Aluminum alloys

Alloying aluminum with various elements markedly improves mechanical properties, strength primarily, at only a slight sacrifice in density, thus increasing specific strength, or strength-to-weight ratio. Traditionally, wrought alloys have been produced by thermo mechanically processing cast ingot into mill products such as billet, bar, plate, sheet, extrusions, and wire. For some alloys, however, such mill products are now made by similarly processing "ingot" consolidated from powder. Such alloys are called PM (powder metal) wrought alloys or simply PM alloys. To distinguish the traditional type from these, they are now sometimes referred to as ingot-metallurgy (IM) alloys or ingot-cast alloys. Another class of PM alloys are those used to make PM parts by pressing and sintering the powder to near-net shape. There are also many cast alloys. All told, there are about 100 commercial aluminum alloys.

There are two principal kinds of wrought alloys: (1) heat-treatable alloys — those strengthened primarily by solution heat treatment or solution heat treatment and artificially aging (precipitation hardening), and (2) non-heat-treatable alloys — those that depend primarily on cold work for strengthening. Alloy designations are a continuation of the four-digit system noted for aluminum, followed with a letter to designate the temper or condition of the alloy: F (as-fabricated condition), O (annealed), H (strain-hardened), W (solution heat-treated and unstable; that is, the alloy is prone to natural aging in air at room temperature), and T (heat-treated to a stable condition). Numerals following T and H designations further distinguish between tempers or conditions. T3, for example, refers to alloys that have been solution heat-treated, cold-worked, and naturally aged to a substantially stable condition. T6 denotes alloys that have been solution heat-treated and artificially aged. H designations are followed by two or three digits. The first (1, 2, or 3) indicates a specific sequence of operations applied. The second (1 to 8) refers to the degree of strain hardening (the higher the number, the greater the amount of strain, or hardening, 8 corresponding to the amount induced by a cold reduction of about 75%). The third (1 to 9) further distinguishes between mill treatments (Table A.10).

The aluminum alloy 2XXX series is characterized by copper (2.3 to 6.3%) as the principal alloying element. Most of these alloys also contain lesser amounts of magnesium and manganese and some may contain small amounts of other ingredients, such as iron, nickel, titanium, vanadium, zinc, and zirconium. 2XXX alloys are strengthened mainly by solution heat treatment, sometimes by solution heat treatment and artificial aging. Among the more common, especially for structural aircraft applications, are aluminum alloys 2014, 2024, and 2219, which can be heat-treated to tensile yield strengths in the range of 276 to 414 MPa. A recently developed alloy for auto body panels is aluminum alloy 2036, which in the T4 temper provides a tensile yield strength of about 195 MPa. Aluminum alloy 2XXX series is not as corrosion resistant as other aluminum alloys and thus is often clad with a thin layer of essentially pure aluminum or a more corrosion-resistant aluminum alloy, especially for aircraft applications.

Manganese (0.5 to 1.2%) is the distinguishing alloying element in the aluminum alloy 3XXX series. Aluminum alloy 3003 also contains 0.12% copper. Aluminum alloys 3004 and 3105 also contain 1 and 0.5% magnesium, respectively. Strengthened by strain hardening, 3XXX alloys provide maximum tensile yield strengths in the range of 186 to 248 MPa and are used for chemical equipment, storage tanks, cooking utensils, furniture, builders’ hardware, and residential siding.

The aluminum alloy 4XXX series is characterized by the addition of silicon: about 12%, for example, in aluminum alloy 4032 and 5% in aluminum alloy 4043. Aluminum alloy 4032, which also contains 1% magnesium and almost as much copper and nickel, is heat-treatable, providing a tensile yield strength of about 315 MPa in the T6 temper. This, combined with its high wear resistance and low thermal expansivity, has made it popular for forged engine pistons. Aluminum alloy 4043, which is alloyed only with silicon, is a strain-hardenable alloy used for welding rod and wire. Some Al-Si alloys are also used for brazing, others for architectural applications. Their appeal for architectural use stems from the dark gray color they develop in anodizing.

