Development of SiC-enriched tool steel by Laser Powder Bed Fusion

Over recent years, the development of new metallic alloys manufactured by Laser Powder Bed Fusion (LPBF) has been the object of increasing attention. Indeed, the ultra-fast heating and cooling rates that characterize LPBF are responsible for the formation of strongly out-of-equilibrium microstructures, involving supersaturated solid solution as well as new metastable phases, thus offering new possibilities in terms of usage properties (hardness, tribological properties…). However, these conditions make difficult the processability of metal alloys such as high carbon tool steels, due to the easy tendency to crack formation that make the final part unusable. On the other hand, the use of LPBF allows to produce complex shape parts implementing internal cooling channels, difficult to obtain by conventional manufacturing methods. Therefore, research is now focusing on the development by LPBF of tool steels with complex chemical composition in order to combine all the advantages of this technology.
This work focuses in particular on the development by LPBF of a low alloy tool steel enriched with silicon carbide (SiC). First manufactured samples exhibit cracks due to the martensitic transformation and the high amount of residual stresses in the final microstructure induced by the high cooling rates during solidification. To obtain a fully dense, defect-free part, the use of specific parameters of the LPBF process is necessary, in particular applying a preheating on the substrate. Indeed, the preheating affects the thermal gradient within the melt pool which allows to change the solidification sequence and to avoid the formation of martensite. Moreover, the complete dissolution of SiC within the melt pool leads to an enrichment with Si and C, thus leading to a strengthening effect due to supersaturated phases in the final microstructure. Furthermore, the in-situ thermal cycles that affect the previous layers during the deposition of a new layer lead to the formation of a complex microstructure depending on the specific phase transformations induced by the local thermal history. The different phases formed with and without preheating such as martensite, austenite, cementite, bainitic ferrite and precipitates were differentiated by combining optical and scanning electron microscopy (including EDS and EBSD analysis), Differential Thermal Analysis, as well as macro- and nano-hardness.