Issue 43

M. Tocci et alii, Frattura ed Integrità Strutturale, 43 (2018) 218-230; DOI: 10.3221/IGF-ESIS.43.17 223 and about 24% lower than the AlSi3. In this case, the main microstructural differences between the AlSi9 and AlSi9CuFe alloys are the presence of intermetallic particles and grain size. Concerning the maximum erosion rate of the three compositions, the increase of eutectic percentage, from AlSi3 to AlSi9, determines a reduction of about 27%, while the presence of Cu and Fe intermetallic does not considerably affect this parameter. As aforementioned, grain size is reported to have a significant influence on mechanical properties. In particular, regarding cavitation erosion, it is reported that fine grain structure leads to an increase in surface density of grain boundaries, which provide a positive supporting action in hindering the movement of dislocation and, therefore, preventing material removal [38]. The presence of coarse grains for AlSi9CuFe alloy reduces the surface density of grain boundaries and this is expected to reduce the material resistance. On the other hand, the presence of intermetallic particles with complex morphology is supposed to be beneficial since they cannot be easily dislodged from the metal matrix, reinforcing the primary phase and thus enhancing the alloy performances [20]. Hence, this suggests that intermetallic particles play a dominant role in comparison with that of grain size in defining cavitation erosion resistance. However, to confirm this assumption, the observation of eroded surfaces is a necessary step to provide a proper interpretation of the erosion mechanism. Figure 5: Mass loss values during cavitation tests for the alloys in as-cast condition. The effect of T6 heat treatment is presented in Fig. 6, where the results for each alloy in as-cast condition are compared with the corresponding performance after heat treatment. As expected [14, 19-20], the heat treatment provides a beneficial effect on cavitation resistance since the mass loss diminishes of approximately 25 % for AlSi9 and of 17 % for AlSi9CuFe alloy. Particularly, the incubation period (Tab. 5) is clearly affected by the heat treatment. In fact, the uniform distribution of fine precipitates in the Al matrix hinders material removal resulting in a longer duration of the incubation stage for the alloys in T6 condition. Finally, it can be clearly noticed how the T6 treatment affects the maximum erosion rate, inducing a reduction of about 25% and 44% for the AlSi9 and AlSi9CuFe respectively. Figure 6: Effect of heat treatment on cavitation erosion resistance for a) AlSi9 and b) AlSi9CuFe alloy.