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Additive manufacturing (AM) of metals and in particular laser powder bed fusion (LPBF) enables a degree of freedom in design unparalleled by conventional subtractive methods. To ensure that the designed precision is matched by the produced LPBF parts, a full understanding of the interaction between the laser and the feedstock powder is needed. It has been shown that the laser also melts subjacent layers of material underneath. This effect plays a key role when designing small cavities or overhanging structures, because, in these cases, the material underneath is feed-stock powder. In this study, we quantify the extension of the melt pool during laser illumination of powder layers and the defect spatial distribution in a cylindrical specimen. During the LPBF process, several layers were intentionally not exposed to the laser beam at various locations, while the build process was monitored by thermography and optical tomography. The cylinder was finally scanned by X-ray computed tomography (XCT). To correlate the positions of the unmolten layers in the part, a staircase was manufactured around the cylinder for easier registration. The results show that healing among layers occurs if a scan strategy is applied, where the orientation of the hatches is changed for each subsequent layer. They also show that small pores and surface roughness of solidified material below a thick layer of unmolten material (>200 mu m) serve as seeding points for larger voids. The orientation of the first two layers fully exposed after a thick layer of unmolten powder shapes the orientation of these voids, created by a lack of fusion.
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.
This study examines the relationship between the magnetic mesostructure with the microstructure of low carbon steel tungsten inert gas welds. Optical microscopy revealed variation in the microstructure of the parent material, in the heat affected and fusion zones, correlating with distinctive changes in the local magnetic stray fields measured with high spatial resolution giant magneto resistance sensors. In the vicinity of the heat affected zone high residual stresses were found using neutron diffraction. Notably, the gradients of von Mises stress and triaxial magnetic stray field modulus follow the same tendency transverse to the weld. In contrast, micro-X-ray fluorescence characterization indicated that local changes in element composition had no independent effect on magnetic stray fields.
The control of residual stress (RS) remains a challenge in the manufacturing of metallic parts using the laser powder bed fusion process (LPBF). This layer-by-layer manufacturing approach gives rise to complex triaxial RS distributions, which require extensive characterization effort for a broader acceptance of LPBF in industry. This study focuses on the distribution of bulk triaxial RS and surface RS in LPBF austenitic steel 316L. The RS are determined by X-ray and neutron diffraction to characterize the RS distribution. Variations in the LPBF parameters interlayer time (ILT) and scanning velocity and their influence on the temperature distribution and resulting RS is investigated using thermographic data from in situ process monitoring. The RS in the LPBF 316L is tensile at the surface and compressive in the bulk. The RS is directly related to the thermal history of the part as shown by the in situ thermography data. Shorter ILT leads to higher temperatures of the part during the manufacturing, which decrease the RS and RS formation mechanisms. Interestingly, the surface RS does not agree with this observation. This study highlights the benefit of using multiple RS determination methods and in situ thermography monitoring to characterize the RS in LPBF processed parts.
Talbot-Lau interferometry provides X-ray imaging techniques with significant enhancement of the radiographic contrast of weakly absorbing objects. The grating based technique allows separation of absorption, refraction and small angle scattering effects. The different efficiency of rectangular and triangular shaped phase gratings at varying detector distances is investigated. The interference patterns (Talbot carpets) are modeled for parallel monochromatic radiation and measured by synchrotron radiation. In comparison to rectangular shapes of phase gratings much higher visibility is obtained for triangular shapes which yield enhanced contrast of a glass capillary test specimen.
The performance of grating interferometers coming up now for imaging interfaces within materials depends on the efficiency (visibility) of their main component, namely the phase grating. Therefore, experiments with monochromatic synchrotron radiation and corresponding simulations are carried out. The visibility of a phase grating is optimized by different photon energies, varying detector to grating distances and continuous rotation of the phase grating about the grid lines. Such kind of rotation changes the projected grating shapes, and thereby the distribution profiles of phase shifts. This yields higher visibilities than derived from ideal rectangular shapes. By continuous grating rotation and variation of the propagation distance, we achieve 2D visibility maps. Such maps provide the visibility for a certain combination of grating orientation and detector position. Optimum visibilities occur at considerably smaller distances than in the standard setup.
The paper focuses on the reformulation of classic Maxwell's (1873) homogenization method for calculation of the residual stresses in matrix composites. For this goal, we equate the far fields produced by a set of inhomogeneities subjected to known eigenstrains and by a fictitious domain with unknown eigenstrain. The effect of interaction between the inhomogeneities is reduced to the calculation of the additional field acting on an inhomogeneity due to the eigenstrains in its neighbors. An explicit formula for residual stresses is derived for the general case of a multiphase composite. The method is illustrated by several examples. The results are compared with available experimental data as well as with predictions provided by the non-interaction approximation (Eshelby solution). It is shown that accounting for interaction can explain many experimentally observed phenomena and is required for adequate quantitative analytical modeling of the residual stresses in matrix composites.
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 degrees) 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.
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 degrees XY-scan, and 67 degrees 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 Kroner 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.
In the present work, electron backscatter diffraction was used to determine the microscopic dislocation structures generated during creep (with tests interrupted at the steady state) in pure 99.8% aluminium. This material was investigated at two different stress levels, corresponding to the power-law and power-law breakdown regimes. The results show that the formation of subgrain cellular structures occurs independently of the crystallographic orientation. However, the density of these cellular structures strongly depends on the grain crystallographic orientation with respect to the tensile axis direction, with (111) grains exhibiting the highest densities at both stress levels. It is proposed that this behaviour is due to the influence of intergranular stresses, which is different in (111) and (001) grains.