Classification of Aluminium Alloys | Metallurgy

In this article we will discuss about the classification of wrought aluminium alloys.

1. Non-Heat Treatable Aluminium Alloys:

These aluminium alloys do not respond to age-hardening heat treatment. It may be that these alloys do not show decrease of solid solubility with the decrease of temperature in their constitution diagram, or do not form coherent precipitates. All these alloys have rather low strength values in the cast, or annealed condition.

These alloys derive their increased strength in the annealed state by solid solution strengthening (Al-Mg), or dispersion hardening (Al-Mn), or both (Al-Mn-Mg), due to the alloying elements present. Further increase in strength of these alloys can be obtained only through the introduction of cold work, such as cold rolling, cold swaging, tube drawing, etc.

Thus, strength is increased from low of 89.7 MN/m² of 1100 to average high of 370 MN/m². Thus, these alloys are used in sheet, bar, plate, wire, extruded, etc. form. These are mainly used where other properties are the prime requirements. These are easily weldable and have high resistance to corrosion in most mediums.

Table 14.2 illustrates some of these alloys:

2. Heat Treatable Aluminium Alloys:

These are precipitation-hardenable type of alloys. Thus, these alloys must be showing characteristics like, decrease of the solid solubility with the fall of temperature; retention of high temperature single phase solid solution by quenching to room temperature as supersaturated solid solution; ageing causes precipitation of coherent/semi-coherent phase/phases.

However, some such alloys (Al-Si, Al-Mn) do not really show precipitation hardening characteristics. The heat treatable aluminium alloys are covered by the following three series. These alloys can be grouped into two classes- Alloys having medium strength but readily weldable, for example, Al-Mg-Si and Al-Zn-Mg alloys; alloys having high strengths but limited weldability. These alloys are primarily for aircraft parts, for example, Al-Cu, Al-Cu-Mg and Al-Zn-Mg-Cu alloys.

(a) Al-Cu and Al-Cu-Mg Alloys (2XXX Series):

For example, alloys like 2011, 2014, 2024, 2025, 2020, etc. There are only few commercial alloys based on binary system, and thus, in actual alloys complex changes occur. Elements like insoluble lead and bismuth in alloy 2011 assist in chip formation to improve the machinability of the alloy. Alloy 2219, after precipitation hardening, has high tensile strength at room temperature, good creep resistance at high temperatures, and good toughness at cryogenic temperatures.

This alloy has been used for fuel tanks for storing liquified gases, which serve as propellants for missiles and space vehicles. Addition of 0.15% Cd and 0.05% Sn in alloy 2021 refines the size of θ’, transition precipitate, thereby increases the strength.

Creep resistance is improved of alloy, Al- 6 Cu-0.3 Mg – 0.6 Mn – 0.2 Gc – 0.1 Si and low iron content, as silicon segregates to θ’ [(MnFe)Al₆]/matrix interface to reduce its coarsening at elevated temperatures and also yields at room temperature a proof stress (0.2%) of 425 MPa UTS of 500 MPa with 12% elongation.

Duralumin (A-3.5 Cu-0.5 Mg 0.5 Mn) is the original A Wilm’s Al-Cu-Mg alloy. Now Duralumin’s have composition, Al, + 2.5-6.0 Cu. 0.4- 2.8 Mg, 0.4-1.0 Mn. Similar alloy 2017 (Al. 3.8-4.8 Cu; 0.4-0.8 Mg; 0.4-0.8 Mn) is used for rivets. As silicon increases response to hardening on artificial ageing, alloy 2014 (AI-4.4 Cu-0.5 Mg- 0.9 Si-0.8 Mn) develops 0.2% proof stress of 320 MPa, and UTS of 485 MPa. Alloy 2024 (Al, 3.8-4.9% Cu, 1.2-1.8 Mg, .3-.9 Mn) has a higher magnesium content (≈ 1.5%) to yield significant hardening in T 3 or T 4 temper.

Such an alloy shows good artificial age hardening, if is cold worked prior to ageing to attain proof (0.2%) stress of 490 MPa and UTS of 520 MPa for T 86 temper. Alloy 2024 is used as sheet, plate and forgings in modern aircrafts. Alloys in 2XXX series have lower fracture toughness, because of large size precipitates of intermetallic compounds formed. The size can be reduced by reducing the amounts of Fe, Si and Cu as has been done in alloy 2124 (Rolls Royce).