The principal alloying element in the aluminum alloy 5XXX series is magnesium, which may range from about 1 to 5%, and is often combined with lesser amounts of manganese and/or chromium. Like 3XXX alloys, the 5XXX are hardenable only by strain hardening and all are available in a wide variety of H tempers. Tensile yield strengths range from less than 69 MPa in the annealed condition to more than 276 MPa in highly strained conditions. Aluminum alloys 5083, 5154, 5454, and 5456 are widely used for welded structures, pressure vessels, and storage tanks, others for more general applications, such as appliances, cooking utensils, builders’ hardware, residential siding, auto panels and trim, cable sheathing, and hydraulic tubing.

The aluminum alloy 6XXX series is characterized by modest additions (0.4 to 1.4%) of silicon and magnesium and can be strengthened by heat treatment. Except for auto sheet aluminum alloys 6009 and 6010, which are typically supplied and used in the T4 temper, the alloys are strengthened by solution heat treatment and artificial aging. As a class, these alloys are intermediate in strength to aluminum alloy 2XXX and aluminum alloy 7XXX but provide good overall fabricability. The auto sheet alloys are relatively new. More traditional among the dozen or so alloys of this kind are aluminum alloy 6061, which is used for truck, marine, and railroad-car structures, pipelines, and furniture, and aluminum alloy 6063 for furniture, railings, and architectural applications. Other applications include complex forgings, high-strength conductors, and screw-machine products.

TABLE A.10

Aluminum Wrought Alloys

Designation	1100		2011		2014		2024		2036	2219
Temper Yield strength (103 psi)	0 5	H18 22	T3 43	T8 45	0 14	T6 60	0 11	T4 47	T4 28	0 11	T62 42
Tensile strength (103 psi)	13	24	55	59	27	70	27	68	49	25	60
Shear strength (103 psi)	9	13	32	35	18	42	18	41	30	—	—
Fatigue limit (103 psi)	5	9	18	18	13	18	13	20	18	—	15
Elongation in 2 in. (%)	45	15	15	12	18	13	22	19	24	-	-
Modulus of elasticity (10s psi)	10.0	10.0	10.2	10.2	10.6	10.6	10.6	10.3	10.6	10.6	—
Melting temperature (°F)	1,190-1,215	1,190-1,215	1,005-1,190	1,005-1,190	945-1,080	945-1,080	935-1,180	935-1,180	1,030-1,200	1,010-1,190	1,010-1,190
Coefficient of thermal expansion (10^in./in.- °F)	13.1	13.1	12.7	12.7	12.8	12.8	12.9	12.9	13.0	12.4	12.4
Thermal conductivity (Btu-in./h-ft3- °F)	1,540	1,510	1,050	1,190	1,340	1,070	1,340	840	1,100	1,190	840
Electrical resistivity (Ohm-cir mil/ft)	18	18	27	23	21	26	21	35	25	24	35

3003		3004		5052		5083		6061		6066		6262	7050	7075
0	HI 8	0	H38	0	H38	0	H321	0	T6	0	T6	T9	T74	T6
6	27	10	36	13	37	21	33	8	40	12	52	55	70	73
16	29	26	41	28	42	42	46	18	45	22	57	58	81	83
11	16	16	21	18	24	25	-	12	30	14	34	35	45	48
7	10	14	16	16	20	-	23	9	14	-	16	13	21	23
40	10	25	6	30	8	22	16	30	17	18	12	10	10	11
10.0	10.0	10.0	10.0	10.2	10.2	10.3	10.3	10.0	10.0	10.0	10.0	10.0	10.3	10.4
,190-1,210	1,190-1,210	1,165-1,210	1,165-1,210	1,125-1,200	1,125-1,200	1,095-1,180	1,095-1,180	1,080-1,205	1,080-1,205	1,045-1,195	1,045-1,195	1,080-1,205	910-1,165	890-1,175
12.9	12.9	13.3	13.3	13.2	13.2	13.2	13.2	13.1	13.1	12.9	12.9	13.0	12.8	13.1
1,340	1,070	1,130	1,130	960	960	810	-	1,250	1,160	1,070	1,020	1,190	1,092	900
21	26	25	25	30	30	36	-	22	24	26	28	24	35	31

