A research team at Lawrence Livermore National Laboratory (LLNL) has developed a plan for minimizing defects in parts printed using laser-based metal additive manufacturing processes. The strategy, which was conducted in collaboration with the Air Force Research Laboratory (AFRL), was conceived using the lab’s high-fidelity computer simulation capability and ultra-high-speed X-ray imaging.
Laser-based powder bed fusion processes produce parts by emitting a laser beam that selectively melts metal powder, layer by layer, until a part is built. Since the process was pioneered, research teams and 3D printer manufacturers have been working to advance it through software and hardware updates, with the aim of improving part quality and process consistency. LLNL recently made a breakthrough on both fronts.
In research conducted by the LLNL, and recently published in the journal Science, the team revealed a previously unknown dynamic within LPBF, in which the process produces “spatter.” As the research team explains, “spatter” consists of small particles or clusters of powder particles that are ejected from the laser’s path and which can land back on the parts. This phenomenon can lead to the formation of pores or defects in final parts. After discovering the spatter interactions, the LLNL team set out to better understand it and find a solution to it.
Using a digital twin computer model, the team conducted virtual experiments of builds at the microscale. These simulations were then compared to experimental data captured using high-speed X-ray and optical imaging under LPBF-AM conditions. This comparison enabled the team to establish a stability criterion and a “power map,” which could help to minimize the defects caused by spatter.
The ‘Power Map’ solution
The power map is a scanning technique that essentially adjusts the laser’s power output along the laser track to stabilize the melt pool. The approach forms the foundation of LLNL’s “intelligent feed-forward” design process, which combines advanced modeling and simulation with experimental analysis so that 3D printers can become more efficient and less defect-prone.
“Spatter is the enemy of building nice parts; it’s not just small particles flying around, they can create a system of classes of spatter that can affect the build in different ways and scenarios,” explained Saad Khairallah, the paper’s lead author and LLNL computational physicist. “People can’t just naively turn on their laser and start scanning, because the scan strategy can create spatter at the start of a track, beyond a size threshold, that can be very bad for the build. The good news is that by using this stability criteria that we describe in the paper, they can modify the scan strategy based on a controlled power map they apply to prevent that large back-spatter.”
The stability criterion can minimize and even eliminate pores, keyholes and other internal defects in printed parts. The research team also realized that using a pre-sintering approach, in which a multibeam laser is run over the powder at a low setting to fuse particles together before the actual build, could result in less spatter. LLNL says its new strategy could result in more reliable printed parts using LPBF.
Simulation and X-Ray
The discovery of spatter particles and the strategy to solve the problem were enabled by LLNL’s advanced simulation capabilities. Specifically, Khairallah and his team developed new capabilities in LLNL’s multi-physics simulation code, ALE3D, in order to monitor the impact of the laser’s rays on the expulsion of particles, as well as to capture other dynamics that create defects in the LPBF process. The combination of this advanced simulation and X-ray diagnostics enabled the team to devise its unique plan for improving the metal AM process.
“X-ray diagnostics provide the only techniques that can simultaneously probe the surface and sub-surface of the metal while also offering the fidelity to track the fast dynamics of laser-induced structural changes,” said co-author Aiden Martin, technical lead for the synchrotron experiments. “The use of X-ray imaging allowed us to experimentally observe the phenomena of spatter formation and shadowing explored in the ALE3D simulations.”
Earlier this year, LLNL hosted a dedication ceremony of the new Advanced Manufacturing Laboratory – AML, a state-of-the-art collaborative facility situated in the recently expanded Livermore Valley Open Campus (LVOC).