Research: Slow curing of metal powder with high-power laser often leads to bubbles

Laser bed powder fusion is a widely used technique for additive manufacturing. Nevertheless, there are a number of challenges that limit the usability of this 3D printing technique. The biggest challenge is small bubbles or pores that can form during the printing process. These lead to weak spots in the final product. In particular, if a high-power laser cures metal powder at a slow speed, such defects arise in many cases.

This is according to research by a series of researchers led by Anthony Rollett, professor of materials science and engineering at US Carnegie Mellon University. Besides Rollett, researchers Tao Sun from the US University of Virginia, Cang Zhao from China's Tsinghua University, Niranjan D. Parab and Kamel Fezzaa from the US Argonne National Laboratory and Xuxiao Li and Wenda Tan from the University of Utah are involved in the project.

Rollet and his team publish their findings in Science. In their research, the team zooms in on the creation of such bubbles and how they can be 'captured' in the final product. "The real practical value of this research is that we can accurately control machines to avoid this problem," Rollet explains.

The formation of bubbles during laser bed powder fusion is a well-known phenomenon also known as the 'keyhole phenomenon'. The phenomenon occurs particularly if a high-power laser melts metal powder at low speed during the printing process. This can create a hole in the material, resembling a keyhole. This hole can lead to defects, often described as bubbles or pores. These can affect the quality of the final product.

To examine pores more closely, the team used a form of X-ray imaging that uses extremely high levels of light. This makes it possible to image bubbles in the keyhole. These bubbles form during fluctuations in the keyhole. "This unstable behaviour generates acoustic waves in the liquid metal, which force the pores away from the keyhole. This causes them to survive long enough to be trapped in the hardening metal," the team writes in a news release on CMU's website.

"If you have a deep keyhole, the walls oscillate strongly," Rollett explains. "These oscillations are strong enough at the bottom of the keyhole to deflect, leaving behind a large bubble. In some cases, this bubble no longer connects with the keyhole. It collapses and creates an acoustic wave. This pushes the remaining pores away from the keyhole."

Important to note here is that these key holes themselves are not errors in the manufacturing process; rather, they increase the efficiency of the laser. Based on their research, the researchers report that there is a clear boundary between stable and unstable keyholes. "As long as you stay outside the risk area (too hot, too cold), the risk of leaving abnormalities is quite small," Rollett states.

Here, he draws a comparison with the speed limit on public roads, although the concept works in reverse. "In this case, the slower you go, the more dangerous it becomes. If you go below the speed limit, you almost certainly create a deviation."

The insight gained from the research should eventually help improve the outcome and quality of printing processes. The researchers therefore expect companies to start integrating their findings into the control of their 3D printers in the near future.

Author: Wouter Hoeffnagel