Improving the hot corrosion resistance of additively manufactured Inconel 718 via recrystallization-based grain boundary engineering induced by its residual stress

Metals containing abundant coherent twin boundaries (TBs) can improve corrosion resistance. Tailoring TBs in additive manufacturing (AM)-fabricated metallic materials with lower stacking fault energy, however, has proven impracticable because the traditional grain boundary engineering (GBE) involved in thermomechanical processing is not appropriate for AM-fabricated alloys with complicated geometric shapes. Here, the AM-fabricated Inconel 718 superalloy was successfully utilized to form abundant TBs induced by the intrinsic residual internal stresses, produced by AM processing via direct heat treatment. The microstructural characteristics and corresponding hot corrosion behaviors of different specimens – initial AM-fabricated, directly aged, and GBE-treated specimens – exposed to a mixed salt of 75 wt.% Na2SO4 + 25 wt.% NaCl at 650 °C were compared and analyzed systematically. The corrosion mechanisms affecting each of the specimens are discussed in detail. The obtained results reveal that the GBE-treated specimen has the most intact surface after the corrosion test, suffering minimum mass loss and having the shallowest corrosion layer depth, exhibiting optimal hot corrosion resistance. This optimal performance is mainly attributed to the penetration of S and O elements that were efficiently impeded by the TBs. This result holds implications for improving hot corrosion resistance in AM-fabricated metallic materials via directly GBE induced by residual stress.