Zinc is the major alloying element in the aluminum alloy 7XXX series, and it is usually combined with magnesium for strengthening by heat treatment. An exception is aluminum alloy 7072, which is alloyed only with 1% zinc, is hardenable by strain hardening, and is used for fin stock or as a clad for other aluminum alloys. The other alloys, such as aluminum alloys 7005, 7049, 7050, 7072, 7075, 7175, 7178, and 7475, contain 4.5 to 7.6% Zn, 1.4 to 2.7 Mg, and, in some cases, may also include copper, magnesium, silicon, titanium, or zirconium. These heat-treatable alloys are the strongest of aluminum alloys, with tensile yield strengths of some exceeding 483 MPa in the T6 temper. They are widely used for high-strength structures, primarily in aircraft.

Cast Aluminum Alloys

Of some 40 or so standard casting alloys, 10 are aluminum die-casting alloys. The others are aluminum sand-casting alloys and/or aluminum permanent-mold-casting alloys. Some of the latter also are used for plaster-mold casting, investment casting, and centrifugal casting. Although the die-casting alloys are not normally heat-treated, those for sand and/or permanent-mold casting often are, usually by solution heat treatment and artificial aging. Both heat-treatable and non-heat-treatable alloys are available. The heat-treatable alloys, which can be solution heat-treated and aged similar to wrought heat-treatable grades, carry the temper designations T2, T4, T5, T6, or T7. Die castings are seldom solution heat-treated because of the danger of blistering.

Table A. 11 shows the nominal compositions of several important casting alloys. The major alloying addition is silicon, which improves the castability of aluminum and provides moderate strength. Other elements are added primarily to increase the tensile strength. Most die castings are made of alloy 413.0 or 380.0. Alloy 443.0 has been very popular in architectural work, and 355.0 and 356.0 are the principal alloys for sand casting. Number 390.0 is employed for die-cast automotive engine cylinder blocks, and alloy F332.0 is used for pistons for internal combustion engines.

Casting alloys are significant users of secondary metal (recovered from scrap for reuse). Thus, casting alloys usually contain minor amounts of a variety of elements; these do no harm as long as they are kept within certain limits. The use of secondary metal is also of increasing importance in wrought alloy manufacturing as producers take steps to reduce the energy that is required in producing fabricated aluminum products.

TABLE A.11

Nominal Composition and Casting Procedure for Common Aluminum Casting Alloys

Alloy	Forma	Si	Cu	Mg
413.0	D	12	—	—
B 443.0b	B, C	5.3	0.15 max	—
F 332.0b	C	9.5	3.0	1.0
355.0	B, C	5.0	1.3	0.5
356.0	B, C	7.0	—	0.3
380.0	D	8.5	3.5	—
390.0	C, D	17	4.5	0.55

^a B = sand casting; C = permanent mold casting; and D = die casting. ^b The letter indicates modifications of alloys of the same general composition or differences in impurity limits, from alloys having the same four-digit numerical designations.

Aluminum alloy 380.0 and its modifications constitute the bulk of die-casting applications. Containing 8.5% Si, 3.5% Cu, and as much as 2% Fe, it provides a tensile yield strength of about 165 MPa, good corrosion resistance, and is quite fluid and free from hot shortness, and thus is readily castable. Engine cylinder heads, typewriter frames, and various housings are among its applications. Aluminum alloy 390.0, a high-silicon (17%) Cu-Mg-Zn alloy, is the strongest of the die-casting alloys, providing a tensile yield strength of about 240 MPa as cast and 260 MPa in the T5 temper. Like high-silicon wrought alloys, it also features low thermal expansivity and excellent wear resistance. Typical applications include auto engine cylinder blocks, brake shoes, compressors, and pumps requiring abrasion resistance. Aluminum alloys 383.0, 384.0, 413.0, and A413.0 provide the best die-filling capacity, and have excellent resistance to hot cracking and die sticking. Aluminum alloy 518.0 provides the best corrosion resistance, machinability, and polishability, but is more susceptible to hot cracking and die sticking than the other alloys.

