Role of the preheating temperature during electron beam powder bed fusion (PBF-EB/M) in precipitation of γ’ and carbides in Inconel 738 superalloy

n 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’ (Ni3Al), Ta-rich MC, and Cr-rich M23C6. 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 M23C6 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.