Aluminium weldingUpdated

HAZ in welded aluminium

 

 

All aluminum alloys are divided into two important types (Fig.. 1):

  • heat-treatable
  • non-heat-treatable.

Heat-treatable alloys are able to increase their strength properties as a result of heat treatment (sequences of heating and cooling). Work hardening of these alloys is only used in combination with heat hardening.. (See T2 states below., T3, T8, T9, T10).

Non-heat-treatable alloys do not have the ability to heat harden or have it in very limited limits.. These alloys are hardened only by cold working..

This distinction is very important when considering the effect of welding parts and structures made of aluminum alloys.. Especially, when considering strength loss in the weld heat-affected zone (HAZ) .

Fig. 1 – Aluminium Alloy Designation System [1]

As seen from the figure 1, aluminum alloys have three main hardening mechanisms:

  • solid-solution strengthening (atoms in solution) (например, for 5xxx series alloys (Fig. 2))
  • strain hardening (work hardening)
  • age hardening (preciption hardening)

Fig. 2 – Hardening effect as a function of the content of alloying element, here Mg [2]

Non-heat-treatable alloys

  • Series 1xxx, 3xxx and 5xxx aluminum alloys are not thermally processed
  • Can only be hardened by cold deformation, often called work hardening. .
  • Work hardening is not usually applied to cast alloys..

Work hardening mechanism

During plastic deformation at room temperature, the number of dislocations in the metal increases. Therefore, it becomes more and more difficult for dislocations to move through the crystal lattice.. The intense movement of dislocations leads to the formation of coils or "forests" of dislocations (Fig.. 3). As a result,, higher loads are required to continue deformation, and the metal loses its ductility. This is the essence of strain hardening..

A decrease in the dislocation density can be achieved by heating the cold-worked metal to a moderately high temperature (annealing). This leads to softening of the metal and restoration of its ductility.. Changes in microstructure, occurring during annealing, called reduction and crystallization (Fig.. 4). The same phenomena occur when the metal is heated in the heat affected zone (HAZ) of the weld..

Thermally unstrengthened alloys achieve optimum mechanical properties as a result of hardening, that is, as a result of cold plastic deformation, and, sometimes, partial annealing (Fig.. 5).


Fig. 3 – In aluminium alloys after a moderate amount of deformation,
the dislocations are not uniformly distributed but instead they form cells,
with walls of tangled dislocations and interior regions of low dislocation density.
Typically, these cells have a diameter of the order of 1µm [4]


Fig. 4 – Dislocations may be removed by heating the cold worked metal (annealing).
This causes the metal to soften and restores ductility (recovery and recrystallisation) [4]


Fig. 5 – Work hardening of non heat treatable aluminium magnesium and
pure aluminium alloy [1]

Heat-treatable aluminum alloys

  • Series 2xxx, 6xxx and 7xxx wrought aluminum alloys are heat-treatable.
  • The 4xxx series includes heat-treatable and non-heat-treatable alloys.
  • Cast alloys of 2xxx.x series, 3xxx.x, 4xxx.x and 7xxx.x are heat-treatable.

Age hardening

This hardening is a two-stage heat treatment:

  • At first, the alloy is processed into a solid solution. It means, that a state of supersaturation of the solid solution is created in the alloy.
  • Secondly, natural aging process, which occurs after quenching, can be accelerated by heating the alloy until, until the precipitation of the smallest coherent particles of the secondary phase occurs. It is these particles that strengthen the alloys due to the fact that, which create obstacles to the movement of dislocations [1, 3] (Fig. 6 и 7).

Fig. 6 – The very small precipitate particles (the GP zones)
responsible for age hardening.
The terms “age hardening” and “precipitation hardening” are synonymous [3]

Fig. 7 – GP zones, with their associated surrounding strain fields,
present impediments to slip by dislocation movement.
As ageing time increases, the zones will increase in size and
slip becomes progressively more difficult
– the alloy age hardens [3]

Solution treatment consists of heating the alloy to a temperature just below the lowest melting point of the alloy system., holding at this temperature until, until a significant amount of alloying elements is dissolved in the base metal (Fig.. 8). The alloy is then rapidly cooled, to keep as many alloying elements as possible in solution and, thus, get a supersaturated solid solution at room temperature.

After solution heat treatment, most heat treatable alloys exhibit natural aging at room temperature.. The rate and degree of natural aging at room temperature differs from alloy to alloy.. for instance, 6063 reaches a relatively stable state in 1-2 weeks. This condition is referred to as T4..

