Aluminum Alloys


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Aluminum Alloys

 

aluminum-based alloys. The first aluminum alloys were produced in the 1850’s. They were alloys of aluminum with silicon and were not very strong or corrosion-resistant. For a long time Si was believed to be deleterious to aluminum alloys. Around 1907 aluminum-copper alloys were developed in the USA (casting alloys with 8 percent Cu and deformation alloys with 4 percent Cu). Ternary alloys of Al-Cu-Mn in the form of castings were proposed in 1910 in England, and two years later aluminum alloys with 10–14 percent Zn and 2–3 percent Cu. The turning point in the development of aluminum alloys came with the work of A. Wilm of Germany (1903–11). He discovered the so-called aging of aluminum alloys, which markedly improves their properties, especially their strength. This improved alloy was named Duralumin. Iu. G. Muzalevskii and S. M. Voronov developed a Soviet variety of Duralumin, the so-called kol’chugaliuminii. In 1921, A. Patch (USA) published a method of “modification” of aluminum-silicon alloys by the addition of small amounts of Na. This addition significantly improved the properties of Al-Si alloys and resulted in their spread and popularity. Extensive research was later conducted to find chemical compounds that would strengthen aluminum alloys by the aging process. New systems of aluminum alloys were developed: corrosion-resistant, decorative, and electrotechnically useful Al-Mg-Si alloys; highest strength Al-Ng-Si-Cu, Al-Zn-Mg, and Al-Zn-Mg-Cu alloys; highest heat-resistant Al-Cu-Mn and Al-Cu-Li alloys; and light, high modular Al-Be-Mg and Al-Li-Mg alloys (see Table 1).

The main advantages of aluminum alloys are low density, high electrical and thermal conductivity, resistance to corrosion, and high specific strength.

Aluminum alloys can be divided into two basic groups according to how articles are produced from them: (1) deformation alloys, including sintered aluminum alloys, for the production through deformation (rolling, forging, and so on) of semifinished products such as sheets, plates, structural shapes, tubes, forgings, and wires and (2) casting alloys, for castings.

Deformation alloys represent about 80 percent (USA, 1967) of all aluminum alloys produced. Semifinished products are produced from ingots of simple form—round, flat, or hollow. Casting of these is relatively less difficult. Chemical composition of the aluminum deformation alloys is determined mainly by the necessity of achieving the optimal set of mechanical, physical, and corrosion-resistant properties. These alloys are characterized by a solid solution structure with the maximum amount of eutectic. Aluminum deformation alloys belong to different groups (see Tables 2a and 2b).

Binary alloys based on the Al-Mg systems (so-called magnalium alloys) are not strengthened by heat treatment. They have high resistance to corrosion and are easily welded. These alloys are widely used in the production of sea and river ships, rockets, hydroplanes, welded containers, pipes, tanks, railroad cars, bridges, refrigerators, and so on.

Alloys of Al-Mg-Si (so-called avial’ alloys) combine good corrosion-resistant properties with a relatively large aging effect. Anodic treatment permits giving many beautiful colors to these alloys.

Table 1. Development of aluminum alloys
SystemStrengthening phaseYear of discovery of the strengthening effectTrade name (USSR)
Al-Cu-Mg..............CuAl2, Al2CuMg1903–11D1, D16, D18, AK4–1
VD-17, D19, M40, VAD1
Al-Mg-Si ..............Mg2Si1915–21AD31, AD33, AV (without Cu)
Al-Mg-Si-Cu ...........Mg2Si, Wphase (Al2CuMgSi)1922AV (with Cu), AK6, AK8
Al-Zn-Mg..............MgZn2, Tphase (Al2Mg3Zn3)1923–24V92, V48–4, 01915, 01911
Al-Zn-Mg-Cu ...........MgZn2, Tphase. (Al2Mg3Zn3)
Sphase (Al2CuMg)
1932V95, V96, V93, V94
Al-Cu-Mn ..............CuAl2, Al12Mg2Cu1938D20,01201
Al-Be-Mg..............Mg2Al31945Alloys of the type ABM
Al-Cu-Li ..............Tphase (Al7, 5Cu4Li)
Tphase (Al2CuLi)
1956VAD23
Al-Li-Mg ..............Al2LiMg1963–6501420

Ternary Al-Zn-Mg alloys have high strength, and are easily welded, but at significant Zn and Mg concentrations they have a tendency to spontaneous corrosion cracking. Alloys of average strength and concentration are reliable.

