Additive Manufacturing Strategies

Processing Parameters in SLM 3D Printing: UK Researchers Test Ti6Al4V Cellular Structures

ST Medical Devices

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In ‘The influence of processing parameters on strut diameter and internal porosity in Ti6Al4V cellular structure,’ UK researchers from the University of Birmingham look further into strut size and porosity issues during bioprinting, and discuss the overall challenges of selective laser melting (SLM) in additive manufacturing. In this research, SLM 3D printing was used to create Ti6Al4V cellular structures, but with a wide range of different parameters.

While porous structures are attractive in many applications today for industries like automotive and aerospace, when created with titanium alloys they ensure strength, corrosion resistance, and the proper amount of density required. Even more importantly, however, lattice structures like Ti6Al4V offer high biocompatibility. Made up of a network of struts that form cells to make lattices, these complex structures are often manufactured with conventional techniques like casting; however, with AM technology, complex geometries can be produced faster and more affordably.

As the researchers point out however, problems can occur in SLM printing when conditions are not properly optimized—resulting in defects due to a ‘mismatch’ between the 3D design and the 3D print. The team set up an experiment for testing parameters and pinpointing a way to improve SLM methods.

Lattice structure fabricated
using SLM

They created a set of structures ranging from 100W to 300W and scan speed ranging 8000 mm/s to 4000 mm/s. Lattices were assessed regarding the effects of input energy on strut diameters, and porosity levels. As they suspected due to compiled data from previous research studies, increased input energy resulted in increased strut diameters:

“This relation is attributed to the fact that inclined struts were built partially on loose powder, which resulted in adhesion of free powder (partially melted powder particles) to the surfaces of the struts. At high input energy condition, the energy transferred to attach powder particles was high enough to result in full melting of the attached powders and hence became part of the fabricated strut.”

Different zones were created based on changes in input energy:

  • Zone 1 – low input energy was directed here, leading to ‘discontinuity’ in the strut. The researchers noted this was due to lack of diffusion between melt pools, along with a balling effect that typically causes defects in SLM.
  • Zone 2 – as the zones ascend in energy, this one is a result of intermediate laser power and scan speed. The researchers noted the formation of irregular defects, again, without diffusion between melt pools. They also noted erratic formation in the struts, resulting in ‘waviness.’
  • Zone 3 – this zone formed with the pairing of higher laser power but low scanning speed, ‘mitigating the previously formed lack of diffusion defects.’

A diagram showing the variation of Strut diameter as a function of increasing linear input energy diameter.

“SLM processing parameters investigated in the current research shows that the input energy density has a significant influence on the strut diameter and porosity morphology within the fabricated struts. Different zones were developed based on changing the input energy,” concluded the researchers.  “Additionally, it was observed that strut diameter size for Ti6Al4V lattice structure increased with increasing the input energy density.”

While much about 3D printing can be deceptively simple, additive manufacturing processes are often more complicated in hardware, software, materials required, and technique. Selective laser melting, although it may offer some challenges, continues to be at the forefront of research projects and studies today, from new procedures created for heat accumulation detection to fabricating steel nuclear components or working with metallic glass. Find out more about strut diameter and porosity issues with this method too here. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

SEM images for struts at different fabrication conditions (a) 100W &4000mm/s (b) 200W & 2400mm/s (c) 300W & 800mm/s

[Source / Images: The influence of processing parameters on strut diameter and internal porosity in Ti6Al4V cellular structure]

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