Thermal cycling effects on the local microstructure and mechanical properties in wire-based directed energy deposition of nickel-based superalloy

In additive manufacturing, intrinsic heat treatments take place during deposition that affect the properties of AM-structures. In this work, the influence of thermal cycling on the local microstructure and mechanical properties of nickel-based superalloy in wire-based electron beam directed energy deposition (EB-DED) was investigated. Structures were fabricated using a continuous deposition strategy (CDS) and discontinuous interpass cooling strategy (ICS) revealing changes in thermal profiles, time-temperature history, and microstructure. An altered morphology along the build-up height and interdendritic zones enriched in Nb are formed. Nb and Mo did not show a clear trend of segregation along the build-up height. Lower fractions of the Laves phase and MCs are found for both configurations. Differences between deposition strategies and locations within AM-structures are found for the gamma" and 8 phase. The higher Nb content in the interdendritic zone promotes the precipitation of gamma" and 8 phase by shortening the aging times compared to wrought materials. The longer deposition times of ICS favour the precipitation of fine gamma" in the interdendritic zone throughout the deposition height. In contrast, the short deposition time of CDS leads to an increase in temperature and a heterogeneous distribution of gamma" along the height, i.e. coarsening of the gamma" followed by a dissolution along the built-up height. The microstructural changes correlate with the mechanical properties. Structures fabricated with ICS exhibit homogeneous mechanical properties throughout, while the graded microstructure of CDS results in graded mechanical properties and decreasing strength throughout.