Investigation the effect of friction stir welding on modification microstructure and mechanical properties of 5052 aluminum alloy welded by gas tungsten arc welding Fusion welding methods like gas-tungsten arc welding (GTAW) have high heat input which can cause problems like porosity in the solidified weld metal and grain growth in the heat affected zone and hence deterioration of the mechanical properties of fusion welded joints. Friction stir processing (FSP) is an effective method to modify the microstructure and mechanical properties of fusion welded joints. In the present study, an Al5052 sheet with the thickness of 2 mm was first welded by GTAW method with AC current parallel to rolling direction. Then, FSP was employed to modify the as-cast microstructure of weld metal and improve the mechanical properties of welded joints. Parameters investigated in the FSP were tool rotational speed (800, 1250 and 1600 rpm) and tool traverse speed (12, 25 and 40 mm/min). Among the joints modified with these parameters, the sample modified with the rotational speed of 800 rpm and traverse speed of 25 mm/min exhibited the best mechanical properties. Results of microstructural studies performed by optical microscopy and scanning electron microscopy, demonstrated that modification of GTA welded joints by FSP leads to the refinement of the dendritic microstructure of GTAW process, and dissolution of precipitates and grain refinement in the stir zone. The hardness of base metal and weld metal after GTAW were respectively 60 and 70 HV, which increased to 95 HV after modification with the traverse speed of 25 mm/min and rotational speed of 800 rpm. Tensile test results showed that the elongation for the base metal and GTA welded joint were respectively 16 and 17.5 % perpendicular to the rolling direction, which increased to 24% after FSP with the traverse speed of 25 mm/min and rotational speed of 800 rpm. Moreover, the value of elongation for the base metal and GTA welded joint were respectively 15 and 21 % in the rolling direction, which increased to 31% after FSP with the traverse speed of 25 mm/min and rotational speed of 800 rpm. Fractography of the fractured surfaces of all samples showed the presence of dimples which indicated that the major fracturing mechanism was ductile. The dimples were finer in the modified samples compared with those observed in the fractured surfaces of base metal and GTA welded specimens. Keywords: Al 5052 alloy, Gas tungsten are welding, Friction stir processing, Microstructure, Mechanical properties, Fractography.
