In the last years, additive manufacturing has become a widespread technology which enables lightweight-design based on topological optimization. Therefore, generation of lattice structures with complex geometries and small thicknesses is allowed. However, a complete metallurgical and mechanical characterization of these materials is crucial for their effective adoption as alternative to conventionally manufactured alloys. Industrial applications require good corrosion resistance and mechanical strength to provide sufficient reliability and structural integrity. Particularly, fatigue behavior becomes a crucial factor since presence of poor surface finishing can decrease fatigue limits significantly. In this work, both the low-cycle-fatigue and high-cycle-fatigue behaviors of Inconel 625, manufactured by Selective Laser Melting, were investigated. Fatigue samples were designed to characterize small parts and tested in the as-built condition since reticular structures are usually adopted without any finishing operation. Microstructural features were studied by light-optical microscopy and scanning-electron microscopy. Finally, fatigue failures were deeply investigated considering fracture mechanics principles with the Kitagawa–Takahashi diagram.

In the as-annealed condition, the nickel-based Alloy 625 has excellent mechanical and corrosion properties compared to those of common stainless steels. This peculiarity enables its exploitation in several industrial fields at cryogenic and high temperatures and in the presence of severely corrosive atmospheres. However, in this alloy, when high-temperature plastic deformation processes and heat treatments are not carefully optimized, the occurrence of excessive grain coarsening can irremediably deteriorate the mechanical strength, possibly leading to incompatibility with the standard requirements. Therefore, this research work investigated the possibility of adopting single- and double-aging treatments aimed at improving such strength loss. Their optimization involved identifying the best compromise between the hardening effect and the loss in corrosion resistance induced by the simultaneous formation of intergranular chromium-rich carbides during aging. The investigation of the aging treatments was performed using hardness, tensile and intergranular corrosion tests considering different time–temperature combinations in a range from 621 °C to 732 °C. Double aging resulted in a considerable acceleration in the hardening response compared to single aging. However, even after its optimization in terms of both temperature and time, the intergranular corrosion resistance remained a critical aspect. Among all the tested conditions, only single aging at 621 °C for 72 h was acceptable in terms of both mechanical and corrosion properties. The influence of longer exposures will be investigated in a future study.

Metallurgical optimization of engineering alloys is traditionally addressed to improve the overall performance from a mechanical point of view. Grain size is one of the most influential and critical parameters to be controlled in nickel alloys, especially in the high-temperature shaping process and final solution treatment, since it can irremediably damage the alloy performance. For this reason, grain coarsening of alloy 625 was investigated in the temperature interval from 980 to 1150 °C ranging from 0.5 to 6 h. The grain-coarsening data were fitted via regression analysis as a function of time and temperature to develop a predictive model. Grain boundary strengthening was studied by hardness and tensile tests, and the relationships between the grain size and the mechanical properties were finally determined by regression analysis. Such equations were included in a thermo-metallurgical model able to predict the mechanical properties after annealing treatment. This predictive model was validated on a forged tube subjected to solution annealing at 1150 °C for 90 min. Then, it was finally used to compare different microstructural conditions in terms of the alloy impact on the environment.

Biphasic stainless steels provide an excellent combination of mechanical and corrosion properties. The occurrence of ferrite spinodal decomposition during processing and heat treatment can induce a dramatic drop in impact energy. In this paper, a forged rod of 480 mm diameter made of 2507 biphasic stainless steel was studied. The spinodal decomposition phenomenon upon aging at 475 °C (748 K) was investigated by macro- and micro-hardness tests on the constituent phases and by Charpy impact tests. Then, the influence of the microstructural constituents and their morphology on the crack path was studied by optical microscopy, noting possible correlations with the impact energy. Successively, the fracture surfaces of selected specimens were analyzed by a scanning electron microscope (SEM). Finally, reversion heat treatments at 550 °C (823 K) and 600 °C (873 K) were investigated to evaluate the possibility of recovering the detrimental effects of the ${\alpha }^{\prime }$ embrittlement. According to the literature, this procedure works well with some biphasic steels, but these steels are characterized by a chromium content lower than that of 2507 grade steel. Regarding the 2507 grade steel studied in this work, complete reversion was obtained by solution-annealing treatment, while reversion heat treatments at both 600 °C (873 K) and 550 °C (823 K) allowed only a partial recovery.