Laser powder bed fusion of M789 on N709 steel substrate: characterization of mechanical and microstructural properties
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Date
2024-06
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University of New Brunswick
Abstract
This study investigates the feasibility of employing laser powder bed fusion (LPBF) of a new commercial alloy, M789 steel, on a wrought N709 steel substrate, potentially enhancing the manufacturing of complex plastic injection moulds and die inserts with improved strength and reduced cycle times. Utilizing an EOS M290 LPBF machine at voestalpine Additive Manufacturing Centre (vAMC) in Mississauga, Ontario, hybrid samples were printed and subsequently subjected to direct aging treatment. This treatment facilitated dislocation annihilation and the formation of the η-phase (Ni3(Ti,Al)) in the M789 section. The electron backscatter diffraction (EBSD) technique revealed random orientations of martensitic grains (in both as-printed and heat-treated states) with traces of reverted austenite that formed after heat treatment. Nanoindentation results (hardness profile across the interface) showed that the post-processing treatment increased the mechanical properties of M789 steel with hardness values comparable to wrought N709 steel. This assessment was validated through tensile testing, where the as-printed material failed in the M789 section, while the heat-treated sample fractured in the N709 section. Failure was detected away from the interface in both conditions, indicating the formation of a strong metallurgical bond.
To further understand the properties of the interface between M789 and N709 steels, thermodynamic and kinetic simulations were performed after the aging treatment, and the results revealed that different sizes and amounts of η-Ni3(Ti,Al) strengthening precipitates exist at the interface, 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 became gradual.
Finally, since M789 steel is a relatively new commercial alloy in the additive manufacturing community, it has been characterized further to determine its high temperature mechanical properties utilizing a Gleeble 563 thermomechanical simulator. By employing the experimental high temperature flow curves, the application of Sellars and Tegart hyperbolic sine model, the Kocks-Mecking model and the Johnson-Mehl-Avrami-Kolmogorov equation facilitated the development of physical material models that accurately depicted the hot deformation behavior of LPBF-fabricated M789 steel. Traditional models, such as the modified Johnson-Cook model, were found inadequate to capture the complexities and metallurgical phenomena associated with high-temperature deformation. The microstructural analysis revealed that dynamic recrystallization (DRX) occurs more readily at higher strain rates and deformation temperatures, whereas at lower temperatures, a lower strain rate was preferred to generate a higher fraction of DRX grains.