A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 2 8 0 3D PRINTSHOP REVISED PROCESS PRINTS GLASS WITH FINE FEATURES A combined team from UC Berkeley and the Albert Ludwig University of Freiburg, Germany, have improved upon a 3D-printing process developed three years ago—computed axial lithography (CAL)—to print much finer features and to print in glass. They dubbed this new system “micro-CAL.” The new method prints glass microstructures faster and produces objects with higher optical quality, design flexibility, and strength. The CAL process is different from other industrial 3D-printing manufacturing processes that build up objects from thin layers of material. CAL 3D prints the entire object simultaneously. Researchers use a laser to project patterns of light into a rotating volume of light-sensitive material, building up a 3D light dose that then solidifies in the desired shape. The layer-less nature of the CAL process enables smooth surfaces and complex geometries. “With micro-CAL, we can print objects in polymers with features down to about 20 millionths of a meter, or about a quarter of a human hair’s breadth,” says Hayden Taylor, principal investigator and professor of mechanical engineering at UC Berkeley. To print the glass, Taylor and his research team collaborated with scientists from the Albert Ludwig University of Freiburg, who have developed a special resin material containing nanoparticles of glass surrounded by a light-sensitive binder liquid. Digital light projections from the printer solidify the binder, then the researchers heat the printed object to remove the binder and fuse the particles together into a solid object of pure glass. “The key enabler here is that the binder has a refractive index that is virtually identical to that of the glass, so that light passes through the material with virtually no scattering,” says Taylor. berkeley.edu. FINDING DEFECTS IN LPBF METAL WITH ULTRASOUND Researchers from Lawrence Liver- more National Laboratory (LLNL) are using laser-based ultrasound to reveal surface and sub-surface defects in laser powder bed fusion (LPBF) metal 3D printing. The all-optical ultrasound technique can perform on-demand characterization of melt tracks and detect formation. “The demonstrated laser-based ultrasound, surface acoustic wave (SAW) system showed excellent sensitivity to surface and near-surface features, including breaks in the LPBF melt line, metal surface splatter, and subsurface air voids,” says LLNL engineer and principal investigator David Stobbe. Surface acoustic waves have historically been used to characterize surface and near-surface features such as cracks, pits, and welds in engineering materials, and are used in geology for detecting subterranean features such as caves. Due to their surface and near-surface sensitivity, SAWs are well-suited for characterizing melt lines in LPBF printing, according to researchers. To test this potential, the LLNL team carried out experiments by producing laser melted lines using a fiber laser directed into a vacuum chamber and produced samples of titanium alloy for analysis with 100, 150, and 350 W powered lasers. Next, they developed a method for producing and detecting surface acoustic waves, using a pulsed laser to generate ultrasound and measured the displacement with a photorefractive Iaser interferometer. “A system like this may find use for rapidly qualifying new LPBF machines and in-service machines after changes to metal powder feedstock or modifications to the melt laser power or scan speed,” says Stobbe. llnl.gov. Surface acoustic waves generated by laser-based ultrasound could find defects in LPBF metal 3D printing. Courtesy of David Stobbe/LLNL. 3D-printed glass lattices, displayed in front of a U.S. penny for scale. Courtesy of Joseph Toombs.
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