Rolling Aluminum: From the Mine Through the Mill PDF

Rolling Aluminum: From the Mine Through the Mill  ACKNOWLEDGEMENTS This manual has been prepared with information and assistance from The Aluminum Association and member companies represented on the Sheet and Plate Division’s Technology Committee....

Rolling Aluminum: From the Mine Through the Mill  ACKNOWLEDGEMENTS This manual has been prepared with information and assistance from The Aluminum Association and member companies represented on the Sheet and Plate Division’s Technology Committee. SHEET & PLATE DIVISION MEMBER COMPANIES Alcoa Inc. JW Aluminum Aleris Rolled Products Kaiser Aluminum and ALSCO Metals Corp. Chemical Corporation AMAG Rolling GmbH Koenig & Vits, Inc. ARCO Aluminum, Inc. Logan Aluminum, Inc. Golden Aluminm Nichols Aluminum Gulf Aluminium Rolling Novelis Inc. Mill Co. Rio Tinto Alcan Impol Aluminum Corp. United Aluminum Corporation Jupiter Aluminum Corporation Wise Metals Group USE OF INFORMATION The use of any information contained herein by any member or non-member of The Aluminum Association is entirely voluntary. The Aluminum Association has used its best efforts in compiling the information contained in this book. While the Association believes that its compilation procedures are reliable, it does not warrant, either expressly or implied, the accuracy or completeness of this information. The Aluminum Association assumes no responsibility or liability for the use of the information herein. All Aluminum Association published standards, data, specifications and other technical materials are reviewed and revised, reaffirmed or withdrawn on a periodic basis. Users are advised to contact The Aluminum Association to ascertain whether the information in this publication has been superseded in the interim between publication and proposed use. The Aluminum Association, Inc. 1525 Wilson Blvd. Suite 600 Arlington, VA 22209 www.aluminum.org © Copyright 2008. Unauthorized reproduction by photocopy or any other method is illegal. First Printing June 1990 Third Edition December 2007 Rolling Aluminum: From the Mine Through the Mill T HOW TO USE THIS PUBLICATION his publication enlarges upon the information presented in The Aluminum Association DVD “Rolling Aluminum: From the Mine Through the Mill”. The expanded explanations of various aspects of aluminum production and rolling appear where they are mentioned in the DVD. The user who wants to learn the general features of aluminum production and rolling will find them summarized in the DVD and the early paragraphs of each section of the manual and in its Self-Test Questions and Answers. For readers who want to understand these subjects more thoroughly, the full text of the manual provides more detailed explanations. The manual may also be used as a ready-reference, by consulting its subheaded Table of Contents and / or alphabetical index. Metric values of the US customary units in the manual have been included for the convenience of the reader and are not intended as precise arithmetic conversions. Where the products (foil / sheet / plate) are defined by their thickness range, the actual metric gauge definition has also been included for reference. Finally, the loose-leaf format permits the easy insertion of product information, additional notes or future informational updates. T ABOUT THE ALUMINUM ASSOCIATION he Aluminum Association is an industry-wide trade organization representing US and foreign producers of aluminum, leading manufacturers of semi-fabricated aluminum products, secondary smelters and suppliers to the aluminum industry. The Association’s aims are to increase public and industrial understanding of alu- minum and the aluminum industry and — through its technical, statistical, marketing and information activities — to serve industries, consumers, financial analysts, educa- tors, government agencies and the public generally. For the aluminum industry and those industries that use aluminum, The Association helps develop standards and designation systems, helps prepare codes and specifica- tions involving aluminum products and studies technical problems of the industry. The Association maintains and periodically issues industry-wide statistics and records which are used as an authoritative source. Association members also join together on a number of commodity and end-market committees to conduct industry-wide market development programs. CONTENTS Rolling Aluminum: From the Mine through the Mill 1 Section-Page INTRODUCTION Advantages of Aluminum........................................1-1 Flat-Rolled Aluminum............................................1-2 Aluminum Sheet Applications....................................1-2 Aluminum Plate Applications.....................................1-2 Aluminum Foil Applications.......................................1-2 Specialty Products..............................................1-2 Definitions: Plate, Sheet, Foil......................................1-4 Rolling Aluminum: General Description.............................1-4 Technological Advances........................................1-5 2 Self-Test Questions..............................................1-7 PRODUCTION OF ALUMINUM AND ITS ALLOYS Bauxite Mining.................................................2-1 Alumina Refining...............................................2-1 Aluminum Reduction: The Hall-Heroult Process......................2-2 Alloying.......................................................2-5 Aluminum Alloy Series...........................................2-5 Heat-Treatable Alloys...........................................2-6 Non-Heat-Treatable Alloys.......................................2-6 Recycled Aluminum Scrap.......................................2-7 Addition of Alloying Elements.....................................2-7 3 Self-Test Questions..............................................2-9 PREPARATION OF ALUMINUM ALLOYS FOR ROLLING Metal Cleaning: Fluxing and Filtration..............................3-1 Fluxing........................................................3-1 Alloy Sample Analysis...........................................3-1 Filtration.......................................................3-2 Additives......................................................3-3 Continuous Casting of Sheet or Plate..............................3-3 DC Ingot Casting...............................................3-6 Electromagnetic Casting (EMC)..................................3-7 Scalping......................................................3-7 Pre-Heating....................................................3-8 Purposes of Pre-Heating.........................................3-8 Pre-Heating Methods............................................3-8 Self-Test Questions..............................................3-9 CONTENTS 6 4 THE ALUMINUM ROLLING MILL Description and Features........................................4-1 Single-Stand and Tandem Mills...................................4-3 Mill Rolls.......................................................4-3 Roll Configurations..............................................4-4 Hot or Cold Rolling..............................................4-6 Hot Rolling: Metallurgical Effects..................................4-7 Cold Rolling: Metallurgical Effects.................................4-7 5 Self-Test Questions..............................................4-9 SHEET ROLLING OPERATIONS Breakdown Mill Parameters......................................5-1 Breakdown Coolant/Lubricant Applications........................5-2 Breakdown Rolling..............................................5-3 Intermediate Hot Rolling.........................................5-3 Sheet Hot Rolling: The Tandem Mill................................5-4 Coiling and Edge Trimming......................................5-5 Annealing.....................................................5-6 Purposes of Annealing...........................................5-6 Full Annealing..................................................5-6 Partial Annealing...............................................5-7 Stabilization Annealing..........................................5-7 Annealing Methods.............................................5-7 Batch Annealing/Treatment......................................5-7 Sheet Annealing/Treatment......................................5-7 Cold Rolling....................................................5-8 Gauge and Profile Control.......................................5-8 Texturing and Embossing........................................5-10 6 Self-Test Questions..............................................5-11 SHEET FINISHING Solution Heat Treating...........................................6-1 Metallurgical Function of Solution Heat Treating.....................6-1 Solution Heat Treatment: “Soaking”...............................6-2 Quenching....................................................6-2 Natural Aging (Precipitation Hardening)...........................6-3 Artificial Aging (Precipitation Heat