# Selective Laser-Melted Alloy 625: Optimization of Stress-Relieving and Aging Treatments
## 4 Conclusions  
The selective laser-melted alloy 625 shows outstanding mechanical and corrosion properties in the as-built condition. Compared to the conventional forged alloy, this difference is the most significant, and it can be ascribed to the improved microstructural fineness of the selective laser-melted material. However, because of the high local thermal gradients induced by the SLM process, residual stresses are present, and their mitigation by suitable  
Figure 11: Metallographic sections of the scan plane of selected corrosion specimens. (**a**,**b**) As-built: 0.5 mm/year; (**c**,**d**) 621 \({}^{\circ}\)C 130 h: 5.7 mm/year; (**e**,**f**) 875 \({}^{\circ}\)C 0.75 h: 5.7 mm/year; (**g**,**h**) 980 \({}^{\circ}\)C 0.5 h: 0.6 mm/year. stress-relieving treatment is normally required to avoid distortions and failures when the component is removed from the build plate, especially in the presence of thin components. The conventional forged alloy 625 is stress-relieved at a minimum temperature of 870 \({}^{\circ}\)C. However, at this temperature, the selective laser-melted material is characterized by the very rapid precipitation of the \(\delta\) phase which detrimentally degrades the deformability and corrosion resistance after short exposure times. Such different behavior depends on the fact that the aging response of the selective laser-melted alloy is significantly accelerated compared to that of the conventional material because of the higher micro-segregation level and the presence of residual stresses. Lower stress-relieving temperatures are not suitable for a satisfying mitigation of residual stresses. For this reason, in this research work, considering the TTP diagram of this alloy, higher stress-relieving temperatures were analyzed by investigating the influence on the mechanical and corrosion properties. In particular, the adoption of a higher temperature, i.e., 980 \({}^{\circ}\)C for 0.5 h, allows us to mitigate residual stresses while maintaining acceptable corrosion resistance. This alternative temperature also permits us to improve tensile deformability, which becomes even better than that in the as-built condition. This is one of the most important findings of this work, since the new stress-relieving treatment at 980 \({}^{\circ}\)C guarantees an optimal compromise among tensile properties, deformability, and corrosion resistance. This result is particularly important from a practical point of view, since it allows for the production of SLM parts with high mechanical properties that are superior to the minimum standard requirements even if slightly lower than those of the as-built condition and improved corrosion properties. Even though the selective laser-melted material is characterized by excellent mechanical properties, this work investigates the possibility of a further strength improvement by single- and double-aging treatments starting from previous results obtained for the conventional forged alloy. Regarding the single-aging treatments at 732 \({}^{\circ}\)C and 621 \({}^{\circ}\)C, compared to the conventional material, a faster precipitation response is obtained in the early stages of formation. On the other hand, the acceleration provided by the double-aging procedure is much more significant in the conventional forged alloy. In fact, in the selective laser-melted material subjected to double-aging treatments, the hardness increments are largely reduced, and this behavior is even more evident when the duration of secondary aging is increased. Therefore, in the selective laser-melted alloy, the beneficial effects of double aging are appreciably lower than those in the conventional alloy. Furthermore, considering that the tensile deformability is already close to the minimum standard prescription in the as-built condition, the adoption of aging treatments encounters some limitations because of deformability loss which is typically associated with the strengthening effect of intermetallic precipitates.