PSI - Issue 7

C. Garb et al. / Procedia Structural Integrity 7 (2017) 497–504

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C. Garb et Al. / Structural Integrity Procedia 00 (2017) 000–000

specimens are shown in Fig. 9 and Fig. 10. In Fig. 9 the different planes are distinguishable and in Fig. 10 the different angles of the planes are recognizable.

Fig. 9 SEM picture of slip bands, different plane levels

Fig. 10 SEM picture of slip bands, different plane angles

Table 4 Results from the fractographic analyse of the micropore sizes of the specimens for room temperature (RT) and 150 °C Specification Fractography RT (µm) Fractography 150 °C (µm) ch AlSi8 Sr T5 Pos. 1 120±67 124±23 cc AlSi8 Sr T6 Pos. 1 85±24 116±37 cc AlSi8 Sr T6 Pos. 2 537±197 495±177 In general, the micropore sizes of ch AlSi8 Sr T5 Pos. 1 and cc AlSi8 Sr T6 Pos. 1 are distinctively smaller than the micropore sizes of cc AlSi8 Sr T6 Pos. 2. Comparing the results from room temperature to the ones tested at 150 °C, the micropore sizes from ch AlSi8 Sr T5 Pos. 1 and cc AlSi8 Sr T6 Pos. 1 slightly increase and the sizes form cc AlSi8 Sr T6 Pos. 2 decrease, but it has to be considered that the scatter band of cc AlSi8 Sr T6 Pos. 2 is significantly enhanced, hence for this specification the decrease is not that distinguished. Regarding Fig. 4, the drop of the fatigue strength is most significant for the specification ch AlSi8 Sr T5 Pos. 1 with the highest amount of slip band induced failures at 150 °C. The cc AlSi8 Sr T6 Pos. 1 reveals also some slip band induced failures and shows a drop in the fatigue strength at 150 °C. The specification cc AlSi8 Sr T6 Pos. 2 exhibits a high porosity with large micropore sizes and stays constant in fatigue strength for 150 °C comparing to the results at room temperature. The failure mechanism caused by defects like micropores appears to be independent of temperature, as long as the defects are large enough. It seems that there exists a lower boundary for defect sizes, at which level a failure mechanism change occurs. At room temperature, no tested specification indicates a failure mechanism change, so the detected defect sizes are located above the lower boundary of failure mechanism change. At 150 °C, the boundary for the failure mechanism change increases in larger defect sizes and a part of the tested specimens of ch AlSi8 Sr T5 Pos. 1 and cc AlSi8 Sr T6 Pos. 1 reaches the lower boundary of the defect sizes and the failure mechanism changes. In general, a slip band induced failure mechanism at room temperature is rare. In absence of micropores in many cases other inhomogenities, like oxides or non-metallic phases, act as crack origin and causes fatigue failure. 5. Conclusions In this investigation, fatigue strength and quasi-static tests are executed and results of aluminium alloy AlSi8Cu3 T5 and T6 castings are compared for room and an elevated temperature of 150°C. The tested specimens are extracted from cylinder heads (AlSi8Cu3 T5) and crankcases (AlSi8Cu3 T6). Every tested sample is fractographically analysed to characterise the failure mechanism and, in case of a defect induced failure, to determine the defect size. • The specifications ch AlSi8 Sr T5 Pos. 1 and cc AlSi8 Sr T6 Pos. 1 exhibit a drop in fatigue strength at 150 °C