Technology Dictionary - ALUMINIUM – GBW

Aluminium and its alloys

(Source: Northeim IfP Westsächsische Hochschule-Zwickau Wollmann, M.)

General:

Aluminium derives its name from the Latin name for alum (double sulphate). Aluminium is the third most abundant element in the earth's crust after oxygen and silicon (7.5%). It was first synthesized in the laboratory by the Danish chemist Oersted in 1825. Shortly afterwards, the renowned German chemist Wöhler repeated this achievement. The first electrolytic deposition of aluminium was demonstrated by Bunsen in 1852. However, it was not until the advent of melt-flow electrolysis that cost-efficiently produced aluminium became a marketable commodity. One basic precondition for this technique was affordable electric power, which became available only after 1866 with the invention of the dynamo (electrical generator). The first patent on melt-flow electrolysis was awarded in 1886. Aluminium could now be produced on an industrial scale. Metallic aluminium was deemed difficult to produce at the time due to the chemical stability of the natural aluminium compound, Al2O31. In refining an aluminium ore, other ore constituents would be much more easily reduced. Today's aluminium production method is therefore based on a chemical refining step to separate off the accompanying elements, followed by melt-flow electrolysis aimed at depositing metallic aluminium of maximum purity. This process yields approx. 99.5 – 99.9% pure aluminium. Super-purity grade aluminium (99.999%) can be obtained by three-layer electrolysis.

Despite the marked increase in aluminium prices (approx. US$ 1800 per tonne  in 2005), the material can compete with steel, even in terms of economic efficiency. Its performance profile is by now comparable with that of a number of steel grades. The best mechanical properties are achieved by age-hardening Cu- and Zn-containing alloys.

Overview of key characteristics:

PARAMETER

VALUE

UNIT

Atomic number

13

 

Relative atomic mass       

26.98154

 

Atomic volume

9.996 x 10-6

m³⁄mol

Density

2.6989 x 10³

kg⁄dm³

Lattice

face-centered cubic

 

Lattice constant

0.40496

Nm

Melting point       

660.3

ºC

Boiling point        

2520

ºC

Thermal conductivity

236        

W⁄m.K

Modulus of elasticity (super-purity grade aluminium)               

67

kN⁄mm²

Electrical resistivity            

2.66 x 10−8

G.m

Specific heat at constant pressure

0.9

kJ⁄kg.K

Coefficient of thermal expansion (0−100ºC)

23.5 x 10−6

1⁄K

Electrical conductivity (pure aluminium)

37

m⁄G.mm²

1 The raw material is bauxite: 55-65 % Al2O3, 28 % Fe2O3, SiO2<28 %, TiO2 and 12-30 % H2O.

From a technological viewpoint, aluminium has three main properties favouring its use:

 

1. Favourable strength-to-density ratio

2. High thermal and electrical conductivity - exceeded only by Ag, Cu and Au

3. Good weather resistance and very good corrosion resistance - stable oxide formation

4. Good technological properties

- Deformability

- Weldability

- Alloyability

Super-purity grade aluminium (99.999%) has a tensile strength of 20 N/mm2. Given its low recrystallization temperature, which lies in the -40 to -60 °C range, it cannot be work-hardened. However, by raising its content of Fe, Si and Ti on the scale of a few hundredths of a percent, the recrystallization temperature can be increased to over 200°C. The material thus becomes work-hardenable. A reduction in purity to 99.5% results in increased yield strength, tensile strength and hardness but is also associated with diminishing ductility:

RP0,2 = 25 N/mm2

Rm = 70 N/mm2

A10 = 50 % (following decrease in purity by alloying)

The accompanying elements provide a fine-grained microstructure. Another reason for the strength increase lies in the presence of solutionized elements in the solid solution. Copper, iron and silicon are most notably associated with this strength gain. Titanium and boron, on the other hand, produce a special grain refinement. Further strength improvements can be achieved by cold-working, although this is again combined with a marked ductility loss:

RP0,2 = 205 N/mm2

Rm = 220 N/mm2

A10 = 7 % (after cold-working)

It follows from the above that a change in mechanical properties is brought about by grain refinement, solid solution strengthening and cold-working. The possibility to attain a further strength increase via cold-forming can be realized only after addition of Fe, Si and Ti. Since the mechanical characteristics are so markedly dependent on the alloying elements, it is clear that the values given in other literature sources may vary. The figures stated here are, above all, intended merely to document how closely the mechanical properties of aluminium depend on the degree of purity.

