Frequently Asked Questions
Included in this section are some commonly asked questions and their answers. If you do not find an answer to your question, or if you need further information on an area, please contact
us.
Topics:
| Alloy and Temper Designations |
Q: Where do the aluminum alloy numbers like 2024 and A356.0 come from?
A: These combinations of numbers and, in some cases, letters are defined in ANSI 35.1 covering the designation of aluminum alloys. The four digit designations are for wrought alloys while the designations with three digits followed by a fourth after the decimal point are for cast alloys. The first digit in both designation systems indicates the alloy group with others digits and in some cases letters defining specific alloy compositions or variations of a base alloy composition. A detailed listing of aluminum alloys that have been registered internationally can be obtained from the Aluminum Association.
Q: What is the difference between wrought and cast products?
A: A wrought product is one that has been subjected to mechanical working by processes such as rolling, extrusion, and forging. A cast product is one in which the shape has been produced by introducing molten aluminum into a mold, and includes processes such as sand casting, permanent mold casting, and die casting.
Q: What are the letters and numbers after the alloy number, like -O, -H19, and -T6?
A: These are the temper designations, which are based on the sequence of basic treatments applied to the alloy. The temper designation follows the alloy designation, separated by a hyphen. Basic temper designations consist of a letter- F, O, H, W, or T -that indicates a type of processing, with one or more subsequent digits used to indicate subdivisions of the basic temper. A detailed listing of tempers that have been registered can be obtained from the Aluminum Association.
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| Aluminum Properties |
Q: What is the difference between mechanical and physical properties?
A: Mechanical properties are defined as those material properties that measure a material's reaction to applied force, like tensile strength, elongation, modulus of elasticity, fracture toughness, and fatigue strength. Physical properties are properties other than mechanical properties that depend on the physics of the material, including that density, thermal and electrical conductivity, and thermal expansion. In the industry, tensile properties are sometimes incorrectly referred to as "physical properties".
Q: What are the tensile properties of a material?
A: Tensile properties are the mechanical properties of the material measured under the influence of a tensile, or pulling stress. In the industry, tensile properties, sometimes just called "tensiles", typically include the ultimate tensile strength, tensile yield strength, and elongation measured over a specified gauge length.
Q: How high a temperature can I use aluminum in my application?
A: While aluminum alloys melt at temperatures typically over 1000°F, their properties can change significantly at much lower temperatures. The answer to this question is dependent not only on the property variation with temperature of the alloy and temper of interest but also on the requirements of the specific application. As a rough rule of thumb, aluminum alloys do not show significant property changes at temperature below 200 , but will lose strength with time at higher temperatures. An excellent resource for properties of specific alloy and temper combinations as a function of temperature is the book Properties of Aluminum Alloys by J. G. Kaufman, available form the Aluminum Association or ASM International.
Q: How do aluminum's properties change at temperatures below room temperature?
A: The many advantages of aluminum are not impaired by exposure to low-temperatures. Aluminum actually gains strength as temperature is reduced, making it a most appropriate material for arctic, space or cryogenic applications. A useful publication is Aluminum Alloys for Cryogenic Applications, available from the Aluminum Association.
Q: How is an alloy's resistance to fracture measured?
A: A number of parameters are used to measure fracture resistance, including impact energy, tear resistance, and fracture toughness. All of these measure the material's resistance to crack extension, but fracture toughness is the test method most commonly specified for evaluating materials that will be used in fracture-critical components. An excellent reference in this area is Fracture Resistance of Aluminum Alloys by J. G. Kaufman, published jointly by The Aluminum Association and ASM International.
Q: What is meant by the fatigue strength of aluminum?
A: Fatigue is the process in which a repeated cyclic stress, most often below the yield strength of the material, can result in damage and subsequent failure. The fatigue strength of an aluminum alloy and temper is the stress below which the material will survive a certain number of cycles, with the later expressed on a log scale. Sometimes the "fatigue endurance limit" is listed, and this is defined as the stress level at 500,000,000 cycles of fully reversed loading.
