Temperature history

Knowing the process temperature is important to understand metallurgical transformations in the rivet during the joining process and the physicochemical changes in the composite, including possible degradation, and changes to its crystallinity and viscosity. Moreover, temperature provides an indication of the heat input under different joining conditions. As explained in Section 5.2.3, IR thermography was adopted to measure the temperature of the expelled composite material, while thermocouples were used to measure the temperature at the interface of the metal and composite.

Figure 6.5-a shows the process temperatures of all the joining conditions evaluated in this work, as measured by thermocouples (TC) within the joint and an infrared thermographic camera (IR). The temperature varied between (413 ± 18) °C and (1064 ± 159) °C when measured by TC and between (453 ± 10) °C and (580 ± 5) °C when measured by IR. Figure 6.5-b illustrates an example of the typical temperature development over time measured by TCs and IR (Appendix D shows all the TCs

and IR curves). The temperatures measured by the TCs were approximately 68 % higher than by IR.

The differences in these measurements may be explained by the fact that the thermocouples, as they are located within the rivet insertion path, were touched by the surface of the hot rivet, capturing its temperature directly. It is worth mentioning that Ti6Al4V has a low thermal conductivity (see Table 5.2) which in turn keeps the temperature localized at the rivet tip high, increasing the TCs measurement. On the other hand, for the thermographic camera, the temperature was only measured in the flash material and therefore was dependent on the amount of composite flowing out of the joining area. It is believed that when the heat input is sufficient to soften a polymeric layer, higher axial forces expel more molten polymer, which improves the acquisition of temperature data.

Moreover, although the flash material has low thermal conductivity, heat is constantly lost through convection, conduction, and radiation, decreasing the measured temperature. From the results, TC was used to further explain the microstructural changes in the joint materials, while IR was used to calculate the heating and cooling rates, as the TCs were damaged by rivet insertion, compromising their measurement of the cooling phase.

Figure 6.5 a) Average process peak temperatures for joining conditions; b) typical process temperature evolution measured by thermocouples (TC) and IR thermography (IR), the inset chart relates to TC

measurement of joining condition 9.

In all conditions, the process temperature was well above the melting temperature of PEEK (Tm, PEEK = 343 °C ± 2 °C) which allowed the melting of PEEK matrix close to the metal rivet.

Moreover, for the majority of the joining conditions a degradation of PEEK matrix directly in contact with the rivet was expected once the decomposition onset temperature (Td, PEEK = 575 °C to 580 °C) [121] is exceeded. However, the inset graph in Figure 6.5-b shows that the polymer was exposed to temperatures above Td, PEEK for a period shorter than 5 s. Therefore, no extensive degradation is expected in the vicinity of the rivet. The highest temperatures achieved in conditions 7 (1064 ± 159) °C and 9 (932 ± 69) °C exceeded the β-transus temperature of Ti6Al4V, suggesting complex

process-induced microstructural transformations in the metal rivet. Such transformations, along with high temperature and strain rate, also affect the flow resistance of Ti6Al4V under hot working conditions, leading to changes in its plastic deformation [204]. The effect of the temperature on the microstructural transformation and plastic deformation of the Ti6Al4V rivet will be reported in detail in Section 6.3.2, while the physicochemical changes of PEEK will be addressed in Section 6.3.3.

Table 6.1 shows the heating rate (HR) and cooling rate (CR) of the joining conditions that developed the lowest (Condition 3, Figure 6.5-a) and the highest (Condition 7, Figure 6.5-a) process temperatures evaluated in this work. The temperature rises faster with higher heat input, represented by an increase in the process temperature, which is intensified by the low thermal conductivity of Ti6Al4V (17.5 W/m·K) and the PEEK (2.0 W/m·K). Moreover, during the plastic deformation of Ti6Al4V at high temperatures and strain rates, as observed in the FricRiveting process, adiabatic heating may also be generated, which raises the actual temperature of the sample and is not conducted away [100]. As reported by Ding, Guo, and Wilson [205], the α→β phase transformation can enhance heat accumulation even more during compressive deformation, which may explain the significant increase of process temperature and consequently HR of Condition 7 in comparison to Condition 3.

Nonetheless, the heating rates calculated for FricRiveting are extremely high in comparison to other friction-based joining processes, such as friction stir welding of Ti6Al4V at 240 °C/s [206], refill friction stir spot welding of aluminum alloys (93 °C/s) [207], and friction spot joining (FSpJ) of AA2024 from (205 ± 0.3) °C/s to (355 ± 0.2) °C/s [208].

Table 6.1 Average heating and cooling rates calculated for joints produced with the lowest (Condition 3) and highest (Condition 7) heat input, i.e. process temperature.

Heating rate [°C/s] Cooling rate [°C/s]

Condition 3 301 ± 9 18 ± 2

Condition 7 1320 ± 22 15 ± 0.01

The measurement of cooling rate showed a moderated decrease in temperature (18 ± 2) °C/s and (15 ± 0.01) °C/s and no significant variation between Conditions 3 and 7. In contrast to the heating phase, during cooling the low thermal conductivity of Ti6Al4V and CF-PEEK tended to inhibit heat dissipation to the environment. The moderated cooling rate after FricRiveting can induce either bimodal or acicular microstructures in Ti6AlV [115] whereas can decrease the PEEK degree of crystallinity. The calculated cooling rates are much higher than the rate (0.08 °C/s [159]) that the woven composite experiences during its manufacturing, which may impair the nucleation and growth of spherulites in the PEEK matrix. These phenomena lead to changes of local mechanical properties throughout the joint, as discussed in Section 6.3.3.