For an engineer about aluminum alloys

Density of aluminium

The most attractive for engineers physical properties of aluminum is its density 2,7 g / cm3, which is only one-third the density of steel.


Figure 1 – Strength per unit density of aluminum compared to other metals and alloys [2]

Aluminum Corrosion resistance

The second most important feature is its good corrosion resistance, although aluminum in terms of chemistry and not too noble metal. This is because, that the "fresh" aluminum (and aluminum alloys) reacts with oxygen and water vapor in the air to form a fine, thick oxide film, which protects the underlying metal from further interaction with the environment. Therefore the technical and most aluminum alloys without copper alloying show very good corrosion resistance in fluids with an acidic pH in the range of from 5 to 8, which correspond to the majority of the atmospheric environment.


Figure 2 – The influence of alloying elements on the corrosion resistance
(and fatigue strength) of aluminum alloys [2]

Thermal expansion of aluminum

Linear thermal expansion of aluminum and its alloys is 24 10-6 on 1 degree Celsius - twice more than that of steel. This should be considered in many designs, in which it is necessary to ensure the free thermal expansion of the elements. When thermal expansion (or contraction) is limited in an aluminum element, stresses arise due to the lower modulus of elasticity, which is the value 2/3 from stress, that would arise in a similar steel member.

Elasticity modulus of aluminum

aluminum Modulus - 70000 MPa, only a third of the modulus of elasticity of steel. This entails significant implications for the design geometry, since deflections of beams, bearing capacity of the columns, ie. their lateral buckling and local buckling are directly dependent on the modulus of elasticity.


Figure 3 – Strength and Young's modulus of some metals [2]


Figure 4 -Tensile Charts for Low Carbon Structural Steel (St52)
and aluminum alloy 6082-T6 [2]

Rigidity aluminum profiles

Many building structures critical parameter profiles is their rigidity. If the steel profiles to replace aluminum with preserving its stiffness, then thicken three times all the walls are not very economically, Since aluminum is lighter than steel just in the same three-fold. However, the relief structures by the use of aluminum - it is a natural desire, both physical, and for economic reasons.

When designing beams, there is a practical and tried-and-true rule of thumb: increase all dimensions except width by 1,4 times and get the cross-sectional moment of inertia with nearly three times more. Then, for a profile with the same stiffness (E I), you will save about 50 % weight. At the same time, to some extent compensated for the loss of rigidity against lateral buckling. Considering, that part of the standard steel profiles are not quite optimal, and you can save more than 50 % weight. This is evident from the figures 5 and 6. If there are no height restrictions, and lateral buckling is not a structural parameter, you can save up to 60 % weight. If the stiffener is not important, and the strength of the steel close to the strength of the aluminum alloy, the savings can be up to 70 %, but this is a final limit the possible weight savings.

sravnenie-zhestkosti-stalnogo-i-aljuminievyx-profilej Figure 5


Figure 6 – four beams, which have the same deflection [2]

This leads to the second important point. If the moment of inertia of the profile increases threefold with increasing profile height only 1,4 fold, the section modulus increase, respectively, in 3: 1.4 = 2.1 times. Therefore, the voltage in the aluminum beam as compared with the steel will be more than two times less. Now it is clear, why the designer do not just "grab" for the high-strength aluminum alloys, and why less alloyed aluminum alloys 6060 and 6063 (AD31) so popular.

aluminum heating

As with other aluminum metal strength decreases with increasing temperature. To a certain temperature, this phenomenon is reversible, i.e., after cooling, the material returns to the same properties, and that before heating. To a temperature of about 80 ° C drop strength can be neglected for all alloys and conditions. Above 80 ° With some design situations may require taking into account the effect of creep.


Figure 7 – The tensile strength of aluminum alloy 2014-T6
at various test temperatures [2]

Thermally strengthened alloys begin to lose strength at temperatures above 110 ° C, the degree of this effect depends on the heating duration.

alloys, not heat-hardened, in the cold-worked condition begin to lose strength at temperatures above 150 ° C, and also depending on the heating duration. After heating the thermally hardenable alloys in the annealed condition "O" irreversible loss of strength does not occur.

It is believed, that a short heating thermally hardened aluminum profiles to a temperature 180-200 ° C for 10-15 minutes, which occurs when "reflowing" Powder Coatings, It does not lead to a serious loss of strength.

Welding of Aluminum Alloys

Much more serious is the loss of strength aluminum alloys during welding. Here, the temperature rises so high because local melting, that the drop in strength near the weld it is necessary to take into account the. Thermally hardenable alloys lose all their strength, obtained by work hardening, and return to the annealed condition "O". Thermally hardenable aluminum alloys in the T6 condition lose about 40 % their strength (figure 8) excluding alloy 7020, which loses only 20 %. All these alloys do not reach the state of complete annealing, inevitable since some effect hardening under cooling seam. The requirements to the strength characteristics of the material in the weld zone is set and controlled by the results of the test samples.

snizhenie-prochnosti-aljuminievogo-splava-pri-svarke

Figure 8 – Effect of heating at welding strength
thermally hardened aluminum alloy (6082-T6) [2]

Sources:

  1. R. Gitter Selection of structural alloys, Brussels 2008
  2. TALAT 2204 – Design Philosophy