Influence of welding parameters on microstructure and mechanical properties of medium and heavy thickness 2219-T8 aluminum alloy BT-FSW joints

Objective To apply 2219 aluminum alloy in the manufacturing of heavy thickness and large diameter rocket fuel tanks, a bobbin tool friction stir welding experiment was conducted on 12 mm thick 2219-T8 aluminum alloy. Methods The influence of welding parameters on the surface morphology and microstructure of the weld was observed by using an optical microscope. The mechanical properties of the joint were characterized by using a tensile testing machine and a hardness tester. The fracture morphology of tensile specimen was examined by using a scanning electron microscope. Results The amount of flash on weld surface and cross-sectional microstructure were related to the welding parameters. Excessive heat input led to overheating oxidation on the advancing side of weld surface and formation of void defects in the weld cross-section. The lowest hardness, approximately 82 HV, was observed in the thermo-mechanically affected zone on the retreating side, where fracture also occurred. Conclusion The joint exhibited the best tensile properties under the parameters of 250 r/min-300 mm/min, with a tensile strength of 336 MPa and a joint efficiency of 80%. The fracture mechanism of all tensile specimen was ductile fracture.