# Plastic Behavior of Laser-Deposited Inconel 718 Superalloy at High Strain Rate and Temperature
## 3 Experimental Procedure
### Temperature Measurement  
Temperature control was performed using three different measuring systems, as illustrated in Figure 2. Two K-type thermocouples were directly welded on the specimen surface at the ends of the gauge length. Their signals were acquired by the National Instruments data acquisition system NI cRIO-9211. Multi-wave infrared pyrometers were used to measure the maximum temperature in the gauge length. Depending on the testing temperature, a specific pyrometer was adopted: the Williamson Pro MW-20-5-C was used for tests with temperature in the range of 150-475 \({}^{\circ}\)C, and the Williamson Pro MW-10-20-C was used for tests with temperature in the range of 475-1750 \({}^{\circ}\)C. The third measuring system was the FLIR X6900 SC high-speed infrared camera. As extensively discussed in [24], each instrument has advantages and disadvantages. For this reason, the adopted solution was the simultaneous use of all of them. This guaranteed the redundancy of the measurements. In addition, it allowed for the possibility of checking the consistency between the performed measurements. The problems related to thermocouples regard the reliability of the measurement: it depends on the quality of the welding and can be affected by the electromagnetic field generated by the heating system. The other important aspect to be considered is that the measurement is local. Hence, in the used configuration, the thermocouples measured  
Figure 3: Sequence of images of a high strain rate test at 850 \({}^{\circ}\)C on an HT specimen: resolution of 52 \(\times\) 128 pixels, framerate of 100,000 fps, shutter of 1/1,000,000 s, time interval between the reported images of 50 \(\upmu\)s.  
the temperature at the ends of the gauge length. Finally, they are slow and consequently not suitable to appreciate the temperature increment that is due to adiabatic self-heating occurring during a dynamic test. The measurement performed with the infrared camera requires setting the emissivity of the material. This property needs to be calibrated. In this study, a high-speed infrared camera was adopted, which can also be suitable to obtain information on the adiabatic heating during the tests. To perform this, proper calibration is needed because the emissivity varies with the temperature; hence, it could change during a dynamic test. Another important advantage of this measuring system is that it outputs the spatial distribution of the temperature over the focused area. This information can be used to check the uniformity of heating along the specimen. The multi-wave infrared pyrometer can provide temperature measurement without knowing the emissivity of the material. The problem, in this case, was that the spot over which the temperature was measured was bigger than the specimen dimensions. This implied that the signal strength had to be adjusted to perform the measurement, but this procedure reduced the range of measurable temperatures. In the used configuration, the pyrometers gave the maximum value of temperature measured over the spot as output; hence, it was a local measurement. In Figure 4, the calibration of the infrared camera is reported. The temperature values measured by the two thermocouples and the pyrometer were averaged and used to calibrate the raw data obtained by the infrared camera. The raw data were expressed in counts that measure the collected energy on each pixel of the detector during the integration time: the more infrared energy incident on the detector, the higher the digital count, which was between 0 and 16,383. The process to obtain the temperature map should imply the conversion of the count values into radiance values, and then, if the camera is properly calibrated, the conversion into temperature values using the known or measured emissivity of the target object. Because of the problems related to emissivity, as previously discussed, the thermocouples and pyrometer data were used to directly convert the counts into a temperature.