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The residual stress distribution in IN718 elongated prisms produced by selective laser melting was studied by means of neutron (bulk) and laboratory X-ray (surface) diffraction. Two deposition hatch lengths were considered. A horizontal plane near the top surface (perpendicular to the building direction) and a vertical plane near the lateral surface (parallel to the building direction) were investigated. Samples both in as-built (AB) condition and removed from the base plate (RE) were characterized. While surface stress fields seem constant for the AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements. Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e., with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the base plate.

The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed.

The relationship between residual stresses and microstructure associated with a laser powder bed fusion (LPBF) IN718 alloy has been investigated on specimens produced with three different scanning strategies (unidirectional Y-scan, 90° XY-scan, and 67° Rot-scan). Synchrotron X-ray energy-dispersive diffraction (EDXRD) combined with optical profilometry was used to study residual stress (RS) distribution and distortion upon removal of the specimens from the baseplate. The microstructural characterization of both the bulk and the near-surface regions was conducted using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). On the top surfaces of the specimens, the highest RS values are observed in the Y-scan specimen and the lowest in the Rot-scan specimen, while the tendency is inversed on the side lateral surfaces. A considerable amount of RS remains in the specimens after their removal from the baseplate, especially in the Y- and Z-direction (short specimen dimension and building direction (BD), respectively). The distortion measured on the top surface following baseplate thinning and subsequent removal is mainly attributed to the amount of RS released in the build direction. Importantly, it is observed that the additive manufacturing microstructures challenge the use of classic theoretical models for the calculation of diffraction elastic constants (DEC) required for diffraction-based RS analysis. It is found that when the Reuß model is used for the calculation of RS for different crystal planes, as opposed to the conventionally used Kröner model, the results exhibit lower scatter. This is discussed in context of experimental measurements of DEC available in the literature for conventional and additively manufactured Ni-base alloys.

Cold cracking resistance is a relevant evaluation criterion for welded joints

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, ...

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