Nagartovka 

Strain-hardened aluminium

 

Work hardening is a natural consequence of most deformation operations of aluminum and its alloys.. Sometimes it is also called “work hardening”. Work hardening further increases the strength of the alloy, which has been achieved as a result of doping and hardening heat treatment. For thermally hardenable alloys strain hardening may also increase the preciption rate of the ageing phase.

Degree of work hardening

Work hardening is used to improve the strength properties of thermally hardened aluminum and aluminum alloys of the 1xxx series., 3xxx and 5xxx. The main parameter of one or another cold-worked state is the degree of work-hardening The degree of work-hardening depends on the amount of plastic cold deformation, which was absorbed by the alloy (figure 1).


Fig. 1 – Influence of strain hardening on mechanical properties [1]

Annealing

Work hardening can be removed or reduced by annealing. Material softening, that is, a decrease in strength and an increase in ductility depend on the selected combination of time-temperature parameters (Figure 2).

Partial annealing

Partial annealing is performed for a partial reduction of strength properties and increase to bring the plastic material in the finished product a predetermined state. It is also called softening or restorative. (Figure 3).

Full annealing

Full annealing is used for the complete removal of peening material and bringing the material to a state with minimum and maximum strength characteristics plastic properties. It is also called recrystallization annealing. During this annealing, a new grain structure is formed (Figure 3)..

Fig. 2 –


Fig. 3 –

Tempers designation

To designate all tempers of aluminum and aluminum alloys (and not only cold-worked ones), the American system is widely used throughout the world, developed at the time of the American Aluminum Association.

  • The initial states of the system of notation deformable aluminum alloys is described in American Standard ANSI H35.1.
  • This system is almost "as is", adopted by the international ISO standard 2107 and European standard EN 515.

State of the material in the hot-pressed state without additional treatments - thermal or strain - designated standards EN and ISO the letter F and no figures for it does not follow.

State fully annealed material designated the letter "O" (not zero) according to the international classification of states of aluminum and aluminum alloys. The letter O with an additional number (for example, O1 refers to annealing with special conditions.

All cold-worked condition designations begin with a letter "H" – Hardened. Behind it can go from 2 to 3 figures.

H1 – Work hardening only

Pure cold-worked condition without additional treatments form a series H1. Fully cold-worked condition of the material, which is designated H18, call state, obtained at cold deformation equivalent relative rolling reduction 0,75. The relative reduction - is the ratio of the difference of the initial and final sheet thickness to the original thickness of the sheet. relative hood 0,75 will be achieved, for example, at an initial thickness 10 mm and the final thickness 2,5 mm: (10 – 2,5)/10 = 0,75. H19 represents the state of the product with an even greater degree of work hardening, than in the state H18. it applies, for example, for a tape thickness 0,30 mm from an aluminum alloy 3104 for body beer cans. state H16, H14 and H12 is obtained with a smaller amount of cold deformation and represent, respectively, trichetvertinagartovannoe, polunagartovannoe and chetvertnagartovannoe states.

H111 and H112 – “almost annealed”

When specifying the requirements for mechanical properties of aluminum and aluminum alloys often use state designation N111 and N112 of the same series H1. N111 state differs from the annealed state of about only a small degree of work hardening, you could get the material when straightening or other technological operations. N112 state differs from the state F only a small degree of work-hardening (hot or cold working), as well as the obligatory control of the mechanical properties.

H2 – Work hardening and partial annealing

H2 series refers to materials, which have been hardened by cold deformation to a higher degree, than it is necessary for the given strength properties, and then reduce this "extra" strength with the help of partial annealing. With increasing degree autofrettage second digit increases from 2 to 8 similar to purely hard-worked states: H22, H24, N26 and N28.

The figure 1 schematically shows the strain-hardened states of the H1 and H2 series at different degrees of hardening and different annealing times at a constant temperature.

  • In states with the same second digit ultimate strength - the same, a yield strength in states with partial annealing below, than the purely work-hardened state.
  • Schedule a strength increase of the degree of cold deformation is convex upward. This reflects the fact, that the first step of cold deformation give maximum strength increase.

Fig. 4 – Deformation hardened states of the H1 and H2 series at various degrees of hardening and
different durations of annealing at a constant temperature [4].

H3 – strain hardening and stabilizing low-temperature annealing

Series H3 - states with work hardening and stabilizing annealing. H32, N34, H36 and N38. This series of conditions only apply for aluminum-magnesium alloys - 5xxx series alloys. The thing is, These alloys for some time in the cold-worked condition may lose, peening achieved strength properties, due to the natural aging mechanism. therefore, if the strength properties of stability is important, they are often heated to moderate temperatures, for example, 220 ° C, to complete the process of aging, thus somewhat reduced strength, but increase the flexibility and, thereby, provide subsequent mechanical stability and performance properties.

Cold deformation and varnishing – H4 series

H4 Series is used for work-hardened products with additional lacquered surface. for instance, in the manufacture of covers beer cans apply thick tape 0,26 mm aluminum alloy 5182 able to H48 – fully strain-hardened and lacquered.

Numbers after H1, H2, H3 и H4

The number after N1, N2, H3 and H4 indicate the degree of work hardening (Figure 5):

  • Most hardened state. Indicated by adding a number 8 (v. it is. HX8).
  • Degree of cold work hardening, which is about half the power for state HX8. Indicated by state HX4.
  • The degree of cold work hardening is approximately halfway between the strength levels for the O state and the HX4 state. Designated HX2.
  • Degree of cold work hardening midway between HX4 and HX8. Designated HX6.
  • figures 1, 3, 5 and 7, likewise, indicate intermediate states between the above.
  • Numeral 9 used to indicate a state, which exceeds the hardening state HX8 by 14 MPa or more.

Table 1 shows the increase in tensile strength of wrought alloys in annealed tempering, when they are processed to HX8 state.

Fig. 5 – Metallurgic states and work hardening [1]

Table 1 – Gains in the tensile strength of wrought alloys in the annealed temper
when they are treated to the HX8 temper [3]

Sources:

  1. The metallurgy of aluminium // Corrosion of aluminium /Chrisian Vargel – Elsevier, 2004
  2. Aluminum and Aluminum Alloys – ASM Handbook – 1996
  3. Introduction to Aluminum Alloys and Tempers – J. Gilbert Kaufman
  4. Design of aluminium structures – Introduction to Eurocode 9 with worked examples – European Aluminium Association – 2020