32 P.L. Clavette et al. Fig. 4.4 AE for 90Ta-10W sprayed on aluminum. Top plot is the WD sensor, bottomis the 30 sensor. Periods correspond to following: (A) nitrogen purge, (B) helium purge, (C) powder deposition using cold He with fairly high amplitude AE, (D) movement of the nozzle away from the surface followed by switching from helium to nitrogen and addition of another 50 g of powder, (E) helium purge in preparation for second run, (F) second powder deposition with hot He, (G) a third powder deposition (hot He) period with no inter-layer purges, (H) movement of the nozzle away from the surface from the surface and conclusion of the test could be used to detect in real time, potential issues related to feedstock powder. Fifty grams of powder were placed into the powder feeder and spraying was allowed to continue until all powder was consumed. Referring to Fig. 4.5, numerous distinct periods were visible, these were as follows: (A) nitrogen purge, (B) helium purge, (C) powder deposition with fairly high amplitude AE, (D) a second period of powder deposition, and (E) movement of the nozzle away from the surface and conclusion of the test. Examining the plots, the AE amplitude from the nitrogen in period A was around 79 dB for the WD sensor and around 72 dB for the 30 sensor. This increased to 86 and 79 dB respectively in period B. For this gas switch, changes to the frequency centroid were seen, but significantly less than the prior run. The reasons for this were unclear. When the coating was built (period C), the AE amplitudes increased substantially: 101 dB for the WD sensor and 98 dB for the 30 sensor. The frequency centroid again exhibited a significant shift lower as the coating built. What should be noted here though are some outlier events with amplitudes in the range 105–112 dB, which contrasts with the prior run without large agglomerates. This was better seen in Fig. 4.6. The outlier events were associated with the impacts of the large agglomerates with the surface. It should be stressed here, that throughout the course of application of the two different coating pass groups, that the overall AE amplitude did not change appreciably. It is also noted that the AE amplitude during the spray process again was higher in the aluminum case as compared to the steel case.
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