Q: What makes aluminum so corrosion resistant?
A: The formation of a tenacious aluminum oxide later on fresh aluminum surfaces provides resistance to attack by the environment and a wide range of other chemical species. Corrosion resistance varies significantly, however, with various alloy and temper combinations as well as environments and secondary conditions such as stress. An excellent reference in this area is Corrosion of Aluminum and Aluminum Alloys by J. R. Davis, available from the Aluminum Association or ASM International.
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| Aluminum Processing Methods |
Q: How is aluminum made?
A: Basically, aluminum is found in the form of an ore called bauxite. Bauxite is refined by the Bayer process to produce alumina, which is turn is converted to aluminum metal via the Hall-Heroult smelting process. The aluminum metal can then be alloyed with other elements or used in the unalloyed form. This serves as the feedstock for casting processes or wrought processing. The latter begins with casting of an ingot or billet that is then worked to shape via processes such as rolling, extrusion, and forging.
Q: What are the differences between plate, sheet, and foil?
A: All of these fall into the category of flat rolled wrought products, with the thickness of the product determining the category in which it belongs. Plate is defined as having a thickness of 0.250" or greater, sheet has a thickness of 0.006" or greater but less than 0.250", and foil has a thickness below 0.006". These three products constitute roughly half of the U.S. Aluminum Industry shipments by volume.
Q: What is an extrusion?
A: An extrusion is a product produced by pushing metal through a die. This simple definition belies the versatility and importance of aluminum extrusions, which make up roughly 15% of the U.S. Aluminum Industry shipments, according to the Aluminum Association.
Q: What are the differences between wire, rod, and bar?
A: All are solid wrought products that are long relative to their cross section, but are distinguished by the shape of the cross section as well as the largest dimension of the cross section. Bar and rod both have at least one perpendicular distance between parallel faces of 0.375" or greater. Rod is defined as having a round cross section, while bar has a square, rectangular, hexagonal, or octagonal cross section. Wire has any of the cross sectional shapes allowed for rod or bar, but its greatest perpendicular distance between parallel faces is less than 0.375". Collectively, these products represent slightly less than 3% of the U.S. Aluminum Industry shipments.
Q: What is a forging?
A: A forging is produced by forming the aluminum into a predetermined shape by hammering, upsetting, closed die forging, for example. Forging shipments comprise about 1% of total U.S. Aluminum Industry shipments by volume.
Q: What processes are used to cast aluminum?
A: A number of processes can be used to produce aluminum castings, with the sand casting, permanent mold casting, and die casting processes representing the majority of product shipments.
Q: What is meant by "Molten Metal Quality"?
A: Molten metal quality is the degree to which an aluminum alloy is free from alkali metal contaminants, non-metallic inclusions, and dissolved hydrogen. Filtration and degassing practices are employed in the cast house as well as the foundry to achieve the required quality levels. For more information on measuring inclusions in aluminum, see the article "Measuring Inclusions in Aluminum Melts" in the Secat Technical Article section.
Q: Why is aluminum heat treated?
A: Heat treatments are applied to aluminum alloys, both wrought and cast, for the purpose of modifying the alloy's mechanical properties. In the non-heat treatable alloys, annealing is used to remove the effects of cold working, resulting in reduced strength but increased ductility or formability. For the heat treatable alloys, the heat treatment sequence of solutionizing, quenching, and aging alloys a wdie range of properties to be produced. The highest strength aluminum alloys are heat treatable. Read a technical article to learn more about some recent advances in aluminum heat treatment.
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| Extrusion Technology |
Q: Where can I find a listing of extrusion suppliers?
A: The Aluminum Extruders Council provides a searchable
online database which allows selection from AEC member companies.
Users can limit their search by selecting forms produced, finishing
services, fabrication services, special services, soft vs. hard
alloy, circle size requirements, and geographic location.
Q: I want to utilize an extrusion, but am concerned about the cost of tooling the die for my special shape. Should I use a "standard" die?
