Despite the many advances in Laser Powder Bed Fusion technology in recent years—including multi-laser capabilities and even support-free builds—powder bed fusion has remained limited by certain factors, namely, consistency. In fact, speaking to many people across the industry, consistency and reliability are two critical points that keep coming up as hurdles to the full industrialization of AM.
Fortunately, the issue of consistency and faultless 3D printed parts is being addressed across the industry, both by metal AM companies and boundary-pushing researchers. In the latter category, Professors Bianca Maria Colosimo and Marco Grasso from the Department of Mechanical Engineering at Politecnico di Milano, have just put forward a compelling new technology that could not only identify inconsistencies in the LPBF process but also fix them during the print.
A patent for the in-situ monitoring process was recently filed (it is currently patent pending), making it easily one of the most promising and cutting edge innovations for laser powder bed fusion AM today. The system, an “open-architecture, highly sensorized LPBF prototype with integrated removal system,” is called PENELOPE. The system is named after Odysseus’ wife in Greek mythology, who became a symbol of fidelity: to avoid her suitors’ advances she promised she would choose one of them to marry when she finished weaving a funeral shroud. However, every night, she would unravel the piece much like the PENELOPE AM system would “undo” a printed layer.
The in-situ monitoring technology being developed by Prof. Colosimo and Grasso can be broken down into three distinct areas: in-situ quality metrology, in-situ thermography and in-situ defect correction. Taken together, the three elements make up a potentially game-changing solution for ensuring the production of faultless, consistent parts using LPBF additive manufacturing. While the system described below is quite complex, one of the most impressive findings so far is that the team demonstrated that the in-situ defect correction does not at all alter the part’s final quality.
In the LPBF process, there are many ways that defects can occur, including in-plane deviations and out-of-plane deviations. For industrial manufacturers, it is not viable to spend time and resources printing a metal part, only to have a defect occur mid-way through. In-situ metrology, therefore, is the first step in identifying if and when a process error is occurring.
The in-situ quality metrology proposed by the Italian research team comprises the first step in the innovative PENELOPE system. It is made up of a high-resolution imaging device, with pre-/post-exposure images and 20 µm/pixel capacity, as well as a reconfigurable illumination system, fitted over the printer’s build chamber.
In short, the in-situ metrology system offers in-line detection of geometrical distortions by capturing layerwise high-res images of the print in different lighting conditions. This enables the system to detect contours using level set-based image segmentation. The imaging system also captures dimensional measurements of a given part’s geometry, meaning it can identify even minute inconsistencies in the printer layers. Finally, Penelope is able to estimate the measurement bias and variability of the printer from build to build.
The second critical element in the patented in-situ monitoring process is thermography, which relies on a high-speed thermography system to thermally map and monitor cooling transitory and fast phenomena, including spatters and plume. The system in question is a 3-5 µm midwave infrared camera (MWIR) with 1000 fps @ 200 µm/pixel and calibration up to 1500 °C.
In short, the aim of the in-situ thermography system is to capture in-line estimation of the part microstructure using video-imaging. This is achieved through the layerwise analysis and modeling of spatial and temporal cooling gradients within the scanned region, and through the estimation of microstructural grain morphology indicators, such as columnar grains or equiavial grains.
According to the researchers, there is still work to be done to improve in-line microstructure estimation. Among the next steps in the research are improving the calibration of the IR camera, which is “difficult or even impossible because of temperature-variant emissivity coefficient and phase variation of material during the LPBF process”; further validating the methodology by conducting tests under different microstructural properties; and monitoring thermal cooling profiles to stabilize the process.
In-situ defect correction
The final part of Colosimo and Grasso’s patent-pending PENELOPE technology is inspired by the possibility of having a LPBF system that is capable of autonomously detecting defects using in-situ monitoring technology and then removing the defective layer—resulting in faultless parts.
Perhaps the most compelling part of the in-situ monitoring solution, the defect correction feature integrates in-situ sending, monitoring and hybrid manufacturing capabilities, which effectively enable the system to grind the surface of a part, erasing the most recent printed layers.
This novel technique could enable LPBF systems to identify defects as they occur, erase them by removing the defective layers, adjust process parameters accordingly and resume the print job.
The system devised to remove the defective layer (or layers) is made up of a surface grinding wheel that is installed in a grinding cart mounted on a linear axis, parallel to the Y-axis. The wheel component is a Borazon abrasive wheel with an average grain size of 120 µm, a maximum speed of 4,000 rpm, a max feed of 30 mm/s and a cutting depth of 10-20 µm.
The researchers have taken into consideration many feasibility questions for the defective layer removal process, including whether it is possible to restore an uncontaminated and uniform powder bed after the grinding wheel removes the top layers of a part, and whether the layer removal affects the overall mechanical performance of the part in question.
In tests, however, the prototype PENELOPE system showed that it was indeed possible to use the grinding tool and still maintain an uncontaminated and homogeneous powder bed. Further tests—CT scans of printed samples—showed that there was no statistical difference in part density between a part that had a layer removed and one that didn’t. Similarly, no anomalous pore concentration was found.
At this stage, the researchers will continue to develop and advance the novel LPBF process with in-situ monitoring and correction capabilities. Future steps in the innovative research project will address the characterization of mechanical performance, additional microstructural analysis, investigations into oxydation effects and more. After addressing these issues, the researchers intend to introduce PENELOPE 2.0, a “fully integrated controller for autonomous defect detection and correction.”