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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1. Introduction Laser powder bed fusion (L-PBF) is an additive manufacturing (AM) process consisting in manufacturing components by the selective melting of a powder bed through the action of a laser and by adding material layer by layer (Yang et al. 2017). Currently, this technique is the most common one used for metal additive manufacturing. Thanks to L-PBF, in fact, a high degree of freedom in design is reached, together with almost fully dense metal components and high mechanical properties. Due to its advantages, L-PBF process is widely used to manufacture components in all the fields where the light-weighting is one of the key themes, for example for vehicle or aerospace parts and medical implants (Khairallah et al. 2016, Herzog et al. 2016, Yeong et al. 2004). In this regard, one of the current challenges in the automotive field is the production of innovative light alloys with high specific properties, which must be also able to guarantee reliability and safety. In recent years, Scalmalloy®, an AlMg alloy modified with scandium and zirconium, has been considered an interesting material in this field. In fact, this innovative alloy exhibits high specific properties, which make it of great interest for high performance applications. In particular, it was found that after heat treatment at 325 °C for 4 h, yield stress reaches about 480 MPa and tensile strength about 520 MPa, together with a ductile behavior (Kuo et al. 2021, Spierings et al. 2017). Therefore, there is a 48% increase in the specific mechanical properties of AlMgScZr alloy compared to the most widely used AlSi ones. These outstanding properties are due to the peculiar microstructure of this alloy. In particular, it exhibits a stable fine-grained structure, together with fine particles that contribute to material hardening (Spierings et al. 2017). These particles form in the aluminum matrix during solidification due to the presence of Sc and Zr (Spierings et al. 2017, Li et al. 2017) and are identified as Al 3 (Sc,Zr) compounds. During the heat treatment, further precipitates form with a smaller size (from 12 to 20 nm), and therefore contributes to the increase in material properties (Kuo et al. 2021, Spierings et al. 2017). In this regard, Kuo et al. (2017) correlated the strengthening effects to the ratio of volume fraction to size, which is very useful when the precipitates spacing is difficult to measure. Moreover, these precipitates, together with the fine-grained structure, determine a low anisotropic behavior of this alloy (Spierings et al. 2017). This is relevant since anisotropy represents a strong limitation for other alloys produced by AM processes. Beside the tensile properties, also the wear behavior of Scalmalloy® was investigated in a few studies. Zhang et al. (2018) performed ball-on-disk tests on AlMgScZr alloy under as-built condition in order to study the coefficient of friction (COF) and the wear rate as the laser scan speed varies during manufacturing. They obtained a COF of 0.61 and a wear rate of 1.74 × 10 -4 mm 3 N -1 m -1 when the scan speed was relatively low, which denotes good wear resistance of the alloy. In order to identify the wear damaging mechanism of annealed Scalmalloy® and its evolution, Tocci et al. (2019) performed pin-on-disk tests with periodical interruptions. They observed an initial stage of sliding wear followed by a tribo-oxidative mechanism as testing distance increases. Moreover, analysis on COF and wear rate values demonstrated significant wear resistance for the material. In addition, they also evaluated cavitation erosion resistance, pointing out good behavior of the alloy in terms of mass loss, but not as high as AlSi10Mg. However, the high temperature wear behaviour of AlMgScZr has not been considered yet, even if it can be very useful for the evaluation of new possible applications, particularly in the automotive field. Therefore, in the present study, pin on disk tests were performed at four different temperatures on AlMgScZr alloy produced via L-PBF and annealed at 325 °C for 4 h, with the aim of evaluating its behaviour and stability in temperature. Material and methods In this study, AlMgScZr (Scalmalloy®) powder with the chemical composition reported in Table 1 was used to produce the samples. Table 1. Chemical composition of the studied alloy (wt%). Mg Sc Zr Al 4.5 0.7 0.3 Bal. 2.