# Selective Laser-Melted Alloy 625: Optimization of Stress-Relieving and Aging Treatments
## 3 Results and Discussion
### Single- and Double-Aging Treatments
by the adoption of temperatures above 875 \({}^{\circ}\)C. In this case, the reduction in residual stresses is ensured, but in order to not activate detrimental microstructural changes, it is necessary to select temperatures far enough from the nose of the TTP curves of the \(\delta\) phase, which is located at about 920 \({}^{\circ}\)C. However, upon increasing the temperature, the loss of mechanical strength due to microstructural recrystallization and coarsening can become significant, and it should be carefully considered in order to identify the condition with the best combination of tensile strength, deformability, and corrosion resistance. The influence of high-temperature annealing on the mechanical strength of the as-built material was investigated from 980 \({}^{\circ}\)C to 1100 \({}^{\circ}\)C. According to the results reported in Figure 7, thermal exposure above 875 \({}^{\circ}\)C determines a loss in mechanical strength compared to the as-built condition. In particular, at 980 \({}^{\circ}\)C and 1038 \({}^{\circ}\)C, the reduction in hardness is limited (10% and 15%, respectively), but this loss becomes more significant at 1100 \({}^{\circ}\)C (30 %). This reduction in mechanical strength upon high-temperature annealing was investigated via optical microscopy, as shown in Figure 8. By increasing the annealing temperature, the melt pool boundaries and the traces of the prior as-built microstructure become less visible, and they disappear almost completely at 1100 \({}^{\circ}\)C. In addition, the presence of recrystallized structures and annealing twins becomes more and more evident up to full recrystallization at 1100 \({}^{\circ}\)C, where the microstructural morphology is more similar to that of conventional forgings. This microstructural transformation and subsequent grain coarsening are associated with the loss in mechanical strength upon high-temperature annealing, as confirmed by the literature [19; 46]. According to the ASTM B446 standard [12], the corrosion resistance of the conventional alloy 625 can be improved by adopting a stabilization heat treatment at 980 \({}^{\circ}\)C after the solution-annealing treatment. For this reason and not to excessively reduce the mechanical strength of the as-built material, the temperature of 980 \({}^{\circ}\)C was selected as a potential alternative to the stress-relieving temperature of 875 \({}^{\circ}\)C. Moreover, according to the time-temperature-precipitation diagram reported in Figure 1, the onset of \(\delta\) phase formation at 980 \({}^{\circ}\)C is delayed compared to that at 875 \({}^{\circ}\)C. The feasibility of this alternative temperature was assessed via tensile and corrosion tests. The tensile properties are reported in Figure 9. The selection of the alternative heat treatment at 980 \({}^{\circ}\)C for 0.5 h is associated with a slight reduction in tensile strength (15 % for yield strength and 3 % for ultimate tensile strength) compared to the as-built condition. However, the tensile properties remain well above the minimum prescriptions of the ASTM B446 standard [12]. An important advantage of this new temperature is represented by the moderate increase in tensile deformability. In fact, in this case, the percentage of elongation after fracture A% satisfies the minimum standard requirement, while the stress-relieving treatment at 875 \({}^{\circ}\)C was associated with insufficient A% values, as shown in Figure 9. In addition, tensile deformability is also slightly improved compared to the as-built condition. Figure 7: Hardness variation after high-temperature annealing from the as-built condition. It is due to the recrystallization phenomena occurring during the heat treatment. The standard deviation values range from 1 to 3 HV30.