The present paper describes microstructure and its stability at elevated temperature of friction stir seam welded dissimilar joints of 5083 and 6061 aluminum alloy plates of 3 mm thickness. Butt-joining of the materials were carried out under several welding conditions varying from 1200 to 2400 rpm and from 60 to 240 mm/min. of rotation speeds and welding speeds, respectively. Microstructure was evaluated by means of optical microscopy, scanning electron microscopy （SEM）, electron dispersive spectroscopy （EDS） and transmission electron microscopy （TEM）. High temperature　stability of joint microstructure was investigated by isothermal annealing specimens at 773 K in a salt bath. Macroscopic observation on cross section of joints revealed that shape of nugget changes depending on welding condition, and voids are observed for the conditions of both higher and lower rotation speeds. Wider range of optimum condition for sound joint with no voids was obtained when 6061Al alloy was fixed on the advancing side. Onion rings were observed to consist of alternative array of two diffrent alloy regions, indicating heterogeneous distribution of constituents although grain size in eack region was almost the same. SEM-EDS analysis clearly showed heterogeneous distribution of Mg in nugget. TEM observation supported the heterogeneity of microstructure by showing two distinct regions with and without precipitates. High temperature annealing at 773 K yielded abnormal grain growth in the nugget. This phenomenon was discussed in relation to the diffusion of each constituent．

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Numerical analysis was carried out to estimate transient temperature characteristic in friction welding of two similar materials of 6061 aluminum alloys. A method used in this study is a practical approach to combine a friction heat input model with a non-steady heat conduction analysis using a finite element method. In this analysis, data of aluminum alloy were used as material property such as specific heat, thermal conductivity. Experiments of friction welding were performed to examine the validity of calculated results. The measured values during the friction welding process were the fundamental characteristics such as rotation speed, pressure, torque and temperature. Transient temperature distribution was calculated using the friction pressure, the upset pressure and the rotation speed. The calculated results were compared with the experimental results about friction heat input and temperature characteristics. As a result, it was confirmed that estimation of transient temperature distribution in a whole friction process was practicable. In addition, it was feasible to simulate the transient friction heat input by incorporating a concept of time lag of first order into the friction heat input model. The maximum temperature distribution near the friction surface was useful in investigating the heat affected zone．

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