# Selective Laser-Melted Alloy 625: Optimization of Stress-Relieving and Aging Treatments
## 1 Introduction  
Nickel alloy 625 was patented in 1964 by [PERSON] and [PERSON]. Its chemical composition is characterized by the presence of high amounts of chromium, molybdenum, and niobium, which provide excellent mechanical strength and corrosion resistance in several environments [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. For such a reason, this alloy grade is adopted in many applications, such as aerospace, chemical, oil and gas extraction, power generation, and automotive [1, 4, 6, 11]. The ASTM B446 standard [12] requires alloy 625 to be heat-treated following two processes: soft annealing and solution annealing. The former requires a minimum temperature of 870 \({}^{\circ}\)C, and it is recommended for applications below 600 \({}^{\circ}\)C, where good mechanical and corrosion properties are required [12]. The latter is instead suggested to improve the creep strength at temperatures above 600 \({}^{\circ}\)C, and it is performed at a minimum temperature of 1093 \({}^{\circ}\)C [12]. In the soft-annealed condition, this alloy is characterized by an austenitic microstructure with a heterogeneous distribution of primary Nb and Ti carbides and nitrides [8, 9, 10, 11, 13]. The thermal exposure above about 600 \({}^{\circ}\)C during processing, heat treatment, and service can activate complex precipitation phenomena that are able to modify the mechanical and corrosion resistance significantly [1, 10]. On the otherhand, special heat treatments can be developed to obtain a hardening effect when required by the service conditions [12]. The time-temperature-precipitation (TTP) curves available in the literature are reported and adapted in Figure 1 for both the conventional alloy and that manufactured by SLM focusing the attention on the \(\delta\) phase that is the most detrimental for the material's toughness. Metal carbides (MC, M\({}_{6}\)C, and M\({}_{23}\)C\({}_{6}\)) and intermetallic (especially \(\gamma^{\prime\prime}\) phase) and Laves phases can be present as well [10]. These compounds can decrease the corrosion resistance (especially when intergranular Cr- and Mo-rich carbides precipitate in the metal matrix), the ductility (especially when Cr- and Mo-rich carbides are created at the grain boundary), and the deformability of the alloy [14; 15; 16; 1; 1; 4; 10]. Finally, niobium, aluminum, and titanium promote the precipitation of the intermetallic hardening \(\gamma^{\prime\prime}\) phase. Nowadays, additive manufacturing is increasingly adopted for producing a variety of components especially when complex geometries are required. In fact, in these cases, the conventional manufacturing processes are typically not suitable because of technological limits and/or shape complexity. Among the wide range of additive technologies, this research work is focused on the selective laser melting (SLM) technique. Starting from a CAD model, this method allows us to generate parts layer by layer with a build direction perpendicular to the base plate. Each powder bed is selectively melted by a laser beam. Once the first layer is completed, the base plate is lowered, and the stacking process is continued. During the process, an inert gas (argon or nitrogen) is adopted to avoid powder oxidation within the chamber. Considering the huge growth in the adoption of additive manufacturing techniques in the last decade, this research work is also aimed at investigating the mechanical properties of the selective laser-melted alloy 625 in comparison with those of the conventionally manufactured one. This is extremely useful for designers in the transition from the conventional 625 alloy to the additive manufactured one with reliable data about the mechanical performance and strategies for further strength enhancement based on tailored heat treatments. The literature data about the mechanical properties of the selective laser-melted alloy 625 are reported in Table 1 and compared to those of the conventional wrought and centrifugally cast alloys.