The residual stress in as-built Laser Powder Bed Fusion IN718 alloy as a consequence of the scanning strategy induced microstructure

Abstract

The effect of two types of scanning strategies on the grain structure and build-up of Residual Stress (RS) has been investigated in an as-built IN718 alloy produced by Laser Powder Bed Fusion (LPBF). The RS state has been investigated by X-ray diffraction techniques. The microstructural characterization was performed principally by Electron Backscatter Diffraction (EBSD), where the application of a post-measurement refinement technique enables small misorientations (< 2°) to be resolved. Kernel average misorientation (KAM) distributions indicate that preferably oriented columnar grains contain higher levels of misorientation, when compared to elongated grains with lower texture. The KAM distributions combined with X-ray diffraction stress maps infer that the increased misorientation is induced via plastic deformation driven by the thermal stresses, acting to self-relieve stress. The possibility of obtaining lower RS states in the build direction as a consequence of the influence of the microstructure should be considered when envisaging scanning strategies aimed at the mitigation of RS. IN718 is a Ni-based superalloy widely used in aviation (e.g., aircraft engines) and energy industries (e.g., power generation turbines) due to its exceptional combination of high-temperature mechanical stability (up to 650 °C), good fatigue life and high resistance to degradation in corrosive or oxidizing environments 1. Nonetheless, nickel alloys are also known for their poor machinability 2. Compared to conventional subtractive routes, Additive Manufacturing (AM) permits the production of 3D complex near-net-shape components with greatly reduced machining, buy-to-fly ratios close to 1 and shorter production times, while also providing comparable material performances 3. Among AM techniques, Laser Powder Bed Fusion (LPBF) consists of a layer-by-layer selective melt of a powder bed using a focused energy source. Unfortunately, one important shortcoming of LPBF is the formation of complex Residual Stress (RS) states, with values in the range of the yield strength of a material when connected to the baseplate. This RS state results from the steep spatial and temporal thermal gradients, locally induced by the laser during processing 4,5. RS is defined as the stress in a body with uniform temperature that is in equilibrium in the absence of external forces 6. In general, RS is undesirable, especially if tensile, as they can negatively affect mechanical properties, distort the design geometry, and induce cracking or delamination 5,7. Considerable amount of work (both experimental and numerical) has been reported to elucidate and mitigate macroscopic RS in AM 3. In general, the RS tends to be tensile towards the surfaces of the sample and compressive at the centre of LPBF parts 5,8-22. Moreover, the largest tensile values tend to occur near the top (last added layers) of the sample, when it remains connected to the baseplate. The magnitude and trend of RS have been shown to vary depending on parameters such as the geometry of the part, ...

On the interplay of microstructure and residual stress in LPBF IN718

“…This relaxation induces an upward b Figure 4 a Illustration of the specimen geometry showing the As-Built and Thin-Baseplate conditions. RS profiles of the X-direction component before and after baseplate thinning for b the Y-scan specimen (replotted from [25]), c the XY-scan specimen, and d the Rot-scan specimen (replotted from [25]). RS profiles of the Y-direction component before and after baseplate thinning for e the Y-scan specimen (replotted from [25]), f the XY-scan specimen, and g the Rot-scan specimen (replotted from [25]).…”

“…RS profiles of the X-direction component before and after baseplate thinning for b the Y-scan specimen (replotted from [25]), c the XY-scan specimen, and d the Rot-scan specimen (replotted from [25]). RS profiles of the Y-direction component before and after baseplate thinning for e the Y-scan specimen (replotted from [25]), f the XY-scan specimen, and g the Rot-scan specimen (replotted from [25]). RS profiles of the Von Mises (VM) component before and after baseplate thinning for h the Y-scan specimen, i the XY-scan specimen, and j the Rot-scan specimen.…”

“…Lowmagnification EBSD orientation maps in the ZY-planes of d the Y-scan specimen, e the XY-scan specimen, and f the Rot-scan specimen. Low-magnification EBSD orientation maps on the XYplane of g the Y-scan specimen (reproduced from [25]), h the XYscan specimen, and i the Rot-scan specimen (reproduced from [25]). In all EBSD maps the scattering vector is perpendicular to the images (i.e., conventionally termed IPF-Z maps).…”

“…With this view, we consider that two fundamental points are so far missing in the AM literature: (i) the textured microstructures resulting from AM necessitates a redefinition of the diffraction elastic constants (DEC) used for the calculation of macro-stresses; and (ii) the formation of RS and microstructure are so far addressed in the literature as two independent results from the selection of the scanning strategy: we propose that in fact they are interconnected, and that this relationship needs to be studied in further detail. In this regard, a previous work by the authors has shown that highly textured columnar grains (i.e., anisotropic grain structure), as opposed to mildly textured elongated grains, can more easily accumulate strain dislocations along the building direction, which is considered to lead to increased RS relaxation [25].…”

“…In LPBF, the surface roughness principally varies depending on the process parameters and on the orientation of the build direction with respect to the recoating blade [21][22][23]. The surface roughness severity is a limiting factor when measuring surface and near-surface RS since it locally relaxes the stress, creating gradient of increasing stress towards the interior [24,25]. Thus, the influence of the process parameters (in our case the scanning strategy) on the surface roughness needs to be considered when measuring surface/near-surface RS.…”