Sand and/or permanent-mold casting alloys are encompassed in each of the aluminum alloy 2XX.X to 8XX.X series. As-cast, tensile yield strengths range from about 97 MPa for 208.0 to 200 MPa for aluminum alloy A390.0. In various solution-treated and aged conditions, several alloys can provide tensile yield strengths exceeding 280 MPa. Strongest of the alloys — 415 MPa in the T7 temper and 435 MPa in the T6 — is aluminum alloy 201.0, which contains 0.7% Ag and 0.25% Ti in addition to 4.6% Cu and small amounts of magnesium and manganese. The T7 temper is suggested for applications requiring stress-corrosion resistance. Applications for 201.0 include aircraft and ordnance fittings and housings; engine cylinder heads; and pistons, pumps, and impellers. Other more commonly used high-strength alloys are aluminum alloys 354.0, 355.0, and 356.0, which are alloyed primarily with silicon, copper, magnesium, manganese, iron, and zinc, and which are used for auto and aircraft components. Premium-quality aluminum-alloy castings are those guaranteed to meet minimum tensile properties throughout the casting or in specifically designated areas.

Aluminum-Lithium Alloys

Aluminum-lithium (Al-Li) alloys are a significant recent development in high-strength wrought aluminum alloys. Lithium is the lightest metal in existence. For each weight percent of lithium added to aluminum, there is a corresponding decrease of 3% in the weight of the alloy. Therefore, Al-Li alloys offer the attractive property of low density. Because of the very low density of lithium, every 1% of this alkali metal can provide a 3% reduction in density and a 10% increase in stiffness-to-density ratio relative to conventional 2XXX and 7XXX alloys. Thus, these alloys, such as aluminum alloys 2090 and 2091 (there are several others, including proprietary ones), are extremely promising for future aircraft applications, see Table A. 12. These alloys are comparable in strength with some of the strongest traditional alloys. Some are ingot-cast products, others PM-wrought alloys.

A second beneficial effect of lithium additions is the increase in elastic modulus (stiffness). Also, as the amount of lithium is increased, there is a corresponding increase in strength due to the presence of very small precipitates that act as strengthening agents to the aluminum. As the precipitates grow during heat treatment, the strength increases to a limit, then begins to decrease. Al-Li alloys therefore come under the classification of precipitation-strengthening alloys.

With the exception of the PM alloy AA 5091 in Table A. 12, all the current commercially available Al-Li alloys are produced by direct-chill casting, and require a precipitation-aging heat treatment to achieve the required properties. In Al-Li alloys containing greater than 1.3% (by weight) of lithium, the intermetallic phase 5-Al3Li precipitates upon natural or artificial aging, but the associated strengthening effect is insufficient to meet the medium or high strength levels usually required (the damage tolerant temper in AA 8090 is an exception).

Cold-working operation is a prime requirement for IM alloys containing relatively high levels of lithium, such as AA 8090 or AA 2090, if optimum medium-strength or high-strength properties are to be achieved. Cold working is readily applied to sheet, plate, and extrusions by stretching, although it must be recognized that in most cases, this can only be carried out by the metal manufacturer.

Alloy AA 5091 is made by the PM process of mechanical alloying (MA). This technology avoids the limitations arising from the need to cold-compress alloys AA 8090 and AA 2090 as forgings, because it is dispersion-strengthened during manufacture. Through careful selection of the Al-Li-Mg base composition, a single-phase solid solution is achieved, and the need for heat treatment is eliminated.