Table 2a. Chemical composition of some aluminum deformation alloys (1 MN/m2 ≈ 0.1 kgf/mm2; 1 kgf/mm2 ≈ 10 MN/m2)
Basic elements (percent by mass)1
Trade name of alloyCuMgZnSIMnSemifinished products2
1 For all alloys Fe and Si are present as impurities; many alloys have, as small additions, Cr, Zr, Ti, Be
2 Semifinished products: L—sheet, Pf—structural shapes, Pr—rod, Pk—forging, Sh—cold worked, Pv—wire, T—tubes, PI—plates, Pn—panels, Ps—bars, F—foils
3 With addition of 1.8–1.3 percent Ni and 0.8–1.3 percent of Fe
4 With addition of 1.2–1.4 percent Li
5 With addition of 1.9–2.3 percent Li
6 With addition of 0.2–0.4 percent Fe
AMg1 ..........<0.010.5–0.8<0.05L
AMg6 ..........<0.15.8–6.8<0.2<0.40.5–0.8L, PI, Pr, Pf
AD31 ...........<0.10.4–0.9<0.20.3–0.7<0.1Pr (L, Pf)
AD33...........0.15–0.40.8–1.2<0.250.4–0.8<0.15Pf (Pr, L)
AV.............0.2–0.60.45–0.9<0.20.5–1.20.15–0.35L, Sh, T, Pr, Pf
AK6............1.8–2.60.4–0.8<0.30.7–1.20.4–0.8Sh, Pk, Pr
AK8............3.9–4.80.4–0.8<0.30.6–1.20.4–1.0Sh, Pk, Pf, L
D1 .............3.8–4.80.4–0.8<0.3<0.70.4–0.8PI (L, Pf, T), Sh, Pk
D16 ............3.8–4.91.2–1.8<0.3<0.50.3–0.9L (Pf, T, Pv)
D19 ............3.8–4.31.7–2.3<0.1<0.50.5–1.0Pf(L)
V65 ............3.9–4.50.15–0.3<0.1<0.250.3–0.5Pv
AK4–13 .........1.9–2.51.4–1.8<0.3<0.35<0.2Pn, Pf (Sh, PI, L)
D20 ............6.0–7.0<0.05<0.1<0.30.4–0.8L, Pf (Pn, Sh, Pk, Pr)
VAD234 .........4.9–5.8<0.05<0.1<0.30.4–0.8Pf (Pr, L)
014205..........<0.055.0–6.0<0.0070.2–0.4L(Pf)
V92 ............<0.053.9–4.62.9–3.6<0.20.6–1.0L (PI, Ps, Pr, Pk), Sh, Pf
0.19156 .........<0.11.3–1.83.4–4.0<0.30.2–0.6L(Pf)
V93 ............0.8–1.21.6–2.26.5–7.3<0.2<0.1Sh (Pk)
V95 ............1.4–2.01.8–2.85.0–7.0<0.50.2–0.6L, PI, Pk, Sh, Pf, Pr
V96 ............2.2–2.82.5–3.57.6–8.6<0.30.2–0.5Pf (Pn, Pk, Sh)

Quaternary Al-Mg-Si-Cu alloys are significantly strengthened by aging but have lower corrosion resistance because of Cu. They are used for structural units sustaining high stress. Quaternary Al-Zn-Mg-Cu alloys have a very

Table 2b. Mechanical properties of some aluminum deformation alloys1
Trade name of alloyTensile strength (σb, MN/m2)Yield strength (σ02, MN/m-)Relative elongation (δ, %)
1 Properties determined on semifinished products, shown without brackets
2 See notes on these alloys from Table 2
AMg1 .............1205027.0
AMg6 .............34017020.0
AD31 .............24022010.0
AD33 .............32026013.0
AV ...............34028014.0
AK6 ..............39030010.0
AK8 ...............47038010.0
D1 ................38022012.0
D16 ...............44029019.0
D19 ..............46034012.0
V65 ...............40020.0
AK4–12 ............4203508.0
D20 ..............40030010.0
VAD232 ............5505004.0
014202 ............44029010.0
V92 ...............45032013.0
0.19152 ............35030010.0
V93 ...............4804402.5
V95 ...............5605307.0
V96 ...............6706307.0

high strength (up to 750 meganewtons per square meter [Mn/m2] or 75 kilograms-force per square millimeter [kgf/mm2]) and resistance to corrosion cracking; they are significantly more sensitive to stress concentrations and repeated stresses than Duralumin (Al-Cu-Mg alloys) and lose their strength after heating above 100°C. The strongest of these alloys become brittle in cryogenic temperatures.

