Microstructure, properties and fatigue fracture of 7050-T7451 aluminum alloy friction stir welded joints

Microstructure, properties and fatigue fracture of 7050-T7451 aluminum alloy friction stir welded joints were mainly studied in this article. Tests of metallographic structure, hardness and fatigue of weldments were carried out, which were analyzed based on the test results.The test results showed that there was a clear boundary between weld nugget zone and thermo-mechanically affected zone, microstructure of surface and different areas of cross-section of the entire joint had obvious differences. Weld nugget zone underwent high-temperature thermal cycles and was subjected to strong stirring, and significant dynamic recrystallization occurred, which resulted in uniform and fine microstructure without obvious directionality. Microstructure of thermo-mechanically affected zone was mainly affected by thermal cycle in friction stir welding process, meanwhile, it was affected by mechanical external force to a certain extent, which resulted in a small plastic deformation, but energy storage was not enough, dynamic recrystallization did not appear in the end. Heat-affected zone was affected by thermal cycle, and its microstructure was larger than that of base material, but there was no obvious plastic deformation, and it still had characteristics of fiber microstructure of base material. The results of hardness test showed that hardness of base material and weld nugget area was relatively higher, while hardness of heat-affected zone was lower, and hardness of thermo-mechanically affected zone was between them.Through analysis of fatigue test, when stress level was 200 MPa, samples fractured on the forward side of heat-affected zone, and surface of fracture was relatively rough. When stress level was 250 MPa, samples fractured in weld nugget area, and surface of fracture was relatively flat. When stress level was 300 MPa, samples fractured at base material, and surface of fracture was very flat.Highlights: 7050-T7451 aluminum alloy welded joints by friction stir welding technology showed good formation. The technology had outstanding advantages and was appropriate for aerospace and other important fields.