In this research, steel alloys based on the Fe-Cr-Mo, Fe-Cr-Mn and Fe-Cr-Mo-Mn-Ni systems have been designed, produced by different atomisation techniques, and processed by laser powder bed fusion (L-PBF) to investigate their microstructural and mechanical behaviour. Both gas atomisation and water atomisation were considered for powder preparation. The resulting different flowability of powders, hence a different densification behaviour during processing, could be compensated by tuning the L-PBF parameters and by the application of a post treatment to improve flowability of the water atomised powders. In agreement with thermodynamic calculations, small-size oxide-based nonmetallic inclusions of the type SiO₂, MnO-SiO₂, Cr₂O₃-SiO₂ were found within the steel matrix and on the fracture surfaces of the water atomised L-PBF alloys, featuring higher amounts of oxygen than the gas-atomised steels. Analyses on microstructure and hardness of the hardenable as-built steels suggested that during laser processing, the multilayer L-PBF structure undergoes an in-situ tempering treatment. Furthermore, the mechanical properties of the L-PBF steels could be widely tuned depending on the post-thermal treatment conditions.

The microstructure and mechanical properties of a 4130-grade steel processed by L-PBF using a feedstock of low-cost water atomized powder have been investigated considering the effects of powder recycling. Chemical analysis of the recycled powder showed a constant amount of alloying elements with a slight reduction in oxygen content. The as-built microstructure was mainly composed of a martensitic structure separated by a high fraction of low-angle grain boundaries, suggesting the application of a direct tempering treatment starting from the as-built condition as a cost-effective post-process thermal treatment rather than the conventional quench and tempering treatment. Moreover, the degree of anisotropy generated by L-PBF in as-built specimens could be reduced after performing either the direct tempering or the quench and tempering treatments. The possible degradation of powder properties on the steel performance was also investigated. After various powder recycling events, no significant deterioration in tensile properties was measured, indicating that the water atomized powder could be a sustainable feedstock candidate for L-PBF.

Wire arc additive manufacturing (WAAM) is highly sought after for producing complex-shaped products with high deposition rates and relatively low cost compared to other additive manufacturing processes. However, the cumulative heat input during processing leads to low-dimensional accuracy, high distortion, and high residual stress, which requires heat control and thermal monitoring. The current work offers a study on the applicability of experimental and numerical thermal monitoring in detecting thermal effects generated during the WAAM processing of a corrosion-resistant SS308L austenitic alloy commonly used in nuclear, aerospace, and marine industries. Temperatures were measured during processing for vertical and horizontal structures by a mono-wavelength pyrometer and coupled with thermo-mechanical simulations. The simulation models were successfully validated through the distortion measured in the substrates of the processed structures, confirming the acquired thermal cycles. Results showed that mono-wavelength pyrometry was only effective for vertical structures away from the deposition area, with emissivity varying from 0.6 to 0.41 for temperatures between 600 and 1700 °C, respectively. For horizontal structures, temperature measurements relied solely on simulations, which predicted heat accumulation and cooling rates. These predictions were validated through secondary dendritic arm spacing and hardness measurements, confirming the impact of intermittent deposition in reducing heat build-up and leading to relatively finer microstructures and less variant hardness profile compared to continuous deposition. Graphical abstract