Results and Discussions As depicted in Figs. 3 and 5, the microstructure of SLM GH3536 exhibits clear anisotropy. The grains in X-Y plane comprise equiaxed crystals with〈001〉and〈101〉preferred orientations, whereas the grains in Y-Z plane are epitaxial columnar crystals. Additionally, the〈001〉crystal direction is roughly parallel to the direction of material addition. Moreover, the SLM specimen exhibits strong {100} texture components in both the X-Y and Y-Z planes. As a result of the extremely rapid cooling rate of SLM, columnar and cellular subgrains are formed that are distinct from those produced via conventional processes. Cracks are also observed in the molten pool; those in the Y-Z plane tend to be parallel to the additive direction, while those in the X-Y plane are randomly distributed. After the HIP treatment, the original melt pool morphology and laser scanning traces on the surface of the specimen completely disappear, and it is difficult to observe the subgrain structure in the specimen (Figs. 3 and 5). Moreover, a large number of precipitated phases exists along the grain boundaries, and EDS results reveal that the precipitated phases are chromium-rich M23C6 carbides and molybdenum-rich M6C carbides (Fig. 4). Cracks in the heat-treated specimens are welded simultaneously. A large number of twin boundaries are formed during recrystallization and are related to the evolution of the low-angle boundaries (LAGBs), as indicated by the grain boundary characteristic diagram (Fig. 6). The tensile properties of the SLM specimens are anisotropic (Fig. 7), with the transverse specimens exhibiting higher strength but less toughness. After heat treatment, the tensile properties of the heat-treated specimens are essentially isotropic; the yield strength of the HIP specimens is decreased, and the toughness is increased as a result of the decrease in LAGBs and the increase in densities. In addition, the fracture morphology (Figs. 8 and 9) reveals that the SLM specimens exhibit the mixed tough-brittle fracture in the transverse direction and the typical ductile fracture in the vertical direction, whereas the HIP specimens exhibit the typical ductile fracture (Fig. 10).
The production of nickel-based high-temperature alloys using selective laser melting (SLM) technology offers significant advantages in terms of material savings, process control, and product performance. However, the use of SLM technology presents several challenges, such as process-induced defects, microsegregation, and anisotropy of the mechanical properties owing to microstructural inhomogeneities. Although several studies on SLM GH3536 have been conducted worldwide, these studies have primarily focused on the effects of process parameters and heat treatment processes on the formability of SLM. Owing to the process characteristics of SLM technology, substantial differences exist in the microstructure and properties of the alloys formed in the additive and laser scanning directions, and these differences have a significant impact on their application. Simultaneously, hot isostatic pressing (HIP) treatment can significantly reduce the defects formed during the SLM process and has a significant impact on the mechanical properties of the material. Therefore, it is necessary to examine the changes in the microstructure and anisotropy of the mechanical properties of SLM-formed alloys before and after HIP. However, there are few systematic studies on the anisotropy of the mechanical properties of SLM-formed GH3536 alloys due to microstructural inhomogeneities, and comparative studies in conjunction with heat treatment are rarely reported in the scientific literature. Using SLM-formed GH3536 specimens as the research object, the microstructure and crystallographic texture characteristics of deposited and HIP specimens are characterized in this study. Additionally, the anisotropy of the mechanical properties of the alloy and its causes are discussed on the basis of the mechanical properties and fracture morphology. Moreover, the effect of HIP on the anisotropic behavior of SLM-formed GH3536 is investigated. This study provides a reference for the practical application of SLM-formed GH3536 alloys.
SLM GH3536 specimens were prepared using SLM equipment, and HIP was applied to the SLM-formed specimens. Using a wire-cutting machine, the prepared samples were then cut into bulk metallographic samples and room-temperature tensile samples. The X-Y and Y-Z planes of the block specimens were polished using sandpaper, followed by diamond polishing. The as-polished samples were etched for 10 s using aqua regia, and their microstructures were observed via optical microscopy (OM) and scanning electron microscopy (SEM). The mechanically polished samples were electropolished with perchloric acid, and the electron backscatter diffraction (EBSD) technique was used to analyze the crystal structures of the materials. Finally, the transverse and vertical tensile specimens were tested at a rate of 5 mm/min using a universal tensile testing machine at room temperature, and the corresponding tensile fractures were observed.
The microstructure and mechanical properties of GH3536 nickel-based superalloy formed via SLM during deposition and subsequent heat treatment are investigated in this study, and the following conclusions are drawn. The deposited specimens exhibit ultrafine grain structures and good tensile properties. The preferred orientations of the equiaxed crystals in the X-Y plane are〈001〉and〈101〉, and the〈001〉direction of columnar crystal structure grown epitaxially in the Y-Z plane is roughly parallel to the additive direction. Considering the influence of grain size and texture strength, the yield and tensile strengths of the transverse specimen are higher than those of the longitudinal specimen, but their ductility is lower. In addition, brittle fracture characteristics and incomplete fusion defects are observed on the fracture surfaces. Following HIP treatment, the columnar grains undergo an equiaxed transformation; the grain orientation is randomly distributed; carbide precipitation is observed along the grain boundary, and the crack heals. Moreover, the tensile strength of the sample decreases; its plasticity increases, and its anisotropy disappears.
