Aug 2, 2017 | By Benedict

Researchers at Lawrence Livermore National Laboratory (LLNL) have used high-speed imaging and computer models to uncover the mechanisms behind material redistribution, or "spatter," a phenomenon that leads to defects in metal 3D printed parts.

Spatter: it’s a problem not only for messy eaters, but also for users of powder-bed fusion additive manufacturing equipment. For years, 3D printer users have had to contend with spatter, a phenomenon in which particles of liquid metal are ejected from the laser's path, ultimately causing defects in the finished part.

Now, however, LLNL researchers may have got to the root of the problem. Using ultrafast imaging of melt-pool dynamics and high-resolution simulations, the researchers found that spatter is not principally caused the laser's recoil pressure, as was previously believed.

Instead, they believe that spatter is a result of the entrainment of metal particles by an ambient gas flow.

“People have been assuming that recoil pressure leads to spatter because that's what the laser welding community has seen,” said Sonny Ly, an LLNL physicist.

According to Ly and colleagues, this explanation can now be shown to be incorrect—or at least insufficient—thanks to LLNL’s high-resolution imaging capabilities.

“We imaged right at the melt pool and you could see particles ejected right from the pool due to recoil, but a majority of particles are swept away and entrained by the gas flow,” Ly said. “The entrained particles can go back into the laser beam and are melted, leading to a more dominant form of spatter.”

The problem with spatter is that, when particles fly out of the path of the laser and land on the printed part, they can contaminate the powder bed and affect the build quality of a layer.

The consequences of a spatter-affected metal print include roughness, porosity, and lack of fusion.

But those negative characteristics might now be sidestepped thanks to LLNL’s use of three kinds of cameras, including a sensor capable of recording up to 10 million frames per second. This setup allowed the researchers to see the gas flow above the powder bed where particles were sucked in.

“It turns out only about 15 percent of the ejections of molten particles are caused by splashing in the melt pool, which was the assumed mechanism,” explained LLNL engineer Gabe Guss. “The rest is primarily cold particles passing through the laser beam above the melt pool and some other factors.”

Guss added that the findings were surprising, because without high-tech imaging equipment it looks like hot ejections come from outward gas pressure rather than inward entrainment.

The visual data gathered using the three cameras was complemented by computer simulations, which showed the incline of the melt pool influences the direction of the spatter.

“The simulations showed a difference in the morphology of the melt pool beneath the laser spot, which allowed us to interpret the experimental observations,” said Saad Khairallah, the LLNL computational engineer/physicist responsible for the simulations. “This is an example where simulations complement experiments and become a key component in a science story.”

The researchers believe their work, which also involved physicists Ibo Matthews and Alexander "Sasha" Rubenchik, could help scientists mitigate the negative effects of spatter and improve flow models for powder-bed fusion 3D printing.

Their findings have been published in the journal Scientific Reports in a paper titled “Metal vapor micro-jet controls material redistribution in laser powder bed fusion additive manufacturing.”

Posted in 3D Printing Technology

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