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Abstract
Friction stir welding of 5083-H321 aluminum alloy using a drilling machine with 1400 rpm maximum rotation speed and 16 mm/min minimum traveling speed is studied. Test specimens were fixed on the table of the drilling machine by a specially designed fixture. The test is carried out at different rotation and traveling speeds, different tool designs, and various insertion depths of the tool shoulder, to determine the suitable friction stir welding conditions. A schedule for the determination of suitable FSW conditions is postulated. The weld joint is evaluated using visual inspection, tension test, hardness test, optical microscopy, and scanning electron microscope. Corrosion resistance of welded samples is also studied.
Preliminary experiments indicate the presence of tunnel-like defect through the welded joint. The cross sectional area of the defect is taken as a measure of the quality of the welded joint. It is found that at a certain rotation speed there is a limiting traveling speed above which no friction stir welding occurs. At 900, 1120, and 1400 rpm rotation speeds, no welding occurs at traveling speeds higher than 40, 63, and 63 mm/min, respectively. Minimum cross-section area of the tunnel-like defect is obtained at high values of the ratio (Rotation Speed / Traveling Speed), i.e., at rotation speed of 1400 rpm and traveling speed of 16 mm/min. Changing shoulder profile from flat to completely grooved decreases the cross-section area to a minimum value. Shoulder plunge depth is changed from 0.2 mm and reach to 0.6 mm. the tunnel-like defect disappears when 0.4 mm shoulder plunge depth is used.
Microstructure investigation of the sound joint reveals a very fine grain size in the nugget zone. Scanning electron micrographs indicate the fragmentation of different phases found in the nugget zone when compared with the base alloy. EDS analysis indicate the presence of Al6(Mn, Fe) particles. Electro back scatter diffraction (EBSD) technique reveals the presence of a subgrain structure in the nugget zone, i.e., the occurrence of dynamic recrystallization. Moreover, the percentage of low angle boundaries is less in the nugget zone than in the thermomechanical zone. This is may be attributed to geometric dynamic recrystallization. However, in the thermomechanical zone limiting area near the heat affected zone static recrystallization takes place due to its stored energy during cold rolling. Static recrystallization leads to equiaxed grains.
The tensile strength of the friction stir welded joint is higher than that using the conventional fusion welding technique. The Hardness distribution profile through the cross-section of the weld joint shows a decrease in the hardness values in the weld zone compared with the parent alloy. The variation in hardness distribution in the weld joint made from present cold worked 5083 alloy (5083-H321) is attributed to the difference in dislocation density between the nugget zone (in 5083-O condition) and the base alloy.
The weld zone has lower corrosion rate than the parent alloy. Also, it is observed that the weld zone and the base alloy are not susceptible to intergranular corrosion, but the weight loss of the weld zone is less than that of the base alloy.