RR (Rolls Royce) 58 (2618) alloys, based on the casting alloy-Y- alloy (Al-4 Cu-1.5 Mg-2 Ni), has a composition, AI-2.2 Cu-1.5 Mg-1 Ni-1 Fe, has high strengthening effects due to higher copper and magnesium, and also due to dispersion hardening of intermetallic compound particles of FcNiAl₉, which are formed due to the presence of iron and nickel.

Addition of 0.2% Si causes finer sizes and dispersion of precipitates in the alloy. Thus, such an alloy can resist creep resistance even when exposed to temperature of 120-150°C for long times, and thus, finds application as sheets in Concorde.

(b) Al-Mg-Si Alloys (6XXX Series):

These are medium strength alloys, mainly used as extrusions as well as sheets, which have good weldability, corrosion resistance and freedom from stress-corrosion cracking. Mg and Si when present in the ratio 1.73:1, forms Mg₂Si, but the excess silicon, when present, may form precipitate of Si.

Thus, these alloys can be divided into three groups:

(i) Alloys having Mg + Si = 0.8 – 1.2%, hut in the balanced ratio of 1.73:1. These alloys can be readily extruded, and the extruded product, when coming out of press, can be directly quenched by water sprays, or may be led to a tank, or even air cooled (when thickness is < 3 mm), then aged at 160-190°C. Alloy 6063 yields a proof stress (0.2%) of 215 MPa and UTS of 245 MPa. These alloys are mainly used for architectural and decorative applications.

(ii) Alloys having Mg + Si > 1.4%, but still has balanced composition. Separate solution treatment and quenching is needed here. These are high strength structural alloys.

(iii) Alloys have silicon content higher than that required to form Mg₂Si. The excess Si refines the size of Mg₂Si precipitates and itself gets precipitated as Si. The fineness of precipitate increases hardness during age hardening, but the presence of Si, as it is reduces the ductility and causes intergranular corrosion. These alloys are used in naturally aged condition, alter the solution treatment at 516° to 545°C and quenched in cold water. Alloy 6061 is aged at room temperature for 48 hours to obtain a T 4 temper. Artificial ageing can be done at 170°C-180°C for 6-10 hours for T 6 temper. Alloy 6061 has composition of Al-1 Mg-0.6 Si-0.25 Cu-0.25 Cr. Here age-hardening is due to CuAl₂ and CuAl₂Mg particles of fine dispersion. Mg₂Si causes dispersion hardening.

The Al-Mg-Si alloys, having more than 1% Mg₂Si, if there is a delay in between quenching and ageing, there is reduction in properties developed (as compared to if had been aged immediately) due to coarser particles. An addition of 0.25% Cu reduces the rate of natural ageing, and thus, reduces the above defects. Reverse happens when Mg₂Si is less than 0.9%.

(c) Al-Zn-Mg Alloys and Al-Zn-Mg-Cu Alloys (7XXX Series):

Al-Zn-Mg alloys are weldable alloys having strength from high to medium levels, depending on the composition. Zn+Mg content should be less than 6% for the weldable alloys but have to compromise with strength and resistance to cracking. The maximum resistance to stress corrosion cracking occurs if Zn/Mg ratio is between 2.7 to 2.9. Weldable alloys are commonly given slower quench, i.e., normally air cooling from solution temperature, and are artificially aged, or even use duplex treatment.

Al-Zn-Mg-Cu alloys show greatest response to age hardening, but normally suffer from stress corrosion cracking. Their high strength to weight ratio led to use of alloys 7075, DTD 683 (British), 7178-T6 (UTS 600 MPa) and 7079-T 6 for the manufacture of civil aircrafts.

The residual stresses developed during faster cold water quenching were seen to be responsible for initiating crack. A milder coolant could be used but required change in composition of the alloy. These alloys are given solution treatment at 480-470°C and artificially aged at 120°-135°C which results in a fine dispersion of MgZn₂, which causes age hardening.

The practice of single ageing treatment (120°- 135°C) of these alloys has been replaced with a duplex ageing treatment (called T 73 temper), in which fine dispersion of η’ (or η) formed from pre-existing GP zones. This treatment, for example in alloy 7075 resulted in about 15% less tensile as well yield strength as compared to in T 6 temper, but showed remarkably improved resistive to stress corrosion cracking.