Treatment)......................6-3 Slitting.........................................................6-4 Tension-Leveling................................................6-4 Surface Finishes................................................6-4 Oiling.........................................................6-5 Degreasing....................................................6-5 Coatings and Markings..........................................6-5 Cutting to Length...............................................6-5 Stretcher-Leveling..............................................6-6 Packaging and Storage.........................................6-6 Self-Test Questions..............................................6-7  CONTENTS 7 7 PLATE ROLLING AND FINISHING Plate Rolling...................................................7-1 Slab Reheating.................................................7-1 Intermediate Edge Trimming.....................................7-1 Plate Finishing..................................................7-1 Solution Heat Treating and Quenching............................7-2 Plate Stretching................................................7-2 Ultrasonic Inspection............................................7-2 Artificial Aging.................................................7-3 Plate Edge Cutting.............................................7-3 Inspection, Superficial Defect Removal and Storage.................7-3 8 Self-Test Questions..............................................7-5 QUALITY AND PROCESS CONTROL Testing and Inspection..........................................8-1 Tensile Strength Testing..........................................8-1 Microscopic Inspection..........................................8-1 Porosity Testing.................................................8-1 Coolant / Lubrication Analysis....................................8-2 Anodizing and Bright-Dipping Tests................................8-2 Dimensional Tolerance Control...................................8-2 Surface Quality Characteristics...................................8-2 Computerized and Centralized Controls...........................8-3 9 Self-Test Questions..............................................8-5 GLOSSARY.................................................9-1 Self-Test Questions..............................................9-7 10 TECHNICAL APPENDICES Appendix A: Aluminum Alloy Designation System....................10A-1 Appendix B: Aluminum Temper Designation System.................10B-1 Appendix C: Typical Aluminum Alloy Properties.....................10C-1 Appendix D: Aluminum Sheet & Plate Standards and Tolerances......10D-1 Appendix E: Self-Test Questions (Technical Appendices).............10E-1 Appendix F: Answers to All Self-Test Questions.......................10F-1 11 REFERENCES...............................................11-1 12 INDEX.......................................................12-1  1 INTRODUCTION Rolling Aluminum: From the Mine through the Mill APPLICATIONS In aircraft and spacecraft... in railcars, autos, trucks, trailers and boats... on skyscrapers and homes... in cans and cooking utensils... strong, lightweight aluminum plate, sheet and foil are vital materials of modern society. T Advantages of Aluminum Heat-conducting—Aluminum is an excel- he properties of aluminum make it one of the lent heat conductor suitable for cooking ware and most advantageous and versatile materials in for heat exchangers; it is more efficient, pound for pound, than copper. Electrical conductivity—Aluminum is also use today. Aluminum is: Lightweight—Aluminum and its alloys weigh only about one-third as much as equal volumes of an excellent conductor of electricity, commonly iron, steel or copper. used in such heavy-duty applications as high-volt- Strong—Given appropriate tempers, some age transmission lines, bus bars and local and building distribution systems. Corrosion resistant—Aluminum, exposed to aluminum alloys equal or surpass the strength of some steels. Strong aluminum alloys can be as much as two or three times stronger than steel for air, forms a transparent natural oxide film which the same weight. seals its surface against further reactions and Cryo-tolerant—In contrast to steel, titanium protects it from corrosion from normal weather exposure. Specific aluminum alloys, treatments and many other materials that become brittle at and/or coatings may be selected to maximize very low (cryogenic) temperatures, aluminum corrosion resistance in particular applications. Non-toxic—Rolled aluminum alloys are non- remains ductile and even gains strength as temper- ature is reduced. This property makes aluminum highly useful in very cold climates and for trans- toxic, easily cleaned, and non-absorbent. For these porting extremely cold materials such as liquefied reasons, they are widely used in food preparation and packaging. Non-combustible—Rolled aluminum does natural gas (-260°F [-162°C]). Ductile and workable—Aluminum alloys can be readily formed and fabricated by all stan- not burn, and it generates no hazardous emissions dard metalworking methods. when exposed to heat. It is safer than many other Joinable—Aluminum alloys can be joined by materials where fire is a potential hazard. all appropriate major methods, including welding, Recyclable—Aluminum’s resistance to cor- mechanical connections, and adhesive bonding. rosion and to reaction with most common materi- Reflective—Aluminum alloys with standard als keeps it in good condition throughout the life- time of most products. Scrap aluminum is widely commercial finishes typically reflect more than 80 recycled, reducing demands for waste disposal percent of visible light and more than 90 percent and the environmental impacts of new material of infrared radiation, making aluminum an effec- production. tive reflector of, or shield against, light, radio waves and radiant heat.  INTRODUCTION 1-2 F A Flat-Rolled Aluminum Aluminum Plate Applications lat-rolled products — sheet, plate and foil — luminum plate can be formed directly into have, for many years, made up by far the strong shapes with uniform thickness. It can largest volume of aluminum products shipped also serve as a “blank” from which complex large- annually in the United States, outstripping other area parts can be machined, such as the ribbed forms such as castings, extrusions, wire, rod and wing plates of advanced aircraft. It can be welded bar, forgings and impacts, and powders and paste. into large, strong, durable structures. Consumption of aluminum sheet and plate in Among many other applications, aluminum North America has generally increased over the plate is used to make railroad gondola and tank past 45 years. In 1960, sheet and plate consump- cars, battle armor for military tanks and vehicles, tion in domestic markets added up to 1.42 billion superstructures of large merchant and military pounds (645 million metric tons), or about 34.6% ships and offshore oil rigs, tanks for storing and of total aluminum product shipments. transporting super-cold liquefied natural gas, air- A By 1980, in a rapidly growing market for alu- craft structural parts, and spacecraft components. Aluminum Foil Applications minum, sheet and plate consumption was 5.56 bil- lion pounds (2,523 million MT) or 46.7%. In 2005, domestic consumption of sheet and luminum foil is familiar to most people in the plate was 9.59 billion pounds (4,349 million MT), popular form of soft, very thin (0.00065 inch 41.5% of the total; including foil, flat-roll prod- [0.0165 mm] thick) household kitchen foil: it’s ucts accounted for just under half (47.7%) of all easy to fold, impenetrable by water and vapor, fire A aluminum products shipped that year. resistant, both heat-conductive and heat-reflective, Aluminum Sheet Applications and inhospitable to molds. But aluminum foil is also made of harder, luminum sheet is a remarkably versatile stronger alloys whose strength may approach that material, not only because of the custom- of steel. tailored characteristics that it can be given in the The variety of properties available in aluminum rolling mill, but also because of its suitability to foils make them useful in applications ranging a wide range of finishing, fabrication and joining from the protective packaging of foods, pharma- processes. ceuticals and many other consumer products, to It can be given a wide variety of surfaces: pat- laminated vapor-barriers and insulation-backing terned, painted, plated, polished, colored, coated, in buildings, to artificial Christmas trees, sound laminated, anodized, etched or textured. It can be transducer/receiver diaphragms, rigid containers machined in different ways: sheared, sawed and and adhesive-bonded structural honeycomb. Specialty Products drilled. It can be easily formed: bent, corrugated, drawn or stamped. It can be readily joined to itself or to other materials: riveted, bolted, clinched, Special types of sheet and plate may be pro- brazed, soldered, welded and adhesively bonded. duced for particular applications. Some examples Its applications are too many to list completely. include: They include such familiar products as: light bulb Anodizing sheet Porcelain enameling bases, beverage and food cans, cooking utensils; Armor plate sheet home appliances; awnings and Venetian blinds; Brazing sheet Reflector sheet siding, roofing, flashing, gutters and curtainwall Decorative panel Rural roofing sheet panels for residential, commercial, industrial and sheet Tapered sheet and utility buildings; highway signs, license plates; Industrial roofing plate heat exchangers; automobile structures and exteri- sheet Tooling plate ors; truck, trailer and van panels; small boat hulls Lithography sheet Trailer roof sheet and aircraft skins. Painted sheet Tread plate Patterned sheet Vinyl-coated sheet  INTRODUCTION 1-3 Just a few of the hundreds of applications of rolled aluminum.  