Laser powder bed fusion of a composition-modified IN738 alloy based on thermodynamic calculations

Abstract

Defects, particularly cracking defects, severely limit the application of Ni-based alloys fabricated via the laser powder bed fusion (LPBF) additive manufacturing process. To address the processability/strength trade-off, in this study a new Ni-based alloy (IN738M) was designed via thermodynamic calculations based on the traditional IN738 alloy. The processability, microstructure, phase precipitation behaviour and mechanical properties were systematically examined. The results demonstrate that the LPBF-fabricated IN738M exhibited excellent LPBF processability, achieving crack-free fabrication even with a high γ′ phase mass fraction. The compositional modifications led to improvements in the microstructure, including the formation of a quasi-continuous carbide network at the interdendritic regions, altered grain orientation and grain refinement. This study also proposes a heat treatment strategy to achieve a bimodal distribution of the γ′ phases for IN738M; the cellular structure was eliminated, with numerous MC-type carbides observed within grains and at the grain boundaries. The IN738M alloy exhibited a superior combination of ultimate tensile strength values (1462 ± 23 MPa) and elongation values (10.2 ± 0.4 %) at room temperature compared to the IN738 alloy (932 ± 35 MPa and 2.6 ± 0.3 %, respectively). These findings will provide valuable guidance for developing Ni-based alloys with enhanced LPBF processability and mechanical properties.