Tailoring the microstructure and mechanical properties of laser-directed-energy-deposited IN738LC alloy via heat treatment

Abstract

The additive manufacturing (AM) of IN738LC, a high-strength Ni-based superalloy, is limited by its inherent crack susceptibility because of its high Al and Ti contents. In this study, crack-suppressed IN738LC components were fabricated via laser-directed energy deposition (DED) using a reduced laser power (300–600 W) and a small beam diameter (0.8 mm), thereby minimizing heat-affected zones. Post-heat treatments were systematically applied, including direct aging (DA), partial (2STEP), and full (3STEP) solution treatments, to investigate their impact on the microstructural evolution and mechanical performance. Electron-backscatter diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy analyses revealed that the γ′ precipitate size and M23C6 carbide distribution were highly sensitive to the degree of homogenization. The use of DA formed ultrafine γ′ precipitates and the highest M23C6 fraction, resulting in a high yield strength (1322.68 MPa at 25 °C) and creep resistance (1109 h at 850 °C) but limited ductility. In contrast, the 3STEP treatment promoted an equiaxed grain morphology and coarsened γ′ precipitates, yielding improved ductility (5.3 %) with moderate strength. These findings demonstrate that precise thermal management during and after DED processing enables both microstructural control and crack suppression in IN738LC alloy, optimizing both tensile and creep properties for high-temperature structural applications.