Precipitation Behaviour at the Interface of an Additively Manufactured M789-N709 Hybrid Alloy

A novel hybrid alloy, designed for plastic injection dies, was fabricated by depositing M789 steel on wrought N709 steel through laser powder bed fusion (LPBF). The M789-N709 interface (with a thickness of about 300 μm) is characterized by the variation of titanium content from 0 wt% in the N709 section to about 1 wt% near the M789 region. A thermodynamic and kinetic simulation after aging treatment was performed to reveal the precipitation behavior in the M789-N709 interface. Different sizes and amounts of ETA-Ni3(Ti,Al) strengthening precipitates exist, with a maximum volume fraction and radius of about 5% and 5 nm, respectively (near the M789 region). Moreover, the volume fraction of precipitates significantly increases with Ti additions of up to 0.6 wt% (i.e., 100 μm from the N709 region);however, beyond this threshold, the increase in the amount of precipitation becomes gradual. The thermodynamic and kinetic simulations performed in the interface are validated by electron diffraction spectroscopy (EDS), electron probe microanalyzer (EPMA) and atomic probe tomography (APT) results;the measured precipitate size from the APT analysis is consistent with the simulation results. Two morphologies of precipitates were detected: elongated and spherical;both contribute to the robustness of the interface. The robust M789-N709 interface formed during the process shows good compatibility between the alloys, which is critical for extended tool life. These new insights about the precipitation at the interface of M789-N709 alloy are crucial in assessing the feasibility of N709 as a cost-effective base material to minimize the use of M789 when printing plastic injection dies.

Deformation‑Induced Strengthening Mechanism in a Newly Designed L‑40 Tool Steel Manufactured by Laser Powder Bed Fusion

The microstructural and mechanical properties of a newly designed tool steel(L-40),specifcally designed to be employed in the laser powder bed fusion(LPBF)technique,were examined.Melt pool boundaries with submicron dendritic structures and about 14%retained austenite phase were evident after printing.The grain orientation after high cooling rate solidifcation is mostly<110>α∥building direction(BD).Then,the heat treatment converted the microstructure into a conventional martensitic phase,reduced the retained austenite to about 1.5%,and increased<111>α∥BD texture.The heat-treated sample exhibits higher tensile strength(1720±14 MPa)compared to the as-printed sample(1540±26 MPa)along the building direction,mainly due to hardening caused by a lower volume fraction of retained austenite phase and precipitation of carbides.As a consequence of the strength-to-ductility trade-of,the heat-treated sample showed lower elongation(10%±2%)than that of the as-printed sample(18%±2%).It was observed that transformation-induced plasticity(TRIP)occurs in both the as-printed and heat-treated samples during tensile testing,which dynamically transforms the retained austenite into martensite,leading to improved ductility.The minimum driving force to initiate the displacive phase transformation is about 6000 J/mol,which was achieved during tensile testing.The strength and ductility of LPBF-produced L-40 were compared with the other LPBF-produced tool steels in literature;the data indicate that heat-treated L-40 has an excellent combination of strength and ductility complemented with high printability.