Effects of Rotation Speed on Microstructure and Mechanical Properties of 2060 Al-Cu-Li Alloy in Friction Stir Welding

Microstructural and mechanical properties of 2.0-mm-thick 2060 Al-Cu-Li alloy joints obtained at different rotation speeds were assessed in this study. The rotation speeds ranged from 400 to 1300 rpm, with welding speed kept at a constant value of 100 mm/min. With the increase in rotation speed, grain coarsening occurred while the density of precipitation decreased in the stir zone (SZ), and great amounts of the Al2CuLi and Al2Cu phases were dissolved while only small amounts of the Al3Zr and Al3Li phases remained. The lowest hardness was found at the interface between the heat-affected zone (HAZ) and thermo-mechanically affected zone (TMAZ); the average hardness value of SZ increased, while the lowest hardness moved in the direction away from the center of joints. The ultimate strength (UTS) of weld first increased rapidly and then slowly decreased as rotation speed increased, with a maximum value of 443 MPa obtained at a rotation speed of 600 rpm, reaching 83.58% of the base metal (BM) strength. Three fracture modes were established to illustrate the growth mechanism of the crack. Analysis of the fracture surface showed that insufficient materials flowed and coarsened secondary phases caused the joint fracture.