Friction stir welding（FSW）experiments of A7N01P-T4 aluminum alloy butt-lap structure were carried out by using different stirring pin offset modes.The bridging characteristics

microstructure and properties of FSW were analyzed.The results show that the butt-lap structure can obtain the satisfied joints under different offset modes.The joint is mainly composed of base material zone

nugget zone

heat affected zone and thermo-mechanically affected zone.However

the joints appears in hook defect

S-line or cold lap defect

which can affect the quality of the joints.When the lap surface is set on the retreating side（RS）

and the stirring pin is inward

the S line can increase the damage to the joints and reduce greatly the load of joints（2.03kN）

while the pin is outward

the size of the cold lap is increased

howeever it has a little effect on the performance of joint（15.86～15.96kN）.When the lap surface is et on the advanced side（AS）

the S line has no obvious effect on the performance of joint.Meanwhile

the pin offset presents slight effects on the hook defects

so it has a little effect on the performance of joint（12.76～12.94kN）.Although the maximum load is lower than the former one in this mode

the stability of joint performance presents more desirable.Furthermore

FSW technology shows a better bridging ability in the welding of aluminum alloy butt-lap configuration.