Issue 43

M. Tocci et alii, Frattura ed Integrità Strutturale, 43 (2018) 218-230; DOI: 10.3221/IGF-ESIS.43.17 228 Some intermetallic particles are visible on the damaged surface both in as-cast and heat-treated condition after 30 min of exposure, as well as after 8 h (Fig. 16). They likely belong to underlying areas of the alloy and are exposed after progressive material removal. In general, it is believed that their abundant presence hinders plastic deformation and material removal, at advanced stage of cavitation erosion, in agreement with the mass loss curves shown in Fig. 5. Nevertheless, for AlSi9CuFe alloy, this contribution is not significantly affecting the incubation period, which is comparable to the values measured for AlSi3 and AlSi9 alloy, contrary to what found in a previous study by the authors [20]. This is likely due to the elongated morphology of the particles in AlSi9CuFe alloy. In fact, according to the experimental evidences [20], coarse and polygonal intermetallic particles were responsible for longer incubation period, besides lower mass loss, in comparison with acicular compounds. In the present case, instead, intermetallic particles are very numerous, but their elongated shape can be responsible for stress concentration [39] and for their easier removal during erosion test in comparison with more rounded particles [40]. Furthermore, it is reported [41] that the direct method used in the present study accelerate the erosion mechanism in comparison with the indirect method used in the previous one. Therefore, this can lead to a general decrease in the incubation period and can make more difficult to identify differences between different alloys. Figure 16: Eroded surfaces after 8 h of cavitation test for a) AlSi9CuFe, b) AlSi9CuFe T6 alloys. C ONCLUSIONS he cavitation erosion resistance of three Al-Si alloys was studied in order to individuate the influence of microstructural parameters and material hardness. It was found that the increase in Si content, and therefore a larger eutectic phase, is beneficial to the alloy performance since this phase is less affected than the Al matrix by the cyclic load associated with bubbles collapse due to cavitation phenomenon. This especially leads to a decrease in total mass loss and maximum erosion rate. In addition, the strengthening of the Al matrix because of heat treatment delays material removal resulting in an increase in the incubation period, with consequent positive effect on the overall material performance. Finally, according to the findings, it is believed that intermetallic particles with complex morphology can provide an additional contribution to improve cavitation erosion resistance of the studied alloys, as demonstrated by the lowest total mass loss measured forAlSi9CuFe alloy. A general trend of decreasing total mass loss with increasing hardness was found, even if a linear correlation between these two parameters cannot be stated. R EFERENCES [1] Davis, J. R., Corrosion of Aluminum and Aluminum Alloys, ASM International, (1999). [2] Okada, T., Iwai, Y., Hattori, S., Tanimura, N., Relation between impact load and the damage produced by cavitation bubble collapse, Wear, 184 (1995) 231-239. [3] Vyas, B., Preece, C. M., Cavitation Erosion of Face Centered Cubic Metals, Metall. Trans. A, 8A (1977) 915-923. [4] Hansson, I., Morch, K. A., The initial stage of cavitation erosion on aluminium in water flow, J. Phys. D: Appl. Phys., 11 (1978) 147-154. [5] Preece, C. M., Macmillan, N. H., Erosion, Ann. Rev. Mater. Sci., 7 (1977) 95-121. T