Microstructure and mechanical properties of 0.3 mm thick aluminum alloy ultra-thin plate by high-speed friction stir welding

Objective The purpose was to explore weld forming and joints’ quality of 0.3 mm thick aluminum alloy ultra-thin plate by friction stir welding. Methods High-speed friction stir welding test of 3003 H19 aluminum alloy ultra-thin plate with thickness of 0.3 mm was conducted by a micro-shoulder mixing head with diameter of 2.5 mm. The surface forming, microstructure and mechanical properties of welded joints under different technological conditions were studied by means of metallographic microstructure observation, tensile test, SEM scanning of fracture and micro-Vickers hardness measurement. Results When rotating speed/welding speed ratio n/ν ≥ 10, weld with better surface forming could be obtained. When rotating speed/welding speed ratio n/ν <10, forming quality of weld surface deteriorated due to insufficient heat input. When welding speed or rotating speed of stirring head was unchanged, tensile strength of welded joints increased first and then decreased with the increase of welding heat input, and when n/ν =10, tensile strength reached the maximum, which were 157 MPa and 159 MPa respectively, and reached 74% and 75% that of base metal. When n/ν <10, tensile strength was the lowest. When 10≤ n/ν ≤12, tensile strength of welded joints was almost the same, and all reached more than 72% that of base metal. The hardness of welded joints was V type distribution, softening phenomenon occurred in and near weld nugget zone, and the hardness dropped from about 65 HV of base material to about 45 HV. Conclusion The micro-shoulder friction stir welding method could realize high-speed welding of aluminum alloy ultra-thin plate, and welding heat input had great influence on forming quality and mechanical properties of weld surface.