Issue 43

M. Tocci et alii, Frattura ed Integrità Strutturale, 43 (2018) 218-230; DOI: 10.3221/IGF-ESIS.43.17 219 liquid micro-jets that are directed toward the surface, leading to its progressive roughening and plastic deformation [3-4]. The repetition of this loading condition can cause material removal and, finally, the failure of the component itself [5]. Typical components that can be exposed to cavitation erosion are hydrofoils, pipelines, hydraulic pumps, and valves [1]. Various studies are available in scientific literature regarding the characterization of cavitation erosion resistance of different alloys, as such as steels [6-7], cast irons [8], titanium and nickel alloys [3, 9-10]. Furthermore, numerous attempts to develop cavitation erosion models based on bulk mechanical properties [11-13] were performed in order to predict the erosion performance of metallic materials. At this regard, some authors report the hardness as an indicator of the erosion resistance. Nevertheless, these models reached limited success in prediction erosion damage, as well as the attempt to correlate cavitation resistance and hardness [14-15]. Damage due to cavitation can occur also on various aluminum automotive components, as such as cylinders and pistons, combustion chambers, etc. [1, 16]. For this reason, over the years, several studies have been performed in order to investigate the cavitation erosion resistance also of aluminum alloys and to correlate it to microstructural and mechanical properties. Concerning the family of wrought alloys, various alloys belonging to the Al-Cu, Al-Mg, Al-Si-Mg or Al-Zn-Mg families were tested in different conditions [14, 17] taking also into account the influence of real erosion environments, as for instance the presence of aggressive fluids or slurries [8, 18]. For casting alloys, in general, it was found that Al alloys resistance is strongly affected by several microstructural properties, as such as grain size, number of interfaces between different phases, presence and morphology of secondary phases [14-15, 19-20]. This is not surprising since various studies report the influence of eutectic [21-22] and intermetallic phase [23-25], as well as the role of grain size [26-27], on other mechanical properties. It should also be mentioned that casting manufacturing produces defects like porosities, non-metallic inclusions, and segregations [28] that could represent preferential site for erosion damage. In order to enhance microstructural and mechanical properties of casting alloys, the effect of Al-Si composites reinforced with particles or fibers of silicon carbide and alumina was also evaluated [29-30]. Furthermore, it is well known that a proper choice of heat treatment parameters can increase several mechanical properties [31-33]. For this reason, the influence of heat treatment on cavitation resistance was investigated and various studies demonstrated the beneficial contribution of age hardening on reducing material damage [14, 19-20]. Considering the erosion mechanism, under continuous exposure to cavitation, it is reported in scientific literature that the initial undulations of the exposed surface gradually develop into craters and material is lost by necking of the rims of the craters, flaking and dislodging of secondary phases [1, 3, 14, 19]. Hence, it appears evident a dependency of cavitation- erosion behavior on the alloy microstructure, not simply related to the heat treatment effect. Nevertheless, a complete characterization of the erosion mechanism of Al casting alloys as a function of eutectic and intermetallic phases is not available in scientific literature. Therefore, in the present study, the cavitation resistance of different Al-Si alloys was evaluated in order to individuate the effect of Si and intermetallic particles on erosion resistance. M ATERIALS AND METHODS he cavitation erosion resistance of three Al-Si-Mg alloys with different contents of alloying elements was evaluated. In particular, the performance of AlSi3Mg and AlSi9Mg alloys, with 3 and 9 wt. % Si respectively, was studied in order to investigate the effect of Si content, i.e. the eutectic content, on erosion resistance. On the other hand, the addition of Cu and Fe to AlSi9Mg alloy allowed the authors to examine also the influence of intermetallic particles. The mean chemical composition of the tested alloys, measured by an optical emission spectrometer, is shown in Tab. 1. Si Mg Cu Fe Mn Ti Al AlSi3Mg 3.39 0.64 -- 0.09 -- 0.09 Balance AlSi9Mg 8.80 0.29 -- 0.08 -- 0.10 Balance AlSi9CuFe 8.80 0.30 0.90 0.53 0.3 0.10 Balance Table 1 : Main alloying elements (wt. %) for the studied alloys. Samples of the alloys were gravity cast in a permanent mould as cylinders, with diameter of 26 mm and height of 60 mm. In all cases, the mould was pre-heated at 180°C and each alloy was poured 50°C above their specific liquidus temperature. The microstructure of samples was observed by means of an optical microscope Leica DMI 5000 M., equipped with LAS image analyser; the samples were polished up to mirror finishing. Regarding microstructural characterization, Secondary T