Friction stir welding of 2060–T8 Al[sbnd]Li alloy. Part I: Microstructure evolution mechanism and mechanical properties

Abstract

2 mm thick rolled 2060–T8 AlLi alloy plates were subjected to friction stir welding (FSW) under rotation rates of 400–1200 rpm and welding speeds of 50–200 mm/min. In the nugget zone (NZ), FSW resulted in the dissolution of the precipitates and the formation of δ′ (Al3Zr) precipitates, as well as the formation of Guinier–Preston (GP) and Guinier–Preston–Bagaryatsky (GPB) zones except for at 400 rpm–200 mm/min. In the heat affected zone (HAZ), FSW led to the dissolution and coarsening of T1 (Al2CuLi) and S′ (Al2CuMg), and the subsequent formation of new θ′ (Al2Cu) and σ (Al5Cu6Mg2) precipitates. The lowest hardness zones (LHZs) of the FSW joints were located at either the NZ or the HAZ under varied welding parameters. Different from the FSW joints of conventional precipitation–hardened aluminum alloys, the ultimate tensile strength of FSW 2060–T8 joints increased as the rotation rate increased from 400 to 800 rpm, but was unchanged with further increasing the rotation rate from 800 to 1200 rpm under a constant welding speed of 200 mm/min. On the other hand, the joint strength increased as the welding speed increased from 50 to 200 mm/min under a constant rotation rate of 1200 rpm. While the FSW joints fractured at the LHZs under 400 rpm–200 mm/min and 1200 rpm–50 mm/min, abnormal fracture at the thermo–mechanically affected zone (TMAZ) was observed under 800 rpm–200 mm/min and 1200 rpm–200 mm/min.