Image Correlation Helps Optimize Laser-Based 3-D Printing

Digital image correlation (DIC) could help prevent warping of metal parts made via laser-based additive manufacturing.

Researchers at Lawrence Livermore National Laboratory used the technique to find correlations between the power and speed of lasers used in powder-bed fusion additive manufacturing and the parts they produce.

“We took time to do a systematic study of residual stresses that allowed us to measure things that were not quantified before,” said lead researcher Dr. Amanda Wu. “Being able to calibrate our AM parameters for residual stress minimization is critical.”

Dr. Amanda Wu images a 3-D-printed part using digital image correlation. Courtesy of Julie Russell/LLNL.

Digital image correlation was performed using a two-camera setup that photographed workpieces before and after they were removed from the build plate. A black-and-white speckle pattern was applied to the parts so that the images could be fed into a software program that produces digital illustrations of high- to low-distortion areas.

In powder-bed fusion, because the laser repeatedly heats and cools the workpiece, there is localized expansion and contraction. This can affect mechanical performance and structural integrity, and may cause shape distortion, detachment from support structures and, potentially, part failure.

Reducing the laser scan vector length, instead of using a continuous laser scan, regulated temperature changes and reduced residual stress, the researchers found. Rotating the laser scan vector relative to the part’s largest dimension also helped reduce residual stress.

The Lawrence Livermore team verified their DIC results by having colleagues at Los Alamos National Laboratory perform residual stress tests using neutron diffraction. This technique, in which a neutron beam detects changes in atomic lattice spacing due to stress, is highly accurate but rarely used because only three federal laboratories in the U.S. have high-energy neutron sources.

The work was published in Metallurgical and Materials Transactions (doi: 10.1007/s11661-014-2549-x).

For more information, visit www.llnl.gov.