**4. Conclusions**

The 5A06 and 6061 aluminium alloys were successfully friction stir welded in both NC and FAC conditions. For all the welded joints, the HAZ of AA6061 was the weakest area and fracture occurred in this region during tensile tests. There were two levels of decrease in the tensile strength of the FSW joints. For the first-level decrease, the tensile strength of the FSW joint was largely weakened due to the destruction of the hardened state (the primary reason), the dissolution of the precipitates and the coarsening of the grains. For the second-level decrease, the tensile strength of the FSW joint was slightly weakened with the increase of the R/T ratio. This is attributed to the further dissolution of the precipitates and further coarsening of the grains, caused by the increase in temperature during FSW. FAC can effectively accelerate the cooling process and reduce the affecting time of high temperature during FSW, suppressing the coarsening of the grains and the dissolution of the precipitates in HAZ of 6061, as well as generally improve the ultimate tensile strength by 10% compared with NC. The nanoindentation hardness contour maps also intuitively illustrate that FSW with forced air cooling is conducive to improving the hardness in the HAZ of AA 6061 and narrowing the weakest area.

**Author Contributions:** Funding acquisition, G.P. and T.Z.; Investigation, G.P., Q.Y., J.H., P.C. and Z.C.; Methodology, J.H. and P.C.; Supervision, G.P. and T.Z.; Writing—original draft, Q.Y.; Writing—review and editing, G.P. and Z.C.

**Funding:** The authors would like to gratefully acknowledge the support of the National Natural Science Foundation of China (Grant Nos. 11772302, 11727803 and 11672356) and Zhejiang Province Public Welfare Technology Application Research Project (2015C31074).

**Conflicts of Interest:** The authors declare no conflict of interest.
