During powder bed fusion, a laser or an electron beam selectively melts a stack of powder layers, recoated and machined in succession one after the other, to form a component.
The density of the solidified material and the surface roughness of the finished components are complex functions of the material characteristics and the process parameters.
When studying the melting and solidification processes, considering individual powder particles reveals details about several physical phenomena, for instance the relationship between capillary effects, wetting conditions and the local stochastic powder configuration. This can help elucidate the fundamental mechanisms responsible for the phenomena involved in pores formation and surface roughness development that are observed during selective beam melting .
Let’s consider the use of electron beam melting to manufacture thin walls using commercially available gas atomized pre-alloyed Ti64 powder. Let’s also assume the powder has a Gaussian particle size distribution between 45μm and 115μm. The electrons are focused and deflected by electromagnetic lenses and release their kinetic energy to the powder particles, which causes them to heat.
The thin vertical walls are built on a stainless steel substrate. The platform is first heated with the defocused electron beam to a temperature of 760C before a layer of Ti–6Al–4V powder is spread over the platform. At this point, the powder-covered platform is again preheated by scanning the defocused electron beam across the entire bed. This procedure sinter the powder particles , immobilizing the powder and increasing thermal and electrical conductivity.
Factors affecting the appearance of vertical thin walls
When investigating the effect of the layer thickness on the resolution and aspect of thin walls during SEBM, results show the quality of the walls decreases dramatically with increasing layer thickness.
For a fixed beam energy:
When the line energy is adapted to the layer thickness in such a way that the total energy input per volume is the same for all layer thicknesses (proportionally smaller energy input for thinner layers):
- Influence of beam energy and scanning speed
For a fixed line energy, the appearance of the walls varies with the scanning speed. When increasing scanning speed and beam power (to keep line energy constant), the wall thickness and the surface roughness increase. The interaction time between beam and powder is reduced as the total energy input remains the same.
The aspect of thin wall built using SEBM is highly sensitive to processing parameters: powder particles, layer thickness, beam energy, scanning speed. The final surface roughness is generally higher than what would be expected from the mean powder particle diameter.
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1] Carolin K¨orner, Andreas Bauereiß and Elham Attar, Fundamental consolidation mechanisms during selective beam melting of powders, Modelling Simul. Mater. Sci. Eng. 21 (2013) 085011 (18pp) doi:10.1088/0965-0393/21/8/085011
 K¨orner C and Attar E 2011 Mesoscopic simulation of selective beam melting processes J. Mater. Process.Technol. 211 978–87
Laser Direct Deposition