Pure aluminium (99 % Al)

Pure aluminium is used as an engineering material. The cold-workability of aluminium is greatly superior to that of steel. Its modulus of elasticity is about 1/3 of steel, while its  thermal expansion is around twice as high. The recrystallization temperature of aluminium is around 300-450°C, which accounts for its low high-temperature strength. Its good deformability is a result of the face-centered cubic lattice. Key applications for pure aluminium lie in the fields of electrical engineering, roofing, product packaging, and tank and process equipment construction.

The material is covered by a firmly adhering and, above all, virtually water-insoluble oxide layer measuring around 0.7 µm in thickness which protects the underlying base metal  from further chemical reactions with its environment.

Due to this protective layer, oxidized aluminium has a much higher standard electrode potential than pure aluminium without this protective layer. The layer is invisible. Aluminium has a microscopic surface roughness. A directed reflection that would make it suitable as a reflector material can only be achieved through appropriate surface treatment. Aluminium with a purity of around 99.9% to 99.99 % can be subjected to anodic or chemical surface finishing to remove this surface roughness. Its surface thus smoothened, the material becomes suitable for use in reflectors, lighting fixtures or ornamental items in automotive engineering or in the fabrication of kitchen equipment. The addition of strength-enhancing alloys, specifically of copper, may reduce the material's corrosion resistance.  

Thanks to its good deformability, pure aluminium is excellently suited for chipless metal forming operations such as deep-drawing or cold extrusion. Its good corrosion resistance makes it the material of choice for various packaging industry applications. It should be noted that the oxidic protection layer is not stable in all media. In particular, aluminium has a low stability in acidic or strongly alkaline solutions. In a corrosive environment, the use of aluminium must be restricted to a 5 to 8 pH range. Where this condition is not met, other anti-corrosion measures have to be implemented. 

Age hardening – age-hardenable aluminium alloys

The strength of some specific materials can be increased substantially by the formation of brittle precipitates. This will usually occur when precipitate particles penetrate the matrix. The distribution pattern of these precipitations depends on the nucleation conditions. If a coarse distribution of precipitates is to be replaced with a fine dispersion pattern, the precipitate particles need to re-form entirely under different nucleation conditions. This mechanism can also be put to work in the case of aluminium.

The hardening process of an alloy thus consists of three phase transformation steps. For reasons of clarity, these shall be explained for a binary system here. The model system comprises the two phases α und β. Let α be the matrix to be strengthened and β the brittle second phase.

1) The initial aim is to dissolve the β-phase. The material is solution-annealed in the monophase range of the α-phase. This involves a temperature at or near T2, as evident from the diagram. At this temperature the β-phase, which mainly consists of type B atoms, becomes fully solutionized in the α-phase.

Fig. 1: Phase diagram of an age-hardenable alloy (source: Bergmann)

2)  Next, the metal is cooled down rapidly enough to suppress diffusion. At T1, a  supersaturated α-solid solution is thus obtained. This α-solid solution contains more B atoms than would correspond to its equilibrium state at this temperature. The degree of supersaturation is expressed by a section of the concentration axis or T1 temperature isoline, respectively. Its starting point is the point of intersection between the T1 isoline and the phase boundary line α/α+β  (this isoline forms the axis on which concentration conditions are represented, i.e., the axis of concentration at temperature T1). The second intersecting point is where the perpendicular through point *) crosses the isoline. In our graph, this would be the section xü.

 

3)  The above is followed by the third step, i.e., the precipitation of particles containing B-atoms or of B-atoms alone. To this end, the diffusion suppressed earlier by rapid cooling is now stimulated in a controlled manner. This treatment is referred to as age-hardening. It results in the formation of finely dispersed precipitate particles which form so-called strain fields in the matrix lattice. Their presence affects the processes associated with dislocation movement and the formation of dislocations. Pre-existing dislocations are blocked; as a result, the material's plastic deformability is impaired. 

