In this study, microstructures and mechanical properties of friction stir welded Gost5632 duplex stainless steels (D) were investigated. Friction stir welding (FSW) was applied to a 1.5 mm thick Gost5632 D plate. Different speciments produced by employing rotation speeds of 400, 600 and 800 rpm and welding speeds of 50, 100 and 150 mm/min. The welding tool was made of a WC-based material, had a shoulder diameters, and the pin length of 16 and 1.25mm, respectively. The weld thermal cycles were determined through thermocouples. The measured temperature at maximum heat input (800 rpm and 50 mm/min) showed that the peak temperature was about 1000?C. Microstructures of the weld area were examined by optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) analysis system. Identity of phases was taken by X-ray diffraction test, vickers microhardness tests at a load of 300 g and mechanical properties by tensile tests were performed. Fractugraphy of tensile samples were investigated finally. Results of XRD showed any harmful phase for example sigma, chi and etc. The reason of this phenomene was high cooling rate of samples during welding. The microstructure features can be separated into three broad regions including the stir zone (SZ), the thermo-mechanically affected zone (TMAZ), and the base metal (BM). Because of severe deformation of material near rotation tool, the grain size of the weld area was reduced. It is generally known that reconstruction of fine and equiaxed grain in the SZ is a result of dynamic recrystallization. Minimum grain size was determined about 1 ?m for 600 rpm rotation and 150 mm/min welding speed. Advancing side of weldment was incurred maximum deformation according to flow materials during FSW. Because of this reason grain size of this region was finer than the other regions. The hardness within the SZs in all conditions of present study was higher than the base metal. The increase of hardness in the stir zone suggests that the hardness profile may be related to the small grain sizes of both phases in the weld zone. The hardness of BM was 284 HV whereas the mean hardness of SZ was about 423 HV. The tensile strength of the weldment (774MPa) was higher than those of BM (680 MPa). The fracture region of all samples were in BM, however fracture was occurred in interfaces of SZ/BM for almost condition of welding. The fracture surface of the weldment was indicated characteristics of ductile dimple fracture except the one with 400 rpm rotation and 50 mm/min welding speed. The results showed that microstructures and mechanical properties of the duplex stainless steel joints were significantly affected by welding conditions. The results of microstructure, tempreture profile, X-ray diffraction and mechanical properties were defined 600 rpm rotation and 100 mm/min welding speed as the optimum conditions for FSW of Gost 5632 DSS. Key word: Friction stir welding, Gost5632 lean duplex stainless steel, Recrystallization dynamic.
