- November 29, 2022
Nov. 2, 2022 - Laser beam melting is a method of additive manufacturing that enables the fabrication of 3D metallic parts with almost unlimited geometrical complexity and excellent mechanical properties. However, its vast potential in the aerospace or medical industries has not yet been exploited due to a lack of process stability and quality management. To address these issues, scientists at RWTH Aachen University in Germany have developed an alternative imaging approach1 that detects errors during the melt process, therefore increasing customer confidence in the quality of intricate parts. Leveraging a 29-Megapixel CCD camera from SVS-Vistek, the groundbreaking imaging solution is accurate, simple to implement and compatible with most commercial laser beam melting systems.
Evaluating layers for error detection
During the laser beam melting process, parts are fabricated through a periodic sequence of powder deposition, layer creation, and the lowering of the building platform for the next powder deposition. A beam source melts the powder locally according to a layer specific scanning pattern. Typical applications today for laser beam melting can be found in the domain of medical implants, FEM (Finite Element Method) optimized lightweight parts, or tool inserts with contoured cooling channels.
In spite of its potential, a wider spread adoption of laser beam melting has not yet occurred. For instance, in areas of high safety requirements like medical engineering and aerospace, customers need proof of the accuracy of produced parts. Overcoming this barrier requires suitable answers be found to these key questions: Which kinds of errors can appear? What is considered a critical process error? How can these critical errors be detected? And which measures can be taken for error debugging?
Acknowledging these questions, scientists at RWTH Aachen University researched and engineered an imaging system able to measure geometrical features determining dimensional accuracy. They began by experimenting with a Laser Beam Melting System from EOS GmbH, and provoking different kinds of process errors by varying the parameters around the recommended standards. Process stability was investigated by building test parts with critical geometrical features. Additionally, some types of errors were induced by manipulating optical components or using different powder qualities.
For analysis of errors, an SVS-Vistek SVCam CCD monochrome Dual GigE Vision camera was mounted in front of the machine window to capture images of the build platform. The SVS-Vistek camera features a 29-megapixel sensor with maximum resolution of 6576 by 4384 pixels at 6.2 frames-per-second. A Hartblei Macro SuperRotater tilt-and-shift lens was installed on the camera to lower perspective distortion, along with a 20mm extension tube to reduce minimum object distance. Field of view was set at 130mm by 114mm.
Diffuse lighting was required to provide homogenous imaging of the mirror-like metallic weld bead structures. This type of lighting minimizes specular reflections that would otherwise saturate a camera’s CCD sensor. Researchers determined that by placing the diffuse lighting close to the working surface and opposite of the camera the best imaging could result.
For each layer of the process, the scientists acquired two images: one after powder deposition and one after laser beam melting. Images of typical errors that the scientists had provoked were also taken using different lighting conditions for evaluation. At the end of testing, it was concluded that the imaging system was able to measure geometrical features and could therefore be commercially used for the control of dimensional accuracy in the laser beam melting processing. Furthermore, analysis of the systematical subdivision of errors found that "superelevations" and poor support connections are the most critical errors that will impact the quality of the fabricated metal part. Researchers believe that a possible solution for debugging this error could be a reduction of heat input in the affected areas. Further studies are planned