PSI - Issue 33

Maria Beatrice Abrami et al. / Procedia Structural Integrity 33 (2021) 878–886 / Structural Integrity Procedia 00 (2019) 000–000

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AlSiMgZr sheets with dimensions of 70 x 30 x 2 mm were manufactured via laser powder bed fusion with the following processing parameters: laser scan speed of 130 cm/s, laser power of 370 W, hatch spacing of 90 µm and layer thickness of 30 µm. After production, samples were annealed at 325 °C for 4 h before testing. According to ASTM G99 standard, pin on disk tests were carried out in dry condition using a THT tribometer (CSM Instruments). The counterpart was a 100Cr6 steel ball with a diameter of 6 mm. Before testing, samples were polished up to mirror finishing in order to reach roughness Ra lower than 0.8 µm. Pin on disk tests allow studying the sliding wear process. A load of 1 N was applied with a linear speed of 0.04 m/s. The wear track diameter was 6 mm. The friction force was detected during the test, allowing the calculation of the coefficient of friction (COF). Tests were performed at four different temperatures (25, 100, 150 and 200 °C) and each test was repeated twice. Every test was stopped after 100 m of sliding distance and concluded after 500 m, to observe the evolution of the worn surface and the damaging mechanism and to determine wear rate in both cases. In particular, the analysis of the worn track was performed by scanning electron microscopy (SEM, Zeiss-LEO EVO 40 equipped with EDS system), while the wear rate was calculated with the following equation: ܹ݁ܽ ݎ ݎ ܽ ݐ ݁ ൌ ௐ௢௥௡ ௩௢௟௨௠௘ ஺௣௣௟௜௘ௗ ௟௢௔ௗ ൈௌ௟௜ௗ௜௡௚ ௗ௜௦௧௔௡௖௘ (1) Worn volume was determined by multiplying the worn area and the track length. In turn, the worn area was obtained with profilometric measurements that allowed recording the cross section of the track. For each wear track, five measurements were performed in different positions and the mean value of wear area together with its standard deviation were calculated. At the end of pin on disk tests, Vickers microhardness measurements were performed on samples with a Mitutoyo HM-200 hardness-testing machine, using an applied load of 200 g and a loading time of 15 s in order to identify any possible change of hardness due to the soaking time at high temperature. In fact, the total exposure time in temperature was of about seven hours, as, in addition to the soaking at test temperature, a preliminary warm-up and a final cooling were also necessary. The mean value and the standard deviation were calculated on the base of 20 hardness measurements. Lastly, samples were etched with a 10% phosphoric acid (H 3 PO 4 ) solution to reveal their microstructure that was analyzed by using an optical microscope (Leica DMI 5000M). 1.1. Wear behavior Coefficient of friction plotted as a function of the sliding distance for every investigated temperature is reported in Fig. 1. In the initial part of the curves, a relatively high COF value (around 0.6) was recorded for every testing temperature. This early stage of sliding is called “running in stage”, during which the polishing of surfaces takes place, removing asperities, dirt, and contaminants (Straffelini 2015). The surfaces wear out until the contact between the two is optimized by removing the asperity peaks. After this early phase of sliding, a transition in the COF can be detected for every test temperature, characterized by a decrease in COF values. This is due to the progressive deformation and fragmentation of the asperities occurring in the previous stage. As can be seen from Fig. 1, the transition stage is longer as the temperature increases. After that, the COF steady value is reached for every test, meaning that the steady conditions are achieved for each temperature investigated after 500 m. Moreover, COF steady value increases with the test temperature (approximately from 0.6 to 0.9); however, a stabilization is detected after 150 °C. In order to gain further information on the changes in the COF curves and on the wear mechanisms, SEM images have been analyzed in the next paragraphs. 3. Results and discussion