Work hardening of aluminium

 

The process of work hardening of aluminum alloys is important both from the point of view of understanding the evolution of the microstructure during plastic deformation, and from a practical point of view, optimizing the formability of aluminum alloys for a wide range of applications. Moreover, work hardening is an effective method of strengthening aluminum alloys and is important for understanding the processes of aluminum deformation processing using large deformations., such as cold extrusion, drawing and rolling.

Working of aluminum

Working of aluminum is the processing of a cast ingot into a product by metal pressure treatment methods by hot and / or cold plastic deformation (Fig.. 1).

Fig. 1 – Process chain of aluminium sheet production [1]

Two important aspects of aluminum deformation processing
• changing shape and
• change in microstructure and mechanical properties.

Shape change

The shape change is expressed by measuring the nominal or true strain [2]:

• Nominal deformation = ((Initial thickness – Final thickness)/Initial thickness) x 100 %
• True strain = ln (initial thickness/final thickness).

Note: ln – natural logarithm.

for instance, when rolling foil thickness 7 µm from slab thickness 300 mm deformation is [2]:

• Nominal deformation = 99,98 %
• True deformation = 10,7

Change in microstructure and mechanical properties

Aluminum as a metal consists of a large number of individual grains or crystals; i.e. they are polycrystalline. for instance, a typical grain or crystal after hot working plus cold working and annealing will have a diameter of about 40 m. So, this typical grain contains many millions of unit cells – order 10¹⁵ pieces [2] (Fig. 2).

Fig. 2 – A typical aluminium grain is of diameter 40μm and
contains many millions of unit cells [2]

In the cast state, primary crystals grow from the liquid phase. The resulting microstructure is usually coarse. When metal is deformed, each grain is deformed due to the movement of linear defects in the crystal lattice. The deformation is carried out by sliding along the slip planes along the shear direction (Fig.. 3). These defects are known as dislocations (Fig.. 4). In crystals, dislocations move along certain crystallographic planes (flat-packed planes), known as slip planes [2].

Fig. 3 – Each grain deforms by the movement of line defects in the crystal lattice;
deformation is by slip on slip planes along the shear direction [2]

Fig. 4 – Deformation by slip of a dislocation [2]

Aluminum hardening mechanisms

Aluminum is ductile and ductile because dislocations can move relatively easily through its crystal lattice.. Industrial aluminum alloys are intentionally strengthened by creating various kinds of obstacles for the movement of dislocations., such as:

• Grain boundaries
• Other dislocations (strain hardening)
• Dissolved atoms (solution hardening)
• Precipitation of secondary phases (SP) (hardening by aging)
• Dispersed particles (dispersoid hardening).
  

Some or all of them can help increase the strength of aluminum and aluminum alloys (Fig.. 5).

Fig. 5 – Obstacles to dislocation movement may contribute to the strength of alloy.
Modified Hall-Petch relationship (Here d is the grain size) [2]

Cold and hot working of aluminium

Deformation of aluminum and its alloys at temperatures below 100 ⁰С is considered cold deformation [4]. For this temperature range, thermally activated processes, which are characteristic of hot deformation, do not play a significant role.

At temperatures above 100 ⁰С plastic deformation of aluminum strongly depends on thermally activated processes. Therefore, the yield strength depends on temperature and strain rate (viscoplasticity). Hot deformation processes are characterized by strong dynamic recovery of dislocation structures and low flow stress.

Cold working of aluminium

Deformation

During plastic deformation of aluminum and aluminum alloys at temperatures below 100 ⁰ Creep phenomena can be neglected. It means, that the process of plastic deformation does not depend on the strain rate. In this case, the yield strength is approximately constant at a given temperature. The figure 6 shows typical room temperature stress-strain curves for Al-Mg alloys. It is seen, that these alloys exhibit a high rate of work hardening with flow stresses from about 100 to 400 MPa.

Fig. 6 – Typical stress-strain curves of Al-Mg alloys [4]

Cold working microstructure

During cold deformation, the number of dislocations increases, and it becomes more and more difficult for dislocations to move through the lattice. As a result, metal hardening or work hardening occurs.. It means, that higher loads are required to continue deformation, and the metal loses its ductility.

Cold plastic deformation is controlled by the movement of dislocations in the crystal lattice. As deformation develops, the density of dislocations in the crystal increases, and the stress to continue deformation increases due to the interaction of mobile dislocations with an increasingly dense network of dislocations and even a dislocation structure (Fig.. 7).

Fig. 7 – In aluminium alloys after a moderate amount of cold deformation,
the dislocations are not uniformly distributed but instead they form cells,
with walls of tangled dislocations and interior regions of low dislocation density [2]

Recovery and recrystallization of work-hardened aluminum

Dislocations can be removed by heating cold worked metal to a moderately high temperature (annealing), which leads to softening of the metal and restoration of its ductility. microstructure changes, occurring during annealing, called reduction and recrystallization.

During hot deformation of aluminum, active dynamic recovery processes occur: dynamic recovery or dynamic recrystallization. As a result of these processes, the metal is not hardened as much, both at room temperature, и, Consequently, smaller loads are required to deform the material (Fig.. 8).

The driving force behind recrystallization is the stored energy, due to the presence of dislocations (Fig.. 9).

The dislocation density can be expressed as the total length of dislocation lines per unit volume of material:

• for annealed aluminum, the dislocation density can be about 10¹⁰ m^-2
• for strongly cold-formed aluminum, the dislocation density increases to approximately 10¹⁵ m^-2.

