Particles-wall friction behaviour, a new way to characterize powder spreadability in L-PBF applications.

  • Stephan, Maxime (Univ. Grenoble Alpes, CEA)
  • Roux, Guilhem (Univ. Grenoble Alpes, CEA, LITEN)
  • Burr, Alexis (Univ. Grenoble Alpes, CEA, LITEN)
  • Ablitzer, Carine (CEA)
  • Garandet, Jean-Paul (Univ. Grenoble Alpes, CEA, LITEN)

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Laser Powder Bed Fusion (L-PBF) is an additive manufacturing (AM) process. Parts are built layer by layer; a laser locally fuses the powder brought by successive powder bed depositions. Final part quality highly depends on powder bed quality, e.g. low surface roughness and high packed density. Powder spreading consists in a recoater, here a blade, pushing a powder heap over a building plate, with a gap under the blade allowing the flow and spread of particles. Powder spreadability is not always linked to powder flowability due to the complexity of particles movement in the powder heap with different flowing zones. Contact between particles and building plate is still unexplored. Due to the fact that it occurs on a previous manufactured layer except for the first one, it involves non-standard surface morphology. A first preliminary work using DEM simulation shows that the contact between particles and building plate has a major effect on the particles deceleration from blade velocity to zero. Moreover, the thicker the deceleration zone, the denser the deposited powder bed. The thickness of the deceleration zone depends on wall sliding and rolling friction coefficients, while flowability is more related to rolling friction. Local friction mechanisms with building plate can be averaged at heap scale with the introduction of a macroscopic wall friction coefficient computed from resultant forces on blade and plate. It is also expressed by a wall friction angle (WFA). Macroscopic wall friction coefficient depends on aforementioned coefficients. It can be experimentally estimated using a FT4 rheometer with friction disks manufactured by L-PBF, and so permits to have a simple experimental set-up to predict spreadability. Moreover, we can identify the powder rolling friction coefficient by comparison with experimental pure sliding (measured using tilted plane experiment). By using different surface roughnesses, it is possible to characterize each powder-surface couple. Indeed, a larger plate roughness increases wall sliding and rolling friction coefficients whereas powder properties are unchanged.