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Cited by 16 publications
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Semi-continuous direct-chill (DC) casting holds a prominent position in commercial aluminium alloy processing, especially in production of large sized ingots.Macrosegregation, which is the non-uniform chemical composition over the length scale of a casting, is one of the major defects that occur during this process. The fact that macrosegregation is essentially unaffected by subsequent heat treatment (hence constitutes an irreversible defect) leaves us with little choice but to control it during the casting stage. Despite over a century of research in the phenomenon of macrosegregation in castings and good understanding of underlying mechanisms, the contributions of these mechanisms in the overall macrosegregation picture; and interplay between these mechanisms and the structure formation during solidification are still unclear. This review attempts to fill this gap based on the published data and own results. The following features make this review unique: results of computer simulations are used in order to separate the effects of different macrosegregation mechanisms. The issue of grain refining is specifically discussed in relation to macrosegregation. This report is structured as follows. Macrosegregation as a phenomenon is defined in the Introduction. In Part 2, direct-chill casting, the role of process parameters and the evolution of structural features in the as-cast billets are described. In Part 3, macrosegregation mechanisms are elucidated in a historical perspective and the correlation with DC casting process parameters and structural features are made. The issue of how to control macrosegregation in direct-chill casting is also dealt with in Part 3. In Part 4, the effect of grain refining on macrosegregation is introduced, the current understanding is described and the contentious issues are outlined. The review is finished with conclusion remarks and outline for the future research.
Semi-continuous direct-chill (DC) casting holds a prominent position in commercial aluminium alloy processing, especially in production of large sized ingots.Macrosegregation, which is the non-uniform chemical composition over the length scale of a casting, is one of the major defects that occur during this process. The fact that macrosegregation is essentially unaffected by subsequent heat treatment (hence constitutes an irreversible defect) leaves us with little choice but to control it during the casting stage. Despite over a century of research in the phenomenon of macrosegregation in castings and good understanding of underlying mechanisms, the contributions of these mechanisms in the overall macrosegregation picture; and interplay between these mechanisms and the structure formation during solidification are still unclear. This review attempts to fill this gap based on the published data and own results. The following features make this review unique: results of computer simulations are used in order to separate the effects of different macrosegregation mechanisms. The issue of grain refining is specifically discussed in relation to macrosegregation. This report is structured as follows. Macrosegregation as a phenomenon is defined in the Introduction. In Part 2, direct-chill casting, the role of process parameters and the evolution of structural features in the as-cast billets are described. In Part 3, macrosegregation mechanisms are elucidated in a historical perspective and the correlation with DC casting process parameters and structural features are made. The issue of how to control macrosegregation in direct-chill casting is also dealt with in Part 3. In Part 4, the effect of grain refining on macrosegregation is introduced, the current understanding is described and the contentious issues are outlined. The review is finished with conclusion remarks and outline for the future research.
Aluminum, Al, is a silver‐white metallic element in Group III of the Periodic Table having an electronic configuration of 1 s 2 2 s 2 2 p 6 3 s 2 3 p 1 . Aluminum exhibits a valence of \documentclass{article}\pagestyle{empty}\begin{document}${+3}$\end{document} in all compounds except for a few high temperature gaseous species in which the aluminum may be monovalent or divalent. Aluminum is the most abundant metallic element on the surfaces of the earth and moon, comprising 8.8% by weight (6.6 atomic %) of the earth's crust. However, it is rarely found free in nature. Nearly all rocks, particularly igneous rocks, contain aluminum as aluminosilicate minerals. Aluminum reflects radiant energy throughout the spectrum. It is odorless, tasteless, nontoxic, and nonmagnetic. Because of its many desirable physical, chemical, and metallurgical properties, aluminum is the most widely used nonferrous metal. The utility of the metal is enhanced by the formation of a stable adherent oxide surface that resists corrosion. Because of high electrical conductivity and lightness, aluminum is used extensively in electrical transmission lines. Its alloys, containing small amounts of other elements, have high strength‐to‐weight ratios. Alloys of aluminum are readily formable by many metalworking processes; they can be joined, cast, or machined and accept a wide variety of finishes. Aluminum, having a density about one‐third that of ferrous alloys, is used in transportation and structural applications where weight‐saving is important. The properties of aluminum vary significantly according to purity and alloying. Aluminum reacts with oxygen, nitrogen, sulfur, and carbon in oxygen‐free atmospheres at high