Aluminum And Aluminum Alloys - NIST

Alloying: Understanding the BasicsJ.R. Davis, p351-416DOI:10.1361/autb2001p351Copyright 2001 ASM International All rights reserved.www.asminternational.orgAluminum andAluminum AlloysIntroduction and OverviewGeneral Characteristics. The unique combinations of propertiesprovided by aluminum and its alloys make aluminum one of the most versatile, economical, and attractive metallic materials for a broad range ofuses—from soft, highly ductile wrapping foil to the most demanding engineering applications. Aluminum alloys are second only to steels in use asstructural metals.Aluminum has a density of only 2.7 g/cm3, approximately one-third asmuch as steel (7.83 g/cm3). One cubic foot of steel weighs about 490 lb;a cubic foot of aluminum, only about 170 lb. Such light weight, coupledwith the high strength of some aluminum alloys (exceeding that of structural steel), permits design and construction of strong, lightweight structuresthat are particularly advantageous for anything that moves—space vehicles and aircraft as well as all types of land- and water-borne vehicles.Aluminum resists the kind of progressive oxidization that causes steel torust away. The exposed surface of aluminum combines with oxygen to forman inert aluminum oxide film only a few ten-millionths of an inch thick,which blocks further oxidation. And, unlike iron rust, the aluminum oxidefilm does not flake off to expose a fresh surface to further oxidation. If theprotective layer of aluminum is scratched, it will instantly reseal itself.The thin oxide layer itself clings tightly to the metal and is colorless andtransparent—invisible to the naked eye. The discoloration and flaking ofiron and steel rust do not occur on aluminum.Appropriately alloyed and treated, aluminum can resist corrosion bywater, salt, and other environmental factors, and by a wide range of otherchemical and physical agents. The corrosion characteristics of aluminumalloys are examined in the section “Effects of Alloying on CorrosionBehavior” in this article.

352 / Light Metals and AlloysAluminum surfaces can be highly reflective. Radiant energy, visiblelight, radiant heat, and electromagnetic waves are efficiently reflected,while anodized and dark anodized surfaces can be reflective or absorbent.The reflectance of polished aluminum, over a broad range of wavelengths, leads to its selection for a variety of decorative and functionaluses.Aluminum typically displays excellent electrical and thermal conductivity, but specific alloys have been developed with high degrees of electrical resistivity. These alloys are useful, for example, in high-torqueelectric motors. Aluminum is often selected for its electrical conductivity,which is nearly twice that of copper on an equivalent weight basis. Therequirements of high conductivity and mechanical strength can be met byuse of long-line, high-voltage, aluminum steel-cored reinforced transmission cable. The thermal conductivity of aluminum alloys, about 50 to 60%that of copper, is advantageous in heat exchangers, evaporators, electrically heated appliances and utensils, and automotive cylinder heads andradiators.Aluminum is nonferromagnetic, a property of importance in the electrical and electronics industries. It is nonpyrophoric, which is important inapplications involving inflammable or explosive-materials handling orexposure. Aluminum is also non-toxic and is routinely used in containersfor food and beverages. It has an attractive appearance in its natural finish,which can be soft and lustrous or bright and shiny. It can be virtually anycolor or texture.The ease with which aluminum may be fabricated into any form is oneof its most important assets. Often it can compete successfully withcheaper materials having a lower degree of workability. The metal can becast by any method known to foundrymen. It can be rolled to any desiredthickness down to foil thinner than paper. Aluminum sheet can bestamped, drawn, spun, or roll formed. The metal also may be hammeredor forged. Aluminum wire, drawn from rolled rod, may be stranded intocable of any desired size and type. There is almost no limit to the different profiles (shapes) in which the metal can be extruded.Alloy Categories. It is convenient to divide aluminum alloys into twomajor categories: wrought compositions and cast compositions. A furtherdifferentiation for each category is based on the primary mechanism ofproperty development. Many alloys respond to thermal treatment based onphase solubilities. These treatments include solution heat treatment,quenching, and precipitation, or age, hardening. For either casting orwrought alloys, such alloys are described as heat treatable. A large numberof other wrought compositions rely instead on work hardening throughmechanical reduction, usually in combination with various annealing procedures for property development. These alloys are referred to as workhardening. Some casting alloys are essentially not heat treatable and are

