Hybrid joining of M789 steel and wrought N709 steel using laser powder bed fusion

Additive manufacturing technology has emerged as a robust and efficient technique for manufacturing different components, especially those with sophisticated geometries and internal structures. Its near-net-shape manufacturing capability offers numerous benefits and advantages over conventional manufacturing technologies. Among the available additive manufacturing technologies, laser powder bed fusion (LPBF) technology has been used extensively to manufacture small parts through selective layer-by-layer melting of metal powder based on the predefined 3D CAD model.

Since maraging steels generally have high strength and good weldability, they have been successfully manufactured using LPBF for different applications. Heat treatment (solution treatment + aging) is normally applied after the LPBF process to influence the formation of precipitates that determines the strengthening capabilities of the maraging steels. Regardless, maraging steels still exhibit low corrosion resistance that significantly lowers their service life, especially in extreme environments. To overcome these shortcomings, novel Grade 250 maraging steel (M789 steel) was developed to improve corrosion resistance while maintaining high printability.

Although the properties of M789 steel manufactured via LPBF have been studied by several researchers, the introduction of hybrid LPBF technology is expected to transform the manufacturing of such steels further. Based on this work hybrid manufacturing was defined as the combining of LPBF with a traditional manufacturing technique (wrought process) using dissimilar steels. Hybrid technology offers several benefits for achieving desired functional mechanical and physical properties and is appropriate for joining different steels, repairing surfaces, and incorporating metal coatings. Despite the growing amount of literature on various maraging steels fabricated via hybrid LPBF, there is no study on hybrid LPBF-M789 steel despite their performance superiority over similar alloys.

To this note, researchers at the University of New Brunswick: PhD candidate Kudakwashe Nyamuchiwa, Professor Kanwal Chadha, Professor Youliang He and Professor Clodualdo Aranas together with Dr. Yuan Tian from voestalpine Additive Manufacturing Centre Ltd. fabricated a hybrid M789-N709 alloy by printing M789 steel onto wrought 13-8 Cr – Mo N709 steel using LPBF technique. Direct aging treatment was applied to allow the formation of η-phase in the M789 section whilst preserving the mechanical properties of the wrought N709 substrate. Specifically, the texture, microstructure and mechanical properties as well as heat treatment of the fabricated M789 steel, were investigated. The work is currently published in the research journal, Materials Science and Engineering A.

The research team demonstrated the formation of a strong metallurgical bond between the N709 and M789 steels. This was evidenced by the fracturing of the hybrid material in the M789 section in the as-printed state during tensile testing. The fracture of the as-printed material in the M789 section was ascribed to β-NiAl precipitates strengthening in the wrought N709 section. Consequently, the failure of the heat-treated material on the N709 section was attributed to the peak strengthening of the M789 part due to η-phase precipitation under direct aging conditions of 500℃ for 3 hours.

Nanoindentation results revealed that the mechanical properties of the M789 steel were considerably enhanced after the post-processing treatment due to the positive contributions of the precipitation strengthening. Nevertheless, the mechanical properties of the base N709 region remained essentially unchanged. After the LPBF process, the resulting M789-N709 hybrid material was observed to have randomly oriented martensitic structure with very weak building direction texture having maximum intensities of 1.73 and 1.34 before and after direct aging, respectively.

In summary, the study assessed the microstructural and mechanical properties of LPBF M789-N709 hybrid material to determine its feasibility for practical use. A series of tensile tests were performed to validate this assessment, where the as-printed and heat-treated samples failed in the M789 section and fractured in the N709 section, respectively. The results demonstrated that the new hybrid steel alloy could be utilized without the effects of the compromised strength induced by the interface. In a joint statement to Advances in Engineering, the authors explained that their findings would promote the application of LPBF M789-N709 hybrid material to address the evolving needs of different industries.