Fig. 8 – When a large amount of cold work is followed by annealing,
new grains are formed by the process of recrystallisation [2]

Fig. 9 – The driving force for recrystallisation is the stored energy
caused by the presence of dislocations [2]

Strain hardening of non-heat treatable alloys

Stress-strain curves

1xxx series aluminum alloys, 3xxx and 5xxx are hardened by adding alloying elements, which do not typically exhibit classical age hardening. The microstructures of these alloys usually consist of [3]:
• aluminum grains with alloying additives in solution
• compound particles 1–5 µm in size, which were formed during solidification and homogenization (Al(Fe,Mn), Al(Mn,Fe)Si и др.) и
• dispersoids 50–250 nm in size, distributed in aluminum grains.

On pic. 10 shows stress-strain curves of high-purity aluminum, aluminum 1100 and aluminum alloy 5005 и 5754, tested at ambient temperature. As you might expect, the yield strength increases with the content of dissolved alloying elements [3].

Fig. 10 – A comparision of (a) the stress-strain response and
(b) the work hardening behaviour for a number of commercial solid solution alloys [3]

Strain hardened tempers

Strain hardening as a technological operation

Work hardening is important for the development of high strength aluminum and aluminum alloys in work hardened states.. Many alloys are used in the strain-hardened state H-temper, e.g. series alloys 1000 in the production of foil, body blanks aluminum beer cans series alloys 3000 and blanks for a cap of a beer can from series alloys 5000. The purpose of these treatments is to achieve high levels of strength by using the alloy in the cold rolled condition or after cold rolling and partial annealing..

Due to the very high levels of stored energy, achieved by cold rolling, dislocation structure can be thermally unstable, while the alloy sheet softens over time at room temperature. To avoid this, cold rolled sheet is often subjected to partial annealing at a low temperature to obtain some softening, but with strength stabilization.

Designation system for strain-hardened tempers

Vacation designations for forged products, hardened by strain hardening, consist of the letter H, followed by two or more digits. The first digit after H indicated a specific sequence of basic operations.

H1, only strain-hardened

This applies to products, ankle-hardened to obtain the required strength without additional heat treatment. The number after H1 indicates the degree of work hardening.

H2, strain-hardened and partially annealed

This applies to products, which were subjected to strain hardening to a greater extent, than specified for the finished product. This “excessive” strength is then reduced to the desired level by partial annealing. The number after H2 indicates the degree of work hardening, remaining after partial annealing of the product.

The ratio of the strain-hardened states H1X and H2X is shown in the figure 11.

Fig. 11 – The ratio of strain-hardened states H1X and H2X [5]

H3, strain-hardened and stabilized

This applies to products, subjected to work hardening, whose mechanical properties are stabilized by low-temperature heat treatment or by heat, introduced during the manufacturing process. Stabilization usually improves ductility. This designation applies only to those alloys, that, if they are not stabilized, gradually soften upon aging at room temperature. The number after H3 indicates the degree of work hardening, remaining after stabilization.

H4, strain-hardened and lacquered or painted

Related to products, which received strain hardening and were subjected to heating during
subsequent painting or varnishing. Number, added to H4, indicates the amount of work hardening, remaining after painting or varnishing

Additional digits after H1, H2, H3, H4

Indicate the degree of strain hardening:

• HX8 – most hardened state. Adding a number 8
• HX4 – hardening degree, equal to about half of the state of HX8
• HX2 – hardening degree midway between the O state and the HX4 state
• HX6 – degree of hardening in the middle between HX4 and HX8
• figures 1, 3, 5 и 7, likewise, indicate intermediate states between the above
• Numeral 9 used to indicate a state, exceeding HX8 state by 14 MPa or more.

Appendix

“Cold-worked” operations in Heat Treating T Tempers

• T1 – Cooled from an elevated temperature shaping process and naturally aged to a substantially stable condition.
• T2 – Cooled from an elevated temperature shaping process, cold worked, and naturally aged to a substantially stable condition.
• T3 – Solution heat treated, cold worked, and naturally aged to a substantially stable condition.
• T4 – Solution heat treated, and naturally aged to a substantially stable condition.
• T5 – Cooled from an elevated temperature shaping process then artificially aged.
• T6 – Solution heat treated then artificially aged.
• T7 – Solution heat treated then overaged/stabilized.
• Q8 – Solution heat treated, cold worked, then artificially aged.
• T9 – Solution heat treated, artificially aged, then cold worked.
• T10 – Cooled from an elevated temperature shaping process, cold worked, then artificially aged.

For tempers T2, T3, T8 and T9 cold working is applied to heat-treatable alloys in order:

• to improve strength after cooling from a hot-working process or solution heat treatment and for which mechanical properties have been stabilized by room-temperature or artifical ageing.

This tempers also applies to products in which the effects of cold work, imparted by flat-tening or straightening, are accounted for in specified property limits.

Sources: :

1. Aluminium sheet fabrication and processing / J. Hirsch // Fundamentals of aluminium metallurgy – Ed. Roger Lumley – 2011
2. TALAT Lecture 1251 – Mechanical Working and Forming of Shapes /M.H. Jacobs – European Aluminium Association, 1999
3. Work hardening in aluminium alloys / J. Pool et al // Fundamentals of aluminium metallurgy – Ed. Roger Lumley – 2011
4. Design of Aluminum Rolling processes for Foil, Sheet, and Plate /J.H. Driver, O. Engler //Encyclopedia of Aluminum and Its Alloys – Eds. J. E. Totten, M. Tiryakioglu, O. Kessler – 2019
5. Design of aluminium structures – Introduction to Eurocode 9 with worked examples – European Aluminium Association – 2020
6. Introduction to Aluminum Alloys and Tempers /J. Gilbert Kaufman – ASM International, 2000