**4. Discussion**

The experimental results presented in this study clearly confirm the hypothesis that high temperatures occur within the joining gap during collision welding processes. The experiments at reduced ambient pressure showed that the temperature increase also takes place in the absence of the surrounding air. Thus, it seems reasonable to relate the temperature increase to the material that is ejected from the process zone. Depending on the collision kinetics and the involved materials, material ejection can either occur as a dense material flow as a jet and/or as a CoP in case of lower impact energies and depending on the collision angle. The thermal energy of the CoP enables and supports all three bonding types that have been reported for collision welding processes so far. Depending on the

impact kinetics at the propagating collision point either solid-state bonding, solid–liquid coexistence state bonding, or liquid-state bonding can occur [44]. The influence of the material properties, surface properties, the collision environment, and the collision kinetics on the characteristic features of the CoP were investigated in three di fferent experimental setups, as discussed and listed below:


One of the key results of the present study is that the temperature in the joining gap at ambient pressure was found to exceed 5600 K. Due to the arrangemen<sup>t</sup> of the spectral measurements it can be concluded that this temperature occurred in the rapidly closing welding gap *in front* of the actual point of collision. This e ffect can be suppressed at reduced ambient pressure. However, to determine the temperature of the CoP under vacuum-like conditions, a di fferent setup with an increased spectral sensitivity in the infrared range would be needed.

*Metals* **2020**, *10*, 1108

Furthermore, the findings of the present study point to the conclusion that the process glare results from a superposition of multiple, di fferent e ffects that depend on the process environment:


To conclude, it should be noted that the process glare alone cannot be used as a su fficient welding criterion because multiple parameters contribute to the light emission, weld formation, and the corresponding side e ffects. In a conventional ambient atmosphere, the lightning e ffect is dominated by the interaction with the surrounding air. Thus, the e ffect of the collision angle, which significantly influences the welding result, is not directly accessible. Nevertheless, the process glare can be seen as a necessary criterion while additional conditions must be fulfilled to ensure a good weld quality. For example, the thermal properties of the involved materials must be suitable to ensure the cooling of the materials after the contact, as described in [26].

## **5. Conclusions, Research Highlights, and Outlook**

The experimental results at normal ambient pressure indicate temperatures more than 5600 K in the joining gap that enable not only solid-state bonding, but also solid–liquid coexistence state bonding or liquid-state bonding. The process glare consists of di fferent components and depends on certain factors. It occurs if the kinetic energy of the moving joining partner is su fficient to extract a certain number of particles from the surfaces by plastic deformation during the collision. The particles accumulate and then form a CoP. The CoP is compressed in the closing joining gap and heats up until it glows. This e ffect can be intensified by small collision angles where high temperatures are reached by a higher compression rate and comparatively more e ffective wall friction, su fficient to melt the surfaces of the joining partners. To form a sound weld, the CoP must leave the joining zone before the joining partners come into contact. Thus, smooth surfaces and vacuum-like conditions are preferable for collision welding. The absence of surrounding air eases the process observation since exothermic oxidation reactions and shock compression of the gas are avoided. Nevertheless, the occurrence of the process glare is not a su fficient welding criterion, but just a necessary condition.

From these findings, technological guidelines can be derived for collision welding processes. For example, the formation of the CoP is facilitated by soft materials; a small collision angle increases the temperature and surface activation; a low surface roughness supports the escape of the CoP.

Nevertheless, an important question that remains is which mechanism is ultimately responsible for the activation of the joining surfaces. On the one hand, it might be the kinetic energy of the flowing CoP that rubs intensively against the surfaces. On the other hand, surface activation could also be attributed to the heat transfer between the compressed CoP and the surfaces. Furthermore, the possible plasma state might as well play an important role for surface activation, since plasma activation is a well-established technology, see [46]. Although plasma formation during collision welding in normal ambient atmosphere was only attributed to the shock-compressed air in [23], it may also occur under vacuum-like conditions. The CoP itself could be transferred into plasma due to the sudden compression and heating in the joining gap. Future investigations should focus on the time-resolved measurement of the temperature in the joining gap to identify the dominating mechanisms during collision welding. Furthermore, to determine the temperature of the CoP under vacuum-like conditions, a di fferent setup with su fficient spectral sensitivity in the IR range would be needed.
