Laser-based 3D printing has revolutionized metal part production by increasing design complexity. However, traditional metal printing laser beams can cause defects and poor mechanical performance.
Lawrence Livermore National Laboratory’s (LLNL) researchers are exploring other shapes for the Gaussian beams commonly used in high-power laser printing processes like laser powder bed fusion (LPBF).
Researchers experimented with Bessel beams, a type of optical beam that resembles bullseye patterns. They have a variety of unique properties, like self-healing or non-diffraction. These beams reduce the risk of pore formation and keyholing, a porosity-inducing phenomenon in LPBF. This was exacerbated when Gaussian beams were used. This work appears on the journal’s September 17th, 2021, cover.
LLNL researchers stated that the research suggests that Bessel beams, a type of Bessel beam, could alleviate the main concerns with the LBPF technique. These include the complex melt pool instability and large thermal gradient at the point where the laser meets the powder. Gaussian beam forms mainly cause these issues that high-power, off-the-shelf laser systems usually produce.
The Tumkur Umath, LLNL researcher and lead author, said that using Gaussian beams is akin to using a flamethrower for cooking food. You need more control over how heat is deposited around it. The fact that a Bessel beam can redistribute energy away from its center allows us to engineer thermal profiles and decrease thermal gradients. This aids microstructural grain refinement, leading to denser parts and smoother surfaces.
Tumkur won first prize during LLNL’s 2019 Postdoc Research Slam. Bessel beams dramatically expand the laser scan parameter range over traditional Gaussian beam forms, according to the competition. Ideal melt pools are shallow enough not to suffer from keyholing when the laser creates a potent vapor that causes a deep cavity in the metal substrate during builds. LLNL researchers previously discovered. Keyholing causes bubbles in the melt pool, leading to pores and degraded mechanical performance of finished parts.
Another drawback to conventional beams is their tendency to scatter (diffraction) as they propagate. Because they are non-diffractive, Bessel beams offer a deeper focus. The authors found that Bessel beams allowed for greater tolerance in the placement of workpieces relative to the laser’s focal points. Industrial systems often rely on sensitive and expensive techniques to place in-progress materials within the beam’s depth. This is because they depend on expensive and delicate methods for identifying them.
Tumkur stated that beam beams are used widely in imaging, microscopy, and other optical applications due to their non-diffractive, self-healing capabilities. However, beam-shaped engineering is not joined in laser-based manufacturing applications. Our work addresses the disconnect between optical physics, materials engineering, and metal additive manufacturing by incorporating designer beam shapes to control melt pool dynamics.
The LLNL team created the beams using two conical lenses to make a donut shape. After that, the laser was passed through additional optics and scanned to create “rings around” the central beam. The experimental setup was installed in a commercial printer at LLNL’s Advanced Manufacturing Laboratory. It allowed researchers to make cubes from stainless steel powder.
High-speed imaging allowed researchers to study the dynamics of the melt pools. They observed a significant reduction in melt pool turbulence and mitigation of “spatter,” which is the molten metal particles that fly out of the laser’s path during builds. This generally leads to pore formation.
The team discovered that Bessel beams had more substantial, denser and more resilient tensile properties when compared to structures made with traditional Gaussian beams.
“Industry has sought to improve control over the LPBF process to minimize defect,” stated Ibo Matthews, who was the project’s principal investigator before becoming LLNL’s Materials Science Division chief. The complex structure can be added to the laser beam to increase flexibility and control laser-material interaction, heat deposition, and quality of prints.
Saad Khairallah, an LLNL computer scientist, used the multiphysics code ALE3D (LLNL-developed) to simulate the interaction between Gaussian beam and Bessel beam laser shapes using single tracks of metal powder material. The team compared the ways and found that the Bessel beam had better thermal gradients than Gaussian beams. This encouraged the formation of microstructures. Bessel beams also allowed for better energy distribution, as they avoided the “hot spot,” which can cause deep melt pools and pores.
Khairallah stated that simulations “allow you to get detailed diagnostics on the physics occurring and allow you to understand the fundamental mechanisms of our experimental findings.”
Beam shaping is one of the many ways to improve the quality of 3D-printed metal parts. It can be done by incorporating simple optical elements at a lower cost than other scanning strategies. It can also reduce the time and expense of post-processing techniques typically required for parts made with Gaussian beams.
Tumkur stated that parts must be solid and free from defects. He also noted that it is essential to be able to print large structures at a low cost. To make 3D printing compatible with industrial standards and to move beyond traditional manufacturing methods, we must address fundamental issues at microstructural and temporal scales. Beam shaping is an excellent option because it can print many metals in one go and be integrated into commercial printing systems without dealing with integrability issues like other techniques.
Researchers at LLNL are experimenting with other beam-shape engineering strategies as part of a continuing partnership with GE Global Research. They plan to explore complex laser beam and polarization shaping approaches for better control over the quality of printed parts.