A: While there are certainly standard dies producing angles, channels, bars, tubes, pipes, etc., you should recognize that the cost of producing a custom die may be offset by the your ability to "put the metal exactly where you want it". Custom dies can be produced rapidly and for relatively low cost. Initial costs and lead times for extruded aluminum are usually a good deal lower than those required for die casting, forming, roll forming, stamping, pultrusion, impact, or vinyl extrusion. The shape of the extruded profile (solid, semi-hollow or hollow) is an important factor. Other factors are: product surface finish requirements, special dimensional tolerances, and special alloy, design or production requirements that could affect die life.
Q: What factors affect the cost of an aluminum extrusion?
A: Production cost for extrusions is a function of size, quantity, wall thickness, weight/foot, alloy, temper, length, packaging, classification of shape (solid, semi-hollow or hollow), dimensional tolerance requirements, marking and shipping quantity tolerance. When possible, follow the design suggestions of the extruder to generate the lowest production cost.
Q: What's the difference between a solid, semi-hollow, and hollow shape?
A: Hollow, semi-hollow and solid are the three general categories of extruded shapes. A shape is described as hollow if a completely enclosed void exists anywhere in its cross-section. In a semi-hollow shape, a void is only partially enclosed. (Your extruder can give you guidance, but by definition, for a shape to be semi-hollow, the area of the partially enclosed void must be substantially greater than the square of the gap's width.) A solid shape is one that is neither hollow nor semi-hollow.
Q: What are the tolerance capabilities for the extrusion process?
A: Aluminum extrusions are manufactured routinely to
the close dimensional tolerances required for parts that must
fit snug with other parts. Cost is minimized when an extruded
product is designed to function well at standard production
tolerances. However, tighter, custom tolerances can be specified
when necessary. Specific information can be found in three publications:
Understanding Aluminum Extrusion Tolerances, 1995, Drafting
Standards for Aluminum Extruded Products, 1998, and American
National Standard Dimensional Tolerances for Aluminum Mill Products,
ANSI H35.2-2000. All of these are available from the Aluminum
Association.
Q: What does the term "metal dimensions" refer to in terms of tolerances?
A: "Metal dimensions" refers to dimensions measured across solid metal, which are easier to produce to tight tolerances than those measured across a gap or angle. Rely on "metal dimensions" as much as possible when designing close-fitted mating parts or other shapes requiring precise tolerances.
Q: I need to find information on extrusion dies and tooling, including a problem-solving resource. Where can I find this information?
A: The Aluminum Extruders Council offers a variety of resources related to the extrusion process generaly and extrusion dies and tooling specifically. See the article on Extrusion Tooling Resources to "Die" For in the Secat Technical Articles section for more information.
Q: What is meant by "circle size" for an extrusion, and how do I determine what circle size my shape requires?
A: The circumscribing circle size is defined as the smallest diameter circle that will completely enclose the entire cross-section of the extruded shape. When determining circle size, the profile should be treated as a triangle. The length of the hypotenuse is the determining factor. For example, if the shape is a rectangular tube, allow one face to be the base of your triangle. Draw the hypotenuse of the triangle from one corner of the base to the opposite corner of the opposite face of the tube. Whatever dimension is achieved should always be rounded up to the nearest whole number.
Q: I was recently asked to indicate the "exposed surfaces" on an extrusion profile. What does this mean?
A: Exposed surfaces are those that have critical cosmetic or functional requirements and require special attention and protection during the extrusion process. Additional precautions can be taken in the extrusion tool design and extrusion run-out to minimize surface blemishes. The narrower an exposed surface, the more uniform the finish will be. Webs, flanges and abrupt changes in metal thickness may show up as marks on the extrusion's opposite surface, particularly on thin sections. If your exposed surface is highly critical, your extruder may be able to suggest a secondary finishing operation such as buffing, brushing or polishing.
Q: Our product requires a variety of finishes. How versatile are extruded aluminum's finishing qualities?