INTRODUCTION 1-4 DEFINITIONS All three are aluminum products that are rolled flat with a rectangular cross-section. They differ mainly in thickness. Plate is aluminum rolled one-quarter inch (6.3 mm) thick or more. produced in coiled or flat form, or in a variety of special shapes or fabrications. Some Aluminum plate is a product that is rectangular in aluminum sheet producers can perform additional cross-section and form, and 0.250 inch (6.3 mm) processing or apply special surface treatments so or more thick. It may have sheared or sawed that the sheet product is suitable for the desired edges. It is usually produced in flat form (but can application. (Notes: When products are ordered be coiled) or in a variety of special shapes or to metric specifications using metric dimensions, fabrications. Some aluminum plate producers can sheet is defined as being > 0.20 mm to ≤ 6 mm perform additional processing or apply special thick. Sheet was previously defined as ≥ 0.006" surface treatments so that the plate product is (0.15 mm) through 0.249" (6.3 mm).) Thinner than that, it’s “foil.” suitable for the desired application. (Note: When products are ordered to metric specifications using Plain aluminum foil is a rolled product that is metric dimensions, plate is defined as being > 6 rectangular in cross section and ≤ to.0079 inch mm thick; however, the common US customary (0.20 mm) thick. It is available in coiled form or in reference thickness for 0.250" is 6.3 mm.) Under one-quarter inch (6.3 mm) down flat sheets. Some aluminum foil producers can per- to eight-thousandths of an inch (0.20 mm) form additional processing or apply special surface thick, it’s called “sheet.” treatments so that the foil product is suitable for the desired application. (Notes: When products are Aluminum sheet is a product that is rectangular in ordered to metric specifications using metric cross-section and form, and of > 0.0079" (0.20 mm) dimensions, foil is defined as being ≤ 0.20 mm thickness through 0.249" (6.3 mm) thickness. It thick. Foil was previously defined as < 0.006" may have sheared, slit or sawed edges. It may be (0.15 mm) thick.) ROLLING ALUMINUM: GENERAL DESCRIPTION Sheet, plate and foil are usually made by rolling thick aluminum between rolls that reduce the thickness and lengthen it, the way a baker rolls out pie crust. But rolling aluminum is a lot more complicated than rolling dough. Tcentral he actual passage of aluminum through a rolling mill is only one step — although the erties and other characteristics specified by each customer. Aluminum Rolling: one — in a comprehensive sequence. Each A A Capsule History flat-rolled product owes its final properties not just to the rolling process itself but to its vital interactions with: the preparatory steps of alloy- luminum is a modern metal — discovered in ing, casting, scalping and pre-heating; intermedi- 1807, first isolated in 1825, produced in tiny ate annealing; and such later finishing steps as amounts as a precious metal in 1845, and widely solution heat treatment or final annealing, stretch- commercialized only after the invention of the ing, leveling, slitting, edge trimming and aging. Hall-Heroult production process in 1886. Since Modern aluminum rolling plants conduct many, then, aluminum has rapidly grown to become the or all, of these operations, selecting, adjusting and second most heavily used metal in the world after coordinating them scientifically to produce flat- iron/steel. rolled products with the precise dimensions, prop-  INTRODUCTION 1-5 Floor plan, typical aluminum smelting plant By the time aluminum arrived on the scene, early as 1917 (on the Junkers J-4 monoplane and mankind had thousands of years’ experience cast- the Dornier DO-1 biplane, forerunner of today’s ing and forging metals and several hundred years “stressed-skin” aircraft structures). of learning to roll them into sheets, plates and A practical method to produce Alclad sheet — even foils. Metal rolling may have been practiced strong aluminum alloy clad with a thin layer of a in developed countries as early as the beginning different aluminum alloy for enhanced corrosion of the 16th century. These familiar metal working resistance — was developed, in the United States, techniques were all adapted and applied to alu- in 1926. Since then, many other clad aluminum minum. products have been developed for such purposes The first aluminum stew pan was stamped in as corrosion protection, surface appearances, or 1890, and most of the aluminum produced before Ttheherolling ease of brazing. Technological Advances 1900 was used in cooking utensils. Aluminum hulled boats were built in the 1890s, including the U.S. racing yacht “Defender”, with fundamental principles and designs of metal aluminum plates, in 1895. mills had been established by the end of By 1903, some 6.5 million pounds (3 million 19th century, but improvements in their kg) of aluminum was consumed annually, about one-third of it in the form of sheet. technology and metallurgy have continued In that same year aluminum foil was first rolled, throughout the 20th century. in France; aluminum foil-rolling in the United Aluminum rolling has gained steadily in both States began in 1913, for wrapping candy and quality and efficiency through advances in its own chewing gum specific technology as well as improvements Thin aluminum skins appeared on aircraft as applied to metal rolling in general.  INTRODUCTION 1-6 Floor plan, typical aluminum rolling plant. Hot and cold continuous sheet rolling mills coolant/lubricant application; and many other came into operation in the United States in 1926. technical advances. The maximum speed of the first U.S. cold “strip” In recent years, aluminum rolling mills have mill was reported to be only about 200 feet per been introducing computerized process control, minute (60 m/min.); modern cold mills operate quality control and inventory tracking, and about 35 times faster! advanced gauge and shape controls, to achieve The rolling industry switched from steam even higher product quality and consistency. engines to electric power; it built specialty mills, Computerization, in turn, is pointing the way to and bigger multi-purpose mills. such further developments as Computer Integrated The standard “two-high” and “four-high” mill Manufacturing (CIM), Statistical Process Control configurations that were brought into early use, (SPC), and Just-In-Time (JIT) production sched- with mill rolls “stacked” vertically, are still basic ules. These recent advances are designed to to the rolling industry. They were later supple- increase product quality and delivery reliability mented, however, by various “cluster” designs and to reduce production costs. Aluminum that starts out two feet (0.6 m) with work rolls supported by two or more back-up thick may travel a mile (1.6 km) through rolls each, as explained in Section 4. large rolling mills, furnaces, cutters and Over the decades, safety, efficiency, cost and stretchers, to emerge as thin sheet only product quality were improved by the introduction a few hundredths of an inch (a few of automatic material handling devices, manipula- tenths of millimeters) thick... And it’s had tors, guides, guards, lifting and tilting tables; auto- a long journey before it even reaches matic roll changers, compact load-measuring cells the rolling mill. on roll bearings, gauge control feedbacks, load equalization and overload protection; programmed  INTRODUCTION 1-7 SELF-TEST QUESTIONS 1.8 Name five applications of aluminum plate. SECTION 1: INTRODUCTION 1.1 Name any five important advantages of rolled aluminum products. 1.9a In US customary units “plate” means a rolled product… a. less than 6 inches thick. b. greater than one-tenth inch thick. c. greater than or equal to one-quarter 1.2 Aluminum resists atmospheric corrosion inch thick. because: d. exactly 0.250 inch thick. a. It is always painted or coated. b. It has a naturally stable oxide film. 1.9b In metric units “plate” means a rolled c. It is a good heat conductor. product… d. It is non-combustible. a. greater than 6 mm thick. b. greater than 2.5 mm thick. 