Role of the preheating temperature during electron beam powder bed fusion (PBF-EB/M) in precipitation of γ’ and carbides in Inconel 738 superalloy
Serena Lerda, B. Luo, Giulio Marchese, Xiangyu Zhao, Sara Biamino, Sasan Dadbakhsh
Abstract
In this work, two tailored electron beam powder bed fusion (PBF-EB/M) preheating conditions were selected to build Inconel 738 (IN738) superalloy. The IN738 samples produced at 1025 °C (R1) and 950 °C (R2) were characterized in the as-built condition to investigate the effect of the preheating temperature on the evolution of their microstructural features. Both samples showed low residual porosity (<0.2 %), and the microstructure displayed columnar grains elongated in the building direction, together with the precipitation of γ’ (Ni 3 Al), Ta-rich MC, and Cr-rich M 23 C 6 . The R1 condition developed a multimodal γ’ size distribution subjected to strong coarsening along the building direction. This scenario provoked a notable microhardness gradient and micro-crack formation due to the severe residual stresses developed. On the contrary, the lower preheating temperature chosen for the R2 condition provoked the precipitation of γ’ in a homogeneous monomodal size distribution with negligible coarsening along the building direction, thus lowering the residual stresses (no micro-cracks) and stabilizing the microhardness The preheating temperature thus had a lower impact on carbide precipitation and evolution compared to the γ’ one. Both conditions experienced the precipitation of Cr-rich M 23 C 6 and Ta-rich MC carbides along the grain boundaries, coupled with fine Ta-rich MC carbide precipitation inside the grains. However, the R2 condition underwent slightly lower precipitation. The current study therefore highlights the beneficial effects of tailoring the PBF-EB/M preheating temperature to control the evolution of the microstructural features and their effect on the development of residual stresses and microhardness stabilization in the as-built condition.