From the Lab: What affects porosity in Laser Powder Bed Fusion additive manufacturing?

Any casual observer of additive manufacturing (AM) is likely to believe that it is a simple process: choose a design, click a button, and moments later, a defect-free metal part appears from nowhere.
 
In reality, the process of using a laser to fuse metallic powder in layers can create defects, such as porosity, that can lead to weakness in the part itself. This is a risk, in particular for the Laser Powder Bed Fusion (PBF) technique that is popular in the marine and offshore energy industries for printing small-scale parts.
 
If we examine the technique itself, the process is predisposed to producing defects. Using a laser to fuse powder can produce spherical voids, or pores, in the resulting part. Using too much or too little energy, and it results in pores of different quantities, shape and sizes. Furthermore, different parameter combinations can create unpredictable results that don’t adhere to a linear relationship. For example, increasing power can sometimes create fewer pores, or even more, all dependent on the combination of build conditions.
 
The question then becomes: what is the “Goldilocks” combination of conditions that can produce a part with acceptable levels of porosity? 
 
Statistical studies have already shown that the most direct way to control porosity is through the control and monitoring of laser PBF processes. These studies have shown that the following parameters have the greatest impact:
  • Laser power
  • Scan speed
  • Layer thickness
  • Hatch distance
In the research article “Hierarchical design principles of Selective Laser Melting for high quality metallic objects,” Igor Yadroitsev et al. offer the following potential hierarchy for process parameter selection:
  1. Simulation – Where the melt pool temperature is between the lower melting point and the boiling point.
  2. Single track level – A continuous track (the path of the laser) with sufficient bonding to substrate.
  3. Single layer level – The height of the track and the re-melted depth combined should be greater than the layer thickness and valley depth combined.
  4. 3D object level – A dense part without chains of pores and interlayer pores
Although a number of optimum process parameter combinations exist to give high density values, the current approach towards parameter selection is mainly based on trial and error, along with the experience of the operator. 
 
How can we mitigate the risks created by the variable nature of these approaches?
 
This is the question behind Lloyd’s Register’s new research. We are sponsoring PhD research that is investigating and measuring the repeatability and reproducibility of AM processes. In particular, this research is focusing on the selection of parameters to ensure manufacturers can leverage a best-practise industry approach. Experimentation has already begun on AM porosity at our Global Technology Centre, in partnership with Nanyang Technological University, in Singapore.
 
Once complete, the research will form part of our “Guidance Notes for Additive Manufacturing of Metallic Parts” and certification requirements. This independent certification framework with provide a method of interrogating and evaluating a designer, programmer and machine operator’s understanding and experience of process variability and parameter selection.
 
Have a viewpoint on porosity, parameter control or Laser PBF? Join our Additive Manufacturing Industry forum here, and help us shape our independent certification service for AM.