Crack elimination and synergistic improvement of strength and ductility in directed energy deposited IN738 superalloy via substrate heating and heat treatment

During the directed energy deposition (DED) process of IN738 superalloy, elevated thermal stresses and low-ductility γ′ phase can lead to crack formation. To tackle the cracking issue, this study developed a substrate heating platform within the DED apparatus, raising the substrate temperature to 300 °C to diminish the temperature gradient and thermal stress, while also optimizing laser power and scanning speed to achieve DED crack-free IN738. On-site thermal imaging and numerical simulations indicated that the temperature gradient and thermal stress in the samples gradually diminished with rising substrate temperatures and optimized process parameters. Moreover, microstructural characterization results revealed that heating the substrate to 300 °C reduced the content of the γ′ phase, while optimizing process parameters further refined the grains. The combination of reduced thermal stress, grain refinement, and diminished γ′ phase volume fraction collectively enabled the DED of crack-free IN738. On the crack-free DED IN738, a solution treatment at 1120°C for two hours notably improved its mechanical properties. The solution treatment simultaneously eliminated the Laves phase and generated finer γ′ phases along with a bimodal structure. These strengthening mechanisms effectively enhanced strength and ductility. Digital image correlation (DIC) and fractographic analyses substantiated that the solution treatment improved the coordinated deformation ability and the solubility of certain carbides reduced defects, further enhancing strength and ductility. Interestingly, a subsequent 24-hour aging treatment at 850°C did not significantly improve properties due to γ′ phase coarsening and δ phase precipitation.