# Selective Laser-Melted Alloy 625: Optimization of Stress-Relieving and Aging Treatments
## 3 Results and Discussion
### Single- and Double-Aging Treatments  
The age-hardening response upon single-aging treatments was investigated in the selective laser-melted material at the same aging temperatures considered by [PERSON] et al. [43; 44] for the conventional wrought alloy, i.e., 732 \({}^{\circ}\)C and 621 \({}^{\circ}\)C. The experimental single-aging curves obtained using the hardness tests are reported in Figure 4 and were compared with those obtained by [PERSON] et al. [43; 44] on the conventional forged and soft-annealed material. The main difference is related to the initial hardness, which is significantly lower in the forged material (soft-annealed condition) compared to the selective laser-melted one (as-built condition). As previously described, this difference can be ascribed to the improved microstructural fineness in the selective laser-melted alloy. For such a reason, the conventional forged and soft-annealed alloy requires a single aging process at 621 \({}^{\circ}\)C for 600 h to attain a hardness value equal to that in the as-built condition for the selective laser-melted material. Regarding the hardness increment, the selective laser-melted material is characterized by a faster response in the early stage of precipitation, especially in the case of single aging at 621 \({}^{\circ}\)C. Then, the rate of hardness increase starts reducing compared to the behavior of the conventional material. Figure 4: Hardness curves after single-aging treatments at 621 \({}^{\circ}\)C (**a**,**c**) and 732 \({}^{\circ}\)C (**b**,**d**) of the selective laser-melted alloy in comparison to the conventional forged one. The hardness increase induced by the aging treatment is significantly less for the SLMed specimens, even if their hardness always remains higher than that of the forged material. The standard deviation values range from 1 to 5 HV30. Successively, starting from the results obtained by [PERSON] et al. [43; 44] on the double aging of the conventional forged alloy, the double-aging response with primary aging at 732 \({}^{\circ}\)C and secondary aging at 621 \({}^{\circ}\)C was also investigated in the selective laser-melted material by varying the exposure times at both the temperature levels. The results, reported in Figure 5, were compared to those obtained in the conventional material. Figure 5b shows that the hardness increase allowed for the addition of the primary aging step at 732 \({}^{\circ}\)C compared to single aging at 621 \({}^{\circ}\)C. According to the results reported in Figure 5b, the acceleration provided by the double-aging procedure is more significant in the conventional forged alloy compared to the selective laser-melted one. In fact, in the latter case, the hardness increment is largely reduced, and this behavior is even more evident when the duration of secondary aging is increased. Therefore, in the selective laser-melted material, the beneficial effects of double aging in terms of mechanical strengthening are appreciably lower than those experimented in the conventional alloy. Such different behavior is probably determined by the faster precipitation response of the additively manufactured material that does not need additional primary aging at higher temperatures to trigger and accelerate the age-hardening process, as is required for the conventional alloy [43; 44]. The influence of the single-aging treatments at 621 \({}^{\circ}\)C on room-temperature mechanical strength was investigated via tensile tests for both the selective laser-melted alloy and the conventional forged one. The results are reported and compared in Figure 6. In the conventional material, because of the presence of insufficient mechanical properties in the soft-annealed condition, the aging treatment is required to satisfy the minimum standard requirements [12]. In particular, in this case, single aging at 621 \({}^{\circ}\)C for 130 h is associated with acceptable tensile properties.