1.3 Aluminum weighs about as much as an c. less than 150 mm thick. equal volume of iron, steel or copper. d. exactly 6 mm thick. a. Three-quarters. b. One-half. 1.10a In US customary units “sheet" means a c. One-third. rolled product... d. One-quarter. a. from.010 to.200 inch thick. b. from.008 to.249 inch thick. 1.4 High-strength aluminum alloy can be... c. from.001 to.249 inch thick. a. up to half as strong as d. from.006 to.490 inch thick. b. up to 90% as strong as c. just about as strong as 1.10b In metric units “sheet” means a rolled d. stronger than product... a. from 0.25 to 5 mm thick. …an equal weight of steel. b. from 0.025 through 6 mm thick. c. from 0.20 through 6 mm thick. 1.5 As temperature is reduced, the strength d. greater than 0.20 up to 12 mm thick. of aluminum... a. decreases. b. remains the same. 1.11 Air conditioner fin stock produced as a c. increases. coil product, for example,.0045-inch (0.115 mm) thick falls into the category 1.6 The largest volume of aluminum products of... shipped annually in the United States a. sheet. consists of… b. plate. a. castings. c. foil. b. rolled products. c. forgings and impacts. Why? d. wire, rod and bar. 1.7 Name five applications of aluminum sheet.  INTRODUCTION 1-8 1.12 The existence of aluminum as a metal 1.15 Aircraft skins made of aluminum was first discovered... sheet appeared as early as... a. in prehistoric times. a. 1903. b. around 700 B.C. b. 1917. c. in 1725 (A.D.). c. 1931. d. in 1807. d. 1942. e. in 1903. 1.16 The basic principles and designs of 1.13 The Hall-Heroult process was metal rolling mills had been invented... established by... a. in 1807. a. the end of the Roman Empire. b. in 1825. b. the end of the Renaissance. c. in 1845. c. the end of the 19th century. d. in 1886. d. the middle of the 20th century. 1.14 Most of the aluminum produced before 1900 was used in: a. Military armor. b. Vehicles. c. Cooking utensils. d. Architecture. e. Boat hulls.  2 PRODUCTION Rolling Aluminum: From the Mine through the Mill of Aluminum Alloys: From Mine to Mill BAUXITE MINING Aluminum ore, the mineral bauxite, is mined in many countries around the world, and is crushed and refined to yield alumina — nearly pure aluminum oxide. A luminum is one of the most abundant ele- ments on earth. It is estimated that the solid portion of the earth's crust to a depth of ten miles which may contain 40 to 60 percent impure hydrated aluminum oxide — that is, aluminum oxide to which water molecules have become is about 8% aluminum, surpassed only by oxygen attached. The other components of bauxite typi- (47%) and silicon (28%). Aluminum is a major cally include various amounts of iron oxide, sili- constituent of clay and almost all common rocks. con oxide, titanium oxide and water. Its texture Aluminum is never found as a pure metal in can range from crumbly to something like lime- nature, but only in chemical compounds with stone, and its color varies from off-white to rusty- other elements — and especially with oxygen, red, depending on its iron oxide (rust) content. with which it combines strongly. The richest and most economical bauxite ores Feldspars, micas and clay contain aluminum ox- are often found close to the earth’s surface in trop- ide (A1203), also known as alumina, in concentra- ical and subtropical areas. Worldwide reserves of tions ranging roughly between 15 and 40 percent bauxite ore are estimated to be very large: enough by weight. high grade ore to last perhaps 300 to 500 years, and enough lower-grade ore for another 500 years at recent consumption rates. Clays and other min- erals could, if necessary, provide an almost limit- less source of alumina. Mining methods may vary, but most bauxite is mined in open pits by conventional digging machin- ery, and then is crushed, washed and dried in prepa- ration for separation of the alumina from the other, undesirable components. Subsequent to the mining operation, the aluminum industry spends consider- able effort to the restore the land and its flora and fauna back to their original condition. M ALUMINUM REFINING ost alumina is refined from bauxite by the Bayer process, patented in Germany by Karl *Others includes 2.1% Magnesium, 2.6% Potassium, Josef Bayer in 1888. This process is carried out in 2.8% Sodium, 3.6% Calcium, 5% Iron, and 0.14% Titanium, Manganese, Nickel, Copper, Zinc and Lead. four steps: Main elements found in the earth’s crust. 1. The crushed, washed and dried bauxite is “digested” with caustic soda (sodium hydroxide) Most aluminum is produced from an ore called at high temperatures and under steam pressure, bauxite (named after the town of Les Baux in dissolving the alumina in a mixture with undis- southern France where it was discovered in 1821), solved impurities called “red mud.”  PRODUCTION 2-2 2. This mixture is fil- tered to remove the red mud, which is discard- ed. The clarified alumi- na solution is trans- ferred to tall tanks called “precipitators.” 3. In the precipitator tank, the hot solution is allowed to cool with the addition of a little aluminum hydroxide to “seed”—that is, stimu- late — the precipitation of solid crystals of alu- minum hydroxide and sodium hydroxide. The aluminum hydroxide Bauxite mining. settles to the bottom and is removed from the tank. granulated sugar but is hard enough to scratch glass. The widely-used abrasives corundum and 4. The separated aluminum hydroxide is washed emery are forms of alumina. to remove residues of caustic soda and then is heated to drive off excess water in long rotating Refined alumina consists of about equal weights kilns called “calciners.” Aluminum oxide (alumi- of aluminum and oxygen, which must be separat- na) emerges as a fine white powder that looks like ed in order to produce aluminum metal. ALUMINUM REDUCTION: THE HALL-HEROULT PROCESS A practical way of breaking down the aluminum oxide — the Hall-Heroult process — was invented in 1886. Another mineral, cryolite, is melted, and the aluminum oxide is dissolved in it. Electric current passed through the bath attracts the oxygen to carbon anodes. The carbon and oxygen form carbon dioxide, which bubbles out of the bath. Molten aluminum is left behind at the bottom of the reducing cell, or “pot.” It is siphoned into a crucible for transport to the casting foundry. The process is continuous, and a typical pot may produce about 1400 pounds of aluminum daily. Hundreds of cells on the same electrical circuit form a potline. A luminum and oxygen form such a strong chemical bond that it takes a very large amount of energy to separate them by “brute Chemical methods of breaking down aluminum oxide were developed in the mid-19th century but were so expensive that metallic aluminum cost as force” methods like heating. Although aluminum much as silver. The small amounts of aluminum as a pure metal melts at about 1220°F (660°C); that were produced were used mainly for jewelry aluminum oxide requires a temperature of about and other luxury items. 3660°F (2015°C) before it will melt. Early researchers thought of using electricity to  PRODUCTION 2-3 separate aluminum from its oxide in solution but were frustrated by seemingly high energy require- ments; the inadequacy of their only sources of elec- tricity — batteries; and the insolubility of alumina in water. The invention of the rotary electric generator, the dynamo, in 1866 solved part of that problem. The other part was not solved until 1886 when Charles Martin Hall in the United States and Paul L.T. Heroult in France discovered the answer almost simultaneously. Hall and Heroult found that alumi- na would dissolve in a molten mineral called cryo- lite (a sodium aluminum fluoride salt) at about Charles Martin Hall (left) and Paul L.T. Heroult. 1742°F (950°C). In solution, the aluminum oxide They found the key to modern aluminum is readily separated into aluminum and oxygen by production. electric current. Other solvents might have worked in theory. Cry- the solution molten. olite, however, has the practical advantages of sta- Oxygen atoms, separated from aluminum oxide, bility under process conditions and a density lower carry a negative electrical charge and are attracted than that of aluminum, allowing the newly-forming to the positive poles in each pot. These poles, or metal to sink to the bottom of the “reduction cell.” anodes, are made of carbon which immediately The Hall-Heroult process takes a lot of electricity combines with the oxygen, forming the gases car- but only a low voltage, so it is practical to connect bon dioxide and carbon monoxide. These gases many reduction cells, or “pots”, in series along one bubble free of the melt, leaving behind the alu- long electrical circuit, forming a “potline.” Modern minum which collects at the bottom of the pot. cells are operated with currents of around 250,000 The process, therefore, steadily consumes the amperes but at only four or five volts each. Such carbon anodes, which must be renewed either by cells use about six or seven kilowatts of electricity regular replacement or by continuous feeding of a per pound of aluminum produced. self baking paste (Soderberg anode). About one- The heat generated by electrical resistance keeps half pound (225 g) of carbon is consumed for every pound (455 g) of aluminum produced. Most alu- minum reduction plants include their own facilities to manufacture carbon anodes, each of which may weigh 600 - 700 pounds (270 - 320 kg) and must be replaced after about 14 days of service. The negative electric pole, or cathode, forms the inner lining of each pot. It is also made of carbon. As a cathode it does not react with the melt, so it has a long service life. As alumina is reduced to aluminum and oxygen, fresh alumina is added to the molten bath to contin- ue the process. Aluminum fluoride is also added when necessary to maintain the bath's chemical composition, replacing aluminum fluoride lost by reactions with caustic soda residues and airborne moisture. Modern smelters capture and recycle flu- orides and other emissions. When sufficient molten aluminum has collected at the bottom of a pot, it is siphoned into a crucible for transport to alloying and casting facilities. The aluminum produced by the Hall-Heroult process is Potline more than 99 percent pure.  