These alloys are used widely in the construction of airplanes and rockets. Al-Cu-Mn alloys have an average strength but resist high and low temperatures well, right down to the temperature of liquid hydrogen. Al-Cu-Li alloys are close in strength to Al-Zn-Mg-Cu but have a lower density and a higher modulus of elasticity and are temperature-resistant. Al-Li-Mg alloys have the same strength as Duralumin but 11 percent lower density and a higher modulus of elasticity. They were discovered and developed in the USSR. Al-Be-Mg alloys have a high specific strength and very high modulus of elasticity; they can be welded and are corrosion-resistant. But their application in structures is connected with a number of restraints.

Aluminum deformation alloys include so-called sintered aluminum alloys (instead of an ingot, a compacted powder briquette is used for further deformation). Their production in the USA in 1967 was 0.5 percent by volume. Two groups of sintered aluminum alloys are used in industry: SAP (sintered aluminum powder) and SAS-1 (sintered aluminum alloy).

SAP is strengthened by dispersion particles of aluminum oxide that is insoluble in aluminum. When the dispersion particles of aluminum powder are ground in ball mills in a nitrogen atmosphere containing a controlled amount of oxygen, a very thin film of aluminum oxides is formed on the surface of the dispersion particles. The grinding is done with the addition of stearic acid. As the acid evaporates during the grinding of the primary powders a fusion of these powders into bigger particles occurs. As a result, so-called heavy powder, noncombustible in the air and having a density above 1,000 kg/m3, is formed. Hot or cold powder is compacted into briquettes, sintered, and then deformed by pressing, rolling, or forging. Strength of SAP increases with increase of the content of primary aluminum oxide (formed on the primary powders) up to 20 -22 percent but decreases if it passes this limit. There are, depending on the content of Al2O3, four trade names of SAP: SAP1 (6 -9percent), SAP2 (9.1–13 percent), SAP3 (13.1–18 percent), SAP4 (18.1–20 percent). Extended heat treatment of SAP below its melting temperature does not influence its strength. For temperatures above 200–250°C especially with extended heat treatments SAP has the greatest strength of all aluminum alloys. For example, at 500°C its tensile strength σb= 50 - 80 Mn/m2 (5–8 kgf/mm2). In the form of sheets, structural shapes, forgings, and extrusion products SAP is applied in parts which require resistance to temperature and corrosion. SAP contains large amounts of moisture, adsorbed and bound strongly to the oxidized surface of the powder particles and compacted briquettes. To dry this alloy, it is heated in a vacuum or neutral atmosphere, a little below the melting temperature of aluminum powders or cold compacted briquettes. Degassing of SAP increases its plasticity and permits argon arc welding.

SAS-1, containing 25 percent Si and 5 percent Ni (or Fe), is produced by pulverization of the liquid alloy, which is then compacted, extruded as rods, and forged. Tiny crystalline particles of Si and FeAl3 (NiAl3) influence the matrix in such a way that the modules of elasticity and plasticity increase while the linear expansion coefficient decreases. The smaller the hard particles and the less the distance between them, the more this effect is pronounced. SAS-1 has a low linear expansion coefficient and quite high modulus of elasticity. Powder alloys in this regard are much superior to corresponding casting aluminum alloys.