A: Extruded aluminum accepts a wide range of finishes unmatched by many materials. The natural protective oxide coating with its attractive metallic hue can be colored with a variety of chemical or anodizing processes. Surface textures can be created, from rough to matte to mirror-smooth. Also, surface coatings such as paint, lacquer, enamel, electroplating or laminates can be applied. More information on specific finishing processes is included in the Product Manufacturing Methods section of these FAQs.
Q: What are the differences between seamless and metallurgically welded hollow shapes?
A: The aluminum extruded hollow can be seamless if made by a pierce-mandrel method or can be metallurgically welded if made by the bridge-porthole process. The seamless shape is preferred for severe bending applications, but in most of the smooth radius end uses, the non-seamless metallurgically welded shape will work. As a rule, as long as you are not seeking to utilize the tube for internal flow where burst pressure becomes a concern, the truly seamless is not required. Tooling, product price and availability are considerably less for the non-seamless product.
Q: What is the minimum bending radius for an extrusion?
A: Extruded aluminum tube, pipe and shapes can be formed on conventional bending equipment. The minimum bending radius of a specific extrusion will depend on its size, alloy and temper, the complexity of its shape, and the characteristics of the available equipment. The Aluminum Association publication Forming and Machining Aluminum contains minimum bend radii for several common tube products.
Q: What research and development activities are underway for aluminum extrusion?
A: Active R&D has been undertaken by private companies, universities, and National Laboratories to advance all aspects of aluminum extrusion technology. A comprehensive collection of past work can be found in the Proceedings from the International Aluminum Extrusion Technology Seminars, known as "ET" for short. These are available from the Aluminum Extruders Council via their Publications list. A summary of some specific ongoing R&D in the extrusion area can be found in the Secat Technical Articles section.
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| Product Manufacturing Methods |
Q: What is the anodizing process?
A: Anodizing is an electrochemical process for forming an artificial oxide coating layer on aluminum. The coating provides increased corrosion protection and wear resistance, and can also produce a rainbow of colors to improve the aesthetic appearance of the product.
Q: What are the differences between sulfuric acid anodizing, hard anodizing, and conversion coating?
A: In the sulfuric acid anodizing process, the aluminum workpiece is placed in an acid-based electrolyte. An electric current is then passed through the part. During the oxidation process, the workpiece is coated with a hard, porous oxide layer. Organic coatings or dyes of various colors are later electrodeposited in the pores of the anodic coating.
The primary difference between the sulfuric acid anodize and hard anodizing processes are the operating temperature and the current density at which the process occurs. Hard anodizing produces a heavier coating with better wear properties than a sulfuric acid anodize.
The conversion coating, also known as chrome phosphate or chrome oxide (yellow chrome), is a process that treats the metal surface chemically when the metal is immersed in, or sprayed with, various solutions. As with anodizing, a thick oxide layer is formed although no electrical current is required in the conversion coating process. This finish provides protection and good adhesion, but would typically be used as a base for another final finish.
Q: I'd like my aluminum product to have a shiny finish. Can this be done?
A: Yes. With a chemical finishing process called bright dipping, a special dip solution made of hot phosphoric acid and nitric acids is applied to the product. The resulting finish is mirror-shiny, providing a highly reflective surface.
Q: How can I be assured of good adhesion when applying paint or lacquer to my aluminum product?
A: As with any other type of material, the surface should always be cleaned and pretreated as required for your specific coating. Before applying the topcoat of paint or lacquer, the aluminum should be given a chemical conversion pretreatment, followed by a primer coat. This not only provides good adhesion, it also gives maximum corrosion protection.
Q: How is aluminum joined together?
A: Aluminum can be joined by a range of welding processes as well as mechanical fastening and adhesive bonding. An excellent reference that describes the range of joining processes is Welding Aluminum: Theory and Practice, a publication of the Aluminum Association. Read a review of the recently issued Fourth Edition.
Q: What is meant by the "formability" of a material?
