Influence of Si3N4 powder additivation on PBF-LB processability of stainless steels and microstructural evolution during PBF-LB and a subsequent HIP-URQ densification process

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Due to the geometric flexibility and resource efficiency potential, the powder bed fusion - laser beam process (PBF-LB) has gained significant interest in both science and industry. However, the variety of available iron-based materials for PBF-LB is ultimately limited to those offering a low susceptibility for cold cracking, because of strong thermal gradients and consequential residual stresses acting during the process. To overcome the limitations in processability of standard feedstock powders and broaden the range of available materials, powder additivation can be a suitable instrument. The main objective of this project is to achieve a fundamental understanding of steel powder additivation with silicon nitride (Si3N4) particles in the context of the production of nitrogen (N) alloyed, corrosion resistant steel grades. The investigation includes the influence of powder additivation on rheological powder properties and reflection behavior, studied by experiments and DEM simulation. The processability in PBF-LB and subsequent hot isostatic pressing with ultra-rapid quenching (HIP-URQ) as well as microstructure formation is addressed in scale-bridging microstructural characterizations and diffusion simulations. In this context, the dissolution and possible re-precipitation of Si3N4 additive particles during PBF-LB and HIP-URQ is of particular interest as the main influencing factor on microstructure and consequentially on material properties. The aim of the present approach is to avoid the complete melting of the Si3N4 additive particles during PBF-LB as this can result in significant outgassing and pore formation. Instead, a complete dissolution of the remaining additive particles is desired upon a subsequent HIP-URQ post-processing, inducing a diffusional homogenization of the elements N and Si into the steel matrix and ultimately resulting in a N alloyed stainless steel. With this approach of “reactive additivation”, the utilization of a potentially beneficial influence of the additive particles on the powder feedstock and its processability by PBF-LB is addressed. Furthermore, a comparatively high N solubility of the solid phase austenite is purposely utilized to introduce considerable amounts of N into the steel for an improvement of the material properties which will be correlated to the microstructural states in corrosion testing as well as static and dynamical mechanical testing.