PRODUCTION 2-4 Main steps in the Bayer alumina-refining process and the Hall-Heroult aluminum-reduction process.  PRODUCTION 2-5 ALLOYING In the melt house, the aluminum is poured into a remelt furnace for alloying and fluxing. Adding small amounts of other elements to pure aluminum produces strong alloys, which can be further conditioned by heating, cooling and deformation treatments called “tempering.” A lloying requires the thorough mixing of aluminum with other elements in liquid — molten — form. This may be done in different Each alloy is assigned a four-digit number, in which the first digit identifies a general class, or “series”, characterized by its main alloying ways, depending on the sources of aluminum elements: 1xxx Series alloys include aluminum of used. Newly-produced aluminum may be transferred, 99 percent or higher purity. They have excellent still liquid, from the Hall-Heroult reduction cells corrosion resistance and thermal and electrical into a “holder” or reservoir furnace where solid conductivity. 2xxx Series alloys have copper as their princi- alloying elements are added, to dissolve and mix into the melt. Solid scrap aluminum, from fabrication opera- pal alloying element, often with smaller amounts tions or recycled products, must be melted in a of manganese and magnesium, and can be strengthened substantially by heat treatment. 3xxx Series alloys have manganese as their “remelt” furnace before its alloy composition is adjusted as necessary. Then the recycled, re- alloyed scrap is transferred to a holder. major alloying element, sometimes with smaller Often, new aluminum is alloyed with scrap alu- amounts of magnesium, and are generally non- minum of known composition. The solid scrap is heat-treatable. added to the molten new aluminum, and then fur- 4xxx Series alloys, alloyed mainly by silicon, ther alloying elements are added to the mixture as are usually non-heat-treatable. 5xxx Series alloys contain magnesium as needed to adjust its final composition. Most melting furnaces are large (capacity as much as 200,000 pounds (90,000 kg) or more) their main alloying element, often with smaller and are oil- or gas-fired. They can accept molten amounts of manganese and/or chromium. These alloys are generally non-heat-treatable. 6xxx Series alloys contain silicon and magne- metal and alloying ingredients in addition to scrap at a temperature around 1400°F (760°C). For certain melting needs, the furnace is smaller, has a sium in proportions that will form magnesium sili- lower melting rate and may be electrically heated cide and are heat-treatable. 7xxx Series alloys are alloyed mainly by zinc, to minimize oxidation of light gauge (thin) scrap or to control emissions. The molten metal is then often with smaller amounts of magnesium and transferred to a holder. sometimes copper, resulting in heat-treatable After alloying, the molten metal is cleaned and alloys of very high strength. The 8xxx Series of alloy numbers is reserved fluxed to remove impurities and hydrogen gas A before being transferred to the casting pit. Aluminum Alloy Series for alloys of various other compositions. luminum alloys are registered according to The aluminum alloy designation system is ex- their composition. For most of the world, alu- plained in greater detail in Section 10, Appendix minum alloy formulations are registered with the A. Section 10, Appendix C contains tables of typi- Aluminum Association under a numerical classifi- cal mechanical properties, physical properties, and cation system. comparative characteristics and applications.  PRODUCTION 2-6 HEAT-TREATABLE ALLOYS Some alloys, usually in the 2xxx, 6xxx, and 7xxx series, are “solution heat treatable”. They can be strengthened by heat- ing and then quenching, or rapid cooling. They may be fur- ther strengthened by “cold working” controlled deformation at room temperature. S ome aluminum alloys can be significantly hardened and strengthened by controlled heat- ing and quenching sequences known as “heat solubility in aluminum with increasing temperature. The increase of strength induced by heat treat- ment can be dramatic. For example, in the fully treatment” followed by natural or artificial aging annealed O-temper, aluminum alloy 2024 has an (“precipitation hardening”). Such alloys are called ultimate yield strength of about 27,000 pounds per “heat-treatable.” square inch (185 MPa). Heat treatment and cold Most heat-treatable aluminum alloys contain working followed by natural aging (T3 temper) magnesium, plus one or more other alloying ele- increases its strength 2 1/2 times, to 70,000 ments such as copper, silicon and zinc. In the pounds per square inch (480 MPa). As strength is presence of those elements, even small amounts increased by heat-treating, formability is affected of magnesium promote precipitation hardening. in the other direction: for example, alloy in the T3 These alloys respond to heat treatment because temper is less formable than “fully soft” alloy in their key alloying elements show increasing solid the O-temper. NON-HEAT-TREATABLE ALLOYS “Non-heat-treatable” alloys can be tempered only by cold working and annealing operations. A lloys which will not gain strength and hard- ness from heat treatment are called “non-heat- treatable.” in non-heat-treatable alloys. For example, the ulti- mate tensile strength of alloy 3003 is increased from about 16,000 pounds per square inch (110 The initial strength of these alloys, usually in MPa) in the O-temper to 29,000 pounds per the 1xxx, 3xxx, 4xxx, and 5xxx series, is provided square inch (200 MPa) in the H18 strain hardened by the hardening effect of their alloying elements. temper. The ultimate tensile strength of alloy 3004 Additional strengthening can be created by cold is increased from about 26,000 psi (180 MPa) in working — deformation which induces strain- its O-temper to about 41,000 psi (280 MPa)in the hardening, denoted by the “H” tempers. Strain- H38 temper (strain-hardened and stabilization hardened alloys containing appreciable amounts heated). of magnesium are usually given a final elevated Both heat-treatable and non-heat-treatable alloys temperature treatment to stabilize their properties. may be deliberately softened and made more Cold-working can increase strength significantly formable by annealing. Aluminum for rolling must be alloyed to exacting specifications. It must be suitable for the planned rolling, for tempering, and for its intended end use. Equally important is freedom from impurities that might weaken the alloy, cause mechanical defects, or damage mill rolls.  PRODUCTION 2-7 RECYCLED ALUMINUM SCRAP In the remelt furnace compatible aluminum scrap is added to the pure aluminum, from “charge buckets” moved by overhead cranes. Recycled scrap makes up more than half of the aluminum alloy processed in the United States; and recycling uses 95 percent less energy than making new aluminum. R ecycled beverage cans are not only a large source of aluminum scrap but also a particular- ly convenient one because of the known uniformi- Aluminum scrap generated within the rolling plant itself is also returned to the melting hearths; this in-house scrap is also very convenient ty of their alloy composition. They can be recy- because its alloy composition is exactly known. cled right back into new sheet ingot for canstock, Similarly, scrap returned from fabricators often as well as other products. Some specialized plants consists of uniform loads of identified alloys. produce aluminum sheet ingot only from used alu- Other scrap from various recycled products is minum beverage cans, providing very efficient, also used for alloying, taking into account its alloy energy-saving “closed-loop” recycling. composition, determined by testing. ADDITION OF ALLOYING ELEMENTS Next, selected elements — such as magnesium, silicon, manganese, zinc or copper — are mixed in to achieve the correct alloy composition. M agnesium, silicon and zinc are usually added as pure ingredients. Chromium, manganese and iron are usually added as briquettes contain- the melt. Copper is added in either pure (shot / chopped wire / etc.) or alloyed forms. Titanium and zirconium are typically added as either 5-10% ing up to 85% pure metal, balance being alu- alloy or as compacted, 95% pure “hockey pucks”. minum foil and binders, to facilitate mixing with  PRODUCTION 2-8 NOTES  PRODUCTION 2-9 SELF-TEST QUESTIONS 2.8 Pure aluminum melts at about… SECTION 2: PRODUCTION a. 660°F b. 660°C c.1220°F 2.1 Aluminum makes up about_____of the d.1220°C earth’s crust. a. 8%. 