A: Formability is a broad term that generally describes the ease with which a material can be shaped by plastic deformation. It can include deformation under bending, stretching, drawing, and other conditions. Aluminum's formability varies significantly depending of the specific alloy, temper, and product form.
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| Aluminum Markets and Applications |
Q: Where is aluminum used?
A: Based on the Aluminum Statistical Review 2000, published by the Aluminum Association, transportation applications consume the greatest percentage of aluminum shipped by U.S. producers at roughly 32%. Containers and packaging make up about one fifth of the shipment volume, while building and construction shipments comprise roughly 13% of the total. Other key market areas are exports, electrical, consumer durables, and machinery and equipment.
Q: Why are automakers so interested in using aluminum?
A: A primary motivation for aluminum usage is the potential to reduce vehicle weight. In fact, a 10%weight reduction of a vehicle by substituting aluminum for heavier materials results in a 6-8% weight savings. In addition, aluminum cars have demonstrated excellent safety performance due to the ability to produce larger, lighter cars. Further benefits of aluminum are its corrosion resistance, ability to be shaped by many processes, and recyclability.
Q: What makes aluminum of interest in aerospace applications?
A: Aluminum has been of interest for aerospace applications from the days of the Wright Brothers, due mostly to its high strength to weight ratio. This is especially true for the highly alloyed alloys of the 2xxx and 7xxx series that have been the main materials of construction for commercial aircraft throughout history. Even today, with improvements in many non-aluminum materials, aluminum still comprises some 80% of the structural weight of the most modern aircraft.
Q: How are aluminum cans made?
A: Aluminum beverage cans, one of the great product development success stories for the industry, are produced in two parts. The body of the can is produced by the draw and ironing process from 3004 alloy sheet, while the lids are produced from 5182 alloy by stamping. They are subsequently crimped together to form the familiar two-piece can. The U.S. aluminum industry annually produces material for over 100 billion cans per year-or about one can per person per day. Aluminum cans are also a recycling success story, with the recycling rate now around 60%. Used aluminum cans are recycled and returned to store shelves as new cans in as few as 60 days.
Q: What information is there on use of aluminum in building and construction applications?
A: Building and construction applications represented
about 13% of aluminum product shipments in 2000. Aluminum is
used in builidng, bridge, and other structural applications
because of its high strength-to-weight ratio, excellent corrosion
resistance, ease of fabrication, and desirable appearance. Guidelines
for structural design are provided in the Aluminum Design Manual
2000, available from the Aluminum Association. In addition,
an excellent book on aluminum structures is available. Read
a review of "Aluminum Structures".
Q: How do I clean aluminum?
A: Cleaning of aluminum depends on the specific surface finish of the aluminum product. Generally, use of mild soaps and detergents as well as non-etching cleaners can be used safely. Other cleaning products including solvent and emulsion cleaners, abrasive cleaners, and etching cleaners are available for aluminum but should be used carefully so that the finish is not damaged. Further detailed advice along with product listings are available in the Aluminum Association publication "Care of Aluminum". Read a review of "Care of Aluminum" in the Secat Technical Articles section.
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| Environment, Health and Safety |
Q: What are the benefits of recycling aluminum?
A: Recycling saves 95% of the energy required to make aluminum from ore and avoids 95% of the associated emissions of CO2, SOx, and NOx. Each pound of aluminum that is recycled saves 6.5-7 kilowatt hours of electricity. At current recycling levels, roughly one third of the industry's metal supply is from recycled metal, saving an equivalent of 90 million barrels of oil in energy.
Q: Are there safety concerns in aluminum recycling?
A: While solid bulk aluminum is a safe and noncombustible material, molten aluminum can be reactive. The quality of aluminum scrap for recycling is an important consideration to ensure that remelting can be carried out safely. The Aluminum Association has published guidelines to assist remelting operations in the safe handling and melting of aluminum. You may want to review one such publication, “Guidelines for Aluminum Scrap Receiving and Inspection Based on Health and Safety Considerations” in the Secat Technical Article Section.
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