Laser beam forming to improve metal 3D printing

To address the porosity and imperfections of metal 3D printing, researchers at Lawrence Livermore National Laboratory experimented with the shape of an exotic optical laser beam called the Bessel beam, which is reminiscent of the bullseye pattern. They found that the beam had unique properties such as self-healing and non-diffraction, reducing the possibility of pore formation and “keyholes”, a phenomenon that induces porosity that is exacerbated by the use of Gaussian beams. Did. Credits: Veronica Chen / LLNL

Laser-based 3D printing technology has revolutionized the manufacture of metal parts by significantly increasing design complexity, but the laser beams traditionally used in metal printing have defects and mechanical performance. There are drawbacks that can lead to degradation.

Researchers at Lawrence Livermore National Laboratory (LLNL) have addressed this issue by investigating alternative shapes for Gaussian beams commonly used in high-power laser printing processes such as laser powder bed melting (LPBF). is.

In a treatise published by Science Advances, Researchers have experimented with an exotic light beam shape known as a Bessel beam (reminiscent of a bullseye pattern). It has many unique properties such as self-healing and non-diffraction. They found that applying these types of beams reduced the likelihood of pore formation and “keyholes”. This is an LPBF porosity-inducing phenomenon that is exacerbated by the use of Gaussian beams. This work appeared on the cover of the journal on September 17, 2021.

According to LLNL researchers, alternative shapes such as Bessel beams are a major concern in LBPF technology, with large temperature gradients and complex molten pool instability occurring where the laser comes into contact with metal powder. It shows that can be reduced. This problem is mainly caused by the Gaussian beam shape that most off-the-shelf high power laser systems normally output.

“Using a Gaussian beam is a lot like cooking with a flamethrower. You don’t have much control over how heat builds up around the material,” said the lead author. Said Thej Tumkur Umanath, a research scientist at LLNL. “The fact that Bessel beams allow some of that energy to be redistributed off-center allows us to design thermal profiles, reduce temperature gradients and assist in microstructure grain miniaturization, and ultimately. It means that you can increase the density of parts and smooth the surface. “

Tumakuru, who also won first place award At LLNL’s 2019 Postdoc Research Slam! Bessel beams are competitive when they significantly expand the laser scan parameter space over traditional Gaussian beam shapes. The result is an ideal melt pool that is not too shallow and is not affected by keyholes. This is a phenomenon in which the laser produces strong vapors that create deep cavities in the metal substrate during the build, as LLNL researchers have done before. found.. Keyholes create air bubbles in the molten pool, which form pores and reduce the mechanical performance of the finished part.

Another drawback of traditional beams is that they tend to diffract (diffuse) as they propagate. The Bessel beam provides a deeper depth of focus due to its non-diffractive properties. As a result, the authors used a Bessel beam to observe an increase in tolerance for workpiece placement with respect to the laser focus. Placement is a challenge for industrial systems that often rely on expensive and sensitive techniques to place an ongoing build within the depth of focus of the focused beam each time a layer of metal powder is deposited.

“Bessel beams are widely used in imaging, microscopy, and other optical applications due to their non-diffractive and self-healing properties, but beam shape engineering approaches are less common in laser-based manufacturing applications.” Explains Tumkur. “Our work addresses the apparent disconnect between optical physics and materials engineering in the metal laminate modeling community by incorporating designer beam geometries to control the dynamics of the molten pool.”

The LLNL team shaped the beam by passing the laser through two conical lenses to create a donut shape, and then passing it through additional optics and a scanner to create a “ring” around the central beam. bottom. Researchers installed on a commercial printing press at LLNL’s Advanced Manufacturing Laboratory used experimental equipment to print cubes and other shapes from stainless steel powder.

Through high-speed imaging, researchers studied meltpool dynamics, observing a significant reduction in meltpool turbulence and a reduction in “spatter,” which is a molten metal particle that scatters from the laser path during build. .. Formation.

In mechanical research and simulation, the team found that parts built with a Bessel beam were denser, stronger, and had stronger tensile properties than structures built with a traditional Gaussian beam.

“The industry has long sought the ability to strengthen control of the LPBF process to minimize defects,” said Ibo Matthews, a senior researcher on the project before becoming the leader of LLNL’s Materials Sciences division. It is. “Introducing complex structures into the laser beam gives us greater flexibility and precise control over laser-material interactions, thermal deposition, and ultimately print quality.”

Saad Khairallah, a computer scientist at LLNL, used the multiphysics code ALE3D developed by LLNL to simulate the interaction of both Gaussian and Bessel beam laser shapes with a single track of metal powder material. .. By comparing the resulting tracks, the team found that the Bessel beam showed a better temperature gradient than the Gaussian beam, facilitating better microstructure formation. It also achieved a better energy distribution with the Bessel beam and avoided the “hot spots” found with the Gaussian beam. This creates a deep melting pool and forms pores.

“Simulations give us a detailed diagnosis of the physics that is occurring, so we can understand the basic mechanism behind the experimental results,” says Khairallah.

It is just one of many ways to improve the quality of 3D printed metal parts being studied at LLNL. Beam shaping is a cheaper option than alternative scanning strategies because it can be performed at low cost by incorporating simple optics, saving money and time. According to Tumakuru, he is involved in the post-processing technology normally required for parts built on Gaussian beams.

“We need to manufacture robust, defect-free parts that can print very large structures in a cost-effective manner,” says Tumakuru. “To make 3D printing truly compatible with industry standards and surpass traditional manufacturing approaches, we need to address some of the fundamental issues that arise on very short timeframes and microstructure scales. Beam shaping can be applied to print a variety of metals ubiquitously and can be incorporated into commercial printing systems without the major integration issues of other alternatives, so it’s a real path to go. I think.”

LLNL researchers are currently experimenting with other beam shape engineering strategies as part of an ongoing partnership with GE Global Research, with complex laser beams and polarization shaping for finer control over the quality of printed components. I plan to investigate the approach.

Reference: Thejaswi U. Tumkur, Thomas Voisin, Rongpei Shi, Philip J. Depond, Tien T. Roehling, Sheldon Wu, Michael F. Crumb, John D. Roehling Diffractive beam shaping “Gabe Guss, Saad A. Khairallah, Manyalibo J. Matthews, September 15, 2021 Science Advances..
DOI: 10.1126 / sciadv.abg9358

Co-authors of this treatise include LLNL scientists and engineers Thomas Voisin, Rongpei Shi, Phil Depond, Tien Roehling, Sheldon Wu, Michael Crumb, John Roehling, and Gabe Guss. A research and development program led by the Institute funded this work. Laser beam forming to improve metal 3D printing

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