Fracture toughness and microstructure damage behavior of thin Al–Cu alloy fabricated by wire-arc directed energy deposition

This study aimed to investigate the factors influencing the fracture toughness of thin Al–Cu alloy fabricated using Arc-Direct Energy Deposition (Arc-DED). By adjusting process parameters, samples with different microstructure characteristics were obtained, including an air-cooled (AC) deposit with a high proportion of equiaxed grains, a water-cooled (WC) deposit with a high proportion of columnar grains, and a post-heat-treated air-cooled deposit (T6) with numerous nano-sized precipitates. The conditional fracture toughness (K_Q)/crack tip opening displacement (CTOD) values for the AC, WC, and T6 samples were 11.7 MPa m^1/2/23.27 μm,11.4 MPa m^1/2/22.70 μm, and 17.6 MPa m^1/2/22.39 μm, respectively. The fracture toughness of the T6 sample was 50.4% and 54.4% higher than that of the AC and WC samples, respectively. The heterogeneity of the equiaxed/columnar microstructure affected the distribution of brittle θ-Al₂Cu phases and pores, thereby influencing crack behavior. While columnar grains exhibited a stronger capacity for accumulating geometrically necessary dislocations (GNDs) and coordinating deformation compared to equiaxed grains, cracks were more likely to propagate along grain boundaries when the loading direction was parallel to the growth direction of columnar grains. Consequently, flat crack paths were observed in columnar grains, while equiaxed grains displayed tortuous paths. Furthermore, the precipitation of nano-sized θ'-Al₂Cu phases contributed to improved fracture toughness. By integrating microstructure characterization, fractographic observation, analysis of uneven grain deformation behavior, and examination of GNDs distribution near crack paths, the study revealed microscopic damage characteristics, fracture behavior, and toughening mechanisms.