Abstract
The 2024 aluminum alloys with 3 mm thickness were welded successfully by friction stir welding (FSW). Microstructures and mechanical properties of joints were observed and analyzed. The test result indicated that the weld nugget zone was composed of refined and equiaxed grains. When the welding traveling speed is 300 mm min^–1, the peak tensile strength of joints can reach to 294 MPa, as much as 70 % of the value for the base materials. The fracture of the weld nugget zone is composed of equiaxed dimples with ductile fracture characteristics. Lots of the second phase particles, such as the CuAl₂ phase, exist in the dimples. However, the upper shoulder affected zone shows the mixed fracture characteristic of cleavage and quasi-cleavage with little dimple. The fracture of the bottom swirl defects zone is a typical mixed fracture mode by the ductile and brittle fracture. The divisional structure of fracture is close related to the mechanical properties and microstructures of the FSW joint.

Key words
aluminum alloys, friction stir welding, defects

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