Defects and Their Fingerprint in PBF Processes
Causes of Defects
The defects and process errors in PBF can be grouped into four major categories, depending on their root causes (Mani et al. 2017, Everton et al. 2016, Grasso and Colosimo 2017): design and job preparation, feedstock material, equipment and process conditions.
Defects Induced by Feedstock Material
Different defects may affect the metal powder used in PBF, ranging from irregular morphologies to the presence of powder-entrapped porosity, satellited or broken particles, etc.
Dimensional and morphological issues affect the apparent density and flowability of the powder, which have a direct impact on the quality and performance of the final part. In addition, metallic powders may be contaminated by moisture, organics, adsorbed gases, oxides and nitride films on particle surfaces (Das 2003). Such contamination, which may vary and increase along consecutive power reuse cycles, may degrade the mechanical properties and the geometrical accuracy of the consolidated component in PBF processes (Das 2003, Tang et al. 2015, Seyda et al. 2012, Hann 2016).
Equipment-induced defects include various types of flaws and process errors that may originate as a result of improper or degraded performance of critical machine components and sub-systems, for example the beam scanning/deflection apparatus, the build chamber environmental control and the powder handling and deposition equipment. In laser powder bed fusion (L-PBF), deviations from the nominal beam profile properties, together with out-of-calibration laser scanner conditions and imperfections or contamination of lenses and mirrors, may result in parts with inaccurate final dimensions, internal defects and other location effects that reduce the part-to-part reproducibility (Moylan et al. 2014b, Foster et al. 2015, Chapter 8). In electron beam powder bed fusion (EB-PBF), sources of process errors and defects include worn cathodes, inaccurate beam calibration settings and electromagnetic interference. Other defects are related to the control of the chamber environment in both L-PBF and EB-PBF. The flow rate and laminarity of the inert gas in L-PBF are known to have a direct impact on the part quality, including porosity and geometrical accuracy (Ferrar et al. 2012). The oxygen content and the chamber pressure also influence the interaction between the laser and the material, the surface chemical properties of the solidified material and the process by-products that may induce further unstable conditions and powder bed contamination (Spears and Gold 2016).
Another critical part of the equipment that can induce and propagate defects within the build area is the powder handling and deposition system. A worn recoating system (Foster et al. 2015), and the occurrence of impacts between the recoating system and super-elevated edges of the printed parts, may yield an inhomogeneous powder bed and uneven layer thickness with consequent internal, geometrical and surface defects in the part. Although recoating errors are, in most cases, a consequence of other issues and powder bed contamination, the linear motion of the recoating system causes a propagation of defects originated in some area of the layer along the recoating direction. This is particularly critical in industrial processes where several parts are stacked in the same build volume to optimise production time and costs.