temperatures. Very high purity aluminum, resistant to attack by most acids, is used in the storage of nitric acid, concentrated sulfuric acid, organic acids, and other chemical reagents. Aluminum is, however, dissolved by aqua regia. Aluminum is attacked by salts of more noble metals. In particular, aluminum and its alloys should not be used in contact with mercury or mercury compounds. Aluminum is not attacked by saturated or unsaturated, aliphatic or aromatic hydrocarbons. The chemical stability of aluminum in the presence of alcohols is very good and stability is excellent in the presence of aldehydes, ketones, and quinones. Aluminum, the third most abundant element in the earth's crust, is usually combined with silicon and oxygen in rock. Bauxite is, with rare exceptions, the starting material for the production of aluminum. Metallurgical grade alumina, Al 2 O 3 , extracted from bauxite by the Bayer process, is generally referred to as the ore. Aluminum is obtained by electrolysis of this purified ore. Tremendous growth has been experienced in the aluminum industry, as compared to these of other nonferrous metals. The principal markets for aluminum in the United States are containers, packaging, building and construction, and transportation. Fluoride emission from aluminum smelting cells has long been an area of great concern. Treatment consists of highly (over 99%) efficient dry scrubbers that catch particulates and sorb HF on alumina that is subsequently fed to the cells. Many of the properties of aluminum alloy products depend on metallurgical structure. In addition to features which are present in virtually all metallic products, the structure of aluminum alloys is characterized by three types of intermetallic particles referred to as constituent particles, dispersoid particles, and precipitate particles. Constituent particles, present in all aluminum alloys, play a role in grain‐size control and negatively affect toughness of high strength alloy products. Dispersoids are present in most aluminum alloy products. Their primary function is to control grain size, grain orientation (texture), and degree of recrystallization. Particles classified as precipitates may confer high strength. The nature of the constituent, dispersoid, and precipitate particles depends strongly on the phase diagrams of the particular alloy. Important binary alloys are Al–Fe, Al–Cu, Al–Mg, and Al–Li. Almost all commercial alloys are of ternary or higher complexity. Alloy type is defined by the nature of the principal alloying additions, and phase reactions in several classes of alloys can be described by reference to ternary phase diagrams. Minor alloying additions may have a powerful influence on properties of the product because of the influence on the morphology and distribution of constituents, dispersoids, and precipitates. Important ternary alloys are Al–Fe–Si, Al–Mg–Si, Al–Mg–Mn, and Al–Mg–Zn. Commercially important alloys among the latter always contain more zinc than magnesium to provide attractive combinations of strength, extrudability, and weldability. Further additions to commercial aluminum alloys usually are made either to modify the metastable strengthening precipitates or to produce dispersoids. Silicon is sometimes added to Al–Cu–Mg alloys to help nucleate S′ precipitates without the need for cold work prior to the elevated temperature aging treatments. Copper is often added to Al–Mg–Si alloys in the range of about 0.25% to 1.0% Cu to modify the metastable precursor to Mg 2 Si. The copper additions provide a substantial strength increase. The highest strength aluminum alloy products are based on the Al–Cu–Mg–Zn system and all are strengthened by precursors to the η‐phase. When combined with magnesium, silver, Ag, has found commercial use as an alloying element in several aluminum alloys for specialized applications. It is an essential component with the new Weldalite series of alloys. The three elements commonly added to precipitation hardenable alloys to form dispersoids are manganese, chromium, and zirconium. Aluminum alloys are subjected during manufacture to a variety of thermal treatments that range from heating to assist fabrication, to heating for control of final properties. Aluminum alloys are commercially available in a wide variety of cast forms and in wrought mill products produced by rolling, extrusion, drawing, or forging. The mill products may be further shaped by a variety of metal‐working and forming processes and assembled by conventional joining procedures into more complex components and structures. Aluminum and aluminum alloys are employed in many applications because of the ability to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. This film is 5–10 nm thick when formed in air; if disrupted it begins to form immediately in most environments. Wrought alloys of the Al, Al–Mn, Al–Mg, and Al–Mg–Si types have excellent corrosion resistance in most weathering exposures, including industrial and seacoast atmospheres. Packaging has replaced the building and construction industry as the largest consumer of aluminum in the United States because aluminum is impermeable to gas, resistant to corrosion, and recyclable. The most prominent has been the use of aluminum for beer and carbonated beverages. The largest market worldwide for aluminum products is in the building and construction industry. Because of aluminum's low density, the field of transportation is another large market for aluminum alloys. Aluminum is an excellent conductor of electricity, having a volume conductivity 62% of that of copper. It also has many applications in the chemical and petrochemical industries as piping and tanks.
