Martensitic stainless steel with precipitation hardening and high mechanical strength for additive manufacturing

The combination of Additive Manufacturing (AM) design freedom and a new generation of high-performance metal powders offers a great opportunity to achieve an industrial leap regarding performance, weight reduction, decarbonization, cost, … To support those challenges, Aubert & Duval is evaluating the AM powder version of MLX®19 grade that has originally been developed for forged mechanical parts.The MLX®19 grade is a precipitation hardening stainless steel combining ultra-high tensile strength and good corrosion resistance. This stainless steel has been designed for severe service conditions with targeted properties similar to 300M performance required for structural aerospace components: ultimate tensile strength (UTS) range between 1800 and 1950 MPa, toughness (KIC) from 50 to 80 MPa.m1/2.In this study, MLX®19 powder produced by gas atomization was processed by Laser Powder Bed Fusion (LPBF). MLX®19 proved its suitability for AM processes, thanks to its absence of cracks. LPBF process gives rise to specific non-equilibrium microstructures with the absence of η-Ni3Ti precipitates. Specific heat treatments were applied to precipitate the hardening η-Ni3Ti phase and optimize mechanical properties. Transmission electron microscopy (TEM) analysis was used to monitor the evolution of precipitation. XRD and EBSD analyses revealed that austenite is finely and homogeneously distributed after heat treatment. These microstructural features lead to an increase in UTS of 80 MPa after heat treatment, compared with conventional processes such as forging. Forged and AM-treated parts show similar corrosion resistance. This study demonstrates that LPBF processing is suitable for MLX®19 parts with satisfactory properties, paving the way for the application of additively manufactured MLX®19 components in various industries.