Contrary to what might be expected, the precipitated B- (or B-containing) particles are present not as finely dispersed β-precipitates but in the form of metastable, coherent or partially coherent transition phases. In a first state, B-type alloying atoms may merely accumulate into coherent clusters. This behaviour is attributable to the interface energy going into the phase formation process. When this interface energy is high, the work done in forming nuclei is likewise high. Under specific age-hardening conditions the nucleation work suffices to produce coherent or partially coherent precipitations. For forming an incoherent, brittle equilibrium phase  β´, however, the nucleation activity to be mustered is too high. This is precisely the reason why metastable phases with monolayer atomic strata may already form at room temperature. The strengthening mechanism caused by their presence is generally referred to as natural ageing. Only at higher temperatures will multilayer strata with a localized superlattice form due to the higher diffusion rates. Finally, if artificial ageing (aging at higher temperatures) continues for too long, the formation of independent brittle phases may set in. The hardenability described here is a key prerequisite for the use of aluminium in a broad technical application range. Aircraft engineering as we know it would probably be inconceivably in its entirety if aluminium did not have this ability. However, since aluminium forms age-hardenable alloys with numerous elements, it meets this precondition. Its strength parameters can thus be set to the desired technological requirements through a heat treatment adjusted to the relevant solubility conditions. In summary, such alloys must meet the following requirements in order to be age-hardenable:

1. Limited miscibility

2. Appreciable decline, with decreasing temperature, of the saturation limit of alloying elements which are present in notable quantities in the solid solution.

3. Possibility to "freeze" a supersaturated state through abrupt quenching.

4. Ability to precipitate strength-enhancing phases through ageing.

Definition of terms:

Heat treatment

The term "heat treatment" refers to a technological activity aimed at controlling (i.e., selectively influencing) the properties of a material by changing its microstructure. 

Age hardening (precipitation hardening) is by far the most important of these processes. However, thermal softening and recrystallization annealing (soft annealing) processes as described below also come under this heading.

High-temperature strength

This term denotes a material's strength at elevated temperatures. Materials exhibiting a special high-temperature strength are used in the ceramics, foundry, iron and steelmaking, iron and steel processing, aerospace and petrochemical industries (Source: German Wikipedia).

Homogenizing

Homogenizing generally refers to the process of creating a homogeneous micro-structure,  i.e., one of maximum uniformity and regularity. It usually refers to the homogeneous intermixing of different constituents which are insoluble in one  another.

(Source: German Wikipedia)

Solution annealing

Objective: creation of a homogeneous solid solution

Action: complete solutionizing of the alloying constituent in the aluminium matrix by heat treatment

At temperature of around 460 – 560 °C the alloying elements, which are irregularly distributed in the grain structure and often precipitated out (as the second phase), are solutionized in the solid aluminium solution (mixed crystal) and thereby homogenized.

Quenching:

Objective: creation of a supersaturated solid solution

Activity: Freezing the material state achieved through annealing. This is achieved by quenching from the annealing temperature to room temperature.

Cooling must be achieved as quickly as possible so that the homogeneity achieved by solution annealing will indeed be "frozen". At the same time, it should take place as slowly as possible to prevent distortion or warping of the product due to internal strains. In the same way, key parameters such as corrosion and toughness are influenced by quenching (quench sensitivity). The cooling process settings must therefore be accurately defined and adapted to the alloy used, as well as to specific type of semi-finished product.

Age hardening:

Equivalent terms: ageing, precipitation hardening

Objective: to precipitate out the hardening phase from the supersaturated solid solution

Action: A distinction is made between natural ageing (at room temperature) and artificial ageing (at around 120 - 200°C). Depending on the duration of the treatment, a concentration shift towards the equilibrium state will be achieved.

Age hardening is an effective strength-enhancing technique. It is based on separation (precipitation) processes that will take place in the solid aluminium solution under specific conditions.

Alloying constituents whose solubility in the solid solution declines with increasing temperature are initially solutionized homogeneously in the solid solution at high temperatures. If the temperature is reduced quickly thereafter, the alloying constituents present in the solid solution cannot be precipitated out rapidly enough; the result is a supersaturated solid solution. Extended storage at room temperature or at a moderately elevated temperature will then cause the foreign atoms to be separated out from the solid solution, which results in the formation of fine, very uniformly formed particles. These particles will now block the slip planes and obstruct dislocation movements within the microstructure – the material's strength is thus increased.

