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Laser Beam Shaping To Improve Metal 3D Printing

While laser-based 3D printing techniques have transformed metal part manufacturing by allowing designers to create parts with greater design complexity, the laser beams that are traditionally used in metal printing have limitations that can result in defects and poor mechanical performance.

A team of scientists at Lawrence Livermore National Laboratory (LLNL) is working to resolve the problem by investigating alternative shapes to the Gaussian beams that are commonly used in high-power laser printing processes such as laser powder bed fusion (LPBF).

Bessel beams are exotic optical beam shapes that are reminiscent of bullseye patterns. They have a number of unique properties, including the ability to heal themselves and to be non-diffracting. In a paper published by Science Advances, researchers described their experiments with exotic optical beam shapes known as Bessel beams. Using these types of beams, they discovered that they reduced the likelihood of pore formation and "keyholing," a porosity-inducing phenomenon in LPBF that was exacerbated by the use of Gaussian beams in the first place. The artwork featured on the journal's cover on September 17, 2021.

Scientists at the Lawrence Livermore National Laboratory (LLNL) have concluded that alternative shapes such as Bessel beams may be effective in alleviating the two primary concerns associated with the LBPF technique: the large thermal gradient and complex melt pool instabilities that occur where the laser meets the metal powder. In most cases, the problems are caused by the Gaussian beam shapes that are typically produced by commercially available high-powered laser systems.

In the words of lead author and LLNL research scientist Thej Tumkur Umanath, "using Gaussian beams is a lot like cooking your food with a flamethrower; you don't have a lot of control over how heat is deposited around the material." We can engineer thermal profiles and reduce thermal gradients with a Bessel beam because we redistribute some of the energy away from the center. This allows us to refine microstructural grain refinement, which ultimately results in denser parts and smoother surfaces.

Bessel beams, according to Tumkur, who also won first place in LLNL's 2019 Postdoc Research Slam! competition for the work, significantly expand the laser scan parameter space when compared to traditional Gaussian beam shapes, according to Tumkur. According to LLNL researchers, the result is ideal melt pools that are not too shallow and do not suffer from keyholing, a phenomenon in which the laser creates a large amount of vapor and causes a deep cavity in the metal substrate during the build process. Keyholing causes bubbles to form in the melt pool, which results in pores forming in the finished part and a reduction in mechanical performance.

Conventional beams suffer from another disadvantage in that they are susceptible to diffraction (spreading) as they propagate. Because of their non-diffractive properties, Bessel beams have a greater depth of focus than other types of beams. As a result, the authors discovered that using Bessel beams, they were able to achieve greater tolerance in the placement of the workpiece with respect to the laser's focal point. Placement is a challenge for industrial systems, which frequently rely on expensive and sensitive techniques for positioning an in-progress build within the depth of focus of a focused beam each time a layer of metal powder is deposited on a workpiece.

Although beam-shape engineering approaches have been used extensively in imaging, microscopy, and other optical applications because of their non-diffractive and self-healing properties, Tumkur noted that they are "relatively uncommon" in laser-based manufacturing applications. "By incorporating designer beam shapes to achieve control over melt pool dynamics, our work addresses the apparent disconnect between optical physics and materials engineering in the metal additive manufacturing community."

For this purpose, the researchers at Lawrence Livermore National Laboratory passed the laser beam through two conical lenses to produce a donut shape, after which it was passed through additional optics and a scanner to create "rings" around a central beam. In the Advanced Manufacturing Laboratory at Lawrence Livermore National Laboratory, the researchers used the experimental setup to print cubes and other shapes from stainless steel powder, which was installed in a commercial printing machine.

High-speed imaging was used to study the dynamics of the melt pool, and the researchers discovered a significant reduction in melt pool turbulence as well as a reduction in "spatter," which is defined as molten metal particles that fly away from the laser's path during a build and which generally results in pore formation.

The team discovered that parts constructed with Bessel beams were denser, stronger, and had more robust tensile properties than structures constructed with conventional Gaussian beams through mechanical studies and computer simulations.

"The ability to increase control of the LPBF process in order to minimize defects has long been sought by industry," said Ibo Matthews, who served as principal investigator on the project prior to becoming director of LLNL's Materials Science Division. Incorporating intricately patterned structure into the laser beam increases the ability to control laser-material interaction, heat deposition, and ultimately the quality of prints with greater precision and control.

Saad Khairallah, an LLNL computer scientist, used the LLNL-developed multiphysics code ALE3D to simulate the interaction of both Gaussian and Bessel beam laser shapes with single tracks of metal powder material at the laboratory's Advanced Light Source. According to the team, when they compared the resulting tracks, they discovered that the Bessel beam had better thermal gradients than the Gaussian beams, which encouraged better microstructure formation. With Bessel beams, they were also able to achieve better energy distribution, avoiding the "hot spot" generation that can occur with Gaussian beams, which results in deep melt pools and the formation of pores.

According to Khairallah, "Simulations allow you to obtain detailed diagnostics of the physics that is taking place and, as a result, allow you to understand the fundamental mechanisms that underlie our experimental findings."

Beam shaping is just one of many approaches being investigated at LLNL for improving the quality of 3D-printed metal parts. It is a less expensive option than alternative scanning strategies because it can be accomplished at a low cost by incorporating simple optical elements and can reduce the expense and time involved in post-processing techniques that are typically required for parts built with Gaussian beams, according to Tumkur.

According to Tumkur, "there is a significant need to produce parts that are robust and defect-free, as well as the ability to print very large structures in a cost-effective manner." We must address some fundamental issues that occur at very short temporal regimes and microstructural scales if we are to make 3D printing truly compatible with industrial standards and move beyond conventional manufacturing approaches." I believe that beam shaping is the best option because it can be used to print on a wide variety of metals and can be integrated into commercial printing systems without posing significant integrability challenges, as other alternative techniques are prone to doing."

For the time being, researchers at Lawrence Livermore National Laboratory are experimenting with other beam shape engineering strategies as part of an ongoing partnership with GE Global Research. They hope to investigate complex laser beam and polarization-shaping approaches in order to gain greater control over the quality of printed parts in the future.

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