While laser-based 3D printing techniques have revolutionised the production of metal parts by greatly expanding design complexity, the laser beams traditionally used in metal printing have drawbacks that can lead to defects and poor mechanical performance.

Researchers at Lawrence Livermore National Laboratory (LLNL) are addressing the issue by exploring alternative shapes to the Gaussian beams commonly used in high-power laser printing processes such as laser powder bed fusion (LPBF).

To address porosity and defects in metal 3D printing, Lawrence Livermore National Laboratory researchers experimented with exotic optical laser beam shapes known as Bessel beams – reminiscent of bullseye patterns. They discovered the beams had unique properties such as self-healing and non-diffraction and reduced the likelihood of pore formation and “keyholing,” a porosity-inducing phenomenon exacerbated by the use of Gaussian beams. Credit: Veronica Chen/LLNL

In a paper published by Science Advances, researchers experimented with exotic optical beam shapes known as Bessel beams, reminiscent of bullseye patterns, which possess a number of unique properties such as self-healing and non-diffraction. They discovered that the application of these types of beams reduced the likelihood of pore formation and “keyholing,” a porosity-inducing phenomenon in LPBF exacerbated by the use of Gaussian beams. The work is featured on the journal’s Sept. 17 cover.

LLNL researchers said the work indicates that alternative shapes such as Bessel beams could alleviate the chief concerns in the LBPF technique: The large thermal gradient and complex melt pool instabilities occurring where the laser meets the metal powder. The issues are predominantly caused by Gaussian beam shapes that most off-the-shelf, high-power laser systems typically output.

“Using Gaussian beams is a lot like using a flamethrower to cook your food; you don’t have a lot of control over how heat is deposited around the material,” said lead author and LLNL research scientist Thej Tumkur Umanath. “With a Bessel beam, the fact that we redistribute some of that energy away from the centre means we can engineer thermal profiles and reduce thermal gradients to aid microstructural grain refinement and, ultimately, result in denser parts and smoother surfaces.”