During Laser-directed energy deposition (L-DED), the metallic powder or wire is melted by a laser and solidifies over a substrate. As new layers are successively generated, the melted material solidifies on top of previously deposited layers. Due to this layer-by-layer process, each new deposition creates a thermal cycle that further modifies the microstructure below. The amplitude of those thermal cycles depends on the distance between the processed zone and the region of interest. Therefore, a complex thermal history is generated in the part, referred to as intrinsic heat treatment (IHT).

In this work, in situ High energy X-ray diffraction (HEXRD), Transmission electron microscopy (TEM) and Atom probe tomography (APT) were combined to analyse and identify the microstructure evolution and particularly the nucleation and coarsening of precipitates during L-DED of a X40CrMoV5-1 steel. The careful investigation of the secondary phases by in situ HEXRD experiments enables the observation of precipitate formation during the L-DED process. Distinct carbide populations are formed in the tool steel, depending on the intensity of the thermal cycles caused by the IHT. During the initial cycles, where the material is melted/solidified, no carbide formation was observed. However, once the material starts to be subjected to thermal cycles in the intercritical regions, V-rich M8C7 and Fe-Cr rich M3C/M7C3 carbides begin to form in the material. However, their formation does not necessarily occur simultaneously. The carbides of type M8C7 begin to form when the peak temperature of the thermal cycle lies between 860 °C (Ac1b) and 960 °C (Ac1e). Once the peak temperatures remain below 860 °C, carbides of type M3C/M7C3 starts to form while the M8C7 content does not further increase. Finally, the chemical composition of these precipitates was investigated by APT and where compared to thermodynamic simulations, what demonstrated that the carbides achieve different levels towards the equilibrium state.