Issue 43

M. Tocci et alii, Frattura ed Integrità Strutturale, 43 (2018) 218-230; DOI: 10.3221/IGF-ESIS.43.17 224 Alloys Incubation period (min) Total mass loss (mg) Maximum erosion rate (mg/h) AlSi3 8 27.5 ± 0.5 11 AlSi9 7 25.5 ± 0.7 8 AlSi9 T6 14 19.0 ± 1.0 6 AlSi9CuFe 8 21.0 ± 1.0 8 AlSi9CuFe T6 12 17.5 ± 1.0 4.5 Table 5: Incubation period, total mass loss and maximum erosion rate for the studied alloys. The evolution of the erosion rate during the test is illustrated in Fig. 7 as a function of testing time. According to ASTM G32 [35], it is possible to identify the five main stages of erosion, which are indicated as incubation, acceleration, maximum rate, deceleration and terminal stage. It is interesting to note that the first three phases take place in the first hour of testing for all the studied alloys, with a steep increase of erosion rate, which subsequently diminishes until it reaches quite constant values during the second half of the test. This indicates a tendency towards a linear erosion mechanism with a constant erosion rate, in the range of 1-3 mg/h, regardless the chemical composition and the hardness for the investigated Al-Si alloys. Figure 7: Evolution of erosion rate during cavitation tests for the studied alloys. Since several authors proposed a correlation between mass loss and material hardness for alloys belonging to the same family, the corresponding values for the studied alloys were plotted, as shown in Fig. 8. A fairly linear relationship between the two parameters can be observed, even though R 2 value is not optimal (0.95), confirming the general observation that a clear correlation cannot be stated [14-15]. In order to thoroughly study the cavitation erosion mechanism, SEM analysis of eroded surfaces was carried out after various exposure duration. This it is necessary to identify the influence of microstructural parameters and provide a complete interpretation of the erosion mechanism. Investigation of eroded surfaces: effect of Si content. First, the effect of different Si content was investigated through the observation of the eroded surfaces of AlSi3, AlSi9 and AlSi9 T6 samples after short time exposures. Particularly, representative images after 2 minutes of testing are shown in Fig. 9. It can be appreciated that AlSi3 alloy (Fig. 9a) exhibits the most deformed surface, with secondary phases emerging slightly from the surface due to the plastic deformation of the Al matrix, as confirmed by EDS analysis (Tab. 6). A similar behavior can be noticed for AlSi9 alloy (Fig. 9b), where the abundant eutectic phase does not exhibit evident modification, while the Al matrix is affected by undulation due to plastic deformation. This is a confirmation that the Al matrix in the primary site for erosion, as reported in previous studies by the authors [9, 10].