# Selective Laser-Melted Alloy 625: Optimization of Stress-Relieving and Aging Treatments
## 1 Introduction
Such peculiarity is fundamental to efficiently exploit topological optimization at the design stage, with the aim of obtaining improved geometries in terms of weight and mechanical performance. However, low productivity and high manufacturing costs remain critical aspects for a broader application of these techniques. Moreover, in the as-built condition, additively manufactured parts are typically associated with poor surface finishing and process-induced defects which can detrimentally affect the fatigue resistance and lead to incompatibility with geometrical tolerances. For this reason, post-processing techniques, such as hot isostatic pressing, shot peening, sand blasting, electropolishing, and machining, can be adopted to improve the surface quality and the defect content. In the as-built condition, selective laser-melted (SLMed) components are characterized by the presence of textured tracks with their orientation dependent on the scanning strategy. Because the process involves high cooling rates, the microstructure is characterized by melt pools composed of very fine dendritic structures with both columnar and cellular morphology. The cellular morphology is induced by the modification of the columnar one due to high cooling rates [33]. The presence of high solidification rates and several local thermal cycles determines the formation of a very fine microstructure with a high level of micro-segregation [17]. Even though such high cooling rates are able to develop metallic glasses, alloy 625 solidifies as a crystalline solid with the possible formation of metastable phases sometimes with an amorphous structure. According to the literature [34; 35], the dendrite cores are mainly enriched with nickel and chromium, while the interdendritic regions are rich in niobium and molybdenum and depleted in chromium. Such enrichment in niobium and molybdenum towards the interdendritic region promotes the formation of the \(\delta\) and \(\gamma^{\prime\prime}\) phases [17]. Therefore, the precipitation response in the selective laser-melted material is significantly enhanced. The comparison among the TTP curves of the conventional and selective laser-melted materials is reported in Figure 1. According to the literature [25], one of the main drawbacks of the SLM process is the formation of thermally induced residual stresses. This phenomenon can determine severe distortion and even failure caused by the accommodation of the residual stresses after the removal of the component from the build plate. Such behavior becomes more and more critical in the presence of thin components. This phenomenon is mainly provoked by the extremely high cooling rates (up to 10\({}^{6}\) K/s) achieved during the SLM process which determine strong thermal gradients between the layers [17]. For this reason, stress-relieving treatment performed before removing the components from the build plate is required to prevent such a detrimental phenomenon. According to the literature [5], the minimum stress-relieving temperature recommended for conventional alloy 625 is 870 \({}^{\circ}\)C (1 h per inch of section). In the selective laser-melted material, as observed by several authors [17; 18; 25], which is different from the wrought alloy, the formation of the detrimental \(\delta\), \(Ni_{3}Nb\) phase at 870 \({}^{\circ}\)C is obtained after very short exposure times (15 min). The precipitation of this phase provokes a significant decrease in toughness, tensile deformability, and corrosion resistance [25]. In fact, the low chromium content along the Ni\({}_{3}\)Nb precipitates of both the \(\gamma^{\prime\prime}\) and \(\delta\) phases creates local anodic regions in the microstructures which are preferentially attacked, leading to an accelerated corrosion rate [36; 37].