Metal 3D Printing Roughness Reduced by 80% Using Dual Laser Technique

Lars Vanmunster, a student at KU Leuven, has developed a process for refining the surface finish of metal parts 3D printed using laser powder bed fusion (LPBF). The technique relies on the application of a second, pulsed laser to reduce roughness by 80 percent. The project was so inspired that it resulted in an award granted to Vanmunster’s work.

Due to the spattering of powder particles during the print process, small bumps form on parts that result in a rough finish. In turn, the part must be removed from the printer and finished using such techniques as grinding and polishing. While in some cases, such as orthopedic implants, a rough surface may be beneficial, this is not the case for the majority of parts that must be further processed upon printing to meet final specifications. This post-processing is time intensive and costly and, in some cases, can be the factor that makes or breaks the decision to use 3D printing for a given component.

A variety of methods have been developed to address surface finish of metal parts during the build process. These include mechanical machining, laser ablation and etching, electrochemical machining and more. Among the issues that arise from these methods is the formation of metal chips that can interfere with the build process.

In his research, Vanmunster and colleagues came across an idea that had typically been an issue for previous LPBF research. When experimenting with pulsed lasers to 3D print parts and faced the unfortunate side effect of shock waves interfering with the build. In other areas, however, such as laser shock peening and foil forming, shock waves from lasers can actually have practical applications.

A visualization of the process. Image courtesy of KU Leuven.

Vanmunster, then a master student in the Department of Mechanical Engineering, Manufacturing Processes and Systems at KU Leuven, joint efforts with colleagues and considered using shockwaves from a pulsed laser as a means of improving surface quality. By taking a 3D Systems ProX DMP 320A with a 500W laser and, using a second, nanosecond Ytterbium fiber laser, they developed a dual laser method for performing surface finishing during the print process automatically. While the first energy source is used to print the part, the second pulsed laser removes residual powder by sending tiny shockwaves to the area. This causes the un-sintered powder to blow away before the primary laser remelts the outer surfaces.

A printed part and cross section BEFORE (above) and AFTER (below) the remelting process. The surface is much smoother, and the stair-stepping effect is removed. Image courtesy of KU Leuven.

This process can reduce surface roughness up to 80 percent, thus cutting postprocessing labor and overall part cost. For his thesis, Vanmunster won the “IE NET master thesis awards” of the Flemish engineers’ association.

When asked from where the inspiration for this process sprung, Vanmunster told 3DPrint.com, “The dual laser setup was developed some years ago at the Additive Manufacturing research group at KU Leuven. The phenomena of selective powder removal by the shock waves of the pulsed laser was discovered by the AM team by intensively testing and experimenting with this novel dual laser setup. My research focused on using this setup to reduce surface roughness, in the context of the PhD research of Jitka Metelkova, who is using the machine for hybrid additive manufacturing.”

Incorporating the technology into an existing metal 3D printer required collaboration with the manufacturer itself, but could be something that could ultimately make it into the market, according to Vanmunster.

“From the start, we have implemented the dual laser setup in a commercially available LPBF machine from 3DSystems-LayerWise,” Vanmunster said. “We have collaborated with their engineers and jointly implemented all hardware and software modifications needed. This was done by modifying a 3DSystems DMP ProX320 machine in the framework of a large infrastructure project funded by the Flemish government. Industrial uptake from our research is hence fairly straightforward. This has been in our DNA ever since our AM team at KU Leuven was created in 1990.”

In addition to being able to perform a smoothing processing during the build itself, the use of shockwaves has other benefits over post-processing techniques. For instance, machining may not always be suited for smoothing out more geometrically complex, 3D printed parts. Laser-based surface treatments that occur after the print is complete require a number of time- and labor-intensive steps: removing the part from the machine, clamping it, performing the laser smoothing, and unclamping it.

“This all results in a substantial extra cost. The advantage of the dual laser setup is that it can do the processing in-situ, which means no manual labor and reclamping is required,” Vanmunster said.

Next, Vanmunster hopes to apply his technique to more complex, real-world objects so that he can demonstrate its utility for industrial products. To do so, he joined the university’s AM team as a PhD researcher under the guidance of Prof. Van Hooreweder who is leading the AM team.

“We are about to kick-off a new project with many industrial partners in which we will further explore to possibilities of our novel dual laser system,” Vanmunster said.