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INTRODUCTION

RELIABLE joints with high strength, long fatigue life, and substantial fracture resistance are vital to the aerospace and the automobile industries that use aluminum alloys. Heat treatable aluminum alloys of 2xxx, 6xxx, and 7xxx series are widely used in these industries.[1] This is due to their high strength to weight ratio, good formability, and acceptable corrosion resistance.[2] All parts of a perfect weld need to have the same properties as the base metal. Conventional welding processes suffer from such defects as distortion, gas porosity, residual stress, dendritic brittle structure, lack of fusion, oxide inclusions, hot cracking, and hot tearing of the weld.[2] Reduction of yield stress, ultimate tensile strength,[3] corrosion resistance,[4] and electrical resistivity in the heat-affected zone (HAZ) region[5] are some common consequences of using traditional welding processes. The applied alloy may lose its mechanical strength due to precipitate dissolution[6] and softening heat effects.[7] Gas tungsten arc welding (GTAW) is traditionally used for joining of the aluminum alloys. 6061 aluminum alloys containing magnesium and silicon have low weldability. Microstructure of the GTAW sample usually includes coarse columnar grains similar to the cast samples. According to previous reports, mechanical properties such as tensile[8] and fatigue properties of these alloys have sensibly reduced after pulsed GTAW.

Also, according to previous investigations, GTAW has caused significant lowering of metal properties like resistance to corrosion in the welded region of the 6061 aluminum alloy as compared to the base metal.[9] Friction stir welding (FSW) is a solid state joining process applicable to Al alloys. As a result of the friction stirring and the movement of the materials due to the welding, a sever deformation zone forms along the centerline of the weld.[10] The weld microstructure consists, hence, of three regions: (a) stirred (nugget) zone, (b) thermo-mechanically affected zone (TMAZ), and (c) HAZ.[11] According to reports, UTS of the joint by FSW is ~80 pct, while that of GTAW is ~67 pct of the base metal.[12] A linear regression relationship was founded between grain size of the weld nugget of FSW and tensile strength in welded 6061 aluminum samples.[13] As Al alloys are of great significance in the industrial applications, finding a sound welding technique for them is indispensable. A comparison of the FSW and the GTAW, as two common welding techniques for these alloys, on morphology and mechanical properties of the aluminum alloys is the key to the best selection of the appropriate welding method. This paper reports on the effects of the FSW as compared to the GTAW process on the microstructure and the mechanical properties of the Al6061 alloy for better achievement of the desirable welded metal properties.

II.

V. FAHIMPOUR, Graduate Student, and S.K. SADRNEZHAAD, Professor, are with the Department of Materials Science and Engineering, Sharif University of Technology (SUT), P.O. Box 11365-9466, Tehr