MICROSTRUCTURES AND MECHANICAL PROPERTIES OF FRICTION STIR WELDED ALUMINIUM ALLOY THICK PLATE

Microstructures of weld nugget zone and mechanical properties of the whole specimen and slices in the thick plate joint obtained by friction stir welding (FSW) were studied. The results show that the tensile strengthb, yield strength �0.2 and elongationdecrease with increasing weld speed (60—100 mm/min) at the rotary speed 400 r/min. The maximumb, �0.2 andlie in the top weld and reach 186.7 MPa, 100.3 MPa and 14.1% respectively, while the minimumb, �0.2 and � appear in the bottom joint and only attain 157.5 MPa, 80.2 MPa and 10.1% separately. The fracture morphologies show that there are lots of meshy dimples and the deepest dimple locates in the top joint. The secondary intercrystalline crack and shallow dimples lie in the root of weld. The microhardness at the top is higher than that at the bottom and presents asymmetrically through the weld centerline. The fine and equiaxial grain size is bigger at the top than that at the bottom. Compared with the bottom, second-phase particle at the top is more homogeneous and small, so the strengthening effect inereases.