NIF Laser Technology Could Revolutionize Metal 3D Printing

An award-winning technology originally developed to smooth out and pattern NIF’s high-energy laser beams now can be used to 3D-print metal objects faster than ever before, according to a new study by LLNL researchers.

A team of Lab scientists reported the finding in an Optics Express article published online on May 10. The new method—Diode-based Additive Manufacturing (DiAM)—uses high-powered arrays of laser diodes, a Q-switched (pulsed) laser, and a specialized laser modulator developed for NIF to flash-print an entire layer of metal powder at a time, instead of raster scanning with a laser across each layer as with conventional laser-based powder-bed fusion additive manufacturing systems.

The result, the researchers said, is the potential for large metal objects to be printed in a fraction of the time needed for metal 3D printers on the market today, expanding possibilities for industries requiring larger metal parts, such as aerospace and automotive manufacturers. The combination of speed and degree of design flexibility afforded through the DiAM method, the team concluded, is potentially "far beyond" that of current powder-bed fusion-based systems.

"By cutting the print time and having the ability to upscale, this process could revolutionize metal additive manufacturing," said Ibo Matthews, the LLNL scientist heading the research and the paper’s lead author. "Hypothetically, a one-cubic-meter build would take 10 years of raster-scanned illumination time using current methods, but would only require hours to weeks with DiAM, because you can image large areas of build all at once. Printing with a grey-scaled image may also allow you to reduce residual stress, because you can tailor the thermal stresses spatially and temporally."

The "magic" of the process, Matthews said, is the implementation of a customized laser modulator called an Optically Addressable Light Valve (OALV), which contains a liquid crystal cell and photoconductive crystal in series. Much like a liquid-crystal-based projector, researchers explained, the OALV is used to dynamically sculpt the high-power laser light according to pre-programmed layer-by-layer images. But unlike a conventional liquid-crystal projector, the OALV is un-pixelated and can handle high laser powers.

The technology originally was designed for and installed in NIF as part of the LEOPARD (Laser Energy Optimization by Precision Adjustments to the Radiant Distribution) system, in collaboration with Meadowlark Optics in Colorado and CEA, the French Alternative Energies and Atomic Energy Commission. LEOPARD was deployed in 2010 and won an R&D 100 award in 2012. In NIF, the OALV is used to optimize the profile of the laser beams and locally shadow and protect optics subjected to higher intensities and fluences, or energy densities.

With LEOPARD, NIF electronically protects regions of its beams containing potentially threatening flaws on its final optics, as identified by the Final Optics Damage Inspection (FODI) system. This enables NIF to continue firing until the schedule allows those optics to be removed, repaired, and reintroduced into the beamline.

The team that first demonstrated the light valve could be used for printing parts was initially led by James DeMuth, a former LLNL researcher. John Heebner, the LLNL scientist who led the development of the OALV, described its use in metal 3D printing as a "natural synergy."

"The DiAM project marries two technologies that we’ve pioneered at the Lab—high-power laser-diode arrays and the OALV," Heebner said. "Given that we put all this time and development into this light valve, it became a natural extension to apply it to this project.

"We went through some calculations and it was clear from the outset that it would work (with 3D printing)," he said. "The ability to change a serial process to a parallel process is critical to ensuring that as parts increase in complexity or size that the patterning process speed can be increased to catch up."

Beside the potential to produce larger parts, using such a valve results in imaging quality that rivals and could exceed today’s metal 3D printers, and the ability to fine-tune gradients in the projected image means better control over residual stress and material microstructure, researchers said.

With DiAM printing, the laser light is sourced by a set of four diode laser arrays and a nanosecond (billionth of a second) pulsed laser. It passes through the OALV, which patterns an image of a two-dimensional "slice" of the desired 3D part. The images go from a digital computer file to the laser in a two-stage liquid-crystal modulation process.

In the first stage the images are sourced from a digitized CAD (computer-aided design) model and imprinted on a low-power blue LED (light-emitting diode) source using an ordinary pixelated liquid-crystal projector. In the second stage the blue images activate the OALV’s photoconductive layer, creating local conductive patches (where blue light is present) that transfer voltage to its liquid-crystal layer. This enables the low-power blue images to modulate the high-power laser beam. The beam is then directed onto a build plane, printing the entire metal layer at once.

For the study the researchers used tin powder, successfully demonstrating the printing of two small 3D models, an impeller (a small turbine blade structure) and the LLNL logo.

While speeding up the metal additive process was a main driver for pursuing the technology at LLNL, the larger build size potentially could have significant value for the Lab’s core mission of stockpile stewardship, the researchers said. The laser diodes, which provide most of the energy compared to the pulsed laser system, are also cheap to purchase, so such a system would be more cost-effective than fiber-laser-based machines on the market today.

LLNL’s Laboratory Directed Research and Development program funded the research. Lab scientists Gabe Guss and Reggie Drachenberg played a central role in producing the parts, with contributions from Josh Kuntz and Eric Duoss.