Aluminum durability
Strength characteristics
Almost any structural material, including, and aluminum, the main mechanical characteristics are:
- ultimate tensile strength,
- yield stress 0,2 %
- relative extension.
Structural aluminum alloys have a minimum tensile strength in the range of 200 to 500 MPa. Maximum strength aerospace alloys have, doped with zinc and copper, for example, alloy 7075 (drawing 1).
Figure 1 – Levels strength aluminum alloys
Unalloyed aluminum is not able to achieve this level of strength even in cold-worked (work-hardened) conditions, which arise as a result of cold plastic deformation. These conditions are achieved, for example, cold rolling and drawing sheets extruded tubes. At the same time, cold deformation of unalloyed aluminum leads to a significant increase in its strength properties (tensile strength Rm and tensile yield strength Rp0,2) compared to unalloyed aluminum in the "soft" state, for example, after annealing (figure 2).
Typical structural aluminum alloys, which are used for load-bearing structures of buildings, are highly alloyed variants thermally neuprochnyaemyh alloys of Al-Mg and Al-Mg-Mn systems and thermally hardenable alloys Al-Mg-Si and Al-Zn-Mg.
Figure 2- The minimum values of yield strength 0,2% and
tensile strength
according to the European Standards EN 485-2, EN 754-2 и EN 755-2
dfor various types of wrought aluminum alloys [1]
Two strengthening mechanism
There are two mechanisms of hardening aluminum alloys, which can complement each other:
- strain hardening (auto-work) and
- thermal hardening (aging).
The starting point for evaluating the degree of growth of strength aluminum alloys as a result of deformation or thermal bonding is typically used quantities strength characteristics of the alloy in the "soft" state, which is its complete state after annealing.
Even in the fully annealed alloy of any strength increases with increasing content of interstitial atoms in the solid solution of aluminum, that is, increasing the content of alloying elements. Examples include alloys of Al-Mg and Al-Mg-Mn, which are shown by circles with plus signs (+) at the bottom of the diagram in the picture 1.
strain hardening
Strain hardening - a plastic deformation, for example, as a result of cold rolling, which creates dislocation in the crystal lattice of aluminum. With more and more confronted with each other as the degree of plastic deformation of these dislocations and, as a result, increased resistance to further deformation and, Consequently, strength level is increased. Strain hardening (work-hardening) manifests itself in the form of a strong increase in the ratio Rp0,2/Rm with a corresponding notable reduction in magnitude of the elongation at fracture of the test specimen.
Figure 3 – Influence of the degree of autofrettage on mechanical properties [3]
This hardening mechanism is used to increase the strength of aluminum and aluminum alloys of the following alloying systems:
- Al
- Al-Mn
- Al-Mg
- Al-Mg-Mn
Soft annealing
recrystallization
Any increase in strength as a result of strain hardening disappears at a temperature above 250 ° C as a result of recrystallization and strength aluminum alloy returns to the level, who was in his "soft" state. Annealing at temperatures below the recrystallization temperature (which depends on the chemical composition of the alloy and the degree of cold plastic deformation) results in less dramatic loss of strength as a result of the recovery process.
Figure 4 – Change in hardness and structure during annealing [3]
Return
This thermal softening (annealing at a temperature 200-250 ºС) apply, for example, for "semisolid" state for a sheet, which is in the "solid" state. For a given level of strength aluminum alloy elongation is significantly higher, than for the same level of strength, which is obtained by treating a cold plastic.
Figure 5 – Isothermal annealing curves alloy 5754 [3]
Heat hardening
Thermal hardening aluminum alloy is mainly due to aging mechanism. Therefore, this hardening mechanism is applicable only to some systems of aluminum alloys., such as:
- Al-Cu
- Al-Cu-Mg
- Al-Mg-Si
- Al-Zn
- Al-Zn-Mg
- Al-Zn-Сu-Mg
Heating for quenching and tempering
Pre-hardening operation for this mechanism is the heating operation to a temperature, in which as much as possible alloying elements turned into solid solution of aluminum. This operation is called a thermal treatment at a solid solution by heating for quenching or. Then the alloy is rapidly cooled by quenching to room temperature, resulting in overcooling of the solid solution of aluminum and "freeze" in its alloying elements in a thermodynamically nonequilibrium state.
Figure 6 – Operations of thermal hardening of alloys of the 6xxx series [3]
Aging
The process of age hardening occurs, if this hardened alloy:
- kept for a long time at room temperature (natural aging) – Figure 7;
- kept at an elevated temperature (about 200 ºС) for several hours (artificial aging) – Figure 8.
Figure 7 – Temperature effect on the natural aging of the alloy 2024
(Rm – tensile strength, Rp0.2 – yield point (0.2%)) [3]
Figure 8 – Typical curves of artificial aging
at different temperatures for alloy 2024 [3]
Tensile strength
The figure 9 shows typical stress-strain curves for uniaxial tensile tests of four different aluminum alloys compared to:
- low carbon steel;
- high-strength steel and
- tytanovыm alloy.
Figure 9 Stress-strain curves of aluminum alloys
in comparison with other construction materials [2]
Aluminum represent:
- 99,5 %-ny aluminum (aluminum grade 1050A according to the international classification – analogue aluminum brand AD0 GOST 4784-97) in the annealed state; well suited for deep stamping;
- aluminum alloy Al-Mg system with 4,5 % magnesium – alloy 5083 (AMg4.5) in a semi-adjusted state (H12); used in marine and welded structures;
- Aluminium alloy 6082 (AD35) Al-Mg-Mn-Si systems, hardened and aged to T6 condition (for maximum strength); used in construction;
- Aluminium alloy 7075 (B95) Al-Zn-Mg-Cu systems in a state of maximum thermal hardening; used in aircraft construction.
(For numerical data on the tensile strength of many aluminum alloys, see. here).
The ratio of strength / weight
As seen from the figure 1, of all high-strength steel represented metals have the highest strength to weight ratio. This is followed by the titanium alloy Ti-6Al-4V and aircraft aluminum alloys, and little more - Aluminum alloy 5083-H12 and 6082-T6.
If we consider the strength, which is achieved per unit weight, dividing the strength by the density, then we get a completely different picture (figure 10). In this approach the most efficient structural material is aluminum alloy 7075, and alloys 5083-H12 and 6082-T6 appear to be more effective, than the low-carbon steel.
Figure 10 - strength per unit density of aluminum alloys, and
other construction materials [2]
Sources:
1. Materials Aluminum Association Germany
2. TALAT 1501
3. Corrosion Aluminium /Ch. dragline – ELSEVIER, 2004
