Effect of oblique angle on the formability, crystal growth behavior and microstructure in laser direct energy deposited thin-wall structure of single-crystal superalloy

Laser directed energy deposition process shows a tremendous potential to additively repair and manufacture single-crystal components in a near-net shape. For the specific non-vertical structure such as the tilted and torsional sidewalls of single-crystal components, achieving complete monocrystalline nature in the manufactured zone is still a great challenge and hazy. In this study, oblique thin-wall structures of single-crystal superalloy were fabricated by coaxial laser directed energy deposition process. The effect of oblique angle on the formability, crystal growth behavior and microstructure formation in thin-wall structures of single-crystal superalloy was discussed. The results showed that the critical oblique angle for good formability of thin-wall structure is 30 degrees. A poor-quality and shapeless deposited bead forms once the oblique angle researches 40 degrees. The thin-wall structure inherits the [001]/<100> crystalline orientation from parent single-crystal substrate through the continuous epitaxial growth of ultrafine columnar dendrites. The competition and alternate development of primary trunks-secondary arms-tertiary arms achieve the multiplication and reproduction of ideal columnar dendrites along the [001]/<100> crystalline orientation in the overhanging region of the thin-wall structure. Periodic segregation forms among the columnar dendrites. Epitaxial-growth continuity of the columnar dendrites with consistent crystalline orientation of substrate can be successfully achieved in large-scale tilted and torsional thin-wall structures of single-crystal superalloy with proper processing parameters. These findings are of great importance in the microstructure control for additively manufacturing complex single-crystal components by laser directed energy deposition process.