Temperature Analysis

Infrared Camera

The emission factor ε was determined to vary between 0.75 and 0.89 for the Ti-6Al-4V and Ti-Gr.1 coatings depending on the individual temperature, in the range of 800 °C to 1300 °C.

The IR camera was placed in front of the FS machine and focused at the rod tip, enabling recording of the rod heating rate and the temperature in the process zone, also during cooling.

The thermal images were captured at 20 frames per second and at a frequency of 80 Hz.

The heating rate was determined for the conducted experiments. It has been observed that the heating rate varies with rotational speed. The heating rate for Ti-Gr.1 was determined for three different rotational speeds. When a low rotational speed (300 min^-1) is introduced, the heating rate is low (216 Ks^-1 ± 19 Ks^-1) and leads to a peak temperature of 971 °C, because a low rotational speed necessitates more time to generate the energy required for the plastification phase. In comparison, when high rotational speeds are employed (3000 min^-1 and 6000 min^-1), heating rates of 404 Ks^-1 and 389 Ks^-1 are observed, respectively. This means that the peak temperature during the process at high rotational speeds is rapidly achieved in approximately 3 s. The process temperature exceeded the β-transus temperature in all experiments. The peak temperature achieved the steady-state condition while processing, and no temperature alteration was observed during deposition. A similar temperature distribution has been reported when depositing copper and tool steel. Once the transversal movement is superimposed, the temperature attains a steady-state condition. The heating rate for copper and tool steel is lower than that for titanium alloys [87]. The discrepancies in the heating rate might be derived from different physical properties of the material, particularly the low thermal conductivity of titanium. Furthermore, it can be concluded that the process temperature in FS cannot be suppressed below the β transformation temperature within the wide range of rotational speeds investigated in the current study. A thermal image captured during the process displays the temperature distribution at the rod tip and in the coating (Figure 5.19). The colour scale represents the corresponding temperature level. The highest temperature measured in the process zone was 1377 °C, and the earliest cooling stage of the coating behind the rod exhibited a temperature of 1180 °C.

Figure 5.19: Temperature distribution measured by an IR camera during the process indicating the temperature in the process zone and at the coating behind the rod (in the initial cooling stage).

x z

1180°C

1377°C

Experiments demonstrating the temperature evolution as a result of the rotational speed for Ti-6Al-4V and Ti-Gr.1 coatings are presented in Figure 5.20. For both alloys, an increase in temperature with rotational speed can be observed first, in which the temperature reaches its maximum at 3000 min^-1 for Ti-Gr.1 and at 2000 min^-1 for Ti-6Al-4V. Following this, the temperature reaches a plateau and remains constant with increasing rotational speed. It is remarkable that the temperature for both alloys achieves its maximum at that rotational speed where the flash formation at the coatings is precluded. This fact correlates very well with the flash formation hypothesis presented in section 5.2 showing that a change in deposition speed affects the temperature only in regime I.

Figure 5.20: Temperature evolution measured at the process zone of Ti-6Al-4V and Ti-Gr.1 coatings as a function of the rotational speed.

Thermocouple Measurements

The maximum temperature and cooling time were recorded at the interface of the Ti-6Al-4V coating generated at 3000 min^-1 as demonstrated in Figure 5.21. The two thermocouples measured maximum temperatures of 1169 °C and 1239 °C. The cooling rates were calculated from the maximum temperature down to 200 °C. The two thermocouples measured cooling rates of 46.4 Ks^-1 and 43 Ks^-1. At both cooling curves, a kink at approximately 800 °C was observed. This temperature range correlates with the martensite start temperature (800 °C) [9].

β-transus for Ti-Gr.1 Flash-free

coatings

β-transus for Ti-6Al-4V

Figure 5.21: Maximum temperature and cooling until 200 °C recorded at the interface of Ti-6Al-4V coatings generated at 3000 min^-1. A kink in both curves demonstrates the beginning of martensite transformation at ~ 800°C.

The temperature measurements employing thermocouples demonstrated differences compared with those employing the IR camera. Measurements by thermocouples were carried out at the coating interface; during the IR camera investigation, temperatures from the entire process zone were captured. Moreover, the exact location of the measurement point is unknown, which might lead to different values. These temperature differences might also be related to the placement height of the thermocouples on the substrate. To examine the exact position of the thermocouples, cross sections were prepared. Figure 5.22 shows the actual position of a thermocouple at the interface between the substrate and coating in the cross section of a sample after deposition.

Figure 5.22: A cross section showing the precise thermocouple location at the Ti-Gr.1 coating/substrate interface generated at a 6000 min^-1 rotational speed,a 16 mm/s deposition speed and a 1.6 mm/s consumption rate (a) and image with high magnification (b).

(a) (b)

Substrate Coating

Thermocouple Martensite start

temperature

y z

It can be observed that the thermocouple is located slightly beneath the coating. During the deposition it is pushed into the heated substrate. Moreover, it can be seen that the thermocouple tip was not covered directly by the coating but rather by the deformed substrate material.