*Article* **Interface Formation during Collision Welding of Aluminum**

#### **Benedikt Niessen 1,\*, Eugen Schumacher 2, Jörn Lueg-Altho** ff **3, Jörg Bellmann 4,5, Marcus Böhme 6, Stefan Böhm 2, A. Erman Tekkaya 3, Eckhard Beyer 4, Christoph Leyens 5,7, Martin Franz-Xaver Wagner 6 and Peter Groche 1**


**\*** Correspondence: niessen@ptu.tu-darmstadt.de; Tel.: +49-6151-16-23148

Received: 3 August 2020; Accepted: 3 September 2020; Published: 8 September 2020

**Abstract:** Collision welding is a high-speed joining technology based on the plastic deformation of at least one of the joining partners. During the process, several phenomena like the formation of a so-called jet and a cloud of particles occur and enable bond formation. However, the interaction of these phenomena and how they are influenced by the amount of kinetic energy is still unclear. In this paper, the results of three series of experiments with two different setups to determine the influence of the process parameters on the fundamental phenomena and relevant mechanisms of bond formation are presented. The welding processes are monitored by different methods, like high-speed imaging, photonic Doppler velocimetry and light emission measurements. The weld interfaces are analyzed by ultrasonic investigations, metallographic analyses by optical and scanning electron microscopy, and characterized by tensile shear tests. The results provide detailed information on the influence of the di fferent process parameters on the classical welding window and allow a prediction of the di fferent bond mechanisms. They show that during a single magnetic pulse welding process aluminum both fusion-like and solid-state welding can occur. Furthermore, the findings allow predicting the formation of the weld interface with respect to location and shape as well as its mechanical strength.

**Keywords:** collision welding; impact welding; magnetic pulse welding; model test rig; welding window; jet; cloud of particles; welding mechanisms
