LLNL explores laser beam shaping to improve metal AM printing

While additive manufacturing (AM) techniques have revolutionized metal parts design complexity, the traditional laser beams could lead to defects and poor mechanical performance.

Lawrence Livermore National Laboratory (LLNL) is exploring alternative shapes to the Gaussian beams commonly employed in high-power laser printing processes such as laser powder bed fusion (LBPF). 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.  

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.

Bessel beams significantly expand the laser scan parameter space over traditional Gaussian beam shapes. The result is ideal melt pools that are not too shallow and don’t suffer from keyholing—a phenomenon in which the laser creates a strong vapor and causes a deep cavity in the metal substrate during builds, as LLNL researchers have previously found. Keyholing creates bubbles in the melt pool that form pores and leads to degraded mechanical performance in finished parts.

Through high-speed imaging, researchers studied the dynamics of the melt pool, observing a substantial reduction in melt pool turbulence and mitigation of “spatter”—the molten particles of metal that fly from the laser’s path during a build—which generally leads to pore formation.

In mechanical studies and simulations, the team found that parts built with Bessel beams were denser, stronger and had more robust tensile properties than structures built with conventional Gaussian beams.

Researchers at LLNL are currently experimenting with other beam shape engineering strategies as part of an ongoing partnership with GE Global Research and are planning to investigate complex laser beam and polarization-shaping approaches for greater control over the quality of printed parts.