Laser additive manufacturing (LAM) based on laser powder bed fusion (LPBF) can be used to make components with complex shapes layer by layer from a CAD drawing or other digital design. The technique has now been used widely in the fields of aerospace, energy, and medicine. However, despite the precision that is achievable it suffers one critically limiting shortcoming when metal components are to be fabricated: the components can end up region regions of porosity, incompletely fuse, or develop cracks.

Materials scientists from the University of Sheffield have now worked with mechanical engineers from University College London and colleagues at The European Synchrotron, in Grenoble, France, have used ultrafast, in situ X-ray synchrotron imaging to look at the sintering processes that take place in LAM to help them understand why porosity arises. Porosity can lead to insidious weaknesses in safety-critical metallic components.

The team has also looked closely at the melt pool dynamics, keyhole porosity, and how spatter occurs during the fabrication process to clarify how this affects the layers as they are built up to make a component. Fundamentally, the new insights in these problems could help those using LAM to make metallic components to tune the processes and timings thereof to reduce the formation of pores. Similarly, their findings should also help reduce the amount of spattering that occurs, this leads to roughening of surfaces, contamination of the melt pool, and ultimately reduced integrity of the layers as they combine to form the final product [Chen, Y. et al. Appl. Mater. Today (2020); DOI: 10.1016/j.apmt.2020.100650].

The team adds that it is the employment of the laser in keyhole mode that is most problematic when it comes to porosity and spatter. "The laser-matter interaction is very complex due to strong vaporization of material from the molten pool and the flow of molten metal in the keyhole, driven by recoil pressure and Marangoni convection," the team explains. They point out that there are significant benefits to using keyhole mode but users must consider the problems that can arise. The new work offers a new perspective on the parameters that might be employed to reduce the problems and allow precision components that have high integrity to be fabricated.

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