AlMgScZr alloys fabricated using selective laser melting (SLM) technology have promising applications in the designing and manufacturing of aircrafts due to their superior mechanical properties. However, during the service of the aircraft, the aluminum-alloy structure encounters severe corrosion issues, particularly in coastal and wet environments. Corrosion in aircraft structures poses a significant risk to flight safety; therefore, the materials used in an aircraft must undergo a stringent review to determine if their performance meets the requirements of airworthiness regulations. Presently, domestic and international researchers have focused primarily on the mechanical properties of SLM-fabricated AlMgScZr alloys, with less attention on corrosion resistance. In the previous study, we successfully prepared an Al-4.8Mg-0.82Sc-0.28Zr alloy using SLM; the ultimate tensile strength and elongation attained 344.20 MPa and 24.5%, respectively. Considering the severity of corrosion, we investigated the electrochemical corrosion (the most common type of corrosion observed in aircrafts) properties of SLM-fabricated AlMgScZr alloys and discussed the effect of the microstructure of these alloys on their electrochemical corrosion performance. We anticipate that our research will serve as a resource for the industrial application of the AlMgScZr alloys formed using SLM.
The AlMgScZr alloy was prepared using SLM at different scanning speeds (800, 1000, 1200, 1400, and 1600 mm/s). Initially, the relative densities of various specimens were measured, and the types and distributions of defects in the alloys were characterized via optical microscopy (OM) and scanning electron microscopy (SEM). Subsequently, the microscopic morphology, precipitation phases, and grain information of the alloy were studied using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD), and the effect of scanning speed on the microstructure of the alloy was analyzed. The electrochemical corrosion performance of the specimens was then evaluated using an electrochemical workstation. Finally, the microstructure of the alloys and the effects of pores, precipitated phases, grain size, and grain boundary misorientation angle distribution on their electrochemical corrosion performance were examined.
The microstructure of the SLM-fabricated AlMgScZr alloy consists of fine equiaxed grains at the melt-pool boundaries and coarse columnar grains in the center of the melt pool (Fig. 4). Additionally, at the boundary of the melt pool, there are Al3(Sc,Zr) particles with high concentration, which can serve as nucleation sites for the Al matrix and are responsible for the formation of equiaxed grains. Only a small amount of oxide particles exist in the center of the melt pool, and the Al matrix grows perpendicular to the contour line of the melt pool after nucleation on the oxide particles due to the temperature gradient. As the scanning speed increases, the temperature of the melt pool decreases, increasing the area where Al3(Sc, Zr) particles can exist and reducing the stress in the alloy during solidification, resulting in a gradual decrease in grain size and an increase in low-angle grain boundaries (Figs. 5 and 6). The electrochemical test results indicate that the SLM-fabricated AlMgScZr alloy undergoes significant passivation in NaCl solution with mass fraction of 3.5%. As the scanning speed increases, the electrochemical corrosion properties of the alloy initially increase and then decrease. When the scanning speed is 1200 mm/s, the corrosion current density of the specimen is the lowest (14.48 μA·cm-2), and the corrosion potential (-1.311 V), pitting potential (-0.645 V), polarization resistance (4165.80 Ω·cm2), and passivation-film resistance (66.99 kΩ·cm2) are the highest, resulting in the highest electrochemical corrosion resistance. The corrosion morphology observations of the specimens reveals that severe corrosion occurs at the boundary of the melt pool and the corrosion area and corrosion depth of the specimens formed at the scanning speeds of 1000 mm/s and 1400 mm/s are significantly larger than those formed at the scanning speed of 1200 mm/s. These observations indicate that the corrosion of the specimens formed at the scanning speed of 1000 mm/s and 1400 mm/s is more severe, which is consistent with the electrochemical test result.
This study involves the fabrication of AlMgScZr alloy via selective laser melting. The defects, precipitates, grain size, and grain boundary misorientation have a significant effect on the electrochemical corrosion properties of the alloy. The AlMgScZr alloy fabricated via SLM exhibits a significant passivation behavior in NaCl solution with mass fraction of 3.5%. Additionally, the porosity of the alloy reduces the stability of the surface passivation film, thereby decreasing its resistance to electrochemical corrosion. However, severe corrosion occurs at the melt-pool boundary due to the high number density of Al3(Sc,Zr) particles at the melt-pool boundary, which could act as micro-cathodes to promote the dissolution of the Al matrix. In addition, as the scanning speed increases, the grain size of AlMgScZr alloy gradually decreases and the low-angle grain boundaries gradually increase. This decrease in grain size leads to an increase in the grain boundary density and number of active atoms at the grain boundaries, thereby improving the growth rate of the passivation film. The increase in low-angle grain boundaries leads to an increase in the grain boundary stability, thereby enhancing the electrochemical corrosion resistance of the alloy.
