5.1 Introduction
An Acton grading spectrometer gives data for a range of spectral wavelengths. This data can be used to guide which wavelengths the 3-color spectrometer approach will use. A different intensity response from the Acton spectrometer is directly correlated with the number of photons that are emitted during LPBF. With higher power, more photons will be released leading to a much stronger signal. With a stronger signal present, fluctuations in the signal will then give some insight into the quality of the part. If a strong signal is not present, the sensing method is not suitable for the detection of defects. This experiment systematically reduced the power level to determine the number of photons released, which helped determine the sensitivity of this process monitoring technique when compared to the processing parameters used. A minimum threshold of power was determined to find a domain where enough photons released adequately represented the quality of the LPBF builds.
The first Acton spectrometer experiment performed by UTC prior to the beginning of this work gave results that the main wavelengths seen with strong spectral signatures in
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alloy 718 are 422 nm, 455 nm, 520 nm, and 530 nm. Figure 6.1 illustrates the results of the Acton spectrometer readings.
Figure 5.1: Original Acton experiment at 400 W (80%) and 1000 mm/s [52].
In Section 2.1.1.5, a review of the different temperature calculation of spectroscopy reveals that taking the ratio of two wavelengths can create a proxy for the changing temperature in LPBF. For this process, two wavelengths of strong spectral signal were used to collect data, which was used as a ratio for the change in temperature. The theory behind the temperature proxy is that when two wavelengths are used to evaluate the build, the wavelengths will fluctuate at different rates. Therefore, the ratio (or difference) between the wavelengths will indicate the change in temperature. In order for the temperature proxy to be accurate, the data must be equal in intensity. This is challenging because due to Planks distribution of blackbody radiation, the intensity will increase with increasing wavelength. Therefore, the two wavelengths will not have the same intensity. To eliminate the
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difference in intensity for the ratio of the two wavelengths to be possible, specific calibration is preformed, as stated in Section 2.1.1.6.
Similar to Figure 5.1, each wavelength that was chosen for the 3-color spectrometer approach appears in the Acton spectrometer data as a strong signal. The parameters of the original Acton spectrometer experiment were 400 W (80% power) and 1000 mm/s scan speed and a powder layer thickness of 50 µm. The single beads and thick-to-thin experiments used parameters that were at lower powers because the layer thickness was decreased to 30 µm. During these experiments, the spectral signatures did not behave as predicted. Therefore, a new experiment with the Acton grading spectrometer was performed.
5.2 Approach
The new Acton spectrometer experiment is used to define the power threshold for the spectral signals in LPBF. The Acton grading spectrometer data was collected for a 10 mm x 10 mm cube, with a rotation strip hatch strategy. The rotation of the hatch was set to 45 degrees and data was collected every layer that was horizontal and vertical. The Acton grading spectrometer collected sixty frames of data that was focused on 520 nm. Only the 520 nm wavelength was considered because when the focus was shifted to the 530 nm wavelength, there was no signal even at max power. The 520 nm wavelength signal was the only signal that appeared in the experimental set up. As the build progressed away from the substrate, data was collected at different power levels. Starting at max power and working down to very low power, the Acton grading spectrometer data evaluated the spectral signal of alloy 718.
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5.3 Results
The first experiment adjusted the power levels to define the threshold for the spectral signals. Figure 5.2 demonstrates the number of counts received at the 520 nm wavelength at 500 (100%), 400 (80%), 375 (75%), 300 (60%), 250 (50%), and 175 (35%) W. The counts are the number of photons collected by the Acton grading spectrometer. More counts are a result of high power causing more photons to be released.
Figure 5.2: Acton Data response of variable laser power at 750mm/s.
When decreasing the power level below 300 W (60% power), the difference between noise in the system and the spectral signal become increasingly similar. When comparing the signals found at 375 W (75% power), the spectral signal is abundant, however once the threshold is crossed into the lower power region of 300 W (60% power), the signal begins to drop off significantly. The signal continues to decrease until the point where the signal and the noise are indistinguishable, such as the 250 W (50% power) line. This point, and arguably sooner, the signal is not strong enough to provide any indication of the part quality.
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5.4 Conclusion
The results from the second Acton spectrometer experiment display the threshold of power that establishes a strong spectral signal. To evaluate the part quality, 375 W (75% power) or high is preferred to collect adequate signals. Based on these results, the experiments that used power levels below the threshold are not able collect spectral signatures that are strong enough to indicate part quality. This does not suggest that the information gained through the lower power experiments is not beneficial, only that the ability to ratio the wavelengths is lost. The power threshold determined in this chapter adds new knowledge of the spectral response, creating more accurate experimental designs leading to desired results from the photodiode data.
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