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-Ni 3 (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.