The MA process is inherently more expensive than direct-chill-casting and is not a practical route for producing large rectangular-section ingots for sheet or plate rolling. However, it is useful for certain parts in which the configuration of the forged components would not allow adequate cold compression to be achieved in alloys AA 8090 or AA 2090.

TABLE A.12

Aluminum-Lithium Alloys

Powder Metallurgy (pm) Alloys

Elements such as iron, cobalt, and nickel have been added to aluminum in large quantities to produce alloys that have relatively stable structures at elevated temperatures and higher elastic moduli than conventional aluminum alloys.

The PM process is a rapid solidification process that involves the transformation of finely divided liquid either in the form of droplets or thin sheets that become solid at high solidification rates (on the order of 104 K/s). Solidification occurs as heat is removed from the molten metal.

Rapid-solidification-process aluminum alloys containing iron and cerium appear to result in alloys that have refined particle-size distribution and improved high-temperature stability. These PM alloys appear competitive with titanium up to 191°C.

Aluminum PM parts are made mainly from aluminum alloys 201 AB and 601 AB. Both are copper, silicon, and magnesium compositions that can be heat-treated to the T4 or T6 tempers after sintering. 201 AB is the stronger, having 324 MPa tensile yield strength for 95% dense material in the T6 temper as compared with about 241 MPa for 601 AB for these conditions.

Mechanical Alloying (ma)

This process circumvents the limitations of conventional ingot casting. Blends of powders are mixed in a ball mill. A drum is mounted horizontally and half-filled with steel balls and blends of elemental metal powders. As the drum rotates, the balls drop on the metal powder. The degree of homogeneity of the powder mixture is determined by the size of the particles in the mixture.

Aluminum-Beryllium Alloys (AlBeMet)

AlBeMetAE is an alloy of aluminum and beryllium. The Al-62Be alloy has the modulus of steel at 200 GPa, and density of 2.1 g/cm3, approximately 20% that of steel, which gives a specific modulus of 950 x 106 cm. Table A.13 shows a comparison of the properties of aluminum, beryllium, Al-Li and AlBeMet.

Powder processing for AlBeMet is well established. First, inert-gas atomization produces prealloyed powder. The powder is cold isostatically pressed to a density of about 80% of theoretical, then consolidated by extrusion.

The alloy varies from 60-65% Be and 35-40% Al. The alloy is formed by melting spherical beryllium and aluminum powder and then is atomized and the metals are mixed. As the metal mixture cools, an alloy with a matrixlike structure is formed. The resulting material is extruded and can be rolled or formed with hot isostatic pressing. AlBeMet also can be dip-brazed or welded.

The extruded bar may be fabricated directly into parts, or cut for cross-rolling into sheet. Sheet and plate thicknesses of 7.6 mm to 1.6 mm are produced by rolling, followed by grinding to final gauge. This sheet has essentially isotropic properties in the sheet plane.

A range of applications is made economically possible by the extrusion process, which has produced extrusions with diameters up to 25 cm. For example, profiled AlBeMet extrusions have been fabricated into computer hard disk drive arms, with over 1 million parts machined from extruded bar stock.

AlBeMet is also produced as hot isostatically pressed block. This is quite useful for prototype parts, and for components in which strength and ductility are of secondary importance to stiffness, density, modulus, thermal conductivity, and thermal expansion. The input powder for hot isostatic pressing is the same as that for wrought product manufacture. The process is carried out after degassing to remove adsorbed species such as water vapor.

TABLE A.13

Properties of selected Beryllium and Aluminum Aerospace Alloys

The physical properties of AlBeMet 162 are largely independent of processing route. However, they are critically dependent on the beryllium content of the alloy. Although AlBeMet 162 with 62 wt% Be is the most common composition, others can be quite useful. The most striking physical properties are modulus and density. The modulus is substantially higher, and the density is significantly lower than conventional materials such as Al-Li, which is typically 2.6 g/cm3.

Unlike its physical properties, the mechanical properties of AlBeMet 162 are strongly dependent upon the processing route.

