Microstructure And Mechanical Property Of The 6061 Aluminum Alloy' Friction Stir Welding Joint

Lightweight of automobile is an important research project for the global automotive industry. One of the key technologies achieving lightweight of automobile is to reduce weight of wheels. As a new solid jointing technology, the method of friction stir welding (FSW) can be used to fabricate aluminum composite wheels and thus achieve lightweight of automobile. The joints of the aluminum composite wheels exhibit excellent mechanical properties, small residual stress and distortion. Therefore, FSW has been widely used in producing aluminum composite wheels in foreign countries. However, the research about aluminum composite wheels fabricated by FSW is just starting in China nowadays.In this paper, 6061 aluminum alloy, a type of material used usually as automotive aluminum wheels, is selected as research object. The microstructure, microhardness and room-temperature tensile property of the FSWed joints of 6061 aluminum alloy prepared by different process parameters and heat treatment conditions were studied experimentally. The forming mechanism of joints microstructure was discussed. Meanwhile, the fracture mechanism of the joints was discussed by characterization of fracture morphology. These research results may provide theory and experiment basis for application of the FSW in automotive aluminum wheels. The main research results obtained are as follows:(1) The microhardness of the FSWed joints of 6061-O aluminum alloy is much higher than that of the base alloy. The weld nugget zone possesses the highest value of the microhardness. The microhardness and mechanical properties of the base alloy and FSWed joints are both much better than those of the alloy prepared under an"O"heat treatment condition, irregardless of the process sequence of the T6 heat treatment and the FSW procedure.(2) The optimized parameter is obtained for the 6061-O aluminum alloy under FSW condition. At a rotation speed of 1200 r·min-1 and a welding speed of 300 mm·min-1, the mechanical properties of the obtained joints are comparable to or little higher than those of the base alloy. The fracture of the obtained joints occurs inside the base alloy. There is obvious necking for the base alloy and the joints. Their fracture mode is ductile fracture with honeycomblike feature.(3) The located stage of the heat treatment, before or after the FSW procedure, has an apparent effect on the mechanical properties of the obtained joints. Under the optimized parameters of 1200 r?min-1 rotation speed and 300 mm?min-1 welding speed, the tensile strength and elongation of the FSWed joints of 6061-O aluminum alloy treated by a T6 heat treatment are 230 MPa and 11%, respectively, achieving 85% and 50% of the base alloy, respectively. However, for FSWed 6061-O aluminum alloy, the fracture of the sample after T6 heat treatment occurs at the center of the weld nugget zone of the joints. Its fracture mode is brittle fracture. The strength of the joints is lower than that of the base alloy, only 82% of the base alloy. Meanwhile, the elongation decreases dramatically, the highest and lowest value of which is 5.5% and 1% of the base alloy, respectively.