Thermal hardening aluminum alloys

“Disperse hardening” or “hardening of 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). Это выделение частиц является металлургическим процессом (фазовым превращением), который происходит по следующим причинам:

  • 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” в твердом растворе алюминия;
  • Длительность нахождения сплава при данной температуре достаточна для диффузионного образования металлургической структуры (микроструктуры) с достижением более равновесного состояния согласно соответствующей фазовой диаграммой, 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, которые являются препятствием для пластической деформации скольжением в данном сплаве;
  • 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 – Фазовая диаграмма алюминиевых сплавов серии 6ххх [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) или просто “aging” (ageing или aging).

Term “aging” (ageing) отражает тот факт, 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, выдержка (для вина) и, 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” в отличие от процесса старения при комнатной (цеховой) температуры, which is called “natural aging”.

Metallurgy of thermal hardening of aluminum

Phase diagram

The main metallurgical property of aluminum alloys, которые способны упрочняться путем термической обработки является:

  • полная растворимость некоторого химического элемента (соединения) в твердом растворе при повышенной температуре, 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, но для теории термического упрочнения представляет интерес только ее часть с большим содержанием алюминия (малым содержанием меди). Consider below an alloy with a chemical composition of Al + 4% Cu.

Figure 2 – “Aluminum” часть фазовой диаграммы Al-Cu [1]

Solid solution

At a temperature 550 градусов Цельсия (точка А на диаграмме) равновесной структурой является полностью растворенные в алюминиевой матрице атомы меди. 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 by 300 градусов Цельсия (рисунок 3). Эта равновесная структура (обозначена буквой D на рисунке 3) является стабильной при комнатной температуре, 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 –
образование кластеров растворенных атомов [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.

Быстрое охлаждение от точки А до точки В (см. 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, что закаленный (охлажденный) сплав находится в метастабильном состоянии переохлажденного твердого раствора. It means, что эта микроструктура является термодинамически неустойчивой и сплав в этом состоянии имеет металлургическую движущую силу для движения (изменения) к равновесной структуре. 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. Атомы меди собираются в кластеры (GP-зоны) и эти кластеры имеют вид выделений из твердого раствора. In Al-Cu alloys, these clusters are in the form of plates, in Al-Zn alloys – spheres, and alloys Al-Mg-Si – стержней (рисунок 4).

Figure 4 – Форма кластеров (GP-зон) в различных системах алюминиевых сплавов [1]

 

Thermal hardening technology

Heating for hardening

Нагрев под закалку: 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 by 550 °C.

Quenching

Закалка: 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. Для закалки на прессе температура сплава на выходе из пресса должна быть выше температуры растворения интерметаллидных частиц (температуры сольвуса). Immediately after leaving the press, the alloy is quenched by cooling with compressed air or water jets.

Aging

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

Старение при комнатной температуре: slow – during few hours, days or weeks – loss of secretions, resulting in an increase in hardness.

artificial aging

Включает нагрев до температуры ниже линии сольвуса; нагрев способствует более эффективному формированию образований и выделений.

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 – Механизм сопротивления кластеров движению дислокаций [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 – Изменение прочности сплава при различных температурах старения [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.

Sources:

  1. TALAT 1204.01

application – Additional Information


Artificial aging of alloys 6xxx - AD31, AD33, AD35