Effect of carbon content on the microstructure, tensile properties and cracking susceptibility of IN738 superalloy processed by laser powder bed fusion

The high cracking susceptibility is the major barrier hindering the broad adoption of additive manufacturing (AM) in traditional non-weldable superalloys. It is particularly important to explore the role of elements during the AM processing and establish a new alloy design criterion. In this study, the effect of carbon, a critical element that influences the solidification and mechanical properties of superalloys, on the printability and mechanical properties of a typical non-weldable Inconel 738 (IN738) alloy processed by laser powder bed fusion (LPBF) was studied. Different from the limitation of low carbon content in traditional superalloy design, it was found that a moderate increase of carbon content from 0.11 wt% (alloy IN738LC) to 0.3 wt% (alloy IN738–0.3 C) and 0.6 wt% (alloy IN738–0.6 C) can produce crack-free LPBF samples in a wide range of processing parameters. The effect of carbon content on the microstructure, tensile properties and cracking susceptibility of the LPBF-printed alloys was evaluated. With the increase of carbon content, more granular monocarbides of no more than 200 nm in size form into a quasi-continuous or continuous network at the boundaries of cellular structures, while the low melting-point phases containing boron and γ/γ′ eutectics at cellular boundaries are significantly reduced. The susceptibility to solidification cracking and liquation cracking is substantially decreased owing to the reduced level of element segregation, elimination of initiation source for liquid films, and decrease of local strain concentration. As a result, LPBF IN738–0.3 C and IN738–0.6 C show excellent ultimate tensile strength (1320 and 1598 MPa) and superior total elongation (14.7% and 9.0%), respectively. The results above demonstrate the different roles of carbon element in AM and the potential for designing AM non-weldable superalloys with improved printability and mechanical properties via alloy composition optimization.