2.9 Aluminum oxide melts at about… b. 15% a. 212°F (100°C). c. 28% b. 660°F (350°C). d. 47% c. l000°F (540°C). d. 3660°F (2015°C). 2.2 Aluminum is_____found as a pure metal in nature. 2.10 The main factor which induces the a. always release of metallic aluminum from b. sometimes aluminum oxide is… c. never a. heat. b. electricity. 2.3 Economically mined bauxite contains c. water. what percentage of aluminum oxide? d. None of the above. a. 40-60% b. 25-40% 2.11 The modern Hall-Heroult reduction c. 15-25% process utilizes... a. heat. 2.4 Most bauxite is mined... b. dissolving medium. a. in deep tunnels. c. electric current. b. in open pits. d. a, c c. in rock quarries. e. b, c d. in shallow river bottoms. f. a, b, and c. 2.5 “Alumina” is another name for... 2.12 In the Hall-Heroult reduction process, a. aluminum. what substance combines with the b. bauxite. oxygen in Al2O3 to leave molten c. aluminum oxide. aluminum? d. aluminum alloy. a. Feldspar b. Cathode 2.6 Most alumina is refined by… c. Carbon a. the Karl process. d. Hydrogen b. the Hall process c. the Heroult process. 2.13 Cryolite is… d. the Hall-Heroult process. a. a popular aluminum alloy. e. the Bayer process. b. a sodium aluminum fluoride salt. c. the material of aluminum reduction 2.7 Refined alumina contains what propor- anodes. tion of aluminum by weight? d. None of the above. a. about two-thirds b. about one-half 2.14 A typical modern Hall-Heroult cell uses c. about one-quarter about how many kilowatts of electricity d. None. to produce one pound (455 g) of alu- minum? a. One or two b. Six or seven c. Ten or twenty d. Trick question: no electricity.  PRODUCTION 2-10 2.15 The cathodic lining of an aluminum 2.22 “Solution heat-treatable” alloys are reduction pot has a long service life those which… because… a. can stand high temperatures without a. it is made of brick. melting. b. it is not in direct contact with the b. can be brightened by heating. melt. c. can be dissolved at high tempera- c. it does not react with the melt. tures. d. it plays no role in the reduction d. can be strengthened by heating and process. quenching. 2.16 Molten aluminum is removed from a 2.23 Solution heat-treatable alloys are usually reduction “pot” by… in the… a. siphoning. a. 2xxx, 6xxx and 7xxx series. b. pouring. b. 2xxx, 4xxx and 5xxx series. c. ladling. c. 1xxx, 5xxx and 6xxx series. d. pumping. d. 2xxx series only. 2.17 Aluminum produced by the Hall-Heroult 2.24 “Non-heat-treatable” alloys are those process is… which… a. a high-strength alloy. a. can be strengthened only by low b. about 75% pure aluminum. temperatures. c. exactly 89% pure aluminum. b. can be strengthened only by cold d. more than 99% pure aluminum. working. c. can be strengthened by annealing. 2.18 “The most important reason for having d. risk cracking when heated. remelt furnaces in a rolling facility is to recycle generated process scrap.” 2.25 Non-heat-treatable alloys are usually in This statement is: the… a. True. a. 1xxx series. b. False. b. 3xxx series. c. 4xxx series. 2.19 What is the approximate temperature in d. 5xxx series. a melting furnace? e. All of the above. a.1000°F (540°C). b.1400°F (760°C). 2.26 An element which never plays a major c.1800°F (980°C). part in the preciptiation hardening of heat-treatable aluminum alloys is… 2.20 Wrought aluminum alloy series numbers a. copper. always have how many digits? b. manganese. a. Three. c. zinc. b. Four. d. silicon. c. Five. e. magnesium. d. As many as necessary. 2.27 “Only solution-heat-treatable alloys can 2.21 The different aluminum alloy series are be made more formable by annealing.” characterized by… This statement is: a. alloy strength. a. True. b. aluminum content. b. False. c. alloy composition. d. (None of the above).  PRODUCTION 2-11 2.28 Remelting and alloying scrap aluminum 2.29 The melt is transferred into a holding uses about as much energy as smelting furnace before casting in order to… new aluminum. a. add more scrap. a. 5% b. adjust alloy composition. b. 50% c. remove hydrogen. c. 100% d. a and b. d. 150% e. b and c. f. a and c.  3 PREPARATION Rolling Aluminum: From the Mine through the Mill of Aluminum Alloys for Rolling METAL CLEANING: FLUXING AND FILTRATION Then the molten alloy is “fluxed”: nitrogen and chlorine gases are injected, and their bubbles bring impurities to the surface to be skimmed off by a furnace operator. M aterials charged into an alloying furnace often carry with them some unavoidable for- eign matter, and other non-metallic materials may this purpose to avoid altering the chemical com- position of the alloy. However, small amounts of chlorine or other reactive gases, typically less than be generated by chemical reactions. These sub- 10 percent, may be mixed with the inert gases stances, which could otherwise cause problems in in order to react with certain impurities such as later operations and impair product quality, are sodium, calcium or lithium and form compounds “F removed from the alloy by fluxing and filtration. which can be removed by skimming or filtration. Fluxing Even though chlorine will have a tendency to react and preferentially remove magnesium, it luxing” refers primarily to the removal does offer the most efficient way to remove of dissolved hydrogen gas from molten hydrogen through a chemical reaction in contrast aluminum alloy by bubbling gases through it. to a much slower diffusion process inherent to However, the bubbling gases also “sweep” some inert gases. Due to environmental and other con- solid impurities to the surface where they collect cerns the use of chlorine is being reduced, and in as a frothy dross which is removed. some regions its use is prohibited. Hydrogen gas — derived from airborne water Typical fluxing techniques include porous plugs vapor, from materials added to the melt, or from located in the bottom of the furnace, spinning furnace walls, tools, or anything else that comes rotors and fluxing wands (ceramic coated metal in contact with the melt — dissolves readily in tubes which incorporate porous plugs at the end of molten aluminum. But it is only about 5% as the tube). All of these techniques are designed to soluble in solid aluminum. If it were allowed to promote uniform distribution of fine bubbles which remain while the molten metal is cast, it would enhance diffusion and chemical reaction processes. emerge during solidification to form tiny bubbles: Fluxing may be done in the holding furnace or in blisters which mar the surface, or internal pores a special in-line device between the furnace and the which weaken the product. casting station, separately or in conjunction with Before casting, dissolved hydrogen is removed filtration. by bubbling dry, hydrogen-free gases through the In-line degassing can be used as the only means molten aluminum. Hydrogen dissolved in the alu- of hydrogen removal, or it can be a part of a com- minum diffuses into these other gases and is car- prehensive approach which includes furnace fluxing ried out of the melt by the rising bubbles. Inert and ceramic foam or other rigid media filtration. (nonreactive) gases, chiefly nitrogen, are used for ALLOY SAMPLE ANALYSIS Now a small sample of the alloy is cast and is analyzed within minutes by the lab. A t the holding hearth and during casting, a small amount of molten alloy is scooped up with a ladle and poured into a small mold where it solid disk about two inches (50mm) in diameter, called a “button”. This “button” is identified with the number of cools and hardens in about 15 to 20 seconds into a its alloy batch and is sent to the plant laboratory  PREPARATION 3-2 for analysis. Laboratory analysis may be done The analyzer may be linked with a computer automatically by a device called a spectrometer which automatically registers the batch number or quantometer. The alloy button is placed in a and its specified alloy composition, compares it chamber in the device. Then a high-voltage spark with the actual sample analysis, and issues a instantly vaporizes a tiny amount of material from “pass” message when the alloy is within specifica- the surface of the sample and momentarily tions or an “off-analysis” message if it is not. excites, or energizes, the atoms in this alloy vapor. The results may be returned automatically to The excited atoms immediately radiate away the the hearth operator by instantaneous electronic excess energy, returning to their normal state. communication. Thus, a “rush” request for analy- Each atom radiates excess energy in a pattern, a sis can be answered within a few minutes from spectrum, unique to its element. By detecting and the time the button sample leaves the hearth area. measuring the emitted energy patterns, the spec- If the alloy is found to be “off analysis”, ele- trometer identifies each element present and its ments may be added to correct its composition. It proportion in the alloy sample. will then be retested for approval. If batch correc- A single “shot” in the high voltage spark spec- tion is not practical, off analysis alloy is recycled. If its composition is correct, the alloy trometer is usually completed in about 25 seconds moves to a holding furnace for cast- including the set-up time. A series of four shots on ing; if not, if is corrected and retested. one sample can be completed in two or three min- utes. FILTRATION Before casting, aluminum flows through a filter to remove any remaining particles. A ny foreign particles or inclusions that have escaped the fluxing process or have devel- oped afterward must be removed from the molten They are typically 16" (400 mm), 20" (500 mm) or 24" (600 mm) square and approximately 2" (50 mm) thick. These filters have pore sizes which aluminum alloy before it is cast into rolling ingot. range from 20 per inch (8 per cm) for capturing Inclusions, hard or soft, might weaken the final coarse impurities to 70 per inch (30 per cm) for product, show up as streaks or flaws in its surface, removal of very fine particulates. or damage work roll surfaces. Filtration is per- During the past 25 years another group of filters formed to remove these potentially harmful parti- was developed. They use spinning nozzle technol- cles. Filtration is sometimes performed in combi- ogy. Each of these filters consists of two or three nation with the fluxing step, within the same chambers. Each chamber is equipped with a process unit. spinning nozzle which delivers a fine dispersion Filtration may utilize deep bed filters which are of gas bubbles. It is very similar to the fluxing used for multiple casts and typically consist of process in the holding furnace except that argon 5 to 15" (125 – 380 mm) layers of media such as with up to approximately 3% chlorine is used for sintered high density alumina flakes or spheres flotation (removal) of small particulates and with diameters in the range of.25 to.75" (6 – 19 hydrogen. While the material is flowing through mm). Molten aluminum flows through the deep the chambers, particulates are captured and float bed which captures and retains nonmetallic and to the surface while at the same time hydrogen is similar fine particulates. also removed. Some of the more commonly used Another method utilizes disposable rigid media types are SNIF, ALPUR and LARS. filters which have the morphology of a sponge.  PREPARATION 3-3 ADDITIVES Along the way, additional elements may be fed in; titanium T is often added to help prevent cracking during solidification or for grain refinement. ately before casting, and the most commonly used he addition of small amounts of titanium additive form is 3/8" (10 mm) diameter rod. (usually in combination with either boron Typical grain refiners are 5%Ti /0.3% Boron or or carbon) to molten aluminum alloy as a grain- 5% Ti/0.15% C alloys. For example, a solid rod of refiner is widely practiced in the rolling industry. these grain refiners may be fed into the molten Dissolved in the alloy, titanium carbide or titani- stream of aluminum at a controlled rate en route um diboride particles provide numerous “nucle- to casting. The addition levels are quite low, ation points” where solidification begins during typically less than 0.01 weight percent. cooling. This enhances uniformity, disperses Grain refiner is added “at the last minute” stresses throughout the solidifying aluminum, because it is most effective if its first five minutes and reduces stress-induced cracking. in the aluminum alloy take place during alloy From its holding hearth, molten aluminum is solidification. Longer residence in molten alloy poured through a trough into the ingot-casting would give the grain refiner time to coalesce into mold. The grain refiner is usually added immedi- larger particles, reducing its effectiveness. CONTINUOUS CASTING OF SHEET OR PLATE Aluminum sheet can be continuously cast for a variety of applications. This production method reduces the steps to produce the sheet. The continuous casting process is used for a variety of common aluminum applications. Currently about 20% of the North American sheet and plate production C is produced by this method. The reduction in hot working is a result of the ontinuous casting takes molten metal and reduced casting thicknesses seen in continuous solidifies it into a continuous strip. A vari- casting. Although actual thicknesses vary by ety of methods are used to solidify the metal, method and producer, “as-cast” continuous cast including roll casters, belt casters and block cast- strip is less than 1 inch (25 mm) thick, compared ers. The common feature for all of these methods to the 30+ inch (760+ mm) thick ingot produced is that sheet is taken directly from molten metal, via the DC ingot process. solidified, and coiled in one operation. In many Because it is much thinner than ingot, continu- cases, the strip is run through a hot mill (which ously cast strip cools and solidifies much faster may have more than one stand). than DC ingot; this may produce greater supersat- Three significant differences between continu- uration of alloying elements and less segregation ous casting and the more traditional DC ingot of these elements within metal grains. In effect, casting (see next section) are: continuously cast strip is partly pre-homogenized. Significantly reduced hot working in continu- In addition, the rapid cooling of continuously ous casting compared to DC ingot casting. cast strip minimizes the tendency of alloying elements to concentrate at the surfaces and makes No homogenization between casting and hot scalping unnecessary, so these products can move rolling. directly to rolling. No scalping between casting and hot rolling. The lack of homogenization and reduced hot working does result in different metallurgical structures even in similar alloys. As a result,  PREPARATION 3-4 T continuously cast alloys may have different Roll Casters capabilities than their DC counterparts. he roll caster is the most common form of strip Producers of continuous cast sheet have estab- casting: molten aluminum flows from nozzles lished practices to ensure comparable performance between a pair of water-cooled rolls. In contact in their chosen markets. In some applications, with the rolls, it solidifies at a cooling rate however, efforts to make continuous cast products between 100 and 1,000°F per second (55 – as formable as their DC ingot counterparts may 555°C/sec.). In addition, the newly solidified significantly reduce the advantages of the continu- metal undergoes some hot deformation as it passes ous casting process. through the roll gap. There are many markets where continuous cast Typical roll casters can produce continuous products are found, including (but not limited to): sheet in the range of 30 to 80 inches (760 – 2030 Building and Construction Applications mm) wide, at linear speeds ranging from about 6 to 10 feet per minute (2 – 3 m/min.). Their pro- Foil and Fin Stock ductivity rates are generally in the range of 60 to Food and Beverage Containers 100 pounds of product per hour, per inch of width S (10 – 18 kg per hour per cm of width). Slab Casters Truck and Trailer applications C General Distribution Types of Continuous Casters lab casters produce continuous aluminum prod- ucts in the plate thickness range. These prod- ontinuous casters are generally described by ucts do not undergo any hot deformation in the the type of product they yield (“strip” or caster. They emerge with as-cast metallurgical “slab” casters), or by the type of equipment used characteristics and must be rolled by hot mills to (for example — roll, belt or block casters). achieve wrought aluminum properties and a sheet Both strip (continuous sheet) and slab (continu- gauge that can be coiled. ous plate) casters have been in use since the They produce slab typically between 10 and 80 1950s. They are less versatile in product thick- inches (255 – 2030 mm) wide, at linear speeds nesses and characteristics than conventional mills around 20 feet per minute (6 m/min.), yielding which flat-roll aluminum ingot, but they require productivities in the range of 1,000 to 1,300 lower capital investment and have found economi- pounds per hour, per inch of width (180-225 kg cal application primarily in conjunction with lim- per hour per cm of width). ited-purpose “minimills.” There, one of their main Slab casters cool and solidify molten aluminum advantages — particularly in slab casting — is the at rates ranging from 1 to 10°F per second (1/2 to ability to produce a continuous band, which can 5°C); solid