Tailoring of grain structure and crystallographic texture in nickel-based alloy fabricated by directed energy deposition via nozzle stand-off distance adjustment

The highly directional thermal gradients and the layer-by-layer stacking feature of direct energy deposition (DED) facilitate the epitaxial growth of columnar dendrites along the dendrites in the previous layer, leading to anisotropy in the mechanical properties. The nozzle stand-off distance affects the interaction between the powder stream and laser beam during DED process. This interaction plays a significant role in determining grain growth. Thus, the texture and the grain structure can be regulated in situ by varying the stand-off distance. Here, an image-based powder measurement (IBPM) system was employed to obtain the powder flow characteristics. The texture formation mechanism of nickel-based alloy specimens fabricated by DED was described based on observations of the microstructure and grain orientation. The interaction mechanism between powder mass concentration and energy density was revealed through experiments and numerical simulations. It was concluded that the nozzle stand-off distance exerts a remarkable influence on the curvature of the molten pool boundary, as well as the subsequent microstructure and texture of the nickel-based alloy. The slight curvature of the molten pool boundary in the specimen prepared at the focus position (FP) promotes the epitaxial growth of columnar dendrites. Meanwhile, in the specimen prepared at the negative defocus position (ND), the intersection angle between the boundaries of adjacent deposit tracks is approximately 90 degrees, forming a zigzag band that inhibits the epitaxial growth of the columnar dendrites. By altering the nozzle stand-off distance, the strategy improves the tensile properties of the specimens through influencing the texture intensity and grain size.