**5.3 Influence of temperature**

334 Recent Trends in Processing and Degradation of Aluminium Alloys

Moreover the analysis by means of fractured surfaces of AlSi7MgCu0.5 of the solid and hollow sample of three LCF strain levels shows assimilable fractured surfaces for each strain level. When decreasing the strain level to lower values, a crack propagation area can be seen beginning at the outside of the specimens. The finite element method shows differences that are of the size of less than one per cent from the maximum axial stress (Minichmayr, 2005).

When heat treated aluminium alloys are exposed to elevated or fluctuating higher temperatures in their service life, they show a temperature- and time-dependent aging behaviour which can much decrease the mechanical properties. To investigate these effects on low cycle and thermo-mechanical fatigue, LCF test series at room and higher temperatures, as well as LCF and TMF test series for pre-aged conditions were conducted. Moreover TMF test series with different dwell times at the maximum temperatures were

The first investigation is the separated effect of pre-aging (at an elevated constant temperature) on the deformation and lifetime behaviour by the means of quasi static tests, alternating LCF tests (strain ratio=-1) and temperature-controlled OP-TMF tests (temperature ratio=-1). Figure 4 (left) shows the hysteresis loops for two different total strain levels for non pre-aged and pre-aged specimens at 250°C for 500 hours. At the same LCF strain-level the pre-aged specimens show stress values that are about the half compared to non pre-aged specimens. When investigating the influence of pre-aging on the deformation behaviour by means of tensile tests and LCF tests, at the non pre-aged specimens a high stress hardening tendency can be seen as compared to the tensile test. Pre-aging at 250°C for 500 hours leads to a striking by smaller lifetime in the lower strained LCF region. The deformation behaviour of pre-aged specimens in the manner of stress-cycle or plastic straincycle plots shows a nearly straight line without any distinctive hardening or softening, but a

**5.2 Influence of pre-aging** 

conducted to additionally investigate creep effects.


25°C

25°C / pre-aged 500 h

**144 s, 0 h**

**Stress** σ **[N/mm2**

Fig. 4. Influence of pre-aging on the LCF hysteresis loops (on the left) and on the OP-TMF

At the same TMF temperature-level pre-aged specimens at 250°C for 500 hours show the analogous deformation behaviour tendency as obtained at the LCF results, namely a decrease of about 50 per cent compared to non pre-aged specimens, see figure 4 right. The influence of the dwell time decreases with increasing time and temperature of pre-aging.

**]**

1,E+00 1,E+01 1,E+02 1,E+03 1,E+04 **Number of cycles N [-]**

**8 s, 500 h**

σ**max**

σ**<sup>m</sup>**

**144 s, 500 h 8 s, 0 h**

 **AlCuBiPb - OP-TMF**

 **Tmax = 250°C, R = -1, tDmax = 8 / 144 s pre-aged: 0 h vs. 500 h at 250°C**

markedly higher plastic strain part.

stress-cycle behaviour (on the right)

 **AlCuBiPb - LCF T = 25°C - R = -1 pre-aged: 0 h vs. 500 h at 250 °C** ε **a,t= 3.75 / 10 ‰**


**Stress** σ **[N/mm 2**

**]**

A constant elevated temperature influences firstly the quasi static material behaviour and secondly has a time-dependent effect because of hardening vs. softening effects during service life. In this section the time-dependent influence of a constant elevated temperature on LCF is investigated by means of non pre-aged specimens.

A constant elevated temperature of 200°C leads to a higher damage of the material with differences in the lifetime of about one decade compared to the room temperature results. At 250°C the effect is even more drastically, as figure 5 (left) shows. Although at the high strained area the lifetime is a little higher than for 200°C, after the point of intersection at about 100 cycles there is a tremendous drop in the lifetime. Figure 5 (right) shows the summarized presentation of the influences of pre-aging, constant elevated temperature and applied mean strain on the LCF deformation behaviour by means of the plastic strain amplitude part. At a constant temperature of 200°C the stress softening phase starts after a few cycles, what can be seen in an increase of the plastic strain part.

Fig. 5. Influence of elevated temperature on the LCF lifetime (on the left) and influence of pre-aging, elevated temperature and mean strain on the LCF deformation behaviour (on the right)
