Laser-directed energy deposition of ZrH2 particles reinforced Al7075 alloy: Cracks elimination and strength enhancement

Metal-based laser additive manufacturing, particularly Laser-Directed Energy Deposition (LDED), holds great promise for producing high-strength aluminum alloy components used in industries such as automotive manufacturing, aerospace, and biomedicine. However, achieving defect-free and superior mechanical strength in high-strength aluminum alloys, especially 7XXX series Al alloys, through LDED remains challenging. In this study, the LDED of ZrH2 particles reinforced Al7075 alloy was investigated. Results indicated that the addition of ZrH2 particles to the Al7075 alloy facilitated the columnar-to-equiaxed transition and effectively prevented hotcrack formation during the solidification process, leading to a crack-free and fine equiaxed-grain microstructure. The as-deposited Zr-modified Al7075 alloy demonstrated a favorable combination of strength and ductility, a yield strength of 266 MPa, a strength of 321 MPa, and an elongation of 21.0% in the Al7075-3%ZrH2 sample, which represents promising performance among the reported LDED-fabricated Al alloys. Contributions of four different strengthening mechanisms to yield strength were calculated, showing that the introduction of ZrH2 particles highlighted the effects of grain refinement and geometrically necessary dislocation strengthening. Moreover, the grain reinforcement mechanism in the Al7075 alloy with ZrH2 particles reinforced was elucidated after adequate material characterizations including SEM, EDS, TEM, and XRD. Needle-like Al3Zr and angular-like Al3Zr were observed in the equiaxed and ultrafine grain regions of the LDED-fabricated Al7075-3%ZrH2 samples, corroborating the truth that Al3Zr serves as heterogeneous nucleation sites during the solidification and plays a significant role in grain refinement. The successful manufacturing of crack-free, equiaxed high-strength 7XXX series Al alloys lays the foundation for LDED production of high-strength aluminum engineering components.