Combined effects of carbon content and heat treatment on the high-temperature tensile performance of modified IN738 alloy processed by laser powder bed fusion

Laser powder bed fusion (LPBF) is an advanced manufacturing technology used in processing nickel-based superalloys, notably for aero-engine components. One such material, the LPBF-fabricated IN738 superalloy, is prone to significant cracking issues. This study found that a change in carbon content (the optimal content of which was also determined) effectively mitigated the cracking. This study has systematically investigated the impact of different heat treatments on microstructural alterations and high-temperature tensile properties. The addition of 0.55 wt% of graphite proved effective in entirely inhibiting cracking in LPBF-fabricated IN738 specimens. Pre-alloyed IN738-M powder with the optimal carbon content was then produced and processed via LPBF to assess its formability. The as-built specimen revealed the presence of continuous carbides along the subgrain boundaries. Heat treatment promoted the transformation of substructured grains into recrystallised grains, accompanied by the precipitations of carbides and the γ′ phase; their morphologies were strongly determined by the solution treatment temperature. Differential scanning calorimetry measurements were employed to elucidate the differing microstructural states following distinct heat-treatment regimens. Under a 900 °C testing condition, stress-relieved (SR) specimens were found to exhibit superior performance, demonstrating an ultimate tensile stress (UTS) value of 843.6 MPa, a yield strength (YS) of 807.3 MPa and an elongation of 8.54 %. Notably, SR specimens also exhibited the highest UTS and YS values at 1000 °C, measuring 380.0 MPa and 346.5 MPa, respectively. This study's findings will furnish valuable insights for researchers who aim to enhance the high-temperature tensile performance of LPBF-fabricated nickel-based superalloys.