Heating above room temperature accelerates the reaction of precipitation of the secondary coherent phase. Therefore, in practice, heat-hardenable alloys are usually “artificially aged” in order to obtain maximum strength properties as quickly as possible.. The temperature range of this treatment is usually 120-180 ° C. The actual temperature depends on such variables, as an alloy, required properties and production capabilities (Fig.. 9).

Fig. 8 – Heating to a solid solution temperature to dissolves of the alloying elements,
then rapidly cooled to retain of the alloying elements in solution [1]

Fig. 9 – The effect of time and temperature on the precipitation process [1]

CTempers of aluminum alloys

State symbols include a series of letters and numbers, which immediately follow the alloy designation and are associated with a hyphen (dash), for example, 6061-T6, 6063-T4, 5052-H32 and 5083-H112.

The designation of the state of the aluminum alloy material begins with the letter H or the letter T. The letter H indicates a freewheeling, and the letter T is for thermal hardening.

Strain hardening

The first number after the letter H indicates one of the four basic cold-worked states of the material:

  • H1 - Only chartered
  • H2 - Caked and partially annealed
  • H3 - hard-worked and stabilized (by special annealing)
  • H4 - Coated and varnished or painted.

The second digit after the letter H indicates the degree of autofrettage:

  • Hx2 - Quartered
  • Hx4 - Half Chipped
  • Hx6 - Three Quartered
  • Hx8 - Fully Chipped
  • Hx9 - Charted over full chartered

Fig. 10 – The effect of cold working and annealing on strength as a function of time at constant temperature [2]

thermal hardening

The first digit after the letter T indicates one of ten basic thermally hardened states of the material:

  • T1 - Naturally aged after cooling from thermoforming temperature (e.g., pressing)
  • T2 - Hard-worked after cooling from hot forming temperature and then naturally aged
  • T3 - Tempered, caked and naturally aged
  • T4 - Hardened and Naturally Aged
  • T5 - Artificially aged after cooling from the temperature of the thermoforming process
  • T6 - Hardened and artificially aged
  • T7 - Hardened and Stabilized (over-aged)
  • T8 - Tempered, caked and artificially aged
  • T9 - Tempered, artificially aged and caked
  • T10 - Hot-formed after cooling from the temperature of the hot forming process and then artificially aged

Additional numbers may indicate residual stress relief., for example, Тх51 or Тхх51 (removal of residual stresses by tensile) or Тх52 or Тхх52 (removal of residual stresses by compression).


Fig. 11 – Natural ageing asymptotically approaches an upper limit (T4).
Strength of T64. T6 and T7 is the function of time at elevated temperature [2]

Heat-Affected Zone in Welded Aluminium Joints

Welding causes much greater strength losses in the material.. In case of welding temperatures are so high, that it is necessary to take into account the effects of strength reduction near the weld (the so-called heat-affected zone, HAZ). Type of material to be welded, t. it is. is it a heat treatable alloy, is essential for strength, achieved after welding (Fig.. 12).

Fig. 12 – The type of the material to be welded is decisive for the strength attained after welding [5]

Cold worked alloys

  • The strength of the welded seam of the joint corresponds to the strength in the annealed state (Fig.. 13).
  • The strength of hard-worked alloys in the welding zone is reduced to the strength in the annealed state.


Fig. 13 – Characteristic Mechanical and Technological Values of
the HAZ of AlMg4,5 Mn [5]

Heat treated alloys

  • Weld strength also corresponds to annealed strength.
  • Heat-strengthened alloys in the T6 temper have a loss of approximately 40 % its strength (Fig.. 14)
  • Alloy is the only exception. 7020, which loses only 20 % its original strength [2].
  • Basically, zone of quarrel, annealed after welding, can be returned to its original aged state by repeated heat treatment, including, solid solution treatment, hardening and aging. The main problem is usually the large size of welded structures., which leads to the need to have large heat treatment furnaces. Another problem is distortion, accompanying hardening.
  • Welding heat causes self-aging effect in Al-Zn-Mg type alloys. Welding heat is sufficient to process the welding zone for solid solution, and rapid cooling is equivalent to quenching. The weld is then aged at room temperature.

Fig. 14- Reduction of strength in the heat affected zone (HAZ) (typical for EN AW-6082) [2]

Sources:

  1. TALAT 1501
  2. Design of aluminium structures – Introduction to Eurocode 9 with worked examples / European Aluminium – 2020
  3. TALAT 1204
  4. TALAT 1251
  5. TALAT 4100