Laser powder bed fusion of M789 maraging steel on Cr–Mo N709 steel: Microstructure, texture, and mechanical properties

Abstract

In this work, M789 steel was deposited (printed) onto wrought N709 steel by a laser powder bed fusion (LPBF) process to fabricate a hybrid M789-N709 alloy. The microstructure of the as-printed, new hybrid material consists of melt pool boundaries in the M789 region. Direct aging treatment was then applied, which allowed the annihilation of dislocations as well as the formation of the η-phase (Ni3(Ti, Al)) in the M789 section. Electron backscatter diffraction (EBSD) revealed essentially random orientations of the 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 to comparable to that of 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. This can be attributed to the presence of the η-phase after heat treatment. Since the fracture location is in the base alloy, a strong metallurgical bond is formed between M789 and N709. Thus, the new hybrid alloy can be employed without a compromised strength due to the interface.