Friction stir welding of low Cu and high strength Al-Zn-Mg-Cu alloy plate was carried out at 300 r/min and 900 r/min.The microstructure and mechanical properties of the welded joints were analyzed by means of metallographic microscope

scanning electron microscope

transmission electron microscope

Vickers hardness and tensile test.The results show that there is absence of obvious change of micrometer AlMnCr phase during the welding process

and the dissolution and coarsening occur in intracrystalline nanoscale MgZn

2

phase and grain boundary submicron MgZn

2

phase.With the increase of welding speed

the degree of precipitate dissolution is increased

and the grain size of welded joint tends to be increased.Under the combined action of grains and precipitates

the hardness of the welded joints of the alloy is characterized by approximate U-shaped distribution.The hardness of the heat affected zone on the advancing side is slightly lower than that on the retreating side

and that of the welding nugget and heat affected zone is relatively low

which is the fracture position of the welded joints

presenting zigzag fracture shape affected by the "S" line defect.The welded joints of low Cu and high strength AlZn-Mg-Cu alloy have the better properties under the low rotational speed of 300 r/min.The maximum tensile strength

yield strength

elongation and welding coefficient reach 393 MPa

261 MPa

12% and 80%

respectively.