Through hardness test

tensile test

fracture scanning electron microscopy（SEM） and welding zone transmission electron microscopy（TEM） observation

the effects of natural aging on the microstructure and properties of 6005 A-T4 friction stir welded joints were investigated. The results indicate that the maximum tensile strength of the friction stir welded specimens after natural aging is 194 MPa

and the elongation is 10.9%

while the minimum tensile strength is 180 MPa

and the elongation is 10.3%. The average tensile strength is 186 MPa

and the elongation is 10.2%

which are 89.0% and 93.5% of the base metal

and increased by 5.2% and 9.7%

respectively

compared with the unaged specimens. After natural aging

the hardness of the weld joints presents a “W”-shaped distribution

of which the heat-affected zone

thermo-mechanical influence zone

and weld nugget zone are all increased

and the weakest area is changed from the thermo-mechanical influence zone after welding to the heat-affected zone after natural aging. The fracture after aging is basically consistent with that of the base material with larger and deeper dimple

which coincides with the elongation result

and closer to the base material. After natural aging

β phases are observed in each area of the weld by TEM

and the heat-affected zone is less affected by the welding heat

resulting in insufficient precipitation power

which is the weakest area after natural aging.