**Guangjian Peng 1,2, Qi Yan 1, Jiangjiang Hu 1,2, Peijian Chen 3, Zhitong Chen 4,\* and Taihua Zhang 5,\***


Received: 29 January 2019; Accepted: 4 March 2019; Published: 7 March 2019

**Abstract:** Friction stir welding (FSW) is a promising welding method for welding dissimilar materials without using welding flux. In the present work, 5A06-H112 and 6061-T651 aluminium alloys were successfully welded by friction stir welding with forced air cooling (FAC) and natural cooling (NC). Nanoindentation tests and microstructure characterisations revealed that forced air cooling, which can accelerate the cooling process and suppress the coarsening of grains and the dissolution of precipitate phases, contributes to strengthening and narrowing the weakest area of the joint. The tensile strength of joints with FAC were commonly improved by 10% compared to those with NC. Scanning electron microscopy (SEM) images of the fracture surface elucidated that FSW with FAC tended to increase the number and reduce the size of the dimples. These results demonstrated the advantages of FSW with FAC in welding heat-sensitive materials and provide fresh insight into welding industries.

**Keywords:** friction stir welding; aluminium alloys; forced air cooling; microstructures; tensile strength; hardness distribution
