@article{UlbrichtMohrAltenburgetal.2021,
  author    = {Ulbricht, Alexander and Mohr, Gunther and Altenburg, Simon J. and Oster, Simon and Maierhofer, Christiane and Bruno, Giovanni},
  title     = {Can potential defects in LPBF be healed from the laser exposure of subsequent layers?},
  series = {Metals : open access journal},
  volume    = {11},
  journal   = {Metals : open access journal},
  number    = {7},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2075-4701},
  doi       = {10.3390/met11071012},
  pages     = {14},
  year      = {2021},
  abstract  = {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.},
  language  = {en}
}
@article{SprengelMohrAltenburgetal.2021,
  author    = {Sprengel, Maximilian and Mohr, Gunther and Altenburg, Simon J. and Evans, Alexander and Serrano-Munoz, Itziar and Kromm, Arne and Pirling, Thilo and Bruno, Giovanni and Kannengießer, Thomas},
  title     = {Triaxial residual stress in Laser Powder Bed Fused 316L},
  series = {Advanced engineering materials},
  volume    = {24},
  journal   = {Advanced engineering materials},
  number    = {6},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1438-1656},
  doi       = {10.1002/adem.202101330},
  pages     = {13},
  year      = {2021},
  abstract  = {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.},
  language  = {en}
}