To meet many of the economic requirements imposed by modern designs, net shape or near-net shape processes are required. Investment casting has long been recognized as a process capable of producing parts to near net shape. However, only recently has the capability of investment casting been developed for Al-Be alloy parts. AlBeCast 910 is the first of a family of Al-Be investment casting alloys.

The properties of AlBeCast 910 include 0.2% yield strength of 124 MPa, ultimate tensile strength of 180 MPa, and elongation of 3%.

In addition to conventional processes such as casting and mechanical working, significant progress has been made in adapting new technologies to AlBeMet. The semisolid processing of Al-Be alloys enables the net shape and high production capability of permanent mold die casting for a material whose high melting point and reactivity preclude conventional die casting. As a prototype, a sabot from a 30-mm projectile was semisolid-formed, and the uniformity and repeatability of the process was proved out in a semi production run of approximately 2500 parts.

The high modulus-to-density ratio, 3.8 times that of aluminum or steel, minimizes flexure and reduces the chance of mechanically induced failure. Therefore, AlBeMet plays a dual role in its avionics applications.

The stiffness, low density, and thermal characteristics of Al-Be alloys combine to produce the highest performance brake components for the demanding Formula I race car circuit, specifically the brake caliper from extrusion. It is the stiffness that is particularly important in such parts, because excessive bending of the caliper leads to reduced braking efficiency. In addition, the thermal management characteristics allow higher and more consistent braking performance than possible with conventional materials.

As a structural material, AlBeMet 162 sheet is being qualified as the rudder material for advanced versions of a U.S. Air Force tactical fighter. In addition, as an essentially isotropic material, it allows simplified design, reduced weight, and increased performance.

Space structures also benefit from AlBeMet, including the OrbComm vehicle. On this satellite, the face sheets for the honeycomb bus (the circular portion of the structure), the brackets that hold the bus to the payload, and the long boom are all fabricated AlBeMet structures.

Aluminum-Scandium Alloys (AlSc)

An AlSc-base alloy, Sc7000, has been developed for use as bicycle frame tubing, shock absorbers, and handlebars. The alloy is 50% stronger than aluminum bicycle alloys and reduces the weight in the bicycle frame by 10 to 12%.

New and Special Alloys

Aluminum alloy 7033-T6 is 50% stronger than AA 6061-T6 and provides higher fatigue performance. Developed to provide improved fracture toughness for forgings, the alloy offers corrosion and stress-corrosion properties nearly equal to those of AA 6061-T6, and superior to those of AA 2014-T6.

Ultimate tensile strength of AA 7033 in the longitudinal direction is 517 MPa, vs. 338 MPa for AA 6061. Fracture toughness in the L-S orientation is 66 MPa m1/2, and in the S-L orientation is 41 MPa m1/2. This compares with 36 MPa m1/2 in the L-S direction and 36 MPa m1/2 in the S-L direction for 6061-T6. Close control of alloy chemistry and processing enables products to develop consistent microstructures with typical grain size of 100 |im or less.

Aluminum Powder

Aluminum powder is produced by drawing a stream of the molten metal through an atomizing nozzle and impinging that stream with compressed air or inert gas, solidifying and disintegrating the metal into small particles, which are then drawn into a collection system and screened, graded, and packaged. Particle sizes range from -325 mesh (fine) to +200 mesh (granules). In the process, the metal reacts with oxygen in the air and moisture, causing a thin film of Al2O3 to form on the surface of the particles. Oxide content, which increases with decreasing particle size, ranges from 0.1 to l.0% by weight.

First used to produce aluminum flake by ball milling for paint pigments, aluminum powder now finds many other uses: ferrous and nonferrous metals production, P/M parts, P/M-wrought aluminum alloy mill products, coatings for steel, asphalt-roof products, spray coatings, and vacuum metalizing. Other applications include rocket fuels and explosives, incendiary bombs, pyrotechnics, signal flares, and heat and magnetic shields. It is also used in permanent magnets, high-temperature lubricants, industrial cements, chalking compounds, printing inks, and cosmetic and medical products.