaller than the macro-scale used so far in our analysis of casting and solidification, but larger than the atomic scale. This scale is usually called micro-scale (micrometer size). Phenomena occurring at this scale determine the microscopic shape of the interface. The two fundamental phenomena that determine interface morphology are grain nucleation and grain growth.
Doru Michael Stefanescu
8. Cellular and Dendritic Growth
Abstract
Many of the alloys used in practice, such as steel, aluminum-copper alloys, nickel-base and copper-base alloys, are single phase alloys, which means that the final product of solidification is a solid solution. Depending on the thermal and compositional field, cellular or, in most practical cases, dendritic morphology will occur. In other cases, even when the room temperature microstructure is mostly eutectic some primary phases solidify before the eutectic. They can be solid solutions, carbides, intermetallic phases, inclusions, etc. Their morphology affects mechanical properties, and thus, understanding how this morphology can be controlled is a mater of significant practical importance. A detailed discussion of primary phase growth, and in particular of dendrite growth, will be provided in the following sections.
Doru Michael Stefanescu
9. Eutectic Solidification
Abstract
Some of the most important casting alloys in terms of tonnage as well as applications, such as cast iron and aluminum-silicon alloys, are essentially two-phase alloys. The eutectic has a fixed composition in terms of species A and B, and is in fact a two-phase solid (α + β). Solidification of a liquid of eutectic composition proceeds by transformation of the liquid into a two-phase solid.
Doru Michael Stefanescu
10. Peritectic and Monotectic Solidification
Abstract
Peritectic solidification is very common in the solidification of metallic alloys. Typical examples of systems with peritectic solidification include Fe-C, Fe-Ni, Cu-Sn and Cu-Zn alloys. A schematic phase diagram with peritectic solidification is presented in Figure 10.1. The different reactions corresponding to various compositions are also shown.
Doru Michael Stefanescu
11. Solidification in the Presence of a Third Phase
Abstract
In the mathematical treatment of microstructure evolution presented so far, only two phases were considered, solid and liquid. However, casting alloys often exhibit inclusions that degrade their mechanical properties. The solid, liquid or gaseous inclusions, that constitute a third phase, can considerably affect solidification morphology. In turn, solidification conditions influence the size and distributions of these inclusions. This chapter will describe two of the most significant phenomena related to the interaction between inclusions and the solidification front, interaction of solid particles with the S/L interface and micro-shrinkage formation.
Doru Michael Stefanescu
12. Atomic Scale Phenomena: Nucleation and Growth
Abstract
Solidification is the result of the formation of stable clusters of solid in the liquid (nucleation), followed by their growth. These are phenomena occurring at the atomic scale (nano meter) level. The present understanding of the beginning of formation of solid crystals from their liquid is based on the classical theory of homogeneous nucleation. This theory uses macroscopic concepts and classic thermodynamics to describe the appearance of the first microscopic crystals in the melt.
Doru Michael Stefanescu
13. Stochastic Modeling of Solidification
Abstract
Deterministic models for microstructure evolution have made tremendous progress rewarded by recognition and acceptance by industry. However, their main shortcoming is their inability to provide a graphic description of the microstructure. Another problem is in the very mathematics that is used. Recent advances in electron microscopy make possible the examination of materials with near-atomic resolution. Such nano-scale structures cannot be described adequately through continuum approximations. As stated by Kirkaldy (1995), “Materials scientists who persist in ignoring the microscopic and mesoscopic physics in favor of mathematics of the continuum will ultimately be seen to have rejected an assured path towards a complete theoretical quantification of their discipline.”
Doru Michael Stefanescu
14. Macro-Micro Modeling of Solidification of Some Commercial Alloys
Abstract
Iron-base alloys, that include steel and cast iron, are some of the oldest man made materials. However, they have enjoyed a remarkable longevity because of their wide range of mechanical and physical properties, coupled with their competitive price. Today, iron-base alloys are by far the most widely used casting material (Figure 14.1). While their development has followed the classic path of perception followed by rationalization, it was only relatively recently that these materials have been studied as mathematical systems.
Doru Michael Stefanescu
Backmatter
Metadaten
Titel
Science and Engineering of Casting Solidification
verfasst von
Professor Doru Michael Stefanescu
Copyright-Jahr
2002
Verlag
Springer US
Electronic ISBN
978-1-4757-6472-7
Print ISBN
978-1-4757-6474-1
DOI
https://doi.org/10.1007/978-1-4757-6472-7