*Article* **Interface Behavior and Impact Properties of Dissimilar Al**/**Steel Keyhole-Free FSSW Joints**

**Zhongke Zhang 1, Yang Yu 2,\*, Huaxia Zhao <sup>3</sup> and Xijing Wang <sup>1</sup>**


Received: 28 April 2019; Accepted: 13 June 2019; Published: 18 June 2019

**Abstract:** This work systematically investigates the interface behavior and impact properties of the keyhole-free friction stir spot welding (FSSW) of a dissimilar metal AA6082-T4 Al alloy and DP600 galvanized steel. The keyhole is eliminated by pin retraction technology. The welding process is in accordance with the welding temperature curve and the maximum temperature of the periphery of the shoulder, measured at about 500 ◦C. The transition layers were formed at the interface, in which the Al, Fe, and Zn elements form an inhomogeneous diffusion. A cloud cluster-like mechanical mixing of the Al and steel components is formed in the stirring zone. The impact toughness of the specimen with a welding parameter of 1000 rpm is the best. To a certain extent, the factors affecting the impact energy are not the maximum impact load but the maximum impact deformation. The maximum impact deformation directly reflects the post-crack propagation energy, which significantly affects its impact toughness. In addition, the impact fracture showed a mixed ductile and brittle fracture mode with a brittle–ductile transition zone. Most of the impact energy was absorbed by the ductile fracture.

**Keywords:** FSSW; dissimilar metals; interface behavior; impact properties
