Issue 43

M. Tocci et alii, Frattura ed Integrità Strutturale, 43 (2018) 218-230; DOI: 10.3221/IGF-ESIS.43.17 220 Dendrite Arm Spacing (SDAS) measurements were performed with the intercept method. Five images at 50x magnification were examined for each alloy and average values and standard deviation were calculated. In addition, grain size was evaluated for the three studied alloys. Samples for grain size measurements were electrochemically etched in a 2% fluoroboric acid (HBF 4 ) solution and observed under polarised light. Finally, the average area fraction of intermetallic particles for AlSi9CuFe alloy was calculated by means of LAS software for image analysis in order to thoroughly characterize the microstructure of the alloys. The effect of heat treatment was additionally studied for samples of AlSi9Mg and AlSi9CuFe alloys [14], in order to compare its contribution on material resistance and the influence of intermetallic particles. Samples of both the alloys were solution treated at 520 °C for 4.5 h, then quenched in water at 65°C and aged at 175 °C for 3 h according to industrial parameters for this family of alloys [34]. The average hardness of all the samples (as-cast and heat-treated) was measured by means of Brinell method with a Galileo Ergotest Comp25 apparatus (LTF Galileo Italy) applying a load of 613 N for 10 s and using a ball of 2.5 mm of diameter as an indenter. The measurement was repeated 5 times for each alloy and average values and standard deviation were calculated. Samples for cavitation tests were machined from the castings as cylinders with diameter of 18 mm and with a proper thread in order to be screwed to the sonotrode, as described in ASTM G32 standard according to the direct method [35]. The test equipment consists of a 20 kHz ultrasonic transducer to which is attached a suitably designed titanium waveguide (Ti6Al4V) and an Inconel 625 horn. During the experiments, specimen and horn were immersed in a tank filled with tap water, whose temperature was maintained at 25 ± 1 °C. A schematic representation of the testing apparatus is shown in Fig. 1. Tests were periodically interrupted and samples were removed in order to measure the mass loss as a function of the exposure time, according to the ASTM G32 standard. Figure 1 : Schematic representation of the experimental set up for cavitation tests. Three samples were tested for each examined condition in order to obtain reliable results. The total test duration was set at 8 h. Then, the collected data were processed in order to obtain incubation period, erosion rate and total mass loss. The incubation period is defined as the initial stage of the erosion process during which the mass loss is zero or negligible compared to later stages [35]. Furthermore, in order to investigate the evolution of the erosion mechanism, exposed surfaces were observed by means of Scanning Electron Microscope (SEM) LEO EVO 40 coupled with an Oxford Energy Dispersive Spectroscopy (EDS) probe for elemental analysis. In particular, in order to study the initial stages of the erosion process, and to estimate a correlation between cavitation-erosion resistance and alloy microstructure, the samples were observed after 2, 5 and 30 minutes of cavitation exposure. Finally, samples were also analyzed at the end of the test to thoroughly characterize the morphology of the eroded surfaces. R ESULTS AND DISCUSSION Microstructural characterization he microstructure of AlSi3 and AlSi9 alloys is shown in Fig. 2. The larger amount of eutectic phase (dark gray in the micrographs) for AlSi9 alloy (Fig. 2b) is due to the higher Si content than in AlSi3 alloy (Fig. 2a), as expected. In fact, according to lever rule, AlSi3 alloy contains approximately 13 wt. % of eutectic phase, while for AlSi9 it raises up to 67 wt. %. After heat treatment, the spheroidization of eutectic Si particles is visible (Fig. 2c). T