Thermal hardening aluminum alloys
hardening by aging
Not all aluminum alloys can be thermally hardened, but only the so-called “thermally hardenable aluminum alloys”. Aluminum alloys, which belong to this class, contain certain additives of some soluble elements. Such elements or combinations thereof are copper, magnesium,, silicon, manganese and zinc, which apply, for example, as alloying elements in wrought alloys of the 2xxx series, 6xxx and 7xxx. Certain other elements can also be added to special alloys to achieve special properties..
As a result of appropriate heat treatment, the atoms of these alloying elements dissolved in aluminum gradually form a kind of clusters in the form of very small particles, which stand out inside the grains of aluminum “die”. This process is called so and is called – “excretion” (precipitation). This particle release is a metallurgical process (phase change), which happens for the following reasons:
- The original aluminum alloy is in a thermodynamic state, far from equilibrium. As a result of abrupt cooling, the alloy is transferred to “hypothermia” – all dissolved alloying elements “freeze” in a solid solution of aluminum;
- The residence time of the alloy at a given temperature is sufficient for the diffusion formation of a metallurgical structure (microstructure) with the achievement of a more equilibrium state according to the corresponding phase diagram, for example, such, which is shown in the figure 1;
- The unbalance of the alloy occurs as a result of rapid cooling. During its subsequent holding at a given temperature, the process of particle release occurs, which are an obstacle to plastic slip deformation in this alloy;
- For some time, in the process of separating these particles, their size and number increase.. As a result of this, the hardness of this alloy increases and its strength occurs. “aging” aluminum alloy.
Figure 1 – Phase diagram of 6xxx series aluminum alloys [1]
This process of thermal hardening of aluminum alloys formally refers to “dispersed hardening” (precipitation hardening). However, usually for aluminum alloys, this process is called “hardening of aging” (age hardening) or just “aging” (ageing или aging).
Term “aging” (aging) reflects the fact, that this hardening process does not occur instantly, as is the case with steel hardening, in slow enough and for some time, hours, days, weeks, months. Note, what's the word “ageing” apart from meaning “aging” there are other meanings – aging, aging (for wine) and, even, growing up, which more positively and optimistically reflect the essence of this process.
The speed and degree of hardening increases, if the alloy is aged at elevated temperatures, for example, 200 degrees Celcius. This process is called “artificial aging” in contrast to the aging process at room (workshop) temperature, which is called “natural aging”.
Metallurgy of thermal hardening of aluminum
Phase diagram
The main metallurgical property of aluminum alloys, which are capable of hardening by heat treatment is:
- complete solubility of a certain chemical element (compound) in a solid solution at an elevated temperature, but only very limited solubility in solid solution at room temperature.
Historically, this was first identified in the Al-Cu alloy. A schematic phase diagram for Al-Cu alloys is shown in the figure. 2. It is a classic eutectoid system, but for the theory of thermal hardening, only its part with a high aluminum content (low copper content) is of interest. Consider below an alloy with a chemical composition of Al + 4% Cu.
Figure 2 – “Aluminum” part of the Al-Cu phase diagram [1]
Solid solution
At a temperature 550 degrees Celsius (point A on the diagram) the equilibrium structure is copper atoms completely dissolved in the aluminum matrix. This structure is designated by the Greek letter “alpha”. This temperature 550 degrees Celsius is often called “quenching temperature”.
Slow cooling from quenching temperature
With very slow cooling of the alloy from temperature 550 degrees Celsius, an equilibrium microstructure is formed, which consists of an alpha phase with a very low content of dissolved copper together with coarse particles of the equilibrium secondary phase CuAl2, which formed mainly at high temperatures during cooling from about between 450 to 300 degrees Celsius (figure 3). This equilibrium structure (denoted by the letter D in the figure 3) is stable at room temperature, but it is not very durable and therefore practically does not represent an engineering interest.
Figure 3 – Isolation of the secondary phase from a supercooled solid solution –
the formation of clusters of dissolved atoms [1]
Rapid cooling from quenching temperature
The situation is changing radically, when the alloy is rapidly cooled in water from the quenching temperature 550 degrees Celsius to room temperature.
Rapid cooling from point A to point B (cm. drawings 2 and 3) “freezes” solid solution in that state, which was at 550 degrees Celsius. Of course, it can no longer be an equilibrium state. they say, that the quenched (cooled) alloy is in the metastable state of a supercooled solid solution. It means, that this microstructure is thermodynamically unstable and the alloy in this state has a metallurgical driving force to move (change) towards an equilibrium structure. At elevated temperatures, for example, 150 degrees Celsius, this process is much faster, than at room temperature, since copper atoms and vacancies in the aluminum matrix diffuse much faster. Copper atoms are collected in clusters (GP-zones) and these clusters have the form of precipitates from a solid solution. In Al-Cu alloys, these clusters are in the form of plates, in Al-Zn alloys – spheres, and alloys Al-Mg-Si – rods (figure 4).
Figure 4 – The shape of clusters (GP-zones) in various systems of aluminum alloys [1]
Thermal hardening technology
Heating for hardening
Heating for quenching: alloy is aged at a temperature, at which a homogeneous single-phase state is achieved.
intermetallic compound, such as Mg2Si in 6xxx or Al alloys2Cu in alloys of the 2xxx series, completely go into solid solution, and the alloy reaches a homogeneous state. Heating temperature for hardening, for example, for 6xxx series alloys, is in the range from 500 to 550 °C.
Quenching
Hardening: rapid cooling of solid solution, for example, immersion in water, directly at the exit from the oven or extrusion press.
In this case, the homogeneous state of the alloy is "frozen" at room temperature – precipitation of intermetallic particles is prevented. This is a very mild state, since the number of barriers to the movement of dislocations – minimally. For press hardening, the temperature of the alloy at the outlet of the press must be higher than the dissolution temperature of intermetallic particles (solvus temperature). Immediately after leaving the press, the alloy is quenched by cooling with compressed air or water jets.
Ageing
A supersaturated single phase solid solution is unstable below the solvus line in the equilibrium diagram. This one-phase state has an "innate" tendency to transition to a two-phase. This can be achieved in two ways..
natural aging
Aging at room temperature: slow – during few hours, days or weeks – loss of secretions, resulting in an increase in hardness.
Artificial aging
Includes heating to a temperature below the solvus line; heating promotes more efficient formation of formations and secretions.
Discharge first, which are coherent with the aluminum matrix - they have the same crystal lattice, as the matrix. These coherent phases cause stress in the matrix, which become effective barriers against plastic deformation through the movement of dislocations. When this discharge gets large enough, they become incoherent and form a separate phase. Stresses are decreasing, and the metal becomes softer again, although in any case remains firmer, than in the solid solution state, due to stresses, which inevitably occur around the discharge.
Aging hardening mechanism
Figure 5 – The mechanism of cluster resistance to dislocation motion [1]
Aging hardening curves
Typical graphs of changes in strength and hardness under various aging modes of a hypothetical Al-Cu alloy are shown in the figure 6.
Figure 6 – The change in the strength of the alloy at different aging temperatures [1]
- natural aging – aging hardening at room temperature – very slow, within weeks and months depending on the type of alloy and the degree of alloying.
- At elevated temperatures – 140 degrees Celcius – aging is much faster.
- For aging at 160 degrees Celsius, the peak strength is reached after 24 o'clock, and then the alloy loses strength, getting softer. For 6xxx series alloys, this maximum is achieved after 8-12 hours depending on alloy.
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