Casting aluminum alloys account for 20 percent by volume of all aluminum alloys (USA, 1967). They are characterized by the following important casting properties: low viscosity and only a slight tendency to form shrinkage and gas cavities, fractures, and voids. A. A. Bochvar established that these properties are improved if there is a relatively high content of alloying ingredients, forming a eutec-tic. But this increases the brittleness of the alloys. The most

Table 3. Chemical composition and mechanical properties of some aluminum casting alloys (1 Mn/m2 ≈ 0.1 kgf/mm2; 1 kgf ≈ 10 Mn/m2)
 Elements (percent by mass)Type of casting1Typical mechanical properties
Trade name of alloyCuMgMnSI Tensile strength (σb, Mn/m2Yield strength (σ22,MN/m2)Relative elongation (δ, %)
1 Types of castings: Z, sand casting; V, casting with subsequently melting models; O, case models casting; K, permanent mold casting; D, casting under pressure
2 Zn 3.5–4.5 percent
AL8 ...............9.5–11.50.10.3Z, V, O32017011.0
AL2 ...............0.80.510–13All types of casting2001103.0
AL9 ...............0.20.2–0.40.56–8” “ “2301307.0
AL4 ...............0.30.17–0.30.25–0.58–10.5 “ “2602004.0
AL5 ...............1.0–1.50.35–0.60.54.5–5.5” “ “2401801.0
AL3 ...............1.5–3.50.2–0.80.2–0.84.0–6.0All types of casting except D2301701.0
AL25 ..............1.5–3.00.8–1.20.3–0.611–13K2001800.5
AL30 ..............0.8–1.50.8–1.30.211–13K2001800.7
AL7 ...............4–50.031.22301505.0
AL1 ...............3.75–4.51.25–1.750.7All types of casting exceptD2602200.5
AL19 ..............4.5–5.320.050.6–1.00.3Z, 0,V3702605.0
AL242 ..............0.21.5–2.00.2–0.50.3Z, O, V2903.0

important casting aluminum alloys contain more than 4.5 percent Si (so-called silumin alloys). Addition of a minute amount of Na (a few hundredths of a percent) permits “modification” of the structure of eutectic and solid solution of silumin alloys. Instead of thick and brittle Si crystals, spheroidal crystals form and plasticity of the alloy is much improved. Silumin alloys (see Table 3) include binary alloys of the system Al-Si (AL2) and alloys based on more complicated systems: Al-Si-Mg (AL9), Al-Si-Cu (AL3, AL6), Al-Si-Mg-Cu (AL5, AL10). Alloys of this group are characterized by good casting properties, relatively high corrosion resistance, high density (tightness), and average strength; they are used for complicated castings. In the struggle with gas cavities in silumin alloys Bochvar and A. G. Spasskii applied an original and effective method of casting crystallization under pressure.

The binary alloy Al-Mg (AL8) and alloys of the Al-Mg-Si system with the addition of Mn (AL13 and AL28) and Be and Ti (AL22) belong to the group of alloys with high contents of Mg (above 5 percent). Alloys of this group are corrosion-resistant and have a high strength and a lowered density. The strongest is the AL8 alloy, but the technology of its preparation is complicated. To decrease the oxidation of the alloy in the liquid state, 0.05–0.07 percent of Be is added; to obtain small grains, Ti is added in the same amount. To suppress a reaction of the metal with moisture, boric acid is added. AL8 alloy is cast mainly in sand molds. AL13 and AL28 alloys have better casting properties but lower strength and cannot be strengthened by heat treatment. They are cast in permanent and sand molds. Long low-temperature heating may decrease the corrosion resistance of aluminum alloys with high Mg content.

Alloys with a high content of Zn (above 3 percent) of the Al-Si-Zn and Al-Zn-Mg-Cu systems, AL11 and AL24 respectively, have an increased density and lowered resistance to corrosion, but they have good casting properties and can be used without heat treatment. They are not widely used.

Alloys with a high content of Cu (over 4 percent)—binary alloys of Al-Cu and ternary alloys Al-Cu-Mn with addition of Ti, AL7 and AL19, respectively—have higher temperature resistance than the three previous groups but have somewhat poorer corrosion resistance, casting properties, and leak tightness.

Alloys of the systems Al-Cu-Mg-Ni and Al-Cu-Mg-Mn-Ni (AL1, AL21) are temperature-resistant but do not machine well.