slab emerges at a temperature of about move immediately through a hot-rolling mill for 1,000°F (540°C), requiring little or no reheating transformation into thinner wrought products in an before hot rolling. unbroken flow. The two main commercial types of slab casters I are belt and block casters. Belt Casters Continuous casting processes are undergoing further technical development aimed at increasing S their speed and versatility. Strip (Continuous Sheet) Casters n the belt caster, molten aluminum flows between two almost-horizontal, continuous, thin trip casters can produce continuous aluminum metal belts that are in constant motion. Metal side sheet thin enough to be coiled immediately dams moving with the belts contain the aluminum after casting, without additional rolling. This while it solidifies. The belts are usually inclined at coiled sheet may be re-rolled later into thinner an angle of 5 to 9 degrees from horizontal, to min- sheet gauges. Alternatively, the cast sheet may imize turbulence in the pool of molten aluminum, be fed immediately through a hot-rolling mill for and belt speed is synchronized with the metal thickness reduction before the product is coiled. flow rate. The hot metal transfers its heat through films of  PREPARATION 3-5 lubricant to the metal belts, which are water-cooled As this loop belt rotates around its drive gears, on their outer sides, producing slabs typically in each block takes its turn lining up with its neigh- the range of 1/2 to one inch (12 – 25 mm) thick. bors to contain the flow of molten aluminum from This continuous slab is usually fed directly into a nozzle. The blocks above and below the alu- low-speed in-line rolling mills which reduce it to minum absorb enough heat to solidify the alloy as coils of thinner sheet stock for eventual re-rolling. they travel along with it. Then, as they reach the farther drive gears, each block lifts away from the aluminum and moves around to the far side of its loop where a cooling system removes the heat before the block swings back for another turn in the traveling mold wall. As with belt casters, the continuous slab pro- duced by the block caster is usually hot-rolled immediately into thinner sheet stock. Belt Caster T Block Casters he block caster operates on the same general principle as the belt caster: that is, the alu- minum slab is formed between upper and lower traveling walls. In place of continuous flat belts, however, the block caster’s traveling walls consist of thick blocks of metal lined up side-by-side so that the exposed surfaces which contact the alu- minum form flat walls about six feet long. The sides of the blocks away from the aluminum are attached to a jointed loop belt, like “caterpillar” vehicle treads. Block Caster  PREPARATION 3-6 DC INGOT CASTING But most sheet and plate is rolled from thick slabs called “Ingots”, cast by the “direct chill” or DC method. Aluminum is poured into a mold that is only about six inches (150 mm) deep. Progressive cooling while in contact with the mold and bottom block provides a solidified shell around the still-molten core of the ingot. Once this solid shell forms, the bottom block is lowered along with the aluminum shell, while more aluminum is poured in, hardening at the walls and extending the shell. The ingot emerges at about a speed of 1 1/2 to 4 inches per minute (40-100 mm/min.) and is chilled by a water spray to complete its cooling. This direct-chill method permits mold walls only six inches (150 mm) deep to produce a retangular ingot over 30 feet (9 m) long, seven-and-a-half feet (2300 mm) wide and over two-and-a-half feet (760 mm) thick, weighing “H about 30 tons(27 metric tons). ot metal” (molten aluminum alloy) is Direct-chill ingot casting takes place in a “DC stored before casting in a holding hearth casting pit” and each casting of ingot(s) is called a at a temperature ranging from ~1290 to 1345°F “drop”. Modern casting operations utilize multiple, (~700 – 730°C). Its temperature decreases by commonly 4 – 6, rectangular DC molds typically about 40 to 50°F (22 – 28°C) upon transfer to the 16 to 30" (40 – 75 mm) thick and 45-80" (1.1 – casting pit or station. 2 m) wide. The walls of the mold collar are water cooled to speed the solidification of the ingot shell. The solid shell cools to a temperature in a range of about 570 to 930°F (300 – 500°C) and contracts as it cools, separating from the mold wall before it emerges. The cooling rate is sharply reduced where wall contact is interrupted by this separa- tion, called the shrinkage gap. This effect may permit an unwanted enrichment of alloying elements in a narrow zone near the surface. The enrichment zone is usually removed before rolling, a procedure called “scalping.” As the bottom block is lowered and the solidify- ing ingot emerges from the mold, the surface is further cooled by water from openings around the inside of the mold. The water is recirculated through cooling towers to maintain a correct temperature for effective ingot cooling. Sensors linked to a computer coordinate the rate Direct-chill ingot casting (schematic) of water flow, molten metal levels, the rate of molten metal addition, and the bottom block speed. Casting speeds may range from one-and-a-half to four inches per minute (37 to 100 mm/min.),  PREPARATION 3-7 Temperature Distribution in an ingot during direct-chill casting. E depending on the aluminum alloy and mold size Electromagnetic Casting involved. lectromagnetic casting (EMC) is a DC ingot Since it is very important to provide a continu- casting variation in which a strong magnetic ous distribution of molten metal, fiberglass socks, field confines the molten aluminum while its shell screens and distribution bags are placed in the initially solidifies. Continuous cooling and elimi- ingot head within the confines of the mold. These nation of the liquated zone inherent to the DC provide uniform molten metal distribution and process eliminates the zone of enrichment thus temperature through the entire perimeter of the minimizing the need for scalping. The aluminum mold thus minimizing surfaces defects such as does not contact mold walls, so there is no cold folds or molten metal breakouts. “shrinkage gap” effect and surface enrichment of At two inches per minute (50 mm/min.), it takes alloying elements is avoided about 2-1/2 hours to cast a 290-inch (7365 mm) Therefore, EMC casting can produce ingots long ingot. An aluminum ingot 24 inches thick, with smoother, metallurgically more homogenous 55 inches wide and 290 inches long (about 2 x 4.5 surfaces that need little or no scalping and/or edge x 24 feet [610 x 1400 x 7365 mm]) weighs about trimming on the hot line. A number of sheet prod- 20 tons (18 metric tons) and contains enough ucts, such as 3xxx and 5xxx beverage stock aluminum, for example, to produce more than alloys, have been cast and rolled without the need 1.5 million beverage can bodies. for scalping DC casting has been used to produce ingots as much as 37 feet (11.25 m) long or longer. SCALPING To create high qualify surfaces for rolling, the broadest sides of the ingot are shaved by a machine with rotating blades called a “scalper.” Sometimes all four of the sides are scalped. S calping is performed on an ingot to remove any irregularities or undesirable chemical composi- tions, such as excess oxides or concentrated alloy- The scalper itself is a specialized milling machine with horizontal blades spanning about five feet (1.5 m) or more and rotating on a vertical axle. ing elements, left in its surface by the casting The scalper generally mills the rolling faces of process. By providing smooth, metallurgically the ingot-that is, its widest surfaces, scalping one homogenous surfaces, scalping promotes high side and then turning the ingot over to scalp the quality rolling and avoids the rolling of “slivers” other side. Some operations utilize two sets of into the surface of plate or sheet. cutters permitting simultaneous scalping of both  PREPARATION 3-8 rolling surfaces. Edges and ends which are intermediate trimming. Therefore, based on ingot trimmed during the rolling operation need not be profile, edges may also be scalped, thus eliminat- scalped. Recently, more emphasis is being placed ing the need for intermediate trims or sawing on minimizing downstream operations such as operations. PREHEATING The scalped ingot must be preheated for hot rolling. The temperature and time is controlled to promote specified ingot properties. There are a variety of methods of preheating ingots. These include “walking-beam” or “pusher” furnaces, car-bottom furnaces and soaking pits. P Idurefurnaces. Purposes of preheating Preheating methods reheating the ingot before hot rolling serves ngots are preheated in temperature-controlled several important purposes, including: Several variations of preheating proce- are available to serve different ingot dimen- 1. Raising the alloy’s temperature above the sions or treatment requirements: recrystallization temperature to prevent cold- working (strain-hardening) from ta