# Selective Laser-Melted Alloy 625: Optimization of Stress-Relieving and Aging Treatments
## 3 Results and Discussion
### Single- and Double-Aging Treatments
Such strength improvement is obtained at the expense of tensile deformability, which is reduced, but it remains well above the minimum standard requirement (30%) [12]. The selective laser-melted alloy 625 is characterized by excellent mechanical strength in the as-built condition, but the percentage of elongation after fracture A% is very close to the minimum standard requirement [12]. In this case, the aging treatment can be exploited to further improve its mechanical strength. Nevertheless, such a strengthening effect is obtained at the expense of deformability. In fact, for the tested aging conditions, the values of the percentage of elongation after fracture A% are lower than the minimum prescription [12]. Figure 5: (**a**) Hardness curves upon double aging at 732 \({}^{\circ}\)C and 621 \({}^{\circ}\)C of the selective laser-melted alloy 625 in the as-built condition; (**b**) comparison of the hardness increments with double aging allowed for the addition of primary aging compared to single aging at 621 \({}^{\circ}\)C. The standard deviation values range from 1 to 3 HV30. In the stress-relieved condition (875 \({}^{\circ}\)C 45 min), the mechanical properties of the selective laser-melted alloy 625 are similar to those in the as-built condition. However, tensile deformability is lower than the minimum prescription of the ASTM B446 standard [12]. In addition, the corrosion rate determined according to the ASTM G28-Method A standard is well above the limit value considered in this research work [38]. As reported in the literature [17; 25; 34; 35; 45], such dramatic degradation of properties is determined by the precipitation of carbides and the \(\delta\) phase starting from the interdendritic regions and moving towards the dendrite center. Therefore, this behavior confirms that the stress-relieving temperature of 875 \({}^{\circ}\)C (recommended for the conventional alloy) is not feasible in the selective laser-melted material because of its faster precipitation response. Also in the conventionally manufactured alloy 625, the formation of the \(\delta\) phase determines a severe loss of tensile deformability [2], but the precipitation of this embrittling phase requires at least 20 h at 875 \({}^{\circ}\)C, according to the time-temperature-precipitation diagram reported in Figure 1. Based on the literature TTP curve shown in Figure 1 for the selective laser-melted alloy 625, the onset of \(\delta\) phase formation at 875 \({}^{\circ}\)C requires only 15 min due to the presence of faster precipitation [17; 18]. According to the literature [25], the stress-relieving treatment at 875 \({}^{\circ}\)C is suitable to obtain a sufficient mitigation of residual stresses, but at the expense of a severe toughness loss due to \(\delta\) phase formation. For this reason, it is required to optimize the stress-relieving temperature by avoiding the simultaneous precipitation of detrimental phases. Such microstructural modifications can be avoided with a sufficient reduction in the stress-relieving temperature, but the mitigation of residual stresses can become insufficient [25]. Another possibility is represented  
Figure 6: Room-temperature tensile properties after single-aging treatments at 621 \({}^{\circ}\)C of the conventional forged and soft-annealed alloy 625 [43; 44] and the selective laser-melted one. Figures (**a,b**) report the yield strength (**a**) and the ultimate tensile strength (**b**) in selected conditions. The mechanical properties of the as-built material are significantly higher than those of the conventional forged alloy. Figures (**c,d**) report the percentage elongation after fracture A% (**c**) and the percentage reduction of area Z% (**d**). Tensile deformability is lower and even below the standard minimum requirement for the aged conditions. The standard deviation values for YS and UTS range from 2 to 13 MPa and from 1 to 2% for A% and Z%.