Properties of casting alloys depend significantly on the casting method. The properties are better if the crystallization and feeding of the crystallizing layer proceed at a higher speed. As a rule the best results are achieved with permanent mold casting. Characteristics of separately cast samples may exceed by 25–40 percent the crystallizing properties of the more slowly or poorly fed parts of the casting. Some alloying ingredients in one type of alloy can be detrimental in another. Silicon lowers strength of Al-Mg alloys and worsens mechanical properties of Al-Si and Al-Cu alloys. Tin and lead, even in amounts of tenths of a percent, significantly lower the melting temperature. Iron has a detrimental influence on the silumin alloys because it forms a brittle eutectic of Al-Si-Fe that crystallizes in the form of platelike particles. Iron content is controlled by the method of casting; it is at maximum in casting under pressure and in permanent molds and at minimum in sand casting. Properties of aluminum alloy castings can be significantly improved by diminishing the amount of detrimental metallic and nonmetallic impurities, use of cleaner raw materials, refining, addition of small amounts of Ti, Zr, and Be, and “modification” of alloys and their heat treatment. Refining is done by blowing through with gas (chlorine, nitrogen, argon), using flux containing chloride and fluoride salts, holding in vacuum, or combining these methods.

Table 4. Use of aluminum alloys by different industries in the USA (in tons)
Industry196219651967
Building ..................613,000846,000862,000
Transportation .............612,000838,000862,000
Articles of extended use290,200383,000381,000
Electrotechnical industry485,000490,000576,000
Machine building and instrumentation .........190,500258,500279,000
Containers and packaging ...175,000298,000397,000
Export....................188,000260,200415,000
Total .................2,554,7003,373,7003,772,000

Demand in different industries for aluminum alloys is growing every year (see Table 4). In five years the use of aluminum alloys in the USA increased roughly 1.6 times and is 10 percent higher (1967) by volume than the use of steels. It is planned that in 1966–70 the USSR will more than double its production of aluminum alloys. Aluminum alloys are used in transportation (aviation, ships, railroad

Table 5. Volume of production of semifinished products from aluminum alloys in the USA (in tons)
Type of semifinished product195519601965
Sheets and plates.........610,000630,0001,238,000
Foils.....................89,900131,100184,100
Other rolled semifinished   
products ..............49,90042,20074,800
Wires ...................28,00025,10038,600
Cables ..................71,20083,000195,200
Wires and cables   
with insulation..........18,00027,40058,700
Compacted semifinished   
products ...............309,500386,000700,000
Extruded tubes ...........30,50027,40037,600
Welded tubes ............11,60011,70042,500
Powders .................16,20014,90027,200
Forgings, cold work .......31,90022,70043,200
Sand casting .............75,00058,900124,500
Permanent mold casting .. .135,200117,000150,000
Casting under pressure161,100175,000365,000
Total ................1,638,0001,752,4003,279,400

cars, automobiles) and in the building and construction industry—window frames, wall panels, suspended ceilings, wallpaper, and so on. Use of aluminum alloys for containers and other packaging and in electrotechnical industry (wire, cables, generator and motor windings) is rapidly increasing.

It is interesting to see how aluminum alloys are divided between different kinds of semifinished products (see Table 5).

REFERENCES

Svarivaiushchiesia aliuminievyie splavy. (Svoistva i primenenie). Leningrad, 1959.
Dobatkin, V. I. Slitki aliuminievykh splavov. Sverdlovsk, 1960.
Fridliander, I. N. Vysokoprochnye deformiruemye aliuminievye splavy. Moscow, 1960.
Kolobnev, I. F. Termicheskaia obrabotka aliuminievykh splavov. Moscow, 1961.
Stroitel’nye konstruktsii iz aliuminievykh splavov. Moscow, 1962. (Collection of articles.)
Aliuminievye splavy, vols. 1–6. Moscow, 1963–69.
Al’tman, M. B., A. A. Lebedev, and M. V. Chukhrov. Plavka i lit’e splavov tsvetnykh metallov. Moscow, 1963.
Voronov, S. M. Metallovedenie legkikh splavov. Moscow, 1965.
Altenpohl, D. Aluminium und Aluminiumlegierungen. Berlin, 1965.
L’aluminium, vols. 1–2. Edited by P. Barrand and R. Gadeau. Paris, 1964.
Aluminium, vols. 1–3. Edited by R. Kent Van Horn. New York, 1967.

I. N. FRIDLIANDER

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