In Situ Monitoring of Powder Bed Fusion Homogeneity in Electron Beam Melting
Abstract
:1. Introduction
2. Materials and Methods
2.1. Powder Bed Imaging Setup
2.2. Image Preprocessing and Merging
- Correction of the image perspective error. This operation can be applied during the camera calibration stage. The reader is referred to [29] for an overview of calibration methods commonly applied in machine vision.
- Image filtering, applied to both input images in each layer. The aim is to reduce the noise of the image, smoothing pixel-wise intensity variations. A high image noise is caused by the lack of an external illumination source, which imposes the adoption of a sufficient integration time combined with a sufficient sensor sensitivity enhancement. Is this study, we advocate the use of a median filter applied to both pre- and post-recoating images. It is a nonlinear filter that is known to be quite effective for a simultaneous reduction in image noise and preservation of contours [30]. The pixel intensity in location is replaced by the median of pixel intensities in a neighborhood of pixels. A larger value of denotes more intense smoothing, with a loss of image details. A smaller value of denotes a higher preservation of edges but lower noise filtering. In this study, we adopted a median filtering with , but the parameter can be easily tuned during the image calibration phase to be carried out just once when exploiting sample images.
- Image transformation. Let be the filtered pre-recoating image in one layer and be the filtered post-recoating image in the same layer. Let and be the intensity of a pixel in location of and , respectively. In a grayscale 8 bit image, the pixel intensity is an integer value in the range 0–255, where 0 corresponds to the lowest intensity (black) and 255 corresponds to the highest intensity (white). The proposed way to merge the information in the two input images consists of applying the following transformation:
2.3. Automated Anomaly Detection
3. Results
3.1. Case Study with Damaged Recoater
- Low-severity damage: only one tooth along one row was manually warped;
- Mid-severity damage: two teeth per row were removed;
- High-severity damage: 2 two per row were removed, and the neighboring teeth were manually warped.
3.2. Other Examples
3.3. Influence of the Choice of the Transfer Function
4. Discussion
5. Conclusions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Sensor | Spatial Resolution | Integration Time | Field of View |
---|---|---|---|
CMOS sensor, grayscale, visible range | 130 µm/pixel | 0.04 s | About 90% of build area |
% of False Alarms | Average Extension of False Alarm Regions | Max Extension of False Alarm Regions |
---|---|---|
9.7 × 10−5 | 4 pixels | 32 pixels |
% of False Alarms | |||
---|---|---|---|
Sigmoid function | |||
7.0 × 10−5 | 9.7 × 10−5 | 2.5 × 10−6 | |
Linear function | |||
9.1 × 10−5 | 1.3 × 10−4 | 5.6 × 10−4 | |
Step function | |||
5.2 × 10−5 | 4.4 | 58.7 |
Defects | |||
---|---|---|---|
Porosity | Geometrical Defects | Other | |
Lack of powder | Possible increase in over-melting porosity as a consequence of increased volumetric energy density | Swelling defects caused by intense remelting | Lack of powder lasting for multiple layers is expected to change the microstructural properties |
Excess of powder | Lack of fusion pores as a consequence of decreased volumetric energy density | Swelling defects caused by a thicker solidified layer; possible delamination in case of very severe lack of fusion | Possible microstructural variations caused by decreased volumetric energy density |
Contamination (large spatters, debris) | Possible lack of fusion pores as a consequence of beam attenuation/decreased volumetric energy density | Possible swelling defects in the presence of very severe contamination |
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Grasso, M. In Situ Monitoring of Powder Bed Fusion Homogeneity in Electron Beam Melting. Materials 2021, 14, 7015. https://doi.org/10.3390/ma14227015
Grasso M. In Situ Monitoring of Powder Bed Fusion Homogeneity in Electron Beam Melting. Materials. 2021; 14(22):7015. https://doi.org/10.3390/ma14227015
Chicago/Turabian StyleGrasso, Marco. 2021. "In Situ Monitoring of Powder Bed Fusion Homogeneity in Electron Beam Melting" Materials 14, no. 22: 7015. https://doi.org/10.3390/ma14227015