This alloy is used for large die-forged critical air craft components. Some changes have been made in alloy composition. Silver (0.25-0.4%) was added to modify the precipitation process in some alloys. Alloy 7475 has less impurities like of Fe and Si, also in alloy 7075. The chromium or manganese has been replaced by 0.25% Zr, to make slower quench effective. The use of thermo-mechanical treatment to 7XXX alloys can achieve 20% increase in strength with no loss of toughness.

The Al-Cu-Mg-Fe-Ni alloys have copper and magnesium as the major alloying elements with small additions of iron and nickel. These alloys of complex nature are also called R.R. (Rolls Royce) alloys. One composition 2016 has Al, 3.5-4.5Cu, 0.4-0.8Mg, 1.8% Ni, 0.5-1.0Fe, 0.5-1.0 Si. Here, precipitation hardening is due to Mg₂Si, Al₂CuMg and Al₆Cu₃Ni. This alloy is solutionised at 530°C, water quenched and then aged at 170°C for 10-16 hours. Sometimes, ageing instead may be done at 190°C for 10 hours.

The two common alloys of Al-Cu-Mn series are (i) Al, 6-7 Cu, 0.4-0.8 Mn, 0.1-0.2 Ti, 0.3 Fe, 0.3 Si, 0.1 Zn, 0.05 Mg, (ii) Al, 6-7% Cu, 0.25-0.45 Mg, 0.4-.8 Mn, 0.1-0.2 Ti, 0.3 Fe, 0.1 Zn. Both the alloys respond well to age hardening treatment. The solution treatment is done at 525°C-535°C. A step heating al 500°C for half the time, particularly for the second alloy is commonly used. The first alloy in the form of sheets is aged at 160-170°C for 10-12 hours, and then air cooled. The pressed products are aged at. 220°C for 6-12 hours, and then air cooled. The common age hardening phases are CuAl₂, Al₁₂Mn₂Cu, Al₃Ti and AlSiMnFe. The second alloy is aged at 180-190°C for 12-16 hours, and then air cooled. In this alloy, Al₂CuMg may form particularly in thick sections.

3. Cast Aluminium Alloys:

Aluminium is one of the most versatile metals used for producing castings by processes such as sand casting, gravity die-casting and pressure die-casting. Shrinkage allowances between 3.5 and 8.5% have to be given in the mould design. Some of the cast alloys also respond to precipitation heat treatment.

However, pressure die-cast parts (LM2 and LM24) in which gases/air is entrapped in castings in dissolved state as casting has been done under pressure, and thus, are normally not given solution heat treatment as gases come out to cause blistering of the surface. Castings, in general, except for creep resistance, have inferior mechanical properties as compared to wrought products.

There is no internationally accepted system of nomenclature for aluminum alloys used for making castings. American Aluminium Association used four digit numerical system, in which the first digit indicates the alloy group, just similar to for wrought products with 0 and I denoting castings and ingot respectively.

The temper designations for castings only too are same as for wrought products. British standard uses prefix LM. Most cast aluminium alloy components are made from only four alloys called LM2, LM 4, LM 6 and LM 21 (See table 14.4). The instability of aluminium alloys decreases in order as 3 XX, 4 XX, 5 XX, 2 XX, and 7 XX. Corrosion resistance depends on the composition, but normally the copper-free alloys have greater resistance than those containing copper.

4. Alloys Containing Higher Silicon Content:

Aluminium-silicon alloys are important aluminium casting alloys, because of high fluidity (as large volume of Al-Si eutectic is present in alloys), high resistance to corrosion, good void ability and low coeffi­cient of thermal expansion due to silicon.

The properties of Al-Si alloys depend on the amount of silicon present. The eutectic composition is best for castability. However, still higher the silicon content, better are the mechanical properties. The eutectic formed has almost pure aluminium (≈ 1% Si) and virtually pure silicon as second phase. Slow cooling of solidifying pure Al-Si alloy produces a very coarse microstructure, in which the eutectic consists of large plates, or needles of silicon in a continuous aluminium matrix.

