Towards deeper carburizing by DEDp and L-PBF additive manufacturing of in-situ TiC-reinforced steel composites: powders and processability

Additive manufacturing (AM) of iron-based metal matrix composites (IMMCs) enables the fabrication of more complex geometries than conventional processes for applications such as brake discs, transmission shafts, cutting tools, and gears. Current challenges associated with AM of IMMCs, including homogeneous reinforcement distribution and reinforcement/matrix interfacial decohesion, have been mitigated in the context of deep carburizing for gear applications. In situ reinforcement by Laser Powder Bed Fusion (L-PBF) was achieved using nano-TiC (35–55 nm) dry coating onto spherical steel powder particles (15–45 µm) through Cyclomix® blade mixing, with a reinforcement content of 5 vol% (3.2 wt%). Powder rheology, powder density measurements, single-track experiments, high-speed imaging, and 3D builds revealed improved flowability due to coating, an increasing of spatters amount despite a low porosity rate (0.26 vol%). Directed Energy Deposition using powder (DEDp) enables additional functionalities such as repair and compositional gradients while accounting for process-induced constraints. However, the powder must be coarser to ensure efficient stream. Turbula® premixed composite powders were used with spherical steel particles (45–125 µm) mixed and 3.2 wt% of micrometric TiC particles (3–8 µm and 15–30 µm). Video imaging, single-track experiments and 3D build showed that unmelted TiC particles could not be completely eliminated and create defects. Hardening comparable to carburizing was achieved, increasing from 350 HV to 600 HV throughout the volume in L-PBF samples and at the surface in DED-p samples. Strengthening was likely induced by carbon diffusion from the reinforcement phase; however, the strong thermal cycling associated with DED-p caused softening in previously deposited layers.