Effect of welding speed on joint feature and mechanical properties of friction stir lap welding assisted by external stationary shoulders

Stationary shoulder technology was used in friction stir lap welding (FSLW) in the present study. Joint formation, microstructure, and lap shear failure load (LSFL) of the traditional FSLW joints and FSLW joints assisted by external stationary shoulders were mainly investigated. To further prove its advantage, the external-assisted stationary shoulders with different diameters were also used. Results show that compared with the traditional FSLW joint, smooth surface with very small flash, without shoulder marks can be attained by the assisted external stationary shoulder. With decreasing the welding speed from 200 to 50 mm/min, effective lap width (ELW) gradually becomes bigger, effective sheet thickness (EST) gradually becomes smaller on the FSLW joint assisted by external stationary shoulders. FSLW joints by the assisted external stationary shoulder own bigger LSFL than the traditional FSLW joints. Using bigger shoulder diameter, bigger LSFL can be obtained. Shear fracture and tensile fracture modes can be observed during the lap shear test. LSFL is found to be connected with the fracture mode and the hook morphology.