Defect formation and prevention in directed energy deposition of high-manganese steels and the effect on mechanical properties

Laser beam directed energy deposition (DED-LB) differs from other metal additive manufacturing (AM) methods as it allows high building rates and manufacturing of multi-material components via in-situ alloying. This is especially compelling in combination with high-manganese steel (HMnS), as the mechanical properties can be influenced significantly by tailoring the chemistry-dependent stacking-fault energy (SFE). In DED, this can be used to design parts based on local deformation behavior. However, the increased affinity of HMnS to oxygen causes high amounts of oxide formation in the manufactured parts, ultimately deteriorating the mechanical properties due to premature cracking. To investigate the responsible mechanisms, two sets of processing parameters resulting in varying melt pool sizes were applied to produce X30Mn23 steel with up to 1 wt% in-situ alloyed Al. The melt pool was first modeled using a finite element method (FEM) approach and correlated with microstructure evolution (OM, SEM, EDS, EBSD, APT) and mechanical properties (tensile test). Interaction of the melt pool with the atmosphere and therefore oxide formation was successfully prevented by reducing the melt pool size, making the application of HMnS in DED feasible. The crack formation by oxides and its prevention are discussed in detail. Finally, the feasibility to manufacture HMnS with an in-situ alloying approach is critically evaluated.