Aluminum and Aluminum Alloys / 353used only in as-cast or in thermally modified conditions unrelated to solution or precipitation effects.Cast and wrought alloy nomenclatures have been developed. TheAluminum Association system is most widely recognized in the UnitedStates. Their alloy identification system employs different nomenclaturesfor wrought and cast alloys, but divides alloys into families for simplification. For wrought alloys a four-digit system is used to produce a list ofwrought composition families as follows: 1xxx: Controlled unalloyed (pure) composition, used primarily in theelectrical and chemical industries2xxx: Alloys in which copper is the principal alloying element,although other elements, notably magnesium, may be specified. 2xxxseries alloys are widely used in aircraft where their high strength(yield strengths as high as 455 MPa, or 66 ksi) is valued.3xxx: Alloys in which manganese is the principal alloying element,used as general-purpose alloys for architectural applications and various products4xxx: Alloys in which silicon is the principal alloying element, used inwelding rods and brazing sheet5xxx: Alloys in which magnesium is the principal alloying element,used in boat hulls, gangplanks, and other products exposed to marineenvironments6xxx: Alloys in which magnesium and silicon are the principal alloying elements, commonly used for architectural extrusions and automotive components7xxx: Alloys in which zinc is the principal alloying element (althoughother elements, such as copper, magnesium, chromium, and zirconium,may be specified), used in aircraft structural components and otherhigh-strength applications. The 7xxx series are the strongest aluminumalloys, with yield strengths 500 MPa ( 73 ksi) possible.8xxx: Alloys characterizing miscellaneous compositions. The 8xxx seriesalloys may contain appreciable amounts of tin, lithium, and/or iron.9xxx: Reserved for future useWrought alloys that constitute heat-treatable (precipitation-hardenable)aluminum alloys include the 2xxx, 6xxx, 7xxx, and some of the 8xxxalloys. The various combinations of alloying additions and strengtheningmechanisms used for wrought aluminum alloys are shown in Table 1. Thestrength ranges achievable with various classes of wrought and cast alloysare given in Tables 2 and 3.Casting compositions are described by a three-digit system followed by adecimal value. The decimal .0 in all cases pertains to casting alloy limits.Decimals .1, and .2 concern ingot compositions, which after melting and processing should result in chemistries conforming to casting specificationrequirements. Alloy families for casting compositions include the following:

354 / Light Metals and Alloys 1xx.x: Controlled unalloyed (pure) compositions, especially for rotormanufacture2xx.x: Alloys in which copper is the principal alloying element. Otheralloying elements may be specified.3xx.x: Alloys in which silicon is the principal alloying element. Theother alloying elements such as copper and magnesium are specified.The 3xx.x series comprises nearly 90% of all shaped castings produced.4xx.x: Alloys in which silicon is the principal alloying element.5xx.x: Alloys in which magnesium is the principal alloying element.6xx.x: Unused7xx.x: Alloys in which zinc is the principal alloying element. Otheralloying elements such as copper and magnesium may be specified.8xx.x: Alloys in which tin is the principal alloying element.9xx.x: UnusedHeat-treatable casting alloys include the 2xx, 3xx, and 7xx series.Tables 4 and 5 list nominal compositions for representative wrought andcast aluminum alloys. It should be noted that the alloy compositions listed in these tables make up a rather small percentage of the total amountof compositions developed. More than 500 alloy designations/compositions have been registered by the Aluminum Association Inc. for aluminum alloys. Composition limits for these alloys can be found in theMetals Handbook Desk Edition, 2nd ed., (see the article “ChemicalCompositions and International Designations on pages 426–436) and inRegistration Records on wrought alloys, castings, and ingots published bythe Aluminum Association.Table 1 Classification of wrought aluminum alloysaccording to their strengthening mechanismAlloy systemAluminum seriesWork-hardenable alloysPure 8xxxPrecipitation-hardenable xx

Aluminum and Aluminum Alloys / 355Applications. Aluminum alloys are economical in many applications.They are used in the automotive industry, aerospace industry, in construction of machines, appliances, and structures, as cooking utensils, as covers for housings for electronic equipment, as pressure vessels forcryogenic applications, and in innumerable other areas. Tables 6 and 7 listtypical applications for some of the more commonly used wrought andcast alloys, respectively.Table 2 Strength ranges of various wrought aluminum x5xxx5xxx6xxx7xxx7xxx8xxxType ofalloycompositionAlAl-Cu-Mg(1–2.5% Cu)Al-Cu-Mg-Si(3–6% Cu)Al-Mn-MgAl-SiAl-Mg(1–2.5% Mg)Al-Mg-Mn(3–6% ningmethodTensilestrength rangeMPaksiCold workHeat treat70–175170–31010–2525–45Heat treat380–52055–75Cold workCold work(some heattreat)Cold 40Cold work280–38040–55Heat treatHeat treatHeat treatHeat –7575–9040–80Table 3 Strength ranges of various cast aluminum alloysTensile strength rangeAlloy system (AA 31033–4340–45138–22120–32Heat treatable sand cast alloys (various tempers)Al-Cu (201–206)Al-Cu-Ni-Mg (242)Al-Cu-Si (295)Al-Si-Cu (319)Al-Si-Cu-Mg (355, 5% Si, 1.25% Cu, 0.5% Mg)Al-Si-Mg (356, 357)Al-Si-Cu-Mg (390, 17% Si, 4.5% Cu, 0.6% Mg)Al-Zn (712, 713)Non-heat treatable die cast alloysAl-Si (413, 443, F temper)Al-Mg (513, 515, 518, F temper)Non-heat treatable permanent mold cast alloysAl-Sn (850, 851, 852, T5 temper)