Abstract: Welding speed has an essential effect on microstructure and precipitations of weld nugget zone, thermal-mechanical zone and heat-affected zone in friction stir weld. As welding speed being increased, there is a considerable change for grain size, precipitation size, distribution and density in weld nugget zone and heat-affected zone; and deformation degree of microstructure in thermal-mechanical zone changes a great. Results show that the microstructure of weld nugget zone plays a leading role in improving mechanical property of joint, which demonstrates that the opportunity exists to manipulate welding speed in order to obtain better microstructure of weld nugget zone and improve joint strength. Mechanical measurement shows that joint has the highest ultimate tensile strength of 331 MPa with welding speed v=37.5 mm/min.