Lawrence Livermore National Laboratory (LLNL) scientists have developed a laser-based ultrasound (LBU) technique to accurately capture defect-producing features in metal 3D printing.
According to the LLNL team, their all-optical ultrasound process utilizes surface acoustic waves (SAW) whose energy once scattered from voids, surface features, and melt lines—the tracks where the laser liquifies metal powder in Laser Powder Bed Fusion (LPBF) printing, helps pin point defects in metal 3D printing,
The engineers have validated their findings using optical microscopy and X-ray computed tomography (CT).
SAWs have been historically used to characterize surface and near-surface features such as cracks, pits and welds in engineering materials.
They are also used by geologists on a much larger length scale to detect subterranean features such as caves.
Going by this, one could as well say that SAWs are well-suited for characterizing melt lines in LPBF printing too.
To test SAWs’ potential in metal 3D printing, Stobbe and his 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-watt, 150-watt and 350-watt powered lasers.
They next 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.
Stobbe’s team also performed simulations to inform the experimental measurements and assist with interpreting the results.
In order to do this, they simulated and measured the displacement from the pulsed laser and showed scattering from the melt line, as well as breaks in the melt line, metal splatter adjacent to the melt line and subsurface air voids under the melt line.
The scientists then measured these features experimentally and observed excellent agreement between simulation and experiment.
Results from the laser-based ultrasound (LBU) experiments were validated with optical microscopy for the surface features, and X-ray computed tomography for the sub-surface features.
According to LLNL engineer and lead author Kathryn Harke, the LBU system—in comparison with X-ray CT—offers better real-time inspection and can acquire and process data at a much faster rate.
Image and content: David Stobbe/LLNL