The silicon appears to be interconnected. Alloys having coarse eutectic exhibit low mechanical properties, particularly low ductility due to brittle nature of large silicon plates. If the alloy is hypereutectic in nature, the proeutectic silicon is coarse too, and is present as cuboids, plates, needles, which reduce the properties further. Rapid cooling, such as in permanent mould casting, greatly refines the microstructure, when silicon assumes fibrous form, and both tensile strength and ductility are improved.

Modification:

The refinement, or modification of the microstructure can also be done by adding ‘modifying agent’ to the melt before casting as a mixture of sodium fluoride and sodium chloride (2/3:1/3), so that it has 0.005 to 0.015% residual metallic sodium (0.001% Na is enough). The mechanical properties are improved due to the refinement of microstructure, and even an original hypereutectic alloy solidifies as hypoeutectic alloy.

The remelting of the alloy destroys the modification effects and the modification treatment has to be done again. If instead of sodium salts, strontium in amounts 0.03 to 0.05% as Al-Sr, or Al-Si-Sr alloy when added retains the modified effect even on remelting.

Sodium depresses the eutectic temperature by as much as 12°C as illustrated in Fig. 14.1. The finer microstructure is produced because the rate of nucleation is higher in the super-cooled condition. Probably by forming a film of sodium silicide (Na₂Si) on silicon, sodium restricts the growth of silicon particles.

Thus, a hyper-eutectic alloy solidifies as hypoeutectic alloy and the eutectic-mixture is fine to result in good mechanical properties. Al-Si alloys like LM 4, LM 8 and LM 25 have excellent fluidity, low tendency to crack after solidification, little chance of hot-tearing, etc., and thus, these alloys find applications in all types of castings. These alloys have good corrosion resistance. However, Al-12% Si alloys is not heat treatable by age-hardening.

Al-Si-Cu Alloys:

Addition of copper increases the response of Al-Si alloys to precipitation-hardening which results in higher strength as well as machinability, but reduces the corrosion resistance, ductility and castability. LM 4 (Al, 5% Si, 3% Cu) is given solution heat treatment at 505-520°C for 6-16 hours, then hot water-quenched (70-80°C).

It is artificially-aged at 150°C-170°C for 6-18 hours. The castings can withstand high static load, and is a general engineering alloy. Alloy (Al, 3% Si, 4% Cu) is artificially aged to get T 5 temper to improve strength and machinability to find use in sand and permanent mould castings.

A piston alloy of low cost but high strength with excellent cast ability (Al, 11% Si, 3% Cu), can be used at high temperatures as it manages high level strength and hardness at elevated temperatures. As cast, the alloy has tensile strength of 190 MPa with 1.5% elongation, but after ageing treatment at 200-210°C for 7-9 hours, attains tensile strength of 250-320 MPa with 2-8% elongation.

Al-Si-Mg Alloys:

Addition of Mg to Al-Si alloys increases response to precipitation-hardening to yield higher strengths. LM 8 (Al, 5.5% Si, 0.6% Mg) and LM 25 (Al, 7% Si, 0.3% Mg) increase strength by precipitation of Mg₂Si in aluminium matrix. Such alloys have good corrosion resistance. Alloys find applications in carburetor parts and pump castings. LM 25 is solution heat treated at 535°C for 2-6 hours, and aged at 150-180°C for 3-5 hours.

Al-Si-Cu-Mg Alloys:

The usual composition lies as: Al, 4.5-5.5% Si, 1.0-1.5% Cu, 0.35-0.6% Mg and some Fe and Zn. These alloys have good casting properties. The alloy, LM16 is given solution treatment at 525°C for 6 hours, and then quenched in boiling water. The room temperature ageing for 5 days yields a UTS of 227 MPa with 0.8% elongation.

If the alloy is aged at 225°C for 5 hours, the ductility improves to yield a percentage elongation of 1.4% with UTS of 270 MPa. The precipitation hardening occurs due to CuAI₂ and Mg₂Si. Normally the alloys having higher copper content have CuAl₂ precipitating, but with higher magnesium (lower copper) content Mg₂Si precipitates. If higher iron content is present, then some dispersion hardening occurs due to the AlCuSiFe phase.

