Sensor Data Analysis

The hatch strategy of the build was on a 45° rotation every layer. This means that every other layer there was a straight line across the part. This simplified the calculation for the diode spatial map. However, the layers where the hatching is on a 45° angle had the effect of geometry on microstructure appear. Figure 6.1 visually represents the 45° angel.

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The contour is visible in the beginning of the graph, then the laser scan started the hatch. It is noticeable to see the very fast hatching of the corners that open to the bulk of the part and the longest scan times.

Figure 6.1: The third layer of the 3mm x 3mm section at a 45° angle. The photodiode data captures the change in geometry and relates that to temperature.

The hatch strategy plays a major role in the part quality as well as microstructure development. The fast laser scans in the corners build up heat that attracts powder particles outside of the part to join the melt pool. This phenomenon causes the corners of the part to swell and increase the height of the corners above the powder level. This results in a layer with no powder on the corners due to the swelling. The swelling is represented in the thermal data. Figure 6.2 represents the complete melt pool database for one build. The melt pools do not cover the entire part due to the frame rate of the camera and the exposure time. The reason for this is to be able to calculate temperature accurately.

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Figure 6.2: Thermal data compiled for Layer 220.

It is possible to extend the exposure time to include more of the laser scan into the image and temperature calculations; however, the calculations become significantly more difficult to accurately represent the melt pool temperature. For this reason, only partial layer melt pool information is used.

Finally, the last ability of the multi sensor approach is the mapping of the diode data. The long exposure time of the visible camera helps determine the topology of the parts surfaces each layer. This information is used in unison with the diode data to help create a reasonable image of the surface based on the diode data. Figure 6.3 displays the long exposure time visible camera data for layer 220 of the big-to-little experiment.

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Figure 6.3: Compiled visible camera data for Layer 220 of the big-to-little build.

The long exposure time is able to determine the intensity of the melt pool over a period of 13500 µs. During that time, the light from the melt pool was recorded in the visible camera each frame. The frames were stitched together to produce a complete layer. The diode data was stitched together in a similar manor. Figure 6.4 represents the stitched diode data for the big section and little section in this experiment. When comparing the diode data map with the long exposure visible data it is clear that much of the data is consistent with the visible camera. The strips in the diode data represent the change in direction of the laser, either moving closer to the sensor or away.

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Figure 6.4: Diode data mapped spatially to represent the high intensity points of the big- to-little experiment reflected in the visible and thermal data.

6.3 Conclusion

Fluctuation in the diode data appear because a change in the system occurs. The little section displays higher overall intensity throughout the build. In Figure 6.4, the high temperatures calculated with the thermal camera are correlated with the diode data. With the results from the data, it is possible to monitor the temperature of a LPBF build with a multi sensor approach. To continue this research, the material characterization of the big- to-little experiment will display the changes in microstructure already observed. Computer Tomography (CT) scans will display the location of possible defect. The sensor data will then be analyzed to determine of the location of the defects is represented in the data.

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Chapter 7: Conclusion

In this work, an approach to determine the effects geometry has on the microstructure of LPBF build part has been reviewed. The sensors used for this work were one visible camera, one thermal camera, one high-speed phantom camera, and three photodiodes focused on three wavelengths: 520 nm, 530 nm, and 640nm. The following conclusions have been made:

1. The photodiodes accurately recorded the behavior of the plasma plume. The behavior of the laser is captured in the diode data, revealing changes in direction, power, speed, and melt pool area.

2. The thermal camera calculations determine the change in temperature between melt pools. The change in geometry delivers larger hotter melt pools that are captured and confirmed by the thermal camera.

3. The high speed and visible cameras capture the topology of the surface, while extended exposure times create a complete image of each layer’s surface. 4. The single beads experiment determined that the direction of the laser changes

the relative intensity of the photodiode data. The topology of the plate significantly affects the photodiode data response leading to very sensitive sensor calibration. The single beads also determined how the change in parameters affects the sensor response, creating a full understanding of the sensor’s capabilities.

5. The thick-to-thin experiment created a geometrical change in the part with constant parameter. This results in longer times in between scan speeds for the bulk section and vastly short scan speeds times in thin extremities. The result

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was a larger thermal gradient and microstructural differences, both determined by the multiple sensors collecting data.

6. The big-to-little experiment explored the possibilities of diode data spatially mapped to determine the effect of defects and geometry on the microstructure of a LPBF part. More work is needed to complete the experiment and data analysis.

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