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Defects are still common in metal components built with Additive Manufacturing (AM). Process monitoring methods for laser powder bed fusion (PBF-LB/M) are used in industry, but relationships between monitoring data and defect formation are not fully understood yet. Additionally, defects and deformations may develop with a time delay to the laser energy input. Thus, currently, the component quality is only determinable after the finished process.
Here, active laser thermography, a non-destructive testing method, is adapted to PBF-LB/M, using the defocused process laser as heat source. The testing can be performed layer by layer throughout the manufacturing process. The results of the defect detection using infrared cameras are presented for a custom research PBF-LB/M machine. Our work enables a shift from post-process testing of components towards in-situ testing during the AM process. The actual component quality is evaluated in the process chamber and defects can be detected between layers.
Great complexity characterizes Additive Manufacturing (AM) of metallic components via laser powder bed fusion (PBF-LB/M). Due to this, defects in the printed components (like cracks and pores) are still common. Monitoring methods are commercially used, but the relationship between process data and defect formation is not well understood yet. Furthermore, defects and deformations might develop with a temporal delay to the laser energy input. The component’s actual quality is consequently only determinable after the finished process.
To overcome this drawback, thermographic in-situ testing is introduced. The defocused process laser is utilized for nondestructive testing performed layer by layer throughout the build process. The results of the defect detection via infrared cameras are shown for a research PBF-LB/M machine.
This creates the basis for a shift from in-situ monitoring towards in-situ testing during the AM process. Defects are detected immediately inside the process chamber, and the actual component quality is determined.
X-Ray Computed Tomography (CT) is applied in industry for flaw detection, flaw evaluation and dimensional measurement. This requires the correct CT system settings for sufficient visibility and detectability of flaws and structure elements. The visibility of indications for human observers on a monitor depends on the square root of the reconstructed flaw area (here pores with diameter < ½ inch) in the cross sectional 2D images, the Contrast to Noise Ratio (CNR) and the Modulation Transfer Function (MTF). This applies also to 2D-projections, meaning also to “normal” digital radiographs. The ASTM guide E 1441 describes three essential functions for prediction of the visibility of small circular indications in slice images of a 3D reconstruction or a 2D reconstruction. This is the Contrast Discrimination Function (CDF), the MTF (see also the revision of ASTM E 1695), and the Contrast Detail Dose (CDD) function. The prediction of the visibility of circular indications in reconstructed slice images can be determined from the Contrast Detail Dose function, which is the combination of CDF and MTF and a physiological factor c. The measurement procedures and formulas for the prediction of the detail visibility by CDD will be corrected, tested and verified by modelling and in a Round Robin Test with modern commercial 3D CT systems of different manufacturers. Conclusions will be reported and recommendations will be given for the correct psychological factor c and the consideration of the influence of artefacts, as e.g. cupping and scatter.