 

Thermal softening

Another method classified as a heat treatment is thermal softening. It is carried out at temperatures in the 150 – 250 °C range. The hardness introduced into the material during a cold-forming process such as rolling or cold impact extrusion has the disadvantage of making the material lose its ductility. It becomes "brittle". Thermal softening, carried out by annealing below the recrystallization temperature, reduces the material's strength down to its three-quarter hard or half-hard state while simultaneously increasing its ductility significantly.

Recrystallization annealing (soft annealing)

To restore the "softness" of cold-worked materials (see above) for further downstream  processing, the material is fully re-softened again. A recrystallization process takes place: in the microstructure deformed substantially by the cold working step, new crystal grains grow from nuclei (each of which is an ideal crystal grain of minimum size). The entire grain structure will thus be formed anew, so that favourable conditions are created for further metalforming. Recrystallization annealing is done at 350 – 450°C

Alloys

The properties of aluminium depend on a whole array of factors. Especially the accompanying elements deliberately added to, or accidentally present in, common aluminium grades play a very important role.

With the exception of super-purity aluminium (Al 99.99), the aluminium grades used in engineering all contain additional elements (and are therefore called alloys). Unalloyed or very low alloyed aluminium is used chiefly in the manufacture of metal films and strips, chemical process equipment, and products intended for the electronics and electrical engineering industries.

The purpose of alloying is to improve the properties of aluminium, specifically its strength (most pure metals are soft) and corrosion resistance.

The most common alloying elements are copper (Cu), silicon (Si), magnesium (Mg), zinc (Zn) and manganese (Mn). Other aluminium alloying metals, although less commonly used, include lead, boron, chromium, nickel, titanium, bismuth and zinc. Even a very small quantity of these elements (i.e., a few percent or even fractions thereof) suffices to improve certain properties of aluminium but may, on the other hand,  impair others, in which case an additional element is added to compensate for this deterioration. Aluminium alloys are made by melting, sintering (production of shaped components from metal powder by baking at high temperatures), or mechanical mixing.

Alloy forms

Cast alloys

Aluminium cast alloys contain up to 20% alloying elements including silicon, magnesium and copper and can exclusively be formed by casting. The base metal is typically secondary aluminium (i.e., aluminium recovered from scrap). The main requirement on these alloys is that they exhibit favourable casting properties. The alloying composition of cast alloys is further adapted to the specific casting method employed (sand casting, gravity die casting, pressure die casting). Alloys with a 5 - 20% silicon content show the best casting behaviour. 

Fig.: Main aluminium cast alloys

Applications:

Cast alloys are typically employed in the production of diverse castings having varying properties depending on their alloying constituents. Cast alloys are used, e.g., in the automotive industry – for cylinder heads, engine blocks, crankcases and brake pads. In general mechanical engineering, they are used in the manufacture of fans (impellers), pump housings, bearing  brackets, etc.

Wrought alloys:

Wrought alloys contain up to 10% alloying elements and are optimized for good metalforming behaviour.

A distinction is made been age-hardenable and naturally hard (non-age-hardenable) wrought alloys.

  • In naturally hard (non-age-hardenable) alloys, the elements are present in fully solutionized form. Such materials are easily deformable. They include pure aluminium, Al Mg, Al Mn and Al Mg Mn type alloys
  • In age-hardenable alloys the alloying elements are present in precipitate form at room temperature. Their distribution determines the strength of the alloy. Solution-annealing as part of a heat treatment process causes these elements to become fully solutionized. This state is "frozen" by quenching.

Adding magnesium results in the formation of non-age-hardenable but sea-water-resistant materials. One example of an age-hardenable grade is the classic Duraluminium developed in 1909 by Alfred Wilm. It comprises copper, manganese and magnesium.

The key criterion for wrought alloys is their plastic deformability. The most commonly used aluminium wrought alloys are those made by adding magnesium, silicon, manganese, copper, and zinc. 

Fig. The main wrought alloys