# Selective Laser-Melted Alloy 625: Optimization of Stress-Relieving and Aging Treatments
## 3 Results and Discussion
### Single- and Double-Aging Treatments
Figure 8: Optical micrographs on the ZY plane at 200X (**a**,**c**,**e**,**g**) and 500X (**b**,**d**,**f**,**h**) of the selective laser melted material after annealing at different temperatures from the as-built condition: (**a**,**b**) 875 \({}^{\circ}\)C 45 min; (**c**,**d**) 980 \({}^{\circ}\)C 0.5 h; (**e**,**f**) 1038 \({}^{\circ}\)C 0.5 h; (**g**,**h**) 1100 \({}^{\circ}\)C 0.5 h.  
The susceptibility to intergranular corrosion was investigated following the ASTM G28--Method A standard [38]. The results obtained on the selective laser-melted material were compared with those obtained on the conventional forged alloy 625 published by [PERSON] et al. in previous research studies [43,44]. All the experimental results were collected in the performance map reported in Figure 10 relating to the tensile and corrosion properties. In Figure 10, the green shaded area corresponds to the acceptability region with respect to the minimum standard requirements for the tensile properties [12]. Since the minimum yield strength requirement is more critical compared to the ultimate tensile strength one, the values associated with the latter property were not included in the performance map. The data labels report the corrosion rate in mm/year according to the ASTM G28--Method A standard. An acceptable limit for industrial applications of this grade is 1.2 mm/year (green label: acceptable; red label: not acceptable). Corrosion resistance and tensile deformability in the case of stress-relieving treatment at 875 \({}^{\circ}\)C are significantly improved after modifying the standard recommended temperature with a higher value, i.e., 980 \({}^{\circ}\)C, at the expense of a slight reduction in mechanical strength. Regarding the single-aging treatment at 621 \({}^{\circ}\)C, the strengthening effect is obtained with a detrimental reduction in corrosion resistance. Similarly, in conventional forged alloy 625, the increase in mechanical strength by aging treatment reduces both tensile deformability and corrosion resistance. In this case, full acceptability is obtained in a very narrow region, as also demonstrated by our previous research studies [43,44]. For this reason, considering the strict requirement for the corrosion resistance and rapid degradation of this property upon aging, the exploitation of aging treatments is significantly limited, and it requires a careful optimization of the aging time. Such limitation is even more critical in the selective laser-melted material because of its faster precipitation response. In this case, an additional obstacle to the adoption of aging treatments is represented by the rapid loss of the tensile deformability below the minimum standard requirement. Figure 6.0 Room-temperature tensile strength (**a**) and tensile deformability (**b**) after heat treatment at 980 \({}^{\circ}\)C for 0.5 h compared to the as-built condition and to those obtained at the stress-relieving temperature recommended by the standard (875 \({}^{\circ}\)C). The mechanical resistance is slightly less than that of the as-built and 850 \({}^{\circ}\)C stress-relieved conditions, but the tensile deformability is improved and superior to the minimum standard requirement. The standard deviation values for YS and UTS range from 3 to 12 MPa and up to 1% for A% and Z%. The corroded profiles were investigated in selected conditions for having a relationship with the corrosion rates and characterize the corrosion mechanism. The melt pool boundaries of the laser tracks are preferentially corroded compared to the inner regions. This behavior is confirmed by the literature data, which address such selective attack to the enhanced precipitation of sensitizing Ni\({}_{3}\)Nb phases in these zones [33; 36; 47]. The corrosion rates are in good agreement with the observed corroded profiles. Figure 11 shows some sections of the specimens subjected to the corrosion tests cut perpendicularly to the surfaces exposed to the corroding solution. Figure 11a,b represent the as-built condition.