Alloy A 390 (Al, 17% Si, 4% Cu, 0.55 Mg) has been used for automobile cylinder blocks. Addition of Ni in A 332 (Al, 12% Si, 1% Cu, 1% Mg, 2% Ni) also results in increased strength with additional harden­ing due to dispersion hardening by stable intermetallic compounds. This alloy, LM13, is used for making pistons of I.C. engines.

Al-Cu Alloys:

Of the binary Al-Cu alloys, now only LM 11 (Al, 4.5% Cu) is used for simple shaped small machine parts of aircraft castings and for castings which have to withstand high stresses. Though this alloy shows good response to age-hardening to result in good mechanical properties and resistance to impact, but suffers from casting problems like hot tears, and less resistance to corrosion. Silicon up to 3% may be added to LM II alloy to increase fluidity and castability.

This alloy is solution treated for a long time of 15-16 hours. The alloy is quenched immediately and aged al 120°-170°C for 12-14 hours. Alloy 238 (Al, 10% Cu, 3% Si, 0.3 Mg) shows excellent high temperature properties, and thus, finds applications for making sole plates of domestic hand irons. This alloy shows precipitation and dispersion hardening. With the addition of Ni and Mg, the alloy LM 14 (Al, 4% Cu, 2% Ni and 1.5% Mg) shows good response to age hardening, and even provides stable strength and hardness at temperatures up to 250°C.

Even the corrosion resistance of the alloy is good, but suffers from casting problems like heat tears, high shrinkage, etc. The alloy shows precipitation as well as dispersion hardening effects. The phases present are NiAl₃, AlCuNi, Mg₂Si, CuAl₂. It is solution treated at 515°C for 3-5 hours. It is naturally age hardened, or given artificial ageing at 100°C, but commonly at 220°C for 10-16 hours. This alloy is used for diesel engine pistons and air cooled cylinder heads for aircraft engines.

Addition of chromium and manganese increases the heat resistance of the alloy, but with decreased plasticity. In, Al, 4.6-6% Cu, 2.6-3.6% Ni, 0.8-1.5% Mg, 0.10-0.25% Cr and 0.18-0.35% Mn, the precipitating phases are Ni (Al₆Cu₃Ni), Ni [Al₃ (CuNi)₂] and chromium and manganese containing phases.

The alloy is step solutionised, first at 500°C for 5 hours and then 525°C for 3-5 hours, and then quenched in hot water. Ageing is done at 300°C for 3-7 hours. Addition of expensive silver in alloy (Al, 4.7% Cu, 0.7% Ag, 0.3% Mg) refines the precipitation process to produce very thin plates of θ to result in 0.2% proof stress of 480 MPa and UTS of 550 MPa. This is highest strength giving cast alloy.

Al-Mg Alloys:

Normally the content of magnesium in binary alloys is 4-10%, and are used as sand-cast. The alloys have high resistance to corrosion, good machinability as well as good anodised appearance. Most of these alloys show little response to precipitation hardening, but alloy (Al, 10% Mg) responds well to develop desired combination of (450 MPa & 10-25% E) high strength, ductility and impact resistance in T 4 temper condition. Cooling after solution treatment should be done slowly to reduce stress corrosion cracking. In tropical climates, the precipitate of Mg₅AI₈ form at grain boundaries to reduce ductility and cause stress corrosion cracking.

LM 10 has precipitating phase β (Mg₂Al₃). It is homogenized at 430°C for 10-20 hours. After solution heat treatment, it is quenched in hot oil and is used in as-quenched condition. The alloy is ductile in this state. As this alloy gets natural ageing, the strength increases but ductility decreases with time and embrittlement effect occurs due to precipitation of β-phase along grain boundaries. This can be avoided by adding 1.5% Zn and by reducing Mg to 8%. Zinc causes precipitation of Mg₃Zn₃Al₂ and reduces natural ageing tendency.

Al-Zn-Mg Alloys:

The alloy Al, 10-14% Zn, 6-8% Si, 0.1-0.3% Mg is normally used in the as-cast condition as it has little response to age hardening. Alloy, to avoid the formation of porosity, is given modification treatment. Then, the alloy has good cast ability.

Alloy Al, 3.5-4.5% Zn, 1.4-1.75% Mg, 0.3-0.6% Cu is solution heat treated at 460-480°C for 3-5 hours, water quenched and then aged at 160°C for 12-16 hours to yield high mechanical properties.