Metal additive manufacturing (AM) is an innovative but already established material processing technology for the production of parts and prototypes based on layer-by-layer build-up [1
]. This technique allows an unrivalled design freedom, not reachable via conventional manufacturing routes, combined with a high quality and outstanding mechanical properties [1
]. Nowadays, metal AM is regularly applied with different engineering materials like stainless steel [6
], Ti, Co, and Ni alloys [9
], and, lately, also Al alloys are used [1
Regarding AM processing of Al alloys, attention has been mainly concentrated on the Al-Si system due to the good compromise between its mechanical properties, corrosion resistance, and processability [15
]. Nevertheless, some studies focused also on an innovative Al-Mg alloy modified with Sc and Zr for additive manufacturing, commercially known as Scalmalloy®
. This alloy showed outstanding strength and ductility, together with very low anisotropy [16
]. In this regard, the alloy is characterized by an overall fine-grained structure, which limits the influence of building direction. The formation of this fine grain structure is due to various factors, as for instance the presence of Sc and Zr. These elements allow the formation of primary Al3
(Sc, Zr) particles that act as nucleants for the Al matrix solidification [17
], which is also favored by the similar crystal lattice constants of Al and Al3
(Sc, Zr) phase. This, together with the high cooling rates typical for the process, leads to the formation of fine and equiaxed grains during the first stages of solidification. Some studies [17
] also report the formation of slightly bigger columnar grains that nucleate on the fine-grained regions and grow radially towards the melt pool boundary. However, other authors [20
] identify only a non-uniform distribution of Al3
(Sc, Zr) particles inside the melting pools, with segregation of these particles along the bottom boundary (boundary between layers), while few particles are observed in the center, without referring to areas with different grain structure.
In general, after solidification, Al3
(Sc, Zr) particles are located at grain boundaries and, therefore, hinder grain growth due to repeated heating of the material during the building process [21
] and eventual post-treatment (annealing or hot isostatic pressing) [16
]. This enhances a stable fine-grained structure of the material. Finally, annealing treatment in the range of 325–350 °C has been demonstrated to improve mechanical properties [16
]. This is due to the further Al3
(Sc, Zr) particle formation inside the Al-matrix: in this case, the precipitates have a smaller size than the primary particles and are coherent with the matrix [16
], contributing to increasing material hardness. From a microstructural point of view, oxides and intermetallic phases containing Fe and Mn can be present, especially at grain boundaries. Among these, Mg-oxides, coming from the initial powder or forming during the solidification of melting pools, are often identified and are believed to behave as nucleants for the formation of primary Al3
(Sc, Zr) from the liquid. Such microstructural features are clearly responsible for the mechanical properties of the alloy, which are also affected by other factors, such as laser scan speed [21
], laser power [19
], or platform temperature [22
Besides tensile, hardness, or corrosion properties, wear resistance is an important parameter to evaluate possible new applications of Scalmalloy®
. In general, wear resistance of AM Al-based alloys has not been widely investigated in the scientific literature and studies are mainly available for the sliding wear mechanism of Al-Si alloys [23
]. In this case, the effect of heat treatment and size of Si particles was studied [24
]. It was found that the material in as-built condition exhibits the lowest wear rate as compared to annealed AM samples and cast samples. This is due to the higher hardness of the as-built material caused by the fine network of Si particles uniformly distributed in the Al matrix. Similarly, also Kang et al. investigated the wear behavior of AlSi12 [25
] and AlSi50 [26
] alloys as a function of building parameters. It was confirmed that Si particle size plays a key role in wear resistance and that optimized values of laser power have to be applied in order to obtain nano-sized Si particles, beneficial for material hardness and wear resistance. Furthermore, density and, therefore, porosity levels were found to strongly affect material performance. In addition, AlSi10Mg-TiC [27
], AlSi10Mg-AlN [29
], and Al-12Si-TNM composites [30
] produced by additive manufacturing technologies exhibited remarkable wear resistance, once the processing parameters were optimized. Nevertheless, to date, only one study reported on the tribological behavior of AlMgSc alloy [20
], produced by selective laser melting (SLM), as a function of scan speed. It was found that this parameter significantly affects the wear rate in dry condition since relatively low scan speed was responsible for good wear properties, while these decrease with increasing scan speed. Accordingly, also the wear mechanism changed, with a prevalent abrasion wear behavior at high scan speed. However, the effect of the annealing treatment was not considered, which is widely applied to AM components as well as suggested by material producers [31
] and can affect the wear resistance of the material.
Furthermore, another important wear phenomenon scarcely investigated for AM alloys is cavitation erosion. This damaging mechanism is critical for several components in the hydraulic (valves, pumps, impellers, etc.) or automotive field (pistons, cylinders, combustion chambers, etc.) [32
], which are all in contact with fluids. Lately, some studies [34
] investigated the cavitation resistance of AlSi10Mg alloy fabricated by SLM, also in comparison with the corresponding casting or wrought alloy. Experimental tests revealed an extremely high cavitation erosion resistance of the AM alloy, especially in as-built condition due to the fine microstructure and high hardness. Given the already recognized performance of the AlMgSc alloy, it appears reasonable to investigate its cavitation erosion resistance in order to provide a wider characterization of its properties in order to evaluate new applications.
Therefore, in this study, sliding wear and cavitation erosion resistance of AlMgSc alloy were investigated. Samples were tested after annealing treatment in order to reproduce the actual use condition of the material. The aim is to identify the damaging mechanisms in order to evaluate possible applications where the AlMgSc alloy can experience this type of damage.
2. Materials and Methods
The mean chemical composition of the Scalmalloy®
powder, used to produce samples for the present study, is reported in Table 1
Morphological and dimensional characterization of the powder used for the AM process was carried out by scanning electron microscopy (SEM) (LEO EVO 40, Carl Zeiss AG, Milan, Italy).
Plates with a thickness of 2 mm along the y
-axis and an area of 21 cm2
in the x
plane were produced by direct metal laser sintering (DMLS) (EOS M 290 machine with Yb-fiber laser, maximum power 400 W, 20 m3
/h inert gas supply, F-theta lens, focus diameter of 100 μm, EOS GmbH Electro Optical System; EOS srl, Krailling, Germany), as shown in Figure 1
. The following parameters were used: laser scan speed of 1300 mm/s, laser power of 370 W, hatch spacing of 90 μm, layer thickness of 30 μm. Before testing, samples were heat treated (annealing) at 325 °C for 4 h [16
Microstructural characterization and microhardness measurements were performed on samples in as-built and annealed conditions, while sliding wear and cavitation erosion tests were carried out only on samples in annealed condition, since annealing is widely applied to AM components, as also suggested by the material producers [31
Samples were analyzed by means of optical (Leica DMI 5000M, Wetzlar, Germany) and scanning electron microscopy (LEO EVO 40, Carl Zeiss AG, Milan, Italy), coupled with EDS (energy dispersive spectroscopy, Oxford Instruments, Wiesbaden, Germany) microprobe, after sample polishing up to mirror finishing. In order to observe melting pools, samples were etched with a 10% phosphoric acid (H3PO4) solution. Vickers microhardness measurements before and after annealing treatment were performed with a Mitutoyo HM-200 hardness testing machine (Mitutoyo Italiana srl, Lainate, Italy) with an applied load of 200 g and a loading time of 15 s. The results are presented as average values of at least 20 measurements per sample, to guarantee reliable statistics.
Pin-on-disk tests were performed in dry condition according to ASTM G99 standard [36
] using a THT tribometer (CSM Instruments, Peseux, Switzerland). Samples for wear test (x
surface, see Figure 1
) were polished up to mirror finishing in order to reach a roughness Ra
lower than 0.8 µm, according to ASTM G-99 standard requirements. The counterpart was a 100Cr6 steel ball with a 6 mm diameter. A linear speed of 4 cm/s and a load of 1 N were applied for a total distance of 100 m. The diameter of wear tracks was 3 mm. During the tests, the friction force was monitored, and the friction coefficient was subsequently obtained. The test was repeated twice. The second test was periodically interrupted to observe the evolution of worn surface by SEM. Additionally, a stylus profilometer (Mitutoyo SJ301, Mitutoyo Italiana srl, Lainate, Italy) was used to record the track profile after each interruption. Five measurements were carried out in different positions for each track and the mean value and standard deviation were then calculated. The same track profile measurements were performed at the end of the test. Then, the worn volume was calculated multiplying the worn area and the track length. The wear rate was determined by the equation reported in [37
]. Finally, this second test was prolonged up to 500 m in order to compare the obtained wear rate with data available in the scientific literature for AlSi10Mg alloy produced by AM technology.
Regarding cavitation tests, the x
surface (Figure 1
) was exposed to cavitation after mirror polishing, as done for sliding wear experiments. Cavitation tests were carried out according to ASTM G32 [38
], following the stationary specimen approach. An ultrasonic device with a vibration frequency of 20.0 kHz, a vibration amplitude of 50 μm, and an electrical peak power of 2 kW was used in the present study. The ultrasonic probe was composed of a Ti6Al4V waveguide and a final amplification horn realized in Inconel 625 [35
]. The specimen was inserted in a properly designed holding system and immersed in a tank containing water, at a distance of 0.50 mm from the horn surface. The test was periodically stopped in order to measure the weight loss and observe the morphology of the eroded surface by means of scanning electron microscopy. The total duration of the test was set at 8 h, according to previous works by the authors [35
]. Results are presented in terms of cumulative mass loss and mass loss rate as a function of exposure time.