While we know that 3D printing gives us the ability to create complex shapes and geometries that would be impossible to achieve with conventional forms of manufacturing, less frequently discussed is the fact that these final objects have different properties than those created using traditional fabrication, which can cause problems in the long run.
This is especially true when it comes to laser additive manufacturing (LAM), which includes directed energy deposition (DED) and laser powder bed fusion (PBF), and has rapid cooling rates: scientists have yet to determine which conditions objects should print under for the best properties, which is keeping the technology from being widely adopted in safety-critical engineering applications, like biomedical and energy storage devices and turbine blades, that could really use 3D printing.
But a collaborative team of researchers from Diamond Light Source, the University of Manchester, and the Central Laser Facility at the Research Complex at Harwell (RCaH) has developed a method for seeing inside the LAM process to gain a better understanding of features like the powder consolidation mechanisms and laser-matter interaction.
Diamond Light Source is the UK’s national synchrotron, with 23 beamlines harnessing electron power to produce extremely bright light for studying everything from fossils and viruses to 3D printing. The Additive Manufacturing Team, based out of RCaH, is working with Diamond Light Source scientists from I12, the Joint Engineering Environment Processing (JEEP) beamline, to build a LAM system that can operate on a beamline. This will enable researchers to see right into the process and discover the physical phenomena that takes place during LAM processes.
“The LAM process is very fast, taking place in milliseconds, and to investigate we need microsecond resolution, which can only be achieved with the brilliance of a synchrotron. It allows us to follow the process from powder, through melting and then solidification back into the final solid shape,” explained project leader Professor Peter D. Lee from the University of Manchester. “On JEEP we are investigating the superalloys used in aeroengines, and we need the high energy, hard X-rays produced there to see inside them.”
The team published a paper on their work, titled “In-situ X-ray imaging of defect and molten pool dynamics in laser additive manufacturing,” in the journal Nature Communications. Co-authors include Dr. Chu Lun Alex Leung and Sebastian Marussi from the university, Robert C. Atwood from Diamond Light Source, Michael Towrie with Central Laser Facility, Philip J. Withers with the university, and Professor Lee.
In the paper, the team describes the novel LAM process replicator, or LAMPR, they developed, which lets them image and quantify how the melt track forms as layers are 3D printing. The LAMPR, which mimics a commercial LAM system, fits on the beamline and has windows that are transparent to X-rays, so the scientists can see into the LAM process as it’s occurring.
“The LAMPR is a unique piece of equipment, and beamline support was utterly essential,” said Dr. Leung, who was the PDRA running the experiments. “We worked with Diamond staff right from formulating the proposal. Diamond helped with the mechanical design, and the optics and integrating the LAMPR into the control systems.”
X-ray radiography with high temporal and spatial resolution was used to discover some of the important mechanisms behind powder consolidation and laser-matter interaction during LAM, including the formation and evolution of spatter patterns and melt tracks, porosity in the deposited layers, and the denuded (powder-free) zone.
The team was able to learn important information about the direction and flow velocities of the pore and spatter movements using their time-resolved quantification, which would have been impossible to get with traditional methods.
Their results made the physics underneath LAM more clear, which will help greatly in terms of development. Before these experiments, the hypothesis was that surface porosity forming on finished objects was occurring because of insufficient liquid feeding or incomplete melting. Now, the team knows this formation is due to a pore-busting mechanism, where pores close to the surface leave behind a surface depression when escaping into the atmosphere.
The experiment showed that many times, the continuous track of melted material can occur ahead of the main track, before merging with it, through pre-melting driven by surface tension.The team also learned that heating of inert gas and metal vapor can potentially cause defects by forming a plume which ejects molten droplets and powder away from the main track, and their experiment provided them with new information on the mechanisms of pore formation, such as migration, dissolution, dispersion, and bursting of pores during the LAM process. By continuing to investigate these areas further, we can learn more about laser-matter interactions.
The LAMPR helped the team study various process conditions of the process, and allowed them to develop a process map, which, according to Diamond Light Source, “illustrates how to tune the LAM process to produce a quality product with minimal trial and error” and is different from the typical mechanism map produced through synchrotron imaging that shows the fundamental physics which limit the process window. The team’s process map will help them alter the alloy, conditions, and even the process itself to surmount restrictions and develop a more efficient processing environment.
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