**5.2 Influence of pre-aging**

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 conducted to additionally investigate creep effects.

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 markedly higher plastic strain part.

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

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. The differences in lifetime for the dwell time of 8 s and 144 s decreases from a factor more than 6 to a factor of 2, when the specimens are pre-aged for 500 h at 250°C before being tested. After extensive pre-aging the influence of dwell time completely disappears in face of the cyclic deformation and the lifetime behaviour (Riedler & Eichlseder, 2004; Riedler et al., 2005)