Aluminum, Al, is a silver‐white metallic element in Group III of the Periodic Table. Aluminum exhibits a valence of \documentclass{article}\pagestyle{empty}\begin{document}${+3}$\end{document} in all compounds except for a few high temperature gaseous species in which the aluminum may be monovalent or divalent. Aluminum is the most abundant metallic element on the surfaces of the earth and moon, comprising 8.8% by weight (6.6 atomic %) of the earth's crust. However, it is rarely found free in nature. Nearly all rocks, particularly igneous rocks, contain aluminum as aluminosilicate minerals. Aluminum reflects radiant energy throughout the spectrum. It is odorless, tasteless, nontoxic, and nonmagnetic. Because of its many desirable physical, chemical, and metallurgical properties, aluminum is the most widely used nonferrous metal. The utility of the metal is enhanced by the formation of a stable adherent oxide surface that resists corrosion. Because of high electrical conductivity and lightness, aluminum is used extensively in electrical transmission lines. Its alloys, containing small amounts of other elements, have high strength‐to‐weight ratios. Alloys of aluminum are readily formable by many metalworking processes; they can be joined, cast, or machined and accept a wide variety of finishes. Aluminum, having a density about one‐third that of ferrous alloys, is used in transportation and structural applications where weight‐saving is important. The properties of aluminum vary significantly according to purity and alloying. Aluminum, the third most abundant element in the earth's crust, is usually combined with silicon and oxygen in rock. Bauxite is, with rare exceptions, the starting material for the production of aluminum. Metallurgical grade alumina, Al 2 O 3 , extracted from bauxite by the Bayer process, is generally referred to as the ore. Aluminum is obtained by electrolysis of this purified ore. Tremendous growth has been experienced in the aluminum industry, as compared to these of other nonferrous metals. The principal markets for aluminum in the United States are transportation containers, packaging, building and construction. Fluoride emission from aluminum smelting cells has long been an area of great concern. Treatment consists of highly (over 99%) efficient dry scrubbers that catch particulates and sorb HF on alumina that is subsequently fed to the cells. Important binary alloys are Al–Fe, Al–Cu, Al–Mg, and Al–Li. Almost all commercial alloys are of ternary or higher complexity. Alloy type is defined by the nature of the principal alloying additions, and phase reactions in several classes of alloys can be described by reference to ternary phase diagrams. Minor alloying additions may have a powerful influence on properties of the product because of the influence on the morphology and distribution of constituents, dispersoids, and precipitates. Important ternary alloys are Al–Fe–Si, Al–Mg–Si, Al–Mg–Mn, and Al–Mg–Zn. Commercially important alloys among the latter always contain more zinc than magnesium to provide attractive combinations of strength, extrudability, and weldability. Further additions to commercial aluminum alloys usually are made either to modify the metastable strengthening precipitates or to produce dispersoids. The highest strength aluminum alloy products are based on the Al–Cu–Mg–Zn system and all are strengthened by precursors to the η‐phase. When combined with magnesium, silver, Ag, has found commercial use as an alloying element in several aluminum alloys for specialized applications. It is an essential component with the Weldalite series of alloys. The three elements commonly added to precipitation hardenable alloys to form dispersoids are manganese, chromium, and zirconium. Aluminum alloys are subjected during manufacture to a variety of thermal treatments that range from heating to assist fabrication, to heating for control of final properties. Aluminum alloys are commercially available in a wide variety of cast forms and in wrought mill products produced by rolling, extrusion, drawing, or forging. The mill products may be further shaped by a variety of metal‐working and forming processes and assembled by conventional joining procedures into more complex components and structures. Aluminum and aluminum alloys are employed in many applications because of the ability to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. Wrought alloys of the Al, Al–Mn, Al–Mg, and Al–Mg–Si types have excellent corrosion resistance in most weathering exposures, including industrial and seacoast atmospheres. Transportation has replaced the building and construction industry as the largest consumer of aluminum in the United States because aluminum is impermeable to gas, resistant to corrosion, and recyclable. The most prominent has been the use of aluminum for beer and carbonated beverages. The largest market worldwide for aluminum products is in the building and construction industry. Because of aluminum's low density, the field of transportation is another large market for aluminum alloys. Aluminum is an excellent conductor of electricity, having a volume conductivity 62% of that of copper. It also has many applications in the chemical and petrochemical industries as piping and tanks.
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