**6. Patents**

The flash measurement system enables the identification of suitable collision conditions and can be used for quality assurance during production. It is patented for di fferent impact welding processes [47,48].

**Author Contributions:** Conceptualization, J.B., J.L.-A., B.N., M.B., E.S., E.B., A.E.T., P.G., M.F.-X.W., S.B.; methodology, data analysis, J.B. (MPW of tubes, design of vacuum chamber for MPW of tubes, flash measurements, witness pins, SEM), J.L.-A. (MPW of tubes, PDV measurements), B.N. (Test rig, high-speed imaging, long time exposures), M.B. (spectroscopic measurements, optical microscopy, EBSD), E.S. (MPW of sheets, design of vacuum chamber for MPW of sheets, SEM, High-speed imaging, long time exposures); Design of experiments, investigation, validation, J.B., J.L.-A., B.N., M.B., E.S.; writing—original draft preparation, visualization, J.B., J.L.-A. (1., 2.3.), B.N. (1., 2.2., 3.1., 4., 5.), M.B. (2.2., 2.3., 2.4., 3.2.), E.S. (2.3., 3.2.); writing—review and editing, supervision, project administration, funding acquisition, resources, E.B., C.L., A.E.T., P.G., M.F.-X.W., S.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), gran<sup>t</sup> number BE 1875/30-3, TE 508/39-3, GR 1818/49-3, WA 2602/5-3, BO 1980/23-1. This work is based on the results of the working group "high-speed joining" of the priority program 1640 ("joining by plastic deformation"). It consists of the subprojects A1, A5, A8, and A9. We acknowledge support by the Open Access Publication Funds of the SLUB/TU Dresden.

**Acknowledgments:** We would like to acknowledge the e ffort for the sample preparation, SEM and EDS analysis at Fraunhofer IWS Dresden. As well, we would like to thank Walter Tutsch of PCO AG for the support in setting up the image intensifier camera. The authors also greatly appreciate the help of Stephan Ditscher of Baumüller who supported the programming of the electronical control system of the test rig. We thank Jeanette Brandt for her e fforts in proofreading the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

## **Appendix A Blackbody Model for Temperature Estimation**

According to Planck's law, power density *M<sup>O</sup>* λ in the range between λ and λ + *d*λ for thermal emission can be calculated as

$$M\_{\lambda}^{\mathcal{O}}(\lambda, T)dA \, d\lambda = \frac{2\pi hc^2}{\lambda^5} \frac{1}{e^{\left(\frac{\hbar\kappa}{\lambda\lambda\_B T}\right)} - 1} dA \, d\lambda \tag{A1}$$

However, the sensitivity of imaging sensors (complementary metal-oxide-semiconductor, CMOS) is proportional to the photon flux, which can be calculated from the power density distribution as

$$J\_{\lambda}^{\text{ph}}(\lambda, T) dA \, d\lambda = \frac{2\pi c}{\lambda^4} \frac{1}{e^{\left(\frac{\lambda c}{\lambda \lambda\_B T}\right)} - 1} dA \, d\lambda \tag{A2}$$

As can be seen in Figure A1 that these two expressions yield di fferent shapes of the thermal emission spectra and subsequently di fferent maxima.

A simple method for estimating the temperature of a thermal emitter is the tracking of those spectral maxima. According to Wien's displacement law, higher temperatures cause a shift of emission maxima to shorter wavelengths in both distributions.

$$
\lambda\_{\text{max}}^M = \frac{2897.8 \,\mu m \cdot K}{T} \lambda\_{\text{max}}^J = \frac{3669.7 \,\mu m \cdot K}{T} \tag{A3}
$$

However, due to the limited spectral range and low resolution of the setup used in this work, tracking of those maxima is only feasible for temperatures above 5400 K. Furthermore, the position of those maxima is difficult to track in the noise-afflicted experimental spectra given the low slope of the theoretical spectra for temperatures above 5000 K (Figure A2).

**Figure A1.** Spectral power density and photon flux density for thermal emission at T = 4500 K.

**Figure A2.** Modeled thermal emission spectra in the investigated spectral range for temperatures between 2000 K and 8000 K.
