Aluminium properties

Aluminium alloys at elevated temperatures

Aluminum reaches maximum strength between -100 and +100 degrees. Its improvement at low temperatures is estimated at 10%, and at high temperatures it drops rapidly. Aluminum alloys have sufficient strength up to 300°C.

Low, normal and high temperatures

Most alloys have a plateau of strength between roughly –100°C and 100°C. Below this range an aluminium alloy has higher strength and above it has lower strength. Ultimate strength of most aluminium alloys increases 30 to 50% below this range, while the yield strength increase at low temperatures is not so dramatic, being on the order of 10%. Both ultimate and yield strengths drop rapidly above 100 °C, dropping to nearly zero at 400°C. Some alloys (such as 2219) retain useful (albeit lower) strengths as high as 300 °C. Figure 1 shows the effect of temperature on strength for various wrought aluminium alloys [1].

Figure 1 – Typical tensile strengths of some aluminum alloys at various temperatures.
Adapted from [1]

 

Elevated temperatures

Tensile Strength

The strength of aluminium alloys decreases with the increase in temperature excluding the effects of age-hardening within narrow temperature ranges for various holding periods. The time of exposure is important in the case of cold worked or heat-treated alloys (Figure 1501.05.09) but has little or no effect on the properties of annealed alloys. The heating time at test temperature is often quoted as 10,000 hrs, but with the
time-temperature dependence of strength it may be necessary for other exposure times to be considered [2].

Figure 2 – Tensile Strength of 2014-T6 at Test Temperatures [2]

Other strengths

Shear, compression, bearing and fatigue strengths vary with temperature in much the same way as tensile strength; ratios of these strengths to tensile strength may be taken as constant.

Heat-hardened alloys

Heating heat-hardened alloys also has an effect on strength. Heating for a long enough period of time reduces the condition of the material to the annealed state, which is the weakest temper for the material. The higher the temperature, the briefer the period of time required produce annealing. The length of time of high temperature exposure causing no more than a 5% reduction in strength is given in Figure 3 for 6061-T6.

Figure 3 – Maximum Time at Elevated Temperatures, 6061-T6 [1]

Exposure to elevated temperatures

The reduction in strength caused by exposure to elevated temperatures can only be regained by heat treatment or cold work or a combination of these processes which is usually impractical in the case of fabricated items. The tensile strength of an AlCu4MgSi alloy, tested at room temperature after exposure at elevated temperature, is shown in Figure 4. After either short term exposure at high temperature or long term exposure at medium temperature the material approaches a super soft annealed condition and the lower limit strength becomes constant.

Figure 4 – Tensile Strength of 2014-T6 Tested at Room
Temperature after Exposure at Elevated Temperature [2]

Modulus of elasticity

The modulus of elasticity of aluminium alloys also decreases as the operating temperature increases but unlike strengths which stabilise at a lower annealed value, the modulus of elasticity returns to its room temperature value after exposure (Figure 5 and Figure 6).

Figure 5 – Modulus of Elasticity of Aluminium
at Various Temperatures [2]

Figure 6 – Relative Modulus of Elasticity for Aluminium Alloys
at High Temperatures [3]

Eurocode 9

For thermal exposure up to 2 hours, the 0,2 % proof strength at elevated temperature of the aluminum alloys listed in tables of Figures 7 and 8, follows from:

Figure 7 – 0,2% proof strength reduction factor for aluminium alloys at elevated temperature
for up to 2 hours thermal exposure period (Table 1a from [4])

Figure 8 – Lower limits of the 0,2% proof strength reduction factor
for wrought aluminium alloys from EN 1999-1-1,
which are not included in the table of figure 7
(such as, 3005, 3103, 5049, 5754, 6005A, 6106, 7020, 8011A),
at elevated temperature for up to 2 hours thermal exposure period (Table 1b from [4])

Figure 9 – Modulus of elasticity of aluminium alloys at elevated temperature
for a two hour thermal exposure period (Table 2 from [4])

Figure 10 – 0,2% proof strength and E ratios
for aluminium alloys at elevated temperature
for up to 2 hours thermal exposure period
(EN-AW 3004 and 6xxx-alloys [4])

Figure 11 – 0,2% proof strength and E ratios
for aluminium alloys at elevated temperature
for up to 2 hours thermal exposure period, 5xxx alloys [4]

Creep

Under a constant stress, the deformation of an aluminum part may increase over time, behavior known as creep. Creep effects increase as the temperature increases. At room temperature, very little creep occurs unless stresses are near the tensile strength. Creep is usually not a factor unless stresses are sustained at temperatures over about 95 °C.

For more information on aluminum creep, see TALAT 1253.

High temperature aluminum alloys

Conventional aluminium alloys

Conventional high temperature аluminum alloys like 4032, 2618, 2014 or 6082 are in use for high performance pistons, aerospace airframe and other components. Above 300°C these alloys lose signifcant strength.

Powder aluminium alloys

To overcome the limitation new Aluminum alloys made via powder processing have been developed. Some of these advanced high temperature aluminum alloys could replace titanium or even steel [5]

These alloy show exceptional thermal stability even after hundreds of hours at service temperature and outperform conventional high temperature aluminum alloys like 2618, significantly (Figure 13).

Most of these new high temperature aluminum alloys require new processing routes like advanced powder processing.

Figure 12 – Chemical composition of aluminium alloy Al-MS89 [5]

Figure 13 – Mechanical properties of aluminium alloy Al-MS89 at elevated temperatures
compared with conventional aluminium alloy 2618
(Exposure time – 1000 hours) [5]

Sources:

  1. Aluminum and Its Alloys / J. Randolph Kissell
  2. TALAT 1501
  3. TALAT 2502
  4. EN 1999-1-2
  5. https://www.amt-advanced-materials-technology.com/materials/aluminum-high-temperature/