Powder bed fusion: factors affecting the surface roughness of thin walls

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 [1].

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 [2], immobilizing the powder and increasing thermal and electrical conductivity.

Factors affecting the appearance of vertical thin walls

1. “Powder” effect: spreading and solidification
   
   

The first layer of our example thin walls is melted directly on the preheated building substrate (760C). Two things occur:

• Recoating: the first 100μm-thick powder layer is rather scarce and inhomogeneous
  
  
• Solidification: the localised melt pool geometries are a bit random.
  
  

After solidification, a new powder layer is applied upon a concave, convex or stochastic geometry. Add to this recoating of powder particles, and the aspect of each layer looks different (see picture). This “powder” (or stochastic) effect has a great impact on single melt tracks aspects and resolution.

1. Layer thickness
   
   

The layer thickness is one of the main process parameters for layer based additive manufacturing processes. For SEBM, the layer thickness typically varies between 50 and 150μm.

[1]

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:

• using layer thicknesses smaller than 70μm doesn’t improve the surface quality.
  
  
• wall width increases as the total energy per volume increases.
  
  

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):

• the wall thickness and surface roughness are larger for thicker (100μm thick) layers;
  
  
• reducing layer thickness (50 / 70um) has only a marginal impact on improving the surface roughness of the thin walls. This is due to the random behaviour of powder particles recoated over random melt pool shapes (see above).
  

1. Influence of beam energy and scanning speed
   
   

The mean thickness of the thin walls increases in tune with increasing line energies.

 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.

As the successive layers get process, instability becomes more visible for higher layers. Strong geometry changes in the melt pools compound withe the homogeneity of the recoated new powder layer to create a snow-ball effect. This favour instabilities during build and affect the surface roughness of the thin walls.

Take-away

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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References

 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

[2]  K¨orner C and Attar E 2011 Mesoscopic simulation of selective beam